feat(tools): h5rs, pure-Rust HDF5 tools (ls, dump, stat, diff, check)

New workspace crate clawhdf5-tools with one binary, h5rs, built only on the
clawhdf5 facade and clawhdf5-format (no libhdf5, no C):

- ls [-r] [-v] FILE[/path]: h5ls's listing (same text in its first two
  columns) plus the datatype; -v adds address, link count, layout and chunk
  index, chunk size, storage, filters, datatype and attributes.
- dump [--json] [-A] [-p] [-d PATH] FILE: h5dump DDL (byte-identical to
  h5dump 1.14.6 on the test files) or hdf5-json.
- stat FILE: h5stat's object/link/rank/layout/filter/attribute counts, raw
  data and total size.
- diff [-r] [-q] [-d D] [-p R] A B [OBJ1 [OBJ2]]: structural and value
  differences, exit 0/1/2 like h5diff.
- check [--data] FILE: walks every object, parses every message, verifies
  the checksums of every v2+ structure (including the fractal heap blocks
  the library never checks), checks chunk indexes against their datasets
  and raw data for out-of-file or overlapping extents; every problem with
  its address.

Values over --max-bytes are reported, not read; dense-storage heaps are
verified before objects are read from them; panics are caught (exit 3).
Tests compare with h5ls, h5stat, h5dump and h5diff and with h5py's values,
and flip the checksum of every checksummed structure in a v1.14-format file.

Co-Authored-By: Claude Opus 5.5 (1M context) <[email protected]>

Cargo.toml

@@ -16,6 +16,7 @@ members = [
     "crates/clawhdf5-cli",
     "crates/clawhdf5-napi",
     "crates/clawhdf5-bench",
+    "crates/clawhdf5-tools",
     "crates/libaec-sys",
 ]
 resolver = "2"

crates/clawhdf5-tools/Cargo.toml

@@ -0,0 +1,23 @@
+[package]
+name = "clawhdf5-tools"
+version = "2.7.0"
+edition = "2024"
+rust-version.workspace = true
+license = "MIT"
+description = "h5rs: pure-Rust HDF5 command-line tools (ls, dump, stat, diff, check) without libhdf5"
+repository = "https://git.redclaw.dev/quantumclaw/clawhdf5"
+keywords = ["hdf5", "h5dump", "h5ls", "cli", "science"]
+categories = ["command-line-utilities", "science"]
+readme = "README.md"
+
+[[bin]]
+name = "h5rs"
+path = "src/main.rs"
+
+[dependencies]
+clawhdf5 = { path = "../clawhdf5", version = "2.7.0" }
+clawhdf5-format = { path = "../clawhdf5-format", version = "2.7.0" }
+serde_json = "1"
+
+[dev-dependencies]
+tempfile = { workspace = true }

crates/clawhdf5-tools/README.md

@@ -0,0 +1,231 @@
+# clawhdf5-tools: `h5rs`
+
+HDF5 command-line tools in pure Rust, built only on the `clawhdf5` facade and
+`clawhdf5-format`. No libhdf5 and no C code, so the binary also builds as a
+fully static executable: `cargo build --release -p clawhdf5-tools --target
+x86_64-unknown-linux-musl` gives a static-pie `h5rs` of 1.8 MB with no
+shared-library dependencies (built and run on tank, 2026-09-26, Rust 1.98).
+
+| Command | Modelled on | What it does |
+|---------|-------------|--------------|
+| `h5rs ls` | `h5ls` | list objects: name, kind, shape, datatype; `-v` adds layout, chunking, storage, filters, attributes |
+| `h5rs dump` | `h5dump` | the file's structure and values as DDL text, or as JSON (hdf5-json layout) |
+| `h5rs stat` | `h5stat` | object, link, rank, layout, filter and attribute counts; raw-data and file size |
+| `h5rs diff` | `h5diff` | structural and value differences between two files or objects |
+| `h5rs check` | `h5check` | structural validator: every object and header message, the checksums of version 2+ structures, chunk index consistency |
+
+```bash
+cargo install --path crates/clawhdf5-tools     # or: cargo build --release -p clawhdf5-tools
+h5rs --help
+h5rs <command> --help
+```
+
+Every command takes `--max-bytes N` where it reads values (default 1 GiB): a
+dataset whose dataspace claims more than that is reported instead of read, so
+a corrupt size cannot exhaust memory.
+
+## `h5rs ls`
+
+```console
+$ h5rs ls -r data.h5            # an extract of the output
+/                        Group
+/external                External Link {other.h5//x}
+/grp                     Group
+/grp/ext2                Dataset {6/Inf, 10/Inf} float32
+/grp/gz                  Dataset {1000} int32
+/hard2                   Group, same as /grp/sub
+/named_t                 Type
+/soft                    Soft Link {/contig}
+```
+
+The first two columns are h5ls's (`h5ls -r` prints the same text); h5rs adds
+the datatype. `FILE/path` lists a group's members or one dataset, as h5ls
+does. `-v` prints, per object:
+
+```console
+$ h5rs ls -v data.h5/grp/gz
+gz                       Dataset {1000/1000}
+    Address:   1120
+    Links:     1
+    Layout:    chunked (fixed array index)
+    Chunks:    {100} 400 bytes
+    Storage:   4000 logical bytes, 1237 allocated bytes, 323.36% utilization
+    Filter-0:  shuffle-2 OPT {4}
+    Filter-1:  deflate-1 OPT {4}
+    Filter-2:  fletcher32-3
+    Type:      32-bit little-endian integer
+```
+
+## `h5rs dump`
+
+```console
+$ h5rs dump data.h5            # DDL, like h5dump
+$ h5rs dump -A data.h5         # no dataset values (attributes still shown), like h5dump -A
+$ h5rs dump -d /grp/gz data.h5 # one dataset
+$ h5rs dump -p data.h5         # also STORAGE_LAYOUT and FILTERS blocks
+$ h5rs dump --json data.h5     # hdf5-json
+```
+
+The DDL output is h5dump's: on the test files of `tests/gen_files.py`
+(compact, contiguous and chunked datasets with every chunk index; v1 and v2
+groups; integers of both byte orders, floats, compound with an array member,
+enum, fixed and variable-length strings; soft, external and hard links; a
+named datatype; compact and dense attributes) `h5rs dump` and `h5rs dump -A`
+print the same bytes as h5dump 1.14.6 — `dump_matches_h5dump` in
+`tests/h5rs_interop.rs` checks this. Not covered by that test: references,
+opaque, bitfield, variable-length sequences and virtual datasets. Floats print at their own precision (a `float32` 0.1 prints as
+`0.1`), which for some values is more digits than h5dump's `%g`. The `-p`
+block is h5rs's own (it names the chunk index), not h5dump's.
+
+### JSON schema
+
+`--json` follows the HDF Group's [hdf5-json](https://github.com/HDFGroup/hdf5-json)
+layout:
+
+```json
+{
+  "apiVersion": "1.1.1",
+  "root": "g-0000000000000060",
+  "groups":    { "<id>": { "alias": ["/grp"], "attributes": [...], "links": [...] } },
+  "datasets":  { "<id>": { "alias": [...], "attributes": [...], "shape": {...},
+                           "type": {...}, "creationProperties": {...}, "value": ... } },
+  "datatypes": { "<id>": { "alias": [...], "attributes": [...], "type": {...} } }
+}
+```
+
+- **ids** are `g-`/`d-`/`t-` plus the object header address in 16 hex digits
+  (hdf5-json uses UUIDs; these are stable for a given file). `alias` lists
+  every path that reaches the object.
+- **links**: `{"class": "H5L_TYPE_HARD", "title", "collection", "id"}`,
+  `{"class": "H5L_TYPE_SOFT", "title", "h5path"}`,
+  `{"class": "H5L_TYPE_EXTERNAL", "title", "file", "h5path"}`,
+  `{"class": "H5L_TYPE_USER_DEFINED", "title", "linkClass"}`.
+- **shape**: `{"class": "H5S_NULL"}`, `{"class": "H5S_SCALAR"}` or
+  `{"class": "H5S_SIMPLE", "dims": [...], "maxdims": [...]}` with
+  `"H5S_UNLIMITED"` for an unlimited dimension.
+- **type**: `{"class": "H5T_INTEGER" | "H5T_FLOAT" | "H5T_BITFIELD", "base":
+  "H5T_STD_I32LE" ...}`, `{"class": "H5T_STRING", "charSet", "strPad",
+  "length": n | "H5T_VARIABLE"}`, `{"class": "H5T_COMPOUND", "fields": [{"name",
+  "type"}]}`, `{"class": "H5T_ARRAY", "base", "dims"}`, `{"class": "H5T_ENUM",
+  "base", "mapping": {"NAME": value}}`, `{"class": "H5T_VLEN", "base"}`,
+  `{"class": "H5T_OPAQUE", "size", "tag"}`, `{"class": "H5T_REFERENCE", "base":
+  "H5T_STD_REF_OBJ" | "H5T_STD_REF_DSETREG" | "H5T_STD_REF"}`.
+- **value**: nested lists in the dataset's shape (a scalar is the bare value,
+  a null dataspace `null`). A compound element is a list of its members, an
+  enum element its integer value, a string a JSON string, opaque/bitfield data
+  a `0x...` hex string, an object reference the referenced object's path,
+  NaN/infinities the strings `"NaN"`, `"Infinity"`, `"-Infinity"`, and an
+  integer beyond 64 bits a decimal string.
+- **creationProperties**: `layout` (`{"class": "H5D_CHUNKED", "dims": [...]}`
+  etc.) and `filters` (`[{"id", "name", "class", "parameters"}]`).
+
+A value that cannot be read is replaced by `"value_error": "<reason>"` and
+the command exits 1.
+
+## `h5rs stat`
+
+Prints h5stat's report sections with the same labels for the facts it
+computes — object and link counts, max links to an object, max objects in a
+group, dataset ranks, layout counts, filter counts, attribute counts, total
+raw data size and total file size (all equal to h5stat's on the test files;
+`stat_matches_h5stat` checks them). It does not break metadata space down by
+structure as h5stat does; it reports metadata and free space as one figure.
+
+## `h5rs diff`
+
+```console
+$ h5rs diff a.h5 b.h5                # whole files
+$ h5rs diff a.h5 b.h5 /grp           # one object and everything below it
+$ h5rs diff a.h5 b.h5 /x /y          # different paths in each
+$ h5rs diff -r a.h5 b.h5 /d          # list every differing element
+$ h5rs diff -d 0.001 a.h5 b.h5       # |a - b| > 0.001 is a difference
+$ h5rs diff -p 0.01 a.h5 b.h5        # |a - b| / |a| > 1% is a difference
+```
+
+Exit status: 0 no differences, 1 differences, 2 error — the same as h5diff's
+on the cases `diff_exit_codes_match_h5diff` runs. Compared: which objects
+exist, their kinds, datatypes and shapes, attribute sets and values, dataset
+values, and soft/external link targets. Two differences from h5diff, on
+purpose: objects that cannot be compared (different shapes or datatype
+classes) count as a difference (h5diff warns and exits 0), and two NaNs are
+equal.
+
+## `h5rs check`
+
+```console
+$ h5rs check data.h5
+checked data.h5: superblock v3, 33 objects (4 groups, 28 datasets, 1 named datatypes), 139 header messages, 34 chunks
+checksums verified: superblock 1, v2 object headers 33, v2 B-trees 3, fractal heaps 3 (+3 blocks), chunk indexes 5
+no problems found
+
+$ h5rs check damaged.h5          # one bit flipped in /grp/ext1's chunk index header
+problem: 0x79c /grp/ext1: chunk index (extensible array): checksum mismatch: expected 0xcfe2391f, computed 0x2dd59bed
+checked damaged.h5: superblock v3, 33 objects (4 groups, 28 datasets, 1 named datatypes), 139 header messages, 27 chunks
+checksums verified: superblock 1, v2 object headers 33, v2 B-trees 3, fractal heaps 3 (+3 blocks), chunk indexes 4
+1 problem found
+```
+
+Each problem line is `problem: <address> <object path>: <what>`; addresses
+are HDF5 addresses (relative to the superblock, as h5dump and h5ls print
+them).
+
+It walks every object reachable from the root group (and the superblock
+extension) and checks:
+
+- the superblock (and its checksum, version 2+) and that the file is not
+  shorter than the superblock's end-of-file address;
+- every object header, with the checksum of version 2 headers and of their
+  continuation chunks, and every header message parsed by type (dataspace,
+  datatype, fill value, layout, filter pipeline, attributes, link info,
+  links, group info, symbol table), including shared messages;
+- groups: the symbol table (v1 B-tree, local heap, symbol nodes), or the
+  links, and for dense storage the fractal heap — header, and every direct
+  and indirect block with its checksum, back-pointer and heap offset — and
+  the v2 B-tree name and creation-order indexes (every node's checksum, and
+  the record count against the header's);
+- dense attribute storage the same way;
+- datasets: the layout against the dataspace and datatype (compact and
+  contiguous sizes, chunk rank), and for chunked datasets the whole chunk
+  index (v1 B-tree, single chunk, implicit, fixed array, extensible array,
+  v2 B-tree, with the checksums of the last three): every chunk's offset must
+  be a multiple of the chunk size and inside the extent, appear once, and
+  have a plausible size;
+- that all raw data (contiguous blocks and chunks) lies inside the file and
+  no two pieces overlap.
+
+`--data` also reads every dataset, decoding every chunk through its filters
+(which catches corrupt compressed data and Fletcher-32 mismatches). Data the
+tool cannot decode (a filter it does not implement, such as szip, or a
+dataset over `--max-bytes`) is a `note:`, not a problem. Every problem is
+printed with the address of the structure involved; the exit status is 0
+when there are none, 1 when there are, 2 for a usage error or a missing
+file.
+
+libhdf5's h5check understands only the HDF5 1.8 file format; `h5rs check`
+also covers the structures HDF5 1.10+ writes (fixed/extensible array and v2
+B-tree chunk indexes, and version 3 superblocks).
+
+What it does not check: free-space manager and shared-message (SOHM) table
+checksums, global heap collections other than those a value read touches,
+and objects reachable only by external links.
+
+## Robustness
+
+A panic is a bug: `h5rs` catches it, prints `internal error`, and exits 3
+(`check` records it against the object and carries on). `scripts/h5rs-fuzz.sh`
+runs every subcommand over every file of a corpus (by default the HDF Group's
+CVE reproducers, fetched by `conformance/fetch-corpus.sh`), optionally with
+byte-flipped copies (`MUTATE=N`), under a timeout and a memory limit, with
+overflow checks on, and fails on any panic, crash or hang.
+`scripts/h5rs-check-ok-files.sh` runs `check --data` over the conformance
+files that both clawhdf5 and h5py read in full, which must all pass.
+
+## Tests
+
+```bash
+CLAWHDF5_PYTHON=.venv/bin/python CLAWHDF5_REQUIRE_INTEROP=1 cargo test -p clawhdf5-tools
+```
+
+The interop tests write their files with h5py and compare with h5ls, h5stat,
+h5dump and h5diff; each skips when what it needs is missing unless
+`CLAWHDF5_REQUIRE_INTEROP=1`.

crates/clawhdf5-tools/src/check.rs

@@ -0,0 +1,825 @@
+//! `h5rs check`: a structural validator.
+//!
+//! It walks every object reachable from the root group (and the superblock
+//! extension), parses every header message, verifies the checksum of every
+//! checksummed (version 2+) structure it meets, checks each chunked
+//! dataset's chunk index against the dataset's shape, and checks that raw
+//! data lies inside the file without overlapping other raw data. Every
+//! problem is reported with the address of the structure involved.
+
+use std::collections::HashSet;
+use std::panic::{self, AssertUnwindSafe};
+
+use clawhdf5_format::attribute_info::AttributeInfoMessage;
+use clawhdf5_format::btree_v2::{BTreeV2Header, collect_btree_v2_records};
+use clawhdf5_format::data_layout::DataLayout;
+use clawhdf5_format::dataspace::{Dataspace, DataspaceType};
+use clawhdf5_format::datatype::Datatype;
+use clawhdf5_format::group_info::GroupInfoMessage;
+use clawhdf5_format::link_info::LinkInfoMessage;
+use clawhdf5_format::message_type::MessageType;
+use clawhdf5_format::object_header::ObjectHeader;
+use clawhdf5_format::symbol_table::SymbolTableMessage;
+
+use crate::cli::{Args, Out};
+use crate::h5::{Error, ErrorKind, H5, Kind};
+use crate::info::{self, DsInfo};
+
+pub const USAGE: &str = "\
+usage: h5rs check [--data] [-q] [--max-bytes N] FILE
+
+Validate FILE's structure: walk every object from the root group, parse
+every header message, verify the checksums of version 2+ structures
+(superblock, object headers and continuation chunks, v2 B-tree nodes,
+fractal heap headers and blocks, extensible/fixed array chunk indexes),
+check each chunked dataset's chunk index against its shape (offsets aligned
+to the chunk size and inside the extent, no duplicates, sizes plausible),
+and check that all raw data lies inside the file without overlaps. Every
+problem is printed with the address of the structure involved.
+
+  --data          also read every dataset, decoding every chunk through its
+                  filters (catches corrupt compressed data and Fletcher-32
+                  mismatches)
+  -q, --quiet     print only the problems, not the summary
+  --max-bytes N   largest dataset read by --data (default 1 GiB)
+
+Exit status: 0 no problems, 1 problems found, 2 usage error or file not
+found, 3 internal error.";
+
+const MAX_CHUNKS_CHECKED: usize = 10_000_000;
+
+#[derive(Default)]
+struct Counts {
+    objects: u64,
+    groups: u64,
+    datasets: u64,
+    datatypes: u64,
+    messages: u64,
+    chunks: u64,
+    datasets_read: u64,
+    sb_checksum: u64,
+    ohdr_v2: u64,
+    btree_v2: u64,
+    heaps: u64,
+    heap_block_checksums: u64,
+    chunk_index_checksummed: u64,
+}
+
+struct Problem {
+    addr: u64,
+    path: String,
+    msg: String,
+}
+
+struct Checker<'a> {
+    h5: &'a H5,
+    read_data: bool,
+    eof: u64,
+    problems: Vec<Problem>,
+    /// Things not checked, which are not problems with the file.
+    notes: Vec<Problem>,
+    counts: Counts,
+    /// Raw data extents: (start, end, owner path).
+    extents: Vec<(u64, u64, String)>,
+    heaps_seen: HashSet<u64>,
+    btrees_seen: HashSet<u64>,
+    panicked: bool,
+}
+
+pub fn run(args: &mut Args, out: &mut Out) -> std::io::Result<i32> {
+    let mut read_data = false;
+    let mut quiet = false;
+    let mut max_bytes = None;
+    let mut file = None;
+    while let Some(a) = args.next() {
+        match a.as_str() {
+            "--data" => read_data = true,
+            "-q" | "--quiet" => quiet = true,
+            "--max-bytes" => match args.number() {
+                Some(n) => max_bytes = Some(n),
+                None => return args.usage_error(out, "--max-bytes needs a number", USAGE),
+            },
+            "-h" | "--help" => {
+                writeln!(out.o, "{USAGE}")?;
+                return Ok(0);
+            }
+            s if s.starts_with('-') && s.len() > 1 => {
+                return args.usage_error(out, &format!("unknown option {a}"), USAGE);
+            }
+            _ if file.is_none() => file = Some(a),
+            _ => return args.usage_error(out, &format!("unexpected argument {a}"), USAGE),
+        }
+    }
+    let Some(file) = file else {
+        return args.usage_error(out, "missing FILE", USAGE);
+    };
+    let path = std::path::Path::new(&file);
+    if !path.is_file() {
+        writeln!(out.e, "h5rs check: {file}: no such file")?;
+        return Ok(2);
+    }
+    let mut h5 = match H5::open(path) {
+        Ok(h) => h,
+        Err(_) => return unopenable(path, out),
+    };
+    if let Some(m) = max_bytes {
+        h5.max_bytes = m;
+    }
+    let mut c = Checker {
+        h5: &h5,
+        read_data,
+        eof: h5.data().len() as u64,
+        problems: Vec::new(),
+        notes: Vec::new(),
+        counts: Counts::default(),
+        extents: Vec::new(),
+        heaps_seen: HashSet::new(),
+        btrees_seen: HashSet::new(),
+        panicked: false,
+    };
+    c.superblock();
+    c.objects();
+    c.overlaps();
+    for p in &c.problems {
+        writeln!(out.o, "problem: {:#x} {}: {}", p.addr, p.path, p.msg)?;
+    }
+    if !quiet {
+        for n in &c.notes {
+            writeln!(out.o, "note: {:#x} {}: {}", n.addr, n.path, n.msg)?;
+        }
+    }
+    if !quiet {
+        c.summary(&file, out)?;
+    }
+    Ok(if c.panicked {
+        3
+    } else if c.problems.is_empty() {
+        0
+    } else {
+        1
+    })
+}
+
+/// The file could not be opened at all: say why, as precisely as possible.
+fn unopenable(path: &std::path::Path, out: &mut Out) -> std::io::Result<i32> {
+    let data = match std::fs::read(path) {
+        Ok(d) => d,
+        Err(e) => {
+            writeln!(out.e, "h5rs check: {}: {e}", path.display())?;
+            return Ok(2);
+        }
+    };
+    let msg = match clawhdf5_format::signature::find_signature(&data) {
+        Err(_) => (
+            0,
+            "no HDF5 signature at offset 0 or any power of two from 512".to_string(),
+        ),
+        Ok(off) => match clawhdf5_format::superblock::Superblock::parse(&data[off..], 0) {
+            Ok(_) => (off as u64, "file cannot be opened".to_string()),
+            Err(e) => (off as u64, format!("superblock: {e}")),
+        },
+    };
+    writeln!(out.o, "problem: {:#x} /: {}", msg.0, msg.1)?;
+    writeln!(
+        out.o,
+        "checked {}: 1 problem found (file cannot be opened)",
+        path.display()
+    )?;
+    Ok(1)
+}
+
+fn msg_name(t: MessageType) -> String {
+    match t {
+        MessageType::Unknown(n) => format!("message type {n:#x}"),
+        t => format!("{t:?} message"),
+    }
+}
+
+impl Checker<'_> {
+    fn problem(&mut self, addr: u64, path: &str, msg: impl Into<String>) {
+        self.problems.push(Problem {
+            addr,
+            path: path.to_string(),
+            msg: msg.into(),
+        });
+    }
+
+    fn err(&mut self, default_addr: u64, path: &str, e: &Error) {
+        // Reading attributes or links fails on a damaged heap or B-tree that
+        // the structure checks have already reported at the same address.
+        if let Some(a) = e.addr
+            && self.problems.iter().any(|p| p.addr == a && p.path == path)
+        {
+            return;
+        }
+        self.problem(e.addr.unwrap_or(default_addr), path, e.msg.clone());
+    }
+
+    /// Run `f`; a panic inside it becomes a problem instead of aborting the
+    /// whole check.
+    fn guarded(&mut self, addr: u64, path: &str, f: impl FnOnce(&mut Self)) {
+        let r = panic::catch_unwind(AssertUnwindSafe(|| f(self)));
+        if r.is_err() {
+            self.panicked = true;
+            self.problem(
+                addr,
+                path,
+                "internal error while checking this object (see stderr)",
+            );
+        }
+    }
+
+    fn superblock(&mut self) {
+        let sb = self.h5.sb().clone();
+        if sb.version >= 2 {
+            // Superblock::parse verified it, or the file would not be open.
+            self.counts.sb_checksum += 1;
+        }
+        // libhdf5 stores the end-of-file address counting the user block
+        // (h5py's `userblock_size` files show it), unlike every other
+        // address, which is relative to the superblock.
+        let file_len = self.eof.saturating_add(self.h5.file.user_block_size());
+        if sb.eof_address > file_len {
+            self.problem(
+                0,
+                "/",
+                format!(
+                    "file is truncated: the superblock's end-of-file address is {:#x} but the file \
+                     is {file_len:#x} bytes long",
+                    sb.eof_address
+                ),
+            );
+        }
+        if sb.root_group_address >= self.eof {
+            self.problem(
+                0,
+                "/",
+                format!(
+                    "root group address {:#x} is past the end of the file",
+                    sb.root_group_address
+                ),
+            );
+        }
+        let undef = if sb.offset_size >= 8 {
+            u64::MAX
+        } else {
+            (1u64 << (8 * u32::from(sb.offset_size))) - 1
+        };
+        if let Some(ext) = sb.superblock_extension_address.filter(|&a| a != undef) {
+            self.guarded(ext, "<superblock extension>", |c| match c.h5.header(ext) {
+                Ok(h) => {
+                    if h.version >= 2 {
+                        c.counts.ohdr_v2 += 1;
+                    }
+                    c.messages(ext, "<superblock extension>", &h);
+                }
+                Err(e) => c.err(ext, "<superblock extension>", &e),
+            });
+        }
+    }
+
+    fn objects(&mut self) {
+        let h5 = self.h5;
+        // Collect first, then check: the walk borrows the file, the checks
+        // borrow `self` mutably.
+        let (items, walk) = h5.walk_collect();
+        if let Err(e) = walk {
+            self.err(h5.root(), "/", &e);
+        }
+        for it in items {
+            let path = it.path;
+            // Soft, external and user-defined links have no header here.
+            let (Some(addr), None, Some(header)) = (it.addr, it.first_path, it.header) else {
+                continue;
+            };
+            if addr >= self.eof {
+                self.problem(
+                    addr,
+                    &path,
+                    "object header address is past the end of the file",
+                );
+                continue;
+            }
+            let h = match header {
+                Ok(h) => h,
+                Err(e) => {
+                    self.err(addr, &path, &e);
+                    continue;
+                }
+            };
+            self.counts.objects += 1;
+            self.guarded(addr, &path, |c| c.object(addr, &path, &h));
+        }
+    }
+
+    fn object(&mut self, addr: u64, path: &str, h: &ObjectHeader) {
+        if h.version >= 2 {
+            self.counts.ohdr_v2 += 1;
+        }
+        self.messages(addr, path, h);
+        match self.h5.attributes(h) {
+            Ok((_, errs)) => {
+                for e in errs {
+                    self.problem(addr, path, format!("attribute: {e}"));
+                }
+            }
+            Err(e) => self.err(addr, path, &e),
+        }
+        let is_root = addr == self.h5.root();
+        match Kind::of(h) {
+            Kind::Group => {
+                self.counts.groups += 1;
+                self.group(addr, path, h);
+            }
+            Kind::Dataset => {
+                self.counts.datasets += 1;
+                self.dataset(addr, path, h);
+            }
+            Kind::Datatype => self.counts.datatypes += 1,
+            Kind::Unknown if is_root => self.counts.groups += 1,
+            Kind::Unknown => {
+                self.problem(
+                    addr,
+                    path,
+                    "object header describes no group, dataset or named datatype",
+                );
+            }
+        }
+    }
+
+    /// Parse every message of a header by type.
+    fn messages(&mut self, addr: u64, path: &str, h: &ObjectHeader) {
+        let (os, ls) = (self.h5.os(), self.h5.ls());
+        for m in &h.messages {
+            self.counts.messages += 1;
+            let data =
+                match clawhdf5_format::shared_message::message_data(self.h5.data(), m, os, ls) {
+                    Ok(d) => d.into_owned(),
+                    Err(e) => {
+                        self.problem(addr, path, format!("shared {}: {e}", msg_name(m.msg_type)));
+                        continue;
+                    }
+                };
+            let r: Result<(), String> = match m.msg_type {
+                MessageType::Dataspace => Dataspace::parse(&data, ls)
+                    .map(drop)
+                    .map_err(|e| e.to_string()),
+                MessageType::Datatype => {
+                    Datatype::parse(&data).map(drop).map_err(|e| e.to_string())
+                }
+                MessageType::FillValue | MessageType::FillValueOld => {
+                    let mut mm = m.clone();
+                    mm.data = data.clone();
+                    clawhdf5_format::fill_value::parse_fill_value(&mm)
+                        .map(drop)
+                        .map_err(|e| e.to_string())
+                }
+                MessageType::DataLayout => DataLayout::parse(&data, os, ls)
+                    .map(drop)
+                    .map_err(|e| e.to_string()),
+                MessageType::FilterPipeline => {
+                    clawhdf5_format::filter_pipeline::FilterPipeline::parse(&data)
+                        .map(drop)
+                        .map_err(|e| e.to_string())
+                }
+                MessageType::Attribute => {
+                    clawhdf5_format::attribute::AttributeMessage::parse_in_file(
+                        &data,
+                        self.h5.data(),
+                        os,
+                        ls,
+                    )
+                    .map(drop)
+                    .map_err(|e| e.to_string())
+                }
+                MessageType::AttributeInfo => match AttributeInfoMessage::parse(&data, os) {
+                    Ok(ai) => {
+                        self.dense_storage(
+                            addr,
+                            path,
+                            "attribute",
+                            ai.fractal_heap_address,
+                            [ai.btree_name_index_address, ai.btree_creation_order_address],
+                        );
+                        Ok(())
+                    }
+                    Err(e) => Err(e.to_string()),
+                },
+                MessageType::LinkInfo => LinkInfoMessage::parse(&data, os)
+                    .map(drop)
+                    .map_err(|e| e.to_string()),
+                MessageType::Link => clawhdf5_format::link_message::LinkMessage::parse(&data, os)
+                    .map(drop)
+                    .or_else(|e| match e {
+                        clawhdf5_format::error::FormatError::InvalidLinkType(t) if t >= 65 => {
+                            Ok(())
+                        }
+                        e => Err(e.to_string()),
+                    }),
+                MessageType::GroupInfo => GroupInfoMessage::parse(&data)
+                    .map(drop)
+                    .map_err(|e| e.to_string()),
+                MessageType::SymbolTable => SymbolTableMessage::parse(&data, os)
+                    .map(drop)
+                    .map_err(|e| e.to_string()),
+                _ => Ok(()),
+            };
+            if let Err(e) = r {
+                self.problem(addr, path, format!("{}: {e}", msg_name(m.msg_type)));
+            }
+        }
+    }
+
+    /// Dense (fractal heap + v2 B-tree) link or attribute storage.
+    fn dense_storage(
+        &mut self,
+        addr: u64,
+        path: &str,
+        what: &str,
+        heap: Option<u64>,
+        btrees: [Option<u64>; 2],
+    ) {
+        if let Some(fh) = heap
+            && self.heaps_seen.insert(fh)
+        {
+            self.counts.heaps += 1;
+            let r = crate::heap_blocks::verify(self.h5, fh);
+            self.counts.heap_block_checksums += r.checksums as u64;
+            for e in r.problems {
+                self.err(fh, path, &e.context(&format!("dense {what} storage")));
+            }
+        }
+        for bt in btrees.into_iter().flatten() {
+            if !self.btrees_seen.insert(bt) {
+                continue;
+            }
+            let (os, ls) = (self.h5.os(), self.h5.ls());
+            let Ok(off) = usize::try_from(bt) else {
+                self.problem(bt, path, format!("{what} index address out of range"));
+                continue;
+            };
+            match BTreeV2Header::parse(self.h5.data(), off, os, ls) {
+                Ok(hdr) => {
+                    self.counts.btree_v2 += 1;
+                    match collect_btree_v2_records(self.h5.data(), &hdr, os, ls) {
+                        Ok(recs) => {
+                            if recs.len() as u64 != hdr.total_records {
+                                self.problem(
+                                    bt,
+                                    path,
+                                    format!(
+                                        "{what} index: v2 B-tree header counts {} records, the tree holds {}",
+                                        hdr.total_records,
+                                        recs.len()
+                                    ),
+                                );
+                            }
+                        }
+                        Err(e) => self.problem(bt, path, format!("{what} index (v2 B-tree): {e}")),
+                    }
+                }
+                Err(e) => self.problem(bt, path, format!("{what} index (v2 B-tree header): {e}")),
+            }
+        }
+        let _ = addr;
+    }
+
+    fn group(&mut self, addr: u64, path: &str, h: &ObjectHeader) {
+        if let Some(m) = h
+            .messages
+            .iter()
+            .find(|m| m.msg_type == MessageType::LinkInfo)
+            && let Ok(li) = LinkInfoMessage::parse(&m.data, self.h5.os())
+        {
+            self.dense_storage(
+                addr,
+                path,
+                "link",
+                li.fractal_heap_address,
+                [li.btree_name_index_address, li.btree_creation_order_address],
+            );
+        }
+        if let Err(e) = self.h5.links(h) {
+            self.err(addr, path, &e.context("cannot list the group"));
+        }
+        if let Some(m) = h
+            .messages
+            .iter()
+            .find(|m| m.msg_type == MessageType::SymbolTable)
+            && let Ok(st) = SymbolTableMessage::parse(&m.data, self.h5.os())
+        {
+            for (a, what) in [
+                (st.btree_address, "B-tree"),
+                (st.local_heap_address, "local heap"),
+            ] {
+                if a >= self.eof {
+                    self.problem(
+                        addr,
+                        path,
+                        format!("symbol table {what} address {a:#x} is past the end of the file"),
+                    );
+                }
+            }
+        }
+    }
+
+    fn dataset(&mut self, addr: u64, path: &str, h: &ObjectHeader) {
+        let info = DsInfo::read(self.h5, path, h);
+        for e in [
+            info.dt.as_ref().err(),
+            info.ds.as_ref().err(),
+            info.layout.as_ref().err(),
+            info.filters.as_ref().err(),
+        ]
+        .into_iter()
+        .flatten()
+        {
+            let e = e.clone();
+            self.err(addr, path, &e);
+        }
+        let (Ok(dt), Ok(ds), Ok(layout)) = (&info.dt, &info.ds, &info.layout) else {
+            return;
+        };
+        let esize = u64::from(dt.type_size());
+        if esize == 0 {
+            self.problem(addr, path, "datatype has size 0");
+            return;
+        }
+        let need = match crate::h5::byte_len(ds, dt) {
+            Ok(n) => n,
+            Err(e) => {
+                self.err(addr, path, &e);
+                return;
+            }
+        };
+        if let Some(max) = &ds.max_dimensions {
+            for (i, (&d, &m)) in ds.dimensions.iter().zip(max).enumerate() {
+                if m != u64::MAX && d > m {
+                    self.problem(
+                        addr,
+                        path,
+                        format!("dimension {i} is {d}, above its maximum {m}"),
+                    );
+                }
+            }
+        }
+        match layout {
+            DataLayout::Compact { data } => {
+                if (data.len() as u64) < need {
+                    self.problem(
+                        addr,
+                        path,
+                        format!(
+                            "compact data holds {} bytes; the dataset needs {need}",
+                            data.len()
+                        ),
+                    );
+                }
+            }
+            DataLayout::Contiguous { address, size } => {
+                if let Some(a) = address {
+                    if !info.external && *size < need {
+                        self.problem(
+                            *a,
+                            path,
+                            format!("contiguous storage is {size} bytes; the dataset needs {need}"),
+                        );
+                    }
+                    self.extent(*a, *size, path);
+                }
+            }
+            DataLayout::Chunked { .. } => self.chunked(addr, path, &info, dt, ds, layout),
+            DataLayout::Virtual { .. } => {
+                let mut l = layout.clone();
+                if let Err(e) = l.resolve_vds_mappings(self.h5.data(), self.h5.ls()) {
+                    self.problem(addr, path, format!("virtual dataset mappings: {e}"));
+                }
+            }
+        }
+        if self.read_data {
+            match self.h5.read_dataset(path, dt, ds) {
+                Ok(_) => self.counts.datasets_read += 1,
+                // Valid data this tool cannot decode is not a problem with
+                // the file.
+                Err(e) if e.kind != ErrorKind::Corrupt => self.notes.push(Problem {
+                    addr,
+                    path: path.to_string(),
+                    msg: format!("data not read: {}", e.msg),
+                }),
+                Err(e) => self.problem(addr, path, format!("reading the data: {}", e.msg)),
+            }
+        }
+    }
+
+    fn chunked(
+        &mut self,
+        addr: u64,
+        path: &str,
+        info: &DsInfo,
+        dt: &Datatype,
+        ds: &Dataspace,
+        layout: &DataLayout,
+    ) {
+        let DataLayout::Chunked {
+            chunk_dimensions,
+            btree_address,
+            chunk_index_type,
+            ..
+        } = layout
+        else {
+            return;
+        };
+        let rank = match ds.space_type {
+            DataspaceType::Simple => ds.dimensions.len(),
+            _ => 0,
+        };
+        if chunk_dimensions.len() != rank + 1 {
+            self.problem(
+                addr,
+                path,
+                format!(
+                    "chunked layout has {} chunk dimensions for a rank-{rank} dataset (expected {})",
+                    chunk_dimensions.len(),
+                    rank + 1
+                ),
+            );
+            return;
+        }
+        let cdims = &chunk_dimensions[..rank];
+        if cdims.contains(&0) {
+            self.problem(addr, path, "a chunk dimension is 0");
+            return;
+        }
+        let Some(index_addr) = *btree_address else {
+            return; // no chunk allocated yet
+        };
+        if index_addr >= self.eof {
+            self.problem(
+                addr,
+                path,
+                format!("chunk index address {index_addr:#x} is past the end of the file"),
+            );
+            return;
+        }
+        let chunks = match info::chunks(self.h5, layout, ds, dt) {
+            Ok(c) => c,
+            Err(e) => {
+                self.err(index_addr, path, &e);
+                return;
+            }
+        };
+        if matches!(chunk_index_type, Some(3..=5)) {
+            self.counts.chunk_index_checksummed += 1;
+        }
+        let filtered = matches!(&info.filters, Ok(Some(p)) if !p.filters.is_empty());
+        let chunk_bytes = cdims.iter().try_fold(u64::from(dt.type_size()), |a, &d| {
+            a.checked_mul(u64::from(d))
+        });
+        let max = ds.max_dimensions.clone();
+        let mut seen: HashSet<Vec<u64>> = HashSet::with_capacity(chunks.len().min(1 << 20));
+        let mut reported = 0usize;
+        for (n, c) in chunks.iter().enumerate() {
+            if n >= MAX_CHUNKS_CHECKED {
+                self.problem(index_addr, path, "too many chunks; stopped checking them");
+                break;
+            }
+            self.counts.chunks += 1;
+            let mut bad = Vec::new();
+            if c.offsets.len() < rank {
+                bad.push(format!(
+                    "has {} coordinates for a rank-{rank} dataset",
+                    c.offsets.len()
+                ));
+            } else {
+                for (i, (&o, &cd)) in c.offsets.iter().zip(cdims).enumerate() {
+                    if o % u64::from(cd) != 0 {
+                        bad.push(format!(
+                            "offset {o} in dimension {i} is not a multiple of the chunk size {cd}"
+                        ));
+                    }
+                    let limit = match max.as_ref().and_then(|m| m.get(i)) {
+                        Some(&u64::MAX) | None => ds.dimensions[i],
+                        Some(&m) => m.max(ds.dimensions[i]),
+                    };
+                    if o >= limit && !(limit == 0 && o == 0) {
+                        bad.push(format!(
+                            "offset {o} in dimension {i} is outside the extent {limit}"
+                        ));
+                    }
+                }
+                if !seen.insert(c.offsets[..rank].to_vec()) {
+                    bad.push("appears twice in the chunk index".into());
+                }
+            }
+            if c.chunk_size == 0 {
+                bad.push("has size 0".into());
+            } else if !filtered
+                && let Some(cb) = chunk_bytes
+                && u64::from(c.chunk_size) != cb
+            {
+                bad.push(format!(
+                    "is {} bytes; an unfiltered chunk is {cb}",
+                    c.chunk_size
+                ));
+            }
+            if !bad.is_empty() {
+                reported += 1;
+                if reported <= 50 {
+                    self.problem(
+                        c.address,
+                        path,
+                        format!("chunk at {:?} {}", c.offsets, bad.join("; ")),
+                    );
+                }
+            }
+            self.extent(c.address, u64::from(c.chunk_size), path);
+        }
+        if reported > 50 {
+            self.problem(
+                index_addr,
+                path,
+                format!("{} more bad chunks not listed", reported - 50),
+            );
+        }
+    }
+
+    /// Record raw data at `[start, start + len)`, checking it is in the file.
+    fn extent(&mut self, start: u64, len: u64, path: &str) {
+        if len == 0 {
+            return;
+        }
+        match start.checked_add(len) {
+            Some(end) if end <= self.eof => self.extents.push((start, end, path.to_string())),
+            _ => self.problem(
+                start,
+                path,
+                format!("raw data ({len} bytes) extends past the end of the file"),
+            ),
+        }
+    }
+
+    fn overlaps(&mut self) {
+        let mut ext = std::mem::take(&mut self.extents);
+        ext.sort_unstable_by_key(|e| (e.0, e.1));
+        let mut reported = 0usize;
+        let mut far: Option<(u64, String)> = None;
+        let mut msgs = Vec::new();
+        for (s, e, p) in &ext {
+            if let Some((end, owner)) = &far
+                && s < end
+            {
+                reported += 1;
+                if reported <= 50 {
+                    msgs.push((
+                        *s,
+                        p.clone(),
+                        format!("raw data at {s:#x} overlaps raw data of {owner}"),
+                    ));
+                }
+            }
+            if far.as_ref().is_none_or(|(end, _)| e > end) {
+                far = Some((*e, p.clone()));
+            }
+        }
+        for (a, p, m) in msgs {
+            self.problem(a, &p, m);
+        }
+        if reported > 50 {
+            self.problem(
+                0,
+                "/",
+                format!("{} more raw data overlaps not listed", reported - 50),
+            );
+        }
+    }
+
+    fn summary(&self, file: &str, out: &mut Out) -> std::io::Result<()> {
+        let c = &self.counts;
+        let sb = self.h5.sb();
+        writeln!(
+            out.o,
+            "checked {file}: superblock v{}, {} objects ({} groups, {} datasets, {} named datatypes), \
+             {} header messages, {} chunks",
+            sb.version, c.objects, c.groups, c.datasets, c.datatypes, c.messages, c.chunks
+        )?;
+        writeln!(
+            out.o,
+            "checksums verified: superblock {}, v2 object headers {}, v2 B-trees {}, \
+             fractal heaps {} (+{} blocks), chunk indexes {}",
+            c.sb_checksum,
+            c.ohdr_v2,
+            c.btree_v2,
+            c.heaps,
+            c.heap_block_checksums,
+            c.chunk_index_checksummed
+        )?;
+        if self.read_data {
+            writeln!(out.o, "datasets read: {}", c.datasets_read)?;
+        }
+        match self.problems.len() {
+            0 => writeln!(out.o, "no problems found"),
+            1 => writeln!(out.o, "1 problem found"),
+            n => writeln!(out.o, "{n} problems found"),
+        }
+    }
+}

crates/clawhdf5-tools/src/cli.rs

@@ -0,0 +1,45 @@
+//! Argument handling and output plumbing shared by the subcommands.
+
+use std::io::Write;
+
+/// Where a subcommand writes: `o` for results, `e` for diagnostics.
+pub struct Out<'a> {
+    pub o: &'a mut dyn Write,
+    pub e: &'a mut dyn Write,
+}
+
+/// The arguments after the subcommand name.
+pub struct Args {
+    cmd: &'static str,
+    rest: std::vec::IntoIter<String>,
+}
+
+impl Args {
+    pub fn new(cmd: &'static str, rest: Vec<String>) -> Self {
+        Self {
+            cmd,
+            rest: rest.into_iter(),
+        }
+    }
+
+    #[allow(clippy::should_implement_trait)]
+    pub fn next(&mut self) -> Option<String> {
+        self.rest.next()
+    }
+
+    /// The value after an option such as `--max-bytes`.
+    pub fn value(&mut self) -> Option<String> {
+        self.rest.next()
+    }
+
+    /// A numeric option value.
+    pub fn number<T: std::str::FromStr>(&mut self) -> Option<T> {
+        self.rest.next().and_then(|s| s.parse().ok())
+    }
+
+    /// Report a usage problem; exit status 2.
+    pub fn usage_error(&self, out: &mut Out, msg: &str, usage: &str) -> std::io::Result<i32> {
+        writeln!(out.e, "h5rs {}: {msg}\n\n{usage}", self.cmd)?;
+        Ok(2)
+    }
+}

crates/clawhdf5-tools/src/diff.rs

@@ -0,0 +1,703 @@
+//! `h5rs diff`: compare two files (or two objects) like h5diff.
+
+use std::cell::OnceCell;
+use std::collections::{BTreeMap, HashMap};
+
+use clawhdf5_format::attribute::AttributeMessage;
+use clawhdf5_format::dataspace::{Dataspace, DataspaceType};
+use clawhdf5_format::datatype::Datatype;
+use clawhdf5_format::object_header::ObjectHeader;
+
+use crate::cli::{Args, Out};
+use crate::h5::{H5, Kind, LinkKind};
+use crate::value::{self, Decoder, Value};
+
+pub const USAGE: &str = "\
+usage: h5rs diff [options] FILE1 FILE2 [OBJ1 [OBJ2]]
+
+Compare FILE1 and FILE2 (or OBJ1 in FILE1 with OBJ2 in FILE2, and everything
+below them): the objects present, their kinds, datatypes, shapes, attribute
+sets and values, and soft/external link targets.
+
+  -r, --report       list every differing element (position, values, difference)
+  -q, --quiet        print nothing; only the exit status
+  -d, --delta D      numbers differ only when |a - b| > D
+  -p, --relative R   numbers differ only when |a - b| / |a| > R
+  -c, --count N      list at most N differing elements per object with -r
+  --max-bytes N      largest dataset read (default 1 GiB); a larger one is an error
+
+Two NaNs compare equal. Unlike h5diff, objects that cannot be compared
+(different kinds, datatype classes or shapes) count as a difference.
+
+Exit status: 0 no differences, 1 differences found, 2 error (a file or
+object could not be opened or read).";
+
+#[derive(Clone, Copy)]
+enum Tol {
+    Exact,
+    Delta(f64),
+    Relative(f64),
+}
+
+struct Opts {
+    report: bool,
+    quiet: bool,
+    tol: Tol,
+    count: usize,
+}
+
+/// What a relative path names in one file.
+#[derive(Clone)]
+enum Entry {
+    Obj(u64, Kind),
+    Soft(String),
+    External(String, String),
+    UserDefined(u8),
+    Broken(String),
+}
+
+struct Side<'a> {
+    h5: &'a H5,
+    label: String,
+    base: String,
+    /// Relative path -> what it is.
+    entries: BTreeMap<String, Entry>,
+    /// Address -> relative path, for comparing references.
+    rel_of: OnceCell<HashMap<u64, String>>,
+}
+
+impl Side<'_> {
+    fn full(&self, rel: &str) -> String {
+        if rel.is_empty() {
+            if self.base.is_empty() {
+                "/".into()
+            } else {
+                self.base.clone()
+            }
+        } else {
+            format!("{}{rel}", self.base)
+        }
+    }
+
+    fn rel_paths(&self) -> &HashMap<u64, String> {
+        self.rel_of.get_or_init(|| {
+            let mut m = HashMap::new();
+            let _ = self.h5.walk(|it| {
+                if let (Some(a), None) = (it.addr, it.first_path) {
+                    m.entry(a).or_insert_with(|| it.path.to_string());
+                }
+            });
+            m
+        })
+    }
+}
+
+struct Diff {
+    opts: Opts,
+    diffs: u64,
+    /// Differences already reported under an object heading.
+    per_object: u64,
+    errors: u64,
+}
+
+pub fn run(args: &mut Args, out: &mut Out) -> std::io::Result<i32> {
+    let mut opts = Opts {
+        report: false,
+        quiet: false,
+        tol: Tol::Exact,
+        count: usize::MAX,
+    };
+    let mut max_bytes = None;
+    let mut pos = Vec::new();
+    while let Some(a) = args.next() {
+        match a.as_str() {
+            "-r" | "--report" => opts.report = true,
+            "-q" | "--quiet" => opts.quiet = true,
+            "-d" | "--delta" => match args.number::<f64>() {
+                Some(d) if d >= 0.0 => opts.tol = Tol::Delta(d),
+                _ => return args.usage_error(out, "--delta needs a number >= 0", USAGE),
+            },
+            "-p" | "--relative" => match args.number::<f64>() {
+                Some(r) if r >= 0.0 => opts.tol = Tol::Relative(r),
+                _ => return args.usage_error(out, "--relative needs a number >= 0", USAGE),
+            },
+            "-c" | "--count" => match args.number() {
+                Some(n) => opts.count = n,
+                None => return args.usage_error(out, "--count needs a number", USAGE),
+            },
+            "--max-bytes" => match args.number() {
+                Some(n) => max_bytes = Some(n),
+                None => return args.usage_error(out, "--max-bytes needs a number", USAGE),
+            },
+            "-h" | "--help" => {
+                writeln!(out.o, "{USAGE}")?;
+                return Ok(0);
+            }
+            s if s.starts_with('-') && s.len() > 1 => {
+                return args.usage_error(out, &format!("unknown option {a}"), USAGE);
+            }
+            _ => pos.push(a),
+        }
+    }
+    if pos.len() < 2 || pos.len() > 4 {
+        return args.usage_error(out, "expected FILE1 FILE2 [OBJ1 [OBJ2]]", USAGE);
+    }
+    let mut files = Vec::new();
+    for f in &pos[..2] {
+        match H5::open(std::path::Path::new(f)) {
+            Ok(mut h) => {
+                if let Some(m) = max_bytes {
+                    h.max_bytes = m;
+                }
+                files.push(h);
+            }
+            Err(e) => {
+                writeln!(out.e, "h5rs diff: {e}")?;
+                return Ok(2);
+            }
+        }
+    }
+    let obj1 = pos.get(2).cloned().unwrap_or_else(|| "/".into());
+    let obj2 = pos.get(3).cloned().unwrap_or_else(|| obj1.clone());
+    let mut sides = Vec::new();
+    for (h5, (obj, f)) in files.iter().zip([(&obj1, &pos[0]), (&obj2, &pos[1])]) {
+        let addr = match h5.resolve(obj) {
+            Ok(a) => a,
+            Err(_) => {
+                writeln!(
+                    out.e,
+                    "h5rs diff: object <{obj}> could not be found in <{f}>"
+                )?;
+                return Ok(2);
+            }
+        };
+        let base = obj.trim_end_matches('/').to_string();
+        let base = if base.is_empty() || base.starts_with('/') {
+            base
+        } else {
+            format!("/{base}")
+        };
+        let mut entries = BTreeMap::new();
+        let walk = h5.walk_from(addr, "", &mut |it| {
+            let e = match (it.link.map(|l| &l.kind), it.header) {
+                (Some(LinkKind::Soft(t)), _) => Entry::Soft(t.clone()),
+                (Some(LinkKind::External { file, path }), _) => {
+                    Entry::External(file.clone(), path.clone())
+                }
+                (Some(LinkKind::UserDefined(t)), _) => Entry::UserDefined(*t),
+                (_, Some(Err(e))) => Entry::Broken(e.to_string()),
+                (_, Some(Ok(h))) => Entry::Obj(it.addr.unwrap_or(0), Kind::of(h)),
+                // A second hard link to an object already compared.
+                (_, None) => return,
+            };
+            entries.insert(it.path.to_string(), e);
+        });
+        if let Err(e) = walk {
+            writeln!(out.e, "h5rs diff: {f}: {e}")?;
+            return Ok(2);
+        }
+        sides.push(Side {
+            h5,
+            label: f.clone(),
+            base,
+            entries,
+            rel_of: OnceCell::new(),
+        });
+    }
+    let (a, b) = (&sides[0], &sides[1]);
+    let mut d = Diff {
+        opts,
+        diffs: 0,
+        per_object: 0,
+        errors: 0,
+    };
+    let mut names: Vec<&String> = a.entries.keys().chain(b.entries.keys()).collect();
+    names.sort();
+    names.dedup();
+    for rel in names {
+        match (a.entries.get(rel), b.entries.get(rel)) {
+            (Some(_), None) => {
+                d.diffs += 1;
+                d.say(
+                    out,
+                    &format!("<{}> exists only in <{}>", a.full(rel), a.label),
+                )?;
+            }
+            (None, Some(_)) => {
+                d.diffs += 1;
+                d.say(
+                    out,
+                    &format!("<{}> exists only in <{}>", b.full(rel), b.label),
+                )?;
+            }
+            (Some(ea), Some(eb)) => d.entry(out, a, b, rel, ea, eb)?,
+            (None, None) => {}
+        }
+    }
+    // Per-object counts were printed with each object; add a total when
+    // they do not already tell the whole story.
+    if !d.opts.quiet && d.diffs > 0 && d.diffs != d.per_object {
+        writeln!(out.o, "{} difference(s) found in total", d.diffs)?;
+    }
+    Ok(if d.errors > 0 {
+        2
+    } else if d.diffs > 0 {
+        1
+    } else {
+        0
+    })
+}
+
+fn kind_word(k: Kind) -> &'static str {
+    match k {
+        Kind::Group => "group",
+        Kind::Dataset => "dataset",
+        Kind::Datatype => "datatype",
+        Kind::Unknown => "object",
+    }
+}
+
+impl Diff {
+    fn say(&self, out: &mut Out, msg: &str) -> std::io::Result<()> {
+        if self.opts.quiet {
+            Ok(())
+        } else {
+            writeln!(out.o, "{msg}")
+        }
+    }
+
+    fn error(&mut self, out: &mut Out, msg: &str) -> std::io::Result<()> {
+        self.errors += 1;
+        writeln!(out.e, "h5rs diff: {msg}")
+    }
+
+    fn entry(
+        &mut self,
+        out: &mut Out,
+        a: &Side,
+        b: &Side,
+        rel: &str,
+        ea: &Entry,
+        eb: &Entry,
+    ) -> std::io::Result<()> {
+        let (pa, pb) = (a.full(rel), b.full(rel));
+        match (ea, eb) {
+            (Entry::Broken(e), _) => self.error(out, &format!("<{pa}> in <{}>: {e}", a.label)),
+            (_, Entry::Broken(e)) => self.error(out, &format!("<{pb}> in <{}>: {e}", b.label)),
+            (Entry::Soft(x), Entry::Soft(y)) => {
+                if x != y {
+                    self.diffs += 1;
+                    self.say(out, &format!("soft link: <{pa}> -> {x} and <{pb}> -> {y}"))?;
+                }
+                Ok(())
+            }
+            (Entry::External(f1, p1), Entry::External(f2, p2)) => {
+                if (f1, p1) != (f2, p2) {
+                    self.diffs += 1;
+                    self.say(
+                        out,
+                        &format!("external link: <{pa}> -> {f1}:{p1} and <{pb}> -> {f2}:{p2}"),
+                    )?;
+                }
+                Ok(())
+            }
+            (Entry::UserDefined(x), Entry::UserDefined(y)) => {
+                if x != y {
+                    self.diffs += 1;
+                    self.say(out, &format!("user-defined link: <{pa}> and <{pb}> differ"))?;
+                }
+                Ok(())
+            }
+            (Entry::Obj(aa, ka), Entry::Obj(ab, kb)) if ka == kb => {
+                let (ha, hb) = match (a.h5.header(*aa), b.h5.header(*ab)) {
+                    (Ok(x), Ok(y)) => (x, y),
+                    (Err(e), _) | (_, Err(e)) => return self.error(out, &format!("<{pa}>: {e}")),
+                };
+                let before = self.diffs;
+                let mut rows = Vec::new();
+                match ka {
+                    Kind::Dataset => self.dataset(out, a, b, &pa, &pb, &ha, &hb, &mut rows)?,
+                    Kind::Datatype => match (a.h5.datatype(&ha), b.h5.datatype(&hb)) {
+                        (Ok(x), Ok(y)) => {
+                            if x != y {
+                                self.diffs += 1;
+                                rows.push("datatypes differ".to_string());
+                            }
+                        }
+                        (Err(e), _) | (_, Err(e)) => self.error(out, &format!("<{pa}>: {e}"))?,
+                    },
+                    _ => {}
+                }
+                self.attributes(out, a, b, &pa, &ha, &hb, &mut rows)?;
+                let n = self.diffs - before;
+                if n > 0 && !self.opts.quiet {
+                    writeln!(out.o, "{}: <{pa}> and <{pb}>", kind_word(*ka))?;
+                    for r in &rows {
+                        writeln!(out.o, "{r}")?;
+                    }
+                    writeln!(out.o, "{n} difference(s) found")?;
+                    self.per_object += n;
+                }
+                Ok(())
+            }
+            _ => {
+                self.diffs += 1;
+                self.say(
+                    out,
+                    &format!(
+                        "Not comparable: <{pa}> is a {} and <{pb}> is a {}",
+                        entry_word(ea),
+                        entry_word(eb)
+                    ),
+                )
+            }
+        }
+    }
+
+    #[allow(clippy::too_many_arguments)]
+    fn dataset(
+        &mut self,
+        out: &mut Out,
+        a: &Side,
+        b: &Side,
+        pa: &str,
+        pb: &str,
+        ha: &ObjectHeader,
+        hb: &ObjectHeader,
+        rows: &mut Vec<String>,
+    ) -> std::io::Result<()> {
+        let got = (|| -> crate::h5::Result<_> {
+            Ok((
+                a.h5.datatype(ha)?,
+                a.h5.resolved_dataspace(pa, ha)?,
+                b.h5.datatype(hb)?,
+                b.h5.resolved_dataspace(pb, hb)?,
+            ))
+        })();
+        let (dta, dsa, dtb, dsb) = match got {
+            Ok(x) => x,
+            Err(e) => return self.error(out, &format!("<{pa}>: {e}")),
+        };
+        if let Some(why) = not_comparable(&dta, &dsa, &dtb, &dsb) {
+            self.diffs += 1;
+            rows.push(format!("Not comparable: {why}"));
+            return Ok(());
+        }
+        let raw =
+            a.h5.read_dataset(pa, &dta, &dsa)
+                .and_then(|x| Ok((x, b.h5.read_dataset(pb, &dtb, &dsb)?)));
+        let (ra, rb) = match raw {
+            Ok(x) => x,
+            Err(e) => return self.error(out, &format!("<{pa}>: {e}")),
+        };
+        self.values(out, a, b, pa, (&dta, &ra), (&dtb, &rb), &dsa, rows)
+    }
+
+    #[allow(clippy::too_many_arguments)]
+    fn values(
+        &mut self,
+        out: &mut Out,
+        a: &Side,
+        b: &Side,
+        what: &str,
+        (dta, ra): (&Datatype, &[u8]),
+        (dtb, rb): (&Datatype, &[u8]),
+        ds: &Dataspace,
+        rows: &mut Vec<String>,
+    ) -> std::io::Result<()> {
+        let n = crate::h5::num_elements(ds).unwrap_or(0) as usize;
+        let (da, db) = (Decoder::new(a.h5), Decoder::new(b.h5));
+        let dims: Vec<u64> = match ds.space_type {
+            DataspaceType::Simple => ds.dimensions.clone(),
+            _ => vec![1],
+        };
+        let mut found = 0u64;
+        let mut header = false;
+        for i in 0..n {
+            let va = da.element(dta, ra, i);
+            let vb = db.element(dtb, rb, i);
+            if let (Value::Error(e), _) | (_, Value::Error(e)) = (&va, &vb) {
+                return self.error(out, &format!("<{what}> element {i}: {e}"));
+            }
+            if self.equal(a, b, &va, &vb) {
+                continue;
+            }
+            found += 1;
+            if self.opts.report && (found as usize) <= self.opts.count {
+                if !header {
+                    header = true;
+                    rows.push(format!(
+                        "{:<24}{:<24}{:<24}{}",
+                        "position", "value 1", "value 2", "difference"
+                    ));
+                    rows.push("-".repeat(80));
+                }
+                let pos = format!("[ {} ]", index(i as u64, &dims));
+                let ta = value::text(&va, &|_| None);
+                let tb = value::text(&vb, &|_| None);
+                let dif = match (number(&va), number(&vb)) {
+                    (Some(x), Some(y)) => value::fmt_float((x - y).abs(), 64),
+                    _ => String::new(),
+                };
+                rows.push(format!("{pos:<24}{ta:<24}{tb:<24}{dif}"));
+            }
+        }
+        self.diffs += found;
+        Ok(())
+    }
+
+    #[allow(clippy::too_many_arguments)]
+    fn attributes(
+        &mut self,
+        out: &mut Out,
+        a: &Side,
+        b: &Side,
+        path: &str,
+        ha: &ObjectHeader,
+        hb: &ObjectHeader,
+        rows: &mut Vec<String>,
+    ) -> std::io::Result<()> {
+        let (la, lb) = match (a.h5.attributes(ha), b.h5.attributes(hb)) {
+            (Ok(x), Ok(y)) => (x, y),
+            (Err(e), _) | (_, Err(e)) => return self.error(out, &format!("<{path}>: {e}")),
+        };
+        for e in la.1.iter().chain(lb.1.iter()) {
+            self.error(out, &format!("<{path}>: attribute: {e}"))?;
+        }
+        let ma: BTreeMap<&str, &AttributeMessage> =
+            la.0.iter().map(|x| (x.name.as_str(), x)).collect();
+        let mb: BTreeMap<&str, &AttributeMessage> =
+            lb.0.iter().map(|x| (x.name.as_str(), x)).collect();
+        let mut names: Vec<&str> = ma.keys().chain(mb.keys()).copied().collect();
+        names.sort_unstable();
+        names.dedup();
+        for n in names {
+            match (ma.get(n), mb.get(n)) {
+                (Some(x), Some(y)) => {
+                    if let Some(why) =
+                        not_comparable(&x.datatype, &x.dataspace, &y.datatype, &y.dataspace)
+                    {
+                        self.diffs += 1;
+                        rows.push(format!("attribute \"{n}\": not comparable: {why}"));
+                        continue;
+                    }
+                    for (m, h5) in [(x, a.h5), (y, b.h5)] {
+                        let need =
+                            crate::h5::byte_len(&m.dataspace, &m.datatype).unwrap_or(u64::MAX);
+                        if need > h5.max_bytes || (m.raw_data.len() as u64) < need {
+                            return self.error(
+                                out,
+                                &format!(
+                                    "<{path}> attribute \"{n}\": value is truncated or too large"
+                                ),
+                            );
+                        }
+                    }
+                    let before = rows.len();
+                    let what = format!("{path}\" attribute \"{n}");
+                    self.values(
+                        out,
+                        a,
+                        b,
+                        &what,
+                        (&x.datatype, &x.raw_data),
+                        (&y.datatype, &y.raw_data),
+                        &x.dataspace,
+                        rows,
+                    )?;
+                    if rows.len() > before {
+                        rows.insert(before, format!("attribute \"{n}\":"));
+                    }
+                }
+                (Some(_), None) => {
+                    self.diffs += 1;
+                    rows.push(format!("attribute \"{n}\" exists only in <{}>", a.label));
+                }
+                (None, Some(_)) => {
+                    self.diffs += 1;
+                    rows.push(format!("attribute \"{n}\" exists only in <{}>", b.label));
+                }
+                (None, None) => {}
+            }
+        }
+        Ok(())
+    }
+
+    fn equal(&self, a: &Side, b: &Side, x: &Value, y: &Value) -> bool {
+        if let (Some(p), Some(q)) = (number(x), number(y)) {
+            let exact_ints = matches!((x, y), (Value::Int(_), Value::Int(_)))
+                || matches!((x, y), (Value::Enum(..), Value::Enum(..)));
+            if exact_ints && matches!(self.opts.tol, Tol::Exact) {
+                return int_of(x) == int_of(y);
+            }
+            return close(p, q, self.opts.tol);
+        }
+        match (x, y) {
+            (Value::Str(p), Value::Str(q)) => p == q,
+            (Value::Bytes(p), Value::Bytes(q)) | (Value::OtherRef(p), Value::OtherRef(q)) => p == q,
+            (Value::Compound(p), Value::Compound(q)) => {
+                p.len() == q.len()
+                    && p.iter()
+                        .zip(q)
+                        .all(|((_, u), (_, v))| self.equal(a, b, u, v))
+            }
+            (Value::Array(p), Value::Array(q)) | (Value::Seq(p), Value::Seq(q)) => {
+                p.len() == q.len() && p.iter().zip(q).all(|(u, v)| self.equal(a, b, u, v))
+            }
+            (Value::Ref(None), Value::Ref(None)) => true,
+            (Value::Ref(Some(p)), Value::Ref(Some(q))) => {
+                // Addresses mean nothing across files: compare the paths the
+                // references lead to.
+                let (pp, qq) = (a.rel_paths().get(p), b.rel_paths().get(q));
+                pp.is_some() && pp == qq
+            }
+            _ => false,
+        }
+    }
+}
+
+fn int_of(v: &Value) -> Option<i128> {
+    match v {
+        Value::Int(i) | Value::Enum(_, i) => Some(*i),
+        _ => None,
+    }
+}
+
+fn number(v: &Value) -> Option<f64> {
+    match v {
+        Value::Int(i) | Value::Enum(_, i) => Some(*i as f64),
+        Value::Float(f, _) => Some(*f),
+        _ => None,
+    }
+}
+
+fn close(a: f64, b: f64, tol: Tol) -> bool {
+    if a.is_nan() || b.is_nan() {
+        return a.is_nan() && b.is_nan();
+    }
+    if a == b {
+        return true;
+    }
+    let d = (a - b).abs();
+    match tol {
+        Tol::Exact => false,
+        Tol::Delta(t) => d <= t,
+        Tol::Relative(r) => a != 0.0 && d / a.abs() <= r,
+    }
+}
+
+fn entry_word(e: &Entry) -> &'static str {
+    match e {
+        Entry::Obj(_, k) => kind_word(*k),
+        Entry::Soft(_) => "soft link",
+        Entry::External(..) => "external link",
+        Entry::UserDefined(_) => "user-defined link",
+        Entry::Broken(_) => "unreadable object",
+    }
+}
+
+/// Why two datasets/attributes cannot be compared element by element.
+fn not_comparable(
+    dta: &Datatype,
+    dsa: &Dataspace,
+    dtb: &Datatype,
+    dsb: &Dataspace,
+) -> Option<String> {
+    let rank = |d: &Dataspace| match d.space_type {
+        DataspaceType::Simple => Some(d.dimensions.clone()),
+        DataspaceType::Scalar => Some(Vec::new()),
+        DataspaceType::Null => None,
+    };
+    let (sa, sb) = (rank(dsa), rank(dsb));
+    if sa != sb {
+        let show = |s: &Option<Vec<u64>>| match s {
+            None => "null".to_string(),
+            Some(d) if d.is_empty() => "scalar".to_string(),
+            Some(d) => format!("{d:?}"),
+        };
+        return Some(format!("shapes differ: {} and {}", show(&sa), show(&sb)));
+    }
+    if !types_comparable(dta, dtb) {
+        return Some(format!(
+            "datatypes differ: {} and {}",
+            crate::dtype::short(dta),
+            crate::dtype::short(dtb)
+        ));
+    }
+    None
+}
+
+fn types_comparable(a: &Datatype, b: &Datatype) -> bool {
+    use crate::dtype::class;
+    let numeric = |t: &Datatype| matches!(class(t), "integer" | "float");
+    if numeric(a) && numeric(b) {
+        return class(a) == class(b);
+    }
+    if class(a) != class(b) {
+        return false;
+    }
+    match (a, b) {
+        (Datatype::Compound { members: ma, .. }, Datatype::Compound { members: mb, .. }) => {
+            ma.len() == mb.len()
+                && ma
+                    .iter()
+                    .zip(mb)
+                    .all(|(x, y)| x.name == y.name && types_comparable(&x.datatype, &y.datatype))
+        }
+        (
+            Datatype::Array {
+                base_type: x,
+                dimensions: dx,
+            },
+            Datatype::Array {
+                base_type: y,
+                dimensions: dy,
+            },
+        ) => dx == dy && types_comparable(x, y),
+        (
+            Datatype::VariableLength { base_type: x, .. },
+            Datatype::VariableLength { base_type: y, .. },
+        ) => class(a) == "string" || types_comparable(x, y),
+        (
+            Datatype::Enumeration { base_type: x, .. },
+            Datatype::Enumeration { base_type: y, .. },
+        ) => types_comparable(x, y),
+        _ => true,
+    }
+}
+
+fn index(mut i: u64, dims: &[u64]) -> String {
+    let mut idx = vec![0u64; dims.len()];
+    for (k, &d) in dims.iter().enumerate().rev() {
+        let d = d.max(1);
+        idx[k] = i % d;
+        i /= d;
+    }
+    idx.iter()
+        .map(|x| x.to_string())
+        .collect::<Vec<_>>()
+        .join(" ")
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn tolerances() {
+        assert!(close(1.0, 1.0, Tol::Exact));
+        assert!(!close(1.0, 1.001, Tol::Exact));
+        assert!(close(1.0, 1.001, Tol::Delta(0.01)));
+        assert!(!close(1.0, 1.1, Tol::Delta(0.01)));
+        assert!(close(100.0, 101.0, Tol::Relative(0.02)));
+        assert!(!close(0.0, 1e-9, Tol::Relative(0.5)));
+        assert!(close(f64::NAN, f64::NAN, Tol::Exact));
+        assert!(!close(f64::NAN, 1.0, Tol::Delta(1e9)));
+    }
+
+    #[test]
+    fn positions() {
+        assert_eq!(index(5, &[3, 4]), "1 1");
+        assert_eq!(index(0, &[1]), "0");
+    }
+}

crates/clawhdf5-tools/src/dtype.rs

@@ -0,0 +1,529 @@
+//! Names for datatypes: a short one for listings, h5ls's long form, h5dump's
+//! DDL and the HDF Group's hdf5-json type objects.
+
+use clawhdf5_format::datatype::{
+    CharacterSet, Datatype, DatatypeByteOrder, ReferenceType, StringPadding,
+};
+use serde_json::{Value as J, json};
+
+fn be(o: &DatatypeByteOrder) -> bool {
+    matches!(o, DatatypeByteOrder::BigEndian)
+}
+
+fn order_suffix(o: &DatatypeByteOrder) -> &'static str {
+    match o {
+        DatatypeByteOrder::LittleEndian => "LE",
+        DatatypeByteOrder::BigEndian => "BE",
+        DatatypeByteOrder::Vax => "VAX",
+    }
+}
+
+fn order_word(o: &DatatypeByteOrder) -> &'static str {
+    match o {
+        DatatypeByteOrder::LittleEndian => "little-endian",
+        DatatypeByteOrder::BigEndian => "big-endian",
+        DatatypeByteOrder::Vax => "VAX-order",
+    }
+}
+
+/// True when a float is laid out exactly as IEEE 754 binary16/32/64.
+pub fn is_ieee(dt: &Datatype) -> bool {
+    let Datatype::FloatingPoint {
+        size,
+        bit_offset,
+        bit_precision,
+        exponent_location,
+        exponent_size,
+        mantissa_location,
+        mantissa_size,
+        exponent_bias,
+        ..
+    } = dt
+    else {
+        return false;
+    };
+    let std = match size {
+        2 => (16, 10, 5, 10, 15),
+        4 => (32, 23, 8, 23, 127),
+        8 => (64, 52, 11, 52, 1023),
+        _ => return false,
+    };
+    *bit_offset == 0
+        && *mantissa_location == 0
+        && (
+            *bit_precision,
+            *exponent_location,
+            *exponent_size,
+            *mantissa_size,
+            *exponent_bias,
+        ) == std
+}
+
+/// Short name used by `ls`: `int32`, `float64-be`, `string[3]`, ...
+pub fn short(dt: &Datatype) -> String {
+    match dt {
+        Datatype::FixedPoint {
+            size,
+            signed,
+            byte_order,
+            ..
+        } => format!(
+            "{}int{}{}",
+            if *signed { "" } else { "u" },
+            u64::from(*size) * 8,
+            if be(byte_order) { "-be" } else { "" }
+        ),
+        Datatype::FloatingPoint {
+            size, byte_order, ..
+        } => format!(
+            "float{}{}",
+            u64::from(*size) * 8,
+            if be(byte_order) { "-be" } else { "" }
+        ),
+        Datatype::Time { size, .. } => format!("time{}", u64::from(*size) * 8),
+        Datatype::String { size, .. } => format!("string[{size}]"),
+        Datatype::BitField { size, .. } => format!("bitfield{}", u64::from(*size) * 8),
+        Datatype::Opaque { size, .. } => format!("opaque[{size}]"),
+        Datatype::Compound { members, .. } => format!(
+            "compound{{{}}}",
+            members
+                .iter()
+                .map(|m| format!("{}: {}", m.name, short(&m.datatype)))
+                .collect::<Vec<_>>()
+                .join(", ")
+        ),
+        Datatype::Reference { ref_type, .. } => match ref_type {
+            ReferenceType::Object | ReferenceType::Object2 => "object-reference".into(),
+            ReferenceType::DatasetRegion | ReferenceType::DatasetRegion2 => {
+                "region-reference".into()
+            }
+            ReferenceType::Attribute => "attribute-reference".into(),
+        },
+        Datatype::Enumeration { base_type, .. } => format!("enum<{}>", short(base_type)),
+        Datatype::VariableLength {
+            is_string: true, ..
+        } => "vlen-string".into(),
+        Datatype::VariableLength { base_type, .. } => format!("vlen<{}>", short(base_type)),
+        Datatype::Array {
+            base_type,
+            dimensions,
+        } => format!(
+            "array[{}]<{}>",
+            dimensions
+                .iter()
+                .map(|d| d.to_string())
+                .collect::<Vec<_>>()
+                .join(","),
+            short(base_type)
+        ),
+    }
+}
+
+fn pad_word(p: &StringPadding) -> &'static str {
+    match p {
+        StringPadding::NullTerminate => "null-terminated",
+        StringPadding::NullPad => "null-padded",
+        StringPadding::SpacePad => "space-padded",
+    }
+}
+
+fn cset_word(c: &CharacterSet) -> &'static str {
+    match c {
+        CharacterSet::Ascii => "ASCII",
+        CharacterSet::Utf8 => "UTF-8",
+    }
+}
+
+/// h5ls -v style description.
+pub fn long(dt: &Datatype) -> String {
+    match dt {
+        Datatype::FixedPoint {
+            size,
+            signed,
+            byte_order,
+            ..
+        } => format!(
+            "{}-bit {} {}integer",
+            u64::from(*size) * 8,
+            order_word(byte_order),
+            if *signed { "" } else { "unsigned " }
+        ),
+        Datatype::FloatingPoint {
+            size, byte_order, ..
+        } => {
+            if is_ieee(dt) {
+                format!(
+                    "IEEE {}-bit {} float",
+                    u64::from(*size) * 8,
+                    order_word(byte_order)
+                )
+            } else {
+                format!(
+                    "{}-bit {} non-IEEE float",
+                    u64::from(*size) * 8,
+                    order_word(byte_order)
+                )
+            }
+        }
+        Datatype::Time { size, .. } => format!("{}-bit time", u64::from(*size) * 8),
+        Datatype::String {
+            size,
+            padding,
+            charset,
+        } => format!(
+            "{size}-byte {} {} string",
+            pad_word(padding),
+            cset_word(charset)
+        ),
+        Datatype::BitField {
+            size, byte_order, ..
+        } => format!(
+            "{}-bit {} bitfield",
+            u64::from(*size) * 8,
+            order_word(byte_order)
+        ),
+        Datatype::Opaque { size, tag } => {
+            let tag = String::from_utf8_lossy(tag);
+            let tag = tag.trim_end_matches('\0');
+            if tag.is_empty() {
+                format!("{size}-byte opaque type")
+            } else {
+                format!("{size}-byte opaque type (tag \"{tag}\")")
+            }
+        }
+        Datatype::Compound { size, members } => {
+            let mut s = String::from("struct {");
+            for m in members {
+                s.push_str(&format!(
+                    "\n                   \"{}\" +{} {}",
+                    m.name,
+                    m.byte_offset,
+                    long(&m.datatype)
+                ));
+            }
+            s.push_str(&format!("\n               }} {size} bytes"));
+            s
+        }
+        Datatype::Reference { ref_type, .. } => match ref_type {
+            ReferenceType::Object | ReferenceType::Object2 => "object reference".into(),
+            ReferenceType::DatasetRegion | ReferenceType::DatasetRegion2 => {
+                "dataset region reference".into()
+            }
+            ReferenceType::Attribute => "attribute reference".into(),
+        },
+        Datatype::Enumeration {
+            base_type, members, ..
+        } => {
+            let vals: Vec<String> = members
+                .iter()
+                .map(|m| format!("{} = {}", m.name, enum_value_text(base_type, &m.value)))
+                .collect();
+            format!("enum {} {{{}}}", long(base_type), vals.join(", "))
+        }
+        Datatype::VariableLength {
+            is_string: true,
+            padding,
+            charset,
+            ..
+        } => format!(
+            "variable-length {} {} string",
+            padding.as_ref().map(pad_word).unwrap_or("null-terminated"),
+            charset.as_ref().map(cset_word).unwrap_or("ASCII")
+        ),
+        Datatype::VariableLength { base_type, .. } => {
+            format!("variable length of {}", long(base_type))
+        }
+        Datatype::Array {
+            base_type,
+            dimensions,
+        } => format!(
+            "[{}] {}",
+            dimensions
+                .iter()
+                .map(|d| d.to_string())
+                .collect::<Vec<_>>()
+                .join(","),
+            long(base_type)
+        ),
+    }
+}
+
+/// The integer an enum member's value bytes hold, as text.
+pub fn enum_value_text(base: &Datatype, bytes: &[u8]) -> String {
+    match crate::value::decode_int(base, bytes) {
+        Some(v) => v.to_string(),
+        None => crate::value::hex(bytes),
+    }
+}
+
+/// h5dump's predefined name for an atomic type, if it has one.
+fn atomic_ddl(dt: &Datatype) -> Option<String> {
+    match dt {
+        Datatype::FixedPoint {
+            size,
+            signed,
+            byte_order,
+            ..
+        } => Some(format!(
+            "H5T_STD_{}{}{}",
+            if *signed { "I" } else { "U" },
+            u64::from(*size) * 8,
+            order_suffix(byte_order)
+        )),
+        Datatype::FloatingPoint {
+            size, byte_order, ..
+        } if is_ieee(dt) => Some(format!(
+            "H5T_IEEE_F{}{}",
+            u64::from(*size) * 8,
+            order_suffix(byte_order)
+        )),
+        Datatype::BitField {
+            size, byte_order, ..
+        } => Some(format!(
+            "H5T_STD_B{}{}",
+            u64::from(*size) * 8,
+            order_suffix(byte_order)
+        )),
+        Datatype::Time { .. } => Some("H5T_TIME".into()),
+        _ => None,
+    }
+}
+
+/// h5dump DDL for a datatype; `ind` is the indentation of the line the type
+/// starts on (continuation lines are indented relative to it).
+pub fn ddl(dt: &Datatype, ind: usize) -> String {
+    let pad = " ".repeat(ind + 3);
+    let end = " ".repeat(ind);
+    if let Some(s) = atomic_ddl(dt) {
+        return s;
+    }
+    match dt {
+        Datatype::FloatingPoint {
+            size,
+            byte_order,
+            bit_offset,
+            bit_precision,
+            exponent_location,
+            exponent_size,
+            mantissa_location,
+            mantissa_size,
+            exponent_bias,
+        } => format!(
+            "H5T_FLOAT {{\n{pad}SIZE {size};\n{pad}ORDER H5T_ORDER_{};\n{pad}OFFSET {bit_offset};\n\
+             {pad}PRECISION {bit_precision};\n{pad}EXPONENT {exponent_location} {exponent_size} \
+             BIAS {exponent_bias};\n{pad}MANTISSA {mantissa_location} {mantissa_size};\n{end}}}",
+            order_suffix(byte_order)
+        ),
+        Datatype::String {
+            size,
+            padding,
+            charset,
+        } => string_ddl(&size.to_string(), padding, charset, &pad, &end),
+        Datatype::VariableLength {
+            is_string: true,
+            padding,
+            charset,
+            ..
+        } => string_ddl(
+            "H5T_VARIABLE",
+            padding.as_ref().unwrap_or(&StringPadding::NullTerminate),
+            charset.as_ref().unwrap_or(&CharacterSet::Ascii),
+            &pad,
+            &end,
+        ),
+        Datatype::VariableLength { base_type, .. } => {
+            format!("H5T_VLEN {{ {} }}", ddl(base_type, ind))
+        }
+        Datatype::Opaque { size, tag } => {
+            let tag = String::from_utf8_lossy(tag);
+            format!(
+                "H5T_OPAQUE {{\n{pad}OPAQUE_SIZE {size};\n{pad}OPAQUE_TAG \"{}\";\n{end}}}",
+                tag.trim_end_matches('\0')
+            )
+        }
+        Datatype::Compound { members, .. } => {
+            let mut s = String::from("H5T_COMPOUND {\n");
+            for m in members {
+                s.push_str(&format!(
+                    "{pad}{} \"{}\";\n",
+                    ddl(&m.datatype, ind + 3),
+                    m.name
+                ));
+            }
+            s.push_str(&end);
+            s.push('}');
+            s
+        }
+        Datatype::Reference { ref_type, .. } => match ref_type {
+            ReferenceType::Object => "H5T_REFERENCE { H5T_STD_REF_OBJECT }".into(),
+            ReferenceType::DatasetRegion => "H5T_REFERENCE { H5T_STD_REF_DSETREG }".into(),
+            _ => "H5T_REFERENCE { H5T_STD_REF }".into(),
+        },
+        Datatype::Enumeration {
+            base_type, members, ..
+        } => {
+            let mut s = format!("H5T_ENUM {{\n{pad}{};\n", ddl(base_type, ind + 3));
+            for m in members {
+                let name = format!("\"{}\"", m.name);
+                s.push_str(&format!(
+                    "{pad}{name:<18} {};\n",
+                    enum_value_text(base_type, &m.value)
+                ));
+            }
+            s.push_str(&end);
+            s.push('}');
+            s
+        }
+        Datatype::Array {
+            base_type,
+            dimensions,
+        } => format!(
+            "H5T_ARRAY {{ {} {} }}",
+            dimensions
+                .iter()
+                .map(|d| format!("[{d}]"))
+                .collect::<String>(),
+            ddl(base_type, ind)
+        ),
+        // Atomic types were handled above.
+        _ => short(dt),
+    }
+}
+
+fn string_ddl(
+    size: &str,
+    padding: &StringPadding,
+    charset: &CharacterSet,
+    pad: &str,
+    end: &str,
+) -> String {
+    let p = match padding {
+        StringPadding::NullTerminate => "H5T_STR_NULLTERM",
+        StringPadding::NullPad => "H5T_STR_NULLPAD",
+        StringPadding::SpacePad => "H5T_STR_SPACEPAD",
+    };
+    let c = match charset {
+        CharacterSet::Ascii => "H5T_CSET_ASCII",
+        CharacterSet::Utf8 => "H5T_CSET_UTF8",
+    };
+    format!(
+        "H5T_STRING {{\n{pad}STRSIZE {size};\n{pad}STRPAD {p};\n{pad}CSET {c};\n{pad}CTYPE H5T_C_S1;\n{end}}}"
+    )
+}
+
+/// hdf5-json type object.
+pub fn json(dt: &Datatype) -> J {
+    match dt {
+        Datatype::FixedPoint { .. } => {
+            json!({"class": "H5T_INTEGER", "base": atomic_ddl(dt)})
+        }
+        Datatype::FloatingPoint { .. } if is_ieee(dt) => {
+            json!({"class": "H5T_FLOAT", "base": atomic_ddl(dt)})
+        }
+        Datatype::FloatingPoint {
+            size, byte_order, ..
+        } => json!({
+            "class": "H5T_FLOAT",
+            "size": size,
+            "order": format!("H5T_ORDER_{}", order_suffix(byte_order)),
+        }),
+        Datatype::BitField { .. } => json!({"class": "H5T_BITFIELD", "base": atomic_ddl(dt)}),
+        Datatype::Time { size, .. } => json!({"class": "H5T_TIME", "size": size}),
+        Datatype::String {
+            size,
+            padding,
+            charset,
+        } => json!({
+            "class": "H5T_STRING",
+            "charSet": cset_json(charset),
+            "strPad": pad_json(padding),
+            "length": size,
+        }),
+        Datatype::VariableLength {
+            is_string: true,
+            padding,
+            charset,
+            ..
+        } => json!({
+            "class": "H5T_STRING",
+            "charSet": cset_json(charset.as_ref().unwrap_or(&CharacterSet::Ascii)),
+            "strPad": pad_json(padding.as_ref().unwrap_or(&StringPadding::NullTerminate)),
+            "length": "H5T_VARIABLE",
+        }),
+        Datatype::VariableLength { base_type, .. } => {
+            json!({"class": "H5T_VLEN", "base": json(base_type)})
+        }
+        Datatype::Opaque { size, tag } => json!({
+            "class": "H5T_OPAQUE",
+            "size": size,
+            "tag": String::from_utf8_lossy(tag).trim_end_matches('\0'),
+        }),
+        Datatype::Compound { members, .. } => json!({
+            "class": "H5T_COMPOUND",
+            "fields": members
+                .iter()
+                .map(|m| json!({"name": m.name, "type": json(&m.datatype)}))
+                .collect::<Vec<_>>(),
+        }),
+        Datatype::Reference { ref_type, .. } => json!({
+            "class": "H5T_REFERENCE",
+            "base": match ref_type {
+                ReferenceType::Object => "H5T_STD_REF_OBJ",
+                ReferenceType::DatasetRegion => "H5T_STD_REF_DSETREG",
+                _ => "H5T_STD_REF",
+            },
+        }),
+        Datatype::Enumeration {
+            base_type, members, ..
+        } => {
+            let mut mapping = serde_json::Map::new();
+            for m in members {
+                let v = crate::value::decode_int(base_type, &m.value)
+                    .and_then(|v| i64::try_from(v).ok())
+                    .map(J::from)
+                    .unwrap_or_else(|| J::from(crate::value::hex(&m.value)));
+                mapping.insert(m.name.clone(), v);
+            }
+            json!({"class": "H5T_ENUM", "base": json(base_type), "mapping": mapping})
+        }
+        Datatype::Array {
+            base_type,
+            dimensions,
+        } => json!({"class": "H5T_ARRAY", "base": json(base_type), "dims": dimensions}),
+    }
+}
+
+fn cset_json(c: &CharacterSet) -> &'static str {
+    match c {
+        CharacterSet::Ascii => "H5T_CSET_ASCII",
+        CharacterSet::Utf8 => "H5T_CSET_UTF8",
+    }
+}
+
+fn pad_json(p: &StringPadding) -> &'static str {
+    match p {
+        StringPadding::NullTerminate => "H5T_STR_NULLTERM",
+        StringPadding::NullPad => "H5T_STR_NULLPAD",
+        StringPadding::SpacePad => "H5T_STR_SPACEPAD",
+    }
+}
+
+/// Class name used to decide whether two datatypes can be compared.
+pub fn class(dt: &Datatype) -> &'static str {
+    match dt {
+        Datatype::FixedPoint { .. } => "integer",
+        Datatype::FloatingPoint { .. } => "float",
+        Datatype::Time { .. } => "time",
+        Datatype::String { .. } => "string",
+        Datatype::BitField { .. } => "bitfield",
+        Datatype::Opaque { .. } => "opaque",
+        Datatype::Compound { .. } => "compound",
+        Datatype::Reference { .. } => "reference",
+        Datatype::Enumeration { .. } => "enum",
+        Datatype::VariableLength {
+            is_string: true, ..
+        } => "string",
+        Datatype::VariableLength { .. } => "vlen",
+        Datatype::Array { .. } => "array",
+    }
+}

crates/clawhdf5-tools/src/dump.rs

@@ -0,0 +1,944 @@
+//! `h5rs dump`: the whole file (or one dataset) as h5dump-style DDL text or
+//! as hdf5-json.
+
+use std::cell::OnceCell;
+use std::collections::HashMap;
+
+use clawhdf5_format::attribute::AttributeMessage;
+use clawhdf5_format::data_layout::DataLayout;
+use clawhdf5_format::dataspace::{Dataspace, DataspaceType};
+use clawhdf5_format::datatype::{Datatype, StringPadding};
+use clawhdf5_format::object_header::ObjectHeader;
+use serde_json::{Map, Value as J, json};
+
+use crate::cli::{Args, Out};
+use crate::h5::{Error, H5, Kind, Link, LinkKind};
+use crate::info::{self, DsInfo};
+use crate::value::{self, Decoder, Value};
+
+pub const USAGE: &str = "\
+usage: h5rs dump [--json] [-A] [-p] [-d PATH] [--max-bytes N] FILE
+
+Print FILE's groups, datasets, named datatypes, links and attributes, with
+their values, as h5dump-style DDL text (default) or as JSON.
+
+  --json            hdf5-json layout (see the crate README for the schema)
+  -A, --header      no dataset values (attribute values are still printed,
+                    as with h5dump -A)
+  -p, --properties  also print each dataset's storage layout and filters
+  -d, --dataset P   dump only the dataset at path P
+  --max-bytes N     largest dataset or attribute decoded (default 1 GiB);
+                    a larger one is reported instead of read
+
+Exit status: 0 dumped, 1 something could not be read (reported on stderr
+and marked in the output), 2 error.";
+
+struct Opts {
+    json: bool,
+    header_only: bool,
+    props: bool,
+}
+
+struct Dump<'a> {
+    h5: &'a H5,
+    opts: Opts,
+    problems: usize,
+    paths: OnceCell<HashMap<u64, String>>,
+}
+
+pub fn run(args: &mut Args, out: &mut Out) -> std::io::Result<i32> {
+    let mut opts = Opts {
+        json: false,
+        header_only: false,
+        props: false,
+    };
+    let mut dataset = None;
+    let mut max_bytes = None;
+    let mut file = None;
+    while let Some(a) = args.next() {
+        match a.as_str() {
+            "--json" | "-j" => opts.json = true,
+            "-A" | "--header" => opts.header_only = true,
+            "-p" | "--properties" => opts.props = true,
+            "-d" | "--dataset" => match args.value() {
+                Some(p) => dataset = Some(p),
+                None => return args.usage_error(out, "-d needs a path", USAGE),
+            },
+            "--max-bytes" => match args.number() {
+                Some(n) => max_bytes = Some(n),
+                None => return args.usage_error(out, "--max-bytes needs a number", USAGE),
+            },
+            "-h" | "--help" => {
+                writeln!(out.o, "{USAGE}")?;
+                return Ok(0);
+            }
+            s if s.starts_with('-') && s.len() > 1 => {
+                return args.usage_error(out, &format!("unknown option {a}"), USAGE);
+            }
+            _ if file.is_none() => file = Some(a),
+            _ => return args.usage_error(out, &format!("unexpected argument {a}"), USAGE),
+        }
+    }
+    let Some(file) = file else {
+        return args.usage_error(out, "missing FILE", USAGE);
+    };
+    let mut h5 = match H5::open(std::path::Path::new(&file)) {
+        Ok(h) => h,
+        Err(e) => {
+            writeln!(out.e, "h5rs dump: {e}")?;
+            return Ok(2);
+        }
+    };
+    if let Some(m) = max_bytes {
+        h5.max_bytes = m;
+    }
+    let mut d = Dump {
+        h5: &h5,
+        opts,
+        problems: 0,
+        paths: OnceCell::new(),
+    };
+    let fname = std::path::Path::new(&file)
+        .file_name()
+        .map(|s| s.to_string_lossy().into_owned())
+        .unwrap_or(file.clone());
+    let code = if d.opts.json {
+        d.json(out, dataset.as_deref())?
+    } else {
+        d.ddl(out, &fname, dataset.as_deref())?
+    };
+    if code != 0 {
+        return Ok(code);
+    }
+    Ok(if d.problems > 0 { 1 } else { 0 })
+}
+
+/// A full path for `name` inside the group at `base`.
+fn join(base: &str, name: &str) -> String {
+    if base == "/" {
+        format!("/{name}")
+    } else {
+        format!("{base}/{name}")
+    }
+}
+
+fn quote(s: &str) -> String {
+    s.replace('\\', "\\\\").replace('"', "\\\"")
+}
+
+impl Dump<'_> {
+    /// Object address -> first path, for printing references.
+    fn paths(&self) -> &HashMap<u64, String> {
+        self.paths.get_or_init(|| {
+            let mut m = HashMap::new();
+            let _ = self.h5.walk(|it| {
+                if let (Some(a), None) = (it.addr, it.first_path) {
+                    m.entry(a).or_insert_with(|| it.path.to_string());
+                }
+            });
+            m
+        })
+    }
+
+    fn problem(&mut self, out: &mut Out, what: &str, e: &Error) -> std::io::Result<()> {
+        self.problems += 1;
+        writeln!(out.e, "h5rs dump: {what}: {e}")
+    }
+
+    // ------------------------------------------------------------------
+    // DDL
+    // ------------------------------------------------------------------
+
+    fn ddl(&mut self, out: &mut Out, fname: &str, only: Option<&str>) -> std::io::Result<i32> {
+        writeln!(out.o, "HDF5 \"{}\" {{", quote(fname))?;
+        if let Some(p) = only {
+            let h = match self.h5.resolve(p).and_then(|a| self.h5.header(a)) {
+                Ok(h) => h,
+                Err(e) => {
+                    writeln!(out.o, "}}")?;
+                    writeln!(out.e, "h5rs dump: {p}: {e}")?;
+                    return Ok(2);
+                }
+            };
+            if Kind::of(&h) != Kind::Dataset {
+                writeln!(out.o, "}}")?;
+                writeln!(out.e, "h5rs dump: {p}: not a dataset")?;
+                return Ok(2);
+            }
+            let full = if p.starts_with('/') {
+                p.to_string()
+            } else {
+                format!("/{p}")
+            };
+            self.ddl_dataset(out, &full, &full, &h, 0)?;
+        } else {
+            let root = self.h5.root();
+            let mut seen = HashMap::new();
+            match self.h5.header(root) {
+                Ok(h) => self.ddl_group(out, "/", "/", root, &h, 0, &mut seen)?,
+                Err(e) => {
+                    writeln!(out.o, "GROUP \"/\" {{\n}}")?;
+                    self.problem(out, "/", &e)?;
+                }
+            }
+        }
+        writeln!(out.o, "}}")?;
+        Ok(0)
+    }
+
+    #[allow(clippy::too_many_arguments)]
+    fn ddl_group(
+        &mut self,
+        out: &mut Out,
+        name: &str,
+        path: &str,
+        addr: u64,
+        h: &ObjectHeader,
+        ind: usize,
+        seen: &mut HashMap<u64, String>,
+    ) -> std::io::Result<()> {
+        let pad = " ".repeat(ind);
+        seen.insert(addr, path.to_string());
+        writeln!(out.o, "{pad}GROUP \"{}\" {{", quote(name))?;
+        self.ddl_attributes(out, path, h, ind + 3)?;
+        let links = match self.h5.links(h) {
+            Ok(l) => l,
+            Err(e) => {
+                self.problem(out, path, &e)?;
+                Vec::new()
+            }
+        };
+        // The DDL recursion follows the group nesting; bound it (the walk used
+        // by the other commands is iterative).
+        if seen.len() > crate::h5::MAX_OBJECTS || ind > 3 * MAX_DDL_DEPTH {
+            let e = Error::new("too many objects, or groups nested too deeply; stopped");
+            self.problem(out, path, &e)?;
+            return writeln!(out.o, "{pad}}}");
+        }
+        for l in &links {
+            self.ddl_link(out, path, l, ind + 3, seen)?;
+        }
+        writeln!(out.o, "{pad}}}")
+    }
+
+    fn ddl_link(
+        &mut self,
+        out: &mut Out,
+        base: &str,
+        l: &Link,
+        ind: usize,
+        seen: &mut HashMap<u64, String>,
+    ) -> std::io::Result<()> {
+        let pad = " ".repeat(ind);
+        let inner = " ".repeat(ind + 3);
+        let name = quote(&l.name);
+        let path = join(base, &l.name);
+        match &l.kind {
+            LinkKind::Soft(t) => writeln!(
+                out.o,
+                "{pad}SOFTLINK \"{name}\" {{\n{inner}LINKTARGET \"{}\"\n{pad}}}",
+                quote(t)
+            ),
+            LinkKind::External { file, path: p } => writeln!(
+                out.o,
+                "{pad}EXTERNAL_LINK \"{name}\" {{\n{inner}TARGETFILE \"{}\"\n{inner}TARGETPATH \"{}\"\n{pad}}}",
+                quote(file),
+                quote(p)
+            ),
+            LinkKind::UserDefined(t) => writeln!(
+                out.o,
+                "{pad}USERDEFINED_LINK \"{name}\" {{\n{inner}LINKCLASS {t}\n{pad}}}"
+            ),
+            LinkKind::Hard(a) => {
+                let h = match self.h5.header(*a) {
+                    Ok(h) => h,
+                    Err(e) => {
+                        self.problem(out, &path, &e)?;
+                        return writeln!(out.o, "{pad}UNKNOWN_OBJECT \"{name}\" {{\n{pad}}}");
+                    }
+                };
+                let kind = Kind::of(&h);
+                if let Some(first) = seen.get(a) {
+                    let word = match kind {
+                        Kind::Group => "GROUP",
+                        Kind::Dataset => "DATASET",
+                        Kind::Datatype => "DATATYPE",
+                        Kind::Unknown => "OBJECT",
+                    };
+                    return writeln!(
+                        out.o,
+                        "{pad}{word} \"{name}\" {{\n{inner}HARDLINK \"{}\"\n{pad}}}",
+                        quote(first)
+                    );
+                }
+                match kind {
+                    Kind::Group => self.ddl_group(out, &l.name, &path, *a, &h, ind, seen),
+                    Kind::Dataset => {
+                        seen.insert(*a, path.clone());
+                        self.ddl_dataset(out, &l.name, &path, &h, ind)
+                    }
+                    Kind::Datatype => {
+                        seen.insert(*a, path.clone());
+                        match self.h5.datatype(&h) {
+                            Ok(dt) => writeln!(
+                                out.o,
+                                "{pad}DATATYPE \"{name}\" {};",
+                                crate::dtype::ddl(&dt, ind)
+                            ),
+                            Err(e) => {
+                                self.problem(out, &path, &e)?;
+                                writeln!(out.o, "{pad}DATATYPE \"{name}\" ?;")
+                            }
+                        }
+                    }
+                    Kind::Unknown => {
+                        seen.insert(*a, path.clone());
+                        writeln!(out.o, "{pad}UNKNOWN_OBJECT \"{name}\" {{\n{pad}}}")
+                    }
+                }
+            }
+        }
+    }
+
+    fn ddl_dataset(
+        &mut self,
+        out: &mut Out,
+        name: &str,
+        path: &str,
+        h: &ObjectHeader,
+        ind: usize,
+    ) -> std::io::Result<()> {
+        let pad = " ".repeat(ind);
+        let inner = " ".repeat(ind + 3);
+        writeln!(out.o, "{pad}DATASET \"{}\" {{", quote(name))?;
+        let info = DsInfo::read(self.h5, path, h);
+        match &info.dt {
+            Ok(dt) => writeln!(out.o, "{inner}DATATYPE  {}", crate::dtype::ddl(dt, ind + 3))?,
+            Err(e) => self.problem(out, path, e)?,
+        }
+        match &info.ds {
+            Ok(ds) => writeln!(out.o, "{inner}DATASPACE  {}", info::dataspace_ddl(ds))?,
+            Err(e) => self.problem(out, path, e)?,
+        }
+        if self.opts.props {
+            self.ddl_properties(out, path, &info, ind + 3)?;
+        }
+        if !self.opts.header_only
+            && let (Ok(dt), Ok(ds)) = (&info.dt, &info.ds)
+        {
+            match self.h5.read_dataset(path, dt, ds) {
+                Ok(raw) => self.ddl_data(out, dt, ds, &raw, ind + 3)?,
+                Err(e) => {
+                    writeln!(out.o, "{inner}DATA {{\n{inner}<not read: {e}>\n{inner}}}")?;
+                    self.problem(out, path, &e)?;
+                }
+            }
+        }
+        self.ddl_attributes(out, path, h, ind + 3)?;
+        writeln!(out.o, "{pad}}}")
+    }
+
+    fn ddl_properties(
+        &mut self,
+        out: &mut Out,
+        path: &str,
+        info: &DsInfo,
+        ind: usize,
+    ) -> std::io::Result<()> {
+        let pad = " ".repeat(ind);
+        let inner = " ".repeat(ind + 3);
+        let Ok(layout) = &info.layout else {
+            if let Err(e) = &info.layout {
+                self.problem(out, path, e)?;
+            }
+            return Ok(());
+        };
+        writeln!(out.o, "{pad}STORAGE_LAYOUT {{")?;
+        match layout {
+            DataLayout::Chunked {
+                chunk_dimensions, ..
+            } => {
+                let rank = chunk_dimensions.len().saturating_sub(1);
+                writeln!(
+                    out.o,
+                    "{inner}CHUNKED ( {} )",
+                    chunk_dimensions[..rank]
+                        .iter()
+                        .map(|d| d.to_string())
+                        .collect::<Vec<_>>()
+                        .join(", ")
+                )?;
+                writeln!(
+                    out.o,
+                    "{inner}INDEX {}",
+                    info::chunk_index_name(layout).to_uppercase()
+                )?;
+            }
+            DataLayout::Contiguous { address, size } => {
+                writeln!(out.o, "{inner}CONTIGUOUS")?;
+                match address {
+                    Some(a) => writeln!(out.o, "{inner}OFFSET {a}")?,
+                    None => writeln!(out.o, "{inner}NOT ALLOCATED")?,
+                }
+                writeln!(out.o, "{inner}SIZE {size}")?;
+            }
+            DataLayout::Compact { data } => {
+                writeln!(out.o, "{inner}COMPACT")?;
+                writeln!(out.o, "{inner}SIZE {}", data.len())?;
+            }
+            DataLayout::Virtual { .. } => writeln!(out.o, "{inner}VIRTUAL")?,
+        }
+        if let Ok(a) = info::allocated_bytes(self.h5, info)
+            && matches!(layout, DataLayout::Chunked { .. })
+        {
+            writeln!(out.o, "{inner}SIZE {a}")?;
+        }
+        writeln!(out.o, "{pad}}}")?;
+        writeln!(out.o, "{pad}FILTERS {{")?;
+        match &info.filters {
+            Ok(Some(p)) if !p.filters.is_empty() => {
+                for f in &p.filters {
+                    writeln!(
+                        out.o,
+                        "{inner}{} {{ ID {}; PARAMS {{ {} }} }}",
+                        info::filter_name(f).to_uppercase(),
+                        f.filter_id,
+                        f.client_data
+                            .iter()
+                            .map(|c| c.to_string())
+                            .collect::<Vec<_>>()
+                            .join(" ")
+                    )?;
+                }
+            }
+            Ok(_) => writeln!(out.o, "{inner}NONE")?,
+            Err(e) => self.problem(out, path, e)?,
+        }
+        writeln!(out.o, "{pad}}}")
+    }
+
+    fn ddl_attributes(
+        &mut self,
+        out: &mut Out,
+        path: &str,
+        h: &ObjectHeader,
+        ind: usize,
+    ) -> std::io::Result<()> {
+        let (attrs, errs) = match self.h5.attributes(h) {
+            Ok(x) => x,
+            Err(e) => return self.problem(out, path, &e),
+        };
+        for e in errs {
+            self.problem(out, path, &Error::new(format!("attribute: {e}")))?;
+        }
+        for a in &attrs {
+            self.ddl_attribute(out, path, a, ind)?;
+        }
+        Ok(())
+    }
+
+    fn ddl_attribute(
+        &mut self,
+        out: &mut Out,
+        path: &str,
+        a: &AttributeMessage,
+        ind: usize,
+    ) -> std::io::Result<()> {
+        let pad = " ".repeat(ind);
+        let inner = " ".repeat(ind + 3);
+        writeln!(out.o, "{pad}ATTRIBUTE \"{}\" {{", quote(&a.name))?;
+        writeln!(
+            out.o,
+            "{inner}DATATYPE  {}",
+            crate::dtype::ddl(&a.datatype, ind + 3)
+        )?;
+        writeln!(
+            out.o,
+            "{inner}DATASPACE  {}",
+            info::dataspace_ddl(&a.dataspace)
+        )?;
+        {
+            match self.checked_attr(a) {
+                Ok(()) => self.ddl_data(out, &a.datatype, &a.dataspace, &a.raw_data, ind + 3)?,
+                Err(e) => {
+                    writeln!(out.o, "{inner}DATA {{\n{inner}<not read: {e}>\n{inner}}}")?;
+                    self.problem(out, &format!("{path} attribute {}", a.name), &e)?;
+                }
+            }
+        }
+        writeln!(out.o, "{pad}}}")
+    }
+
+    /// An attribute's value holds as many bytes as its dataspace says.
+    fn checked_attr(&self, a: &AttributeMessage) -> crate::h5::Result<()> {
+        let need = crate::h5::byte_len(&a.dataspace, &a.datatype)?;
+        if need > self.h5.max_bytes {
+            return Err(Error::new(format!(
+                "attribute is {need} bytes, over the {} byte limit (--max-bytes)",
+                self.h5.max_bytes
+            )));
+        }
+        if (a.raw_data.len() as u64) < need {
+            return Err(Error::new(format!(
+                "attribute holds {} bytes, its dataspace needs {need}",
+                a.raw_data.len()
+            )));
+        }
+        Ok(())
+    }
+
+    /// An h5dump DATA block: elements row by row, each line starting with
+    /// the index of its first element, wrapped where h5dump wraps (78 columns).
+    fn ddl_data(
+        &mut self,
+        out: &mut Out,
+        dt: &Datatype,
+        ds: &Dataspace,
+        raw: &[u8],
+        ind: usize,
+    ) -> std::io::Result<()> {
+        let pad = " ".repeat(ind);
+        writeln!(out.o, "{pad}DATA {{")?;
+        let dims: Vec<u64> = match ds.space_type {
+            DataspaceType::Null => {
+                return writeln!(out.o, "{pad}}}");
+            }
+            DataspaceType::Scalar => vec![1],
+            DataspaceType::Simple => ds.dimensions.clone(),
+        };
+        let n = crate::h5::num_elements(ds).unwrap_or(0) as usize;
+        let dec = Decoder::new(self.h5);
+        let last = dims.last().copied().unwrap_or(1).max(1) as usize;
+        let paths = |a: u64| self.paths().get(&a).cloned();
+        let mut line = String::new();
+        let mut errors = 0usize;
+        for i in 0..n {
+            let v = dec.element(dt, raw, i);
+            if matches!(v, Value::Error(_)) {
+                errors += 1;
+            }
+            let comma = if i + 1 < n { "," } else { "" };
+            if let Value::Compound(members) = &v {
+                // h5dump prints each compound element as a block, one
+                // member per line.
+                if !line.is_empty() {
+                    writeln!(out.o, "{line}")?;
+                    line.clear();
+                }
+                writeln!(out.o, "{pad}({}): {{", index_text(i as u64, &dims))?;
+                for (k, (_, m)) in members.iter().enumerate() {
+                    let sep = if k + 1 < members.len() { "," } else { "" };
+                    writeln!(out.o, "{pad}      {}{sep}", value::text(m, &paths))?;
+                }
+                writeln!(out.o, "{pad}   }}{comma}")?;
+                continue;
+            }
+            let t = match dt {
+                // h5dump shows a null-padded string's padding.
+                Datatype::String {
+                    padding: StringPadding::NullPad,
+                    size,
+                    ..
+                } if !matches!(v, Value::Error(_)) => {
+                    let sz = *size as usize;
+                    let b = raw.get(i * sz..(i + 1) * sz).unwrap_or_default();
+                    format!("{}{comma}", value::quote_bytes(b))
+                }
+                _ => format!("{}{comma}", value::text(&v, &paths)),
+            };
+            let row_start = i % last == 0;
+            if row_start || line.len() + 1 + t.len() > 77 {
+                if !line.is_empty() {
+                    writeln!(out.o, "{line}")?;
+                }
+                line = format!("{pad}({}): {t}", index_text(i as u64, &dims));
+            } else {
+                line.push(' ');
+                line.push_str(&t);
+            }
+        }
+        if !line.is_empty() {
+            writeln!(out.o, "{line}")?;
+        }
+        if errors > 0 {
+            self.problems += 1;
+            writeln!(out.e, "h5rs dump: {errors} element(s) could not be decoded")?;
+        }
+        writeln!(out.o, "{pad}}}")
+    }
+
+    // ------------------------------------------------------------------
+    // JSON
+    // ------------------------------------------------------------------
+
+    fn json(&mut self, out: &mut Out, only: Option<&str>) -> std::io::Result<i32> {
+        let mut groups = Map::new();
+        let mut datasets = Map::new();
+        let mut datatypes = Map::new();
+        let mut doc = Map::new();
+        doc.insert("apiVersion".into(), json!("1.1.1"));
+        if let Some(p) = only {
+            let full = if p.starts_with('/') {
+                p.to_string()
+            } else {
+                format!("/{p}")
+            };
+            let r = self
+                .h5
+                .resolve(&full)
+                .and_then(|a| Ok((a, self.h5.header(a)?)));
+            match r {
+                Ok((a, h)) if Kind::of(&h) == Kind::Dataset => {
+                    let obj = self.json_dataset(out, &full, &h)?;
+                    datasets.insert(obj_id(Kind::Dataset, a), obj);
+                }
+                Ok(_) => {
+                    writeln!(out.e, "h5rs dump: {p}: not a dataset")?;
+                    return Ok(2);
+                }
+                Err(e) => {
+                    writeln!(out.e, "h5rs dump: {p}: {e}")?;
+                    return Ok(2);
+                }
+            }
+        } else {
+            doc.insert("root".into(), json!(obj_id(Kind::Group, self.h5.root())));
+            let (items, walk) = self.h5.walk_collect();
+            if let Err(e) = walk {
+                self.problem(out, "/", &e)?;
+            }
+            // Aliases: every path an object is reachable by.
+            let mut aliases: HashMap<u64, Vec<String>> = HashMap::new();
+            for it in &items {
+                if let Some(a) = it.addr {
+                    aliases.entry(a).or_default().push(it.path.clone());
+                }
+            }
+            for it in items {
+                let path = it.path;
+                let (Some(addr), None, Some(header)) = (it.addr, it.first_path, it.header) else {
+                    continue;
+                };
+                let h = match header {
+                    Ok(h) => h,
+                    Err(e) => {
+                        self.problem(out, &path, &e)?;
+                        continue;
+                    }
+                };
+                let kind = Kind::of(&h);
+                let alias = aliases.remove(&addr).unwrap_or_default();
+                let mut obj = match kind {
+                    Kind::Dataset => self.json_dataset(out, &path, &h)?,
+                    Kind::Datatype => {
+                        let mut m = Map::new();
+                        match self.h5.datatype(&h) {
+                            Ok(dt) => {
+                                m.insert("type".into(), crate::dtype::json(&dt));
+                            }
+                            Err(e) => self.problem(out, &path, &e)?,
+                        }
+                        m.insert("attributes".into(), self.json_attributes(out, &path, &h)?);
+                        J::Object(m)
+                    }
+                    _ => {
+                        let mut m = Map::new();
+                        m.insert("attributes".into(), self.json_attributes(out, &path, &h)?);
+                        let links = match self.h5.links(&h) {
+                            Ok(l) => l,
+                            Err(e) => {
+                                if kind == Kind::Group {
+                                    self.problem(out, &path, &e)?;
+                                }
+                                Vec::new()
+                            }
+                        };
+                        let mut jl = Vec::new();
+                        for l in &links {
+                            jl.push(self.json_link(l));
+                        }
+                        m.insert("links".into(), J::Array(jl));
+                        J::Object(m)
+                    }
+                };
+                if let J::Object(m) = &mut obj {
+                    m.insert("alias".into(), json!(alias));
+                }
+                let id = obj_id(kind, addr);
+                match kind {
+                    Kind::Dataset => datasets.insert(id, obj),
+                    Kind::Datatype => datatypes.insert(id, obj),
+                    _ => groups.insert(id, obj),
+                };
+            }
+            doc.insert("groups".into(), J::Object(groups));
+        }
+        doc.insert("datasets".into(), J::Object(datasets));
+        if only.is_none() {
+            doc.insert("datatypes".into(), J::Object(datatypes));
+        }
+        let text = serde_json::to_string_pretty(&J::Object(doc)).map_err(std::io::Error::other)?;
+        writeln!(out.o, "{text}")?;
+        Ok(0)
+    }
+
+    fn json_link(&self, l: &Link) -> J {
+        match &l.kind {
+            LinkKind::Hard(a) => {
+                let kind = self
+                    .h5
+                    .header(*a)
+                    .map(|h| Kind::of(&h))
+                    .unwrap_or(Kind::Unknown);
+                let coll = match kind {
+                    Kind::Dataset => "datasets",
+                    Kind::Datatype => "datatypes",
+                    _ => "groups",
+                };
+                json!({"class": "H5L_TYPE_HARD", "title": l.name, "collection": coll,
+                       "id": obj_id(kind, *a)})
+            }
+            LinkKind::Soft(t) => json!({"class": "H5L_TYPE_SOFT", "title": l.name, "h5path": t}),
+            LinkKind::External { file, path } => json!({"class": "H5L_TYPE_EXTERNAL",
+                "title": l.name, "file": file, "h5path": path}),
+            LinkKind::UserDefined(t) => json!({"class": "H5L_TYPE_USER_DEFINED",
+                "title": l.name, "linkClass": t}),
+        }
+    }
+
+    fn json_dataset(&mut self, out: &mut Out, path: &str, h: &ObjectHeader) -> std::io::Result<J> {
+        let info = DsInfo::read(self.h5, path, h);
+        let mut m = Map::new();
+        match &info.dt {
+            Ok(dt) => {
+                m.insert("type".into(), crate::dtype::json(dt));
+            }
+            Err(e) => self.problem(out, path, e)?,
+        }
+        match &info.ds {
+            Ok(ds) => {
+                m.insert("shape".into(), shape_json(ds));
+            }
+            Err(e) => self.problem(out, path, e)?,
+        }
+        let mut cp = Map::new();
+        if let Ok(l) = &info.layout {
+            let mut lj = Map::new();
+            lj.insert(
+                "class".into(),
+                json!(match l {
+                    DataLayout::Compact { .. } => "H5D_COMPACT",
+                    DataLayout::Contiguous { .. } => "H5D_CONTIGUOUS",
+                    DataLayout::Chunked { .. } => "H5D_CHUNKED",
+                    DataLayout::Virtual { .. } => "H5D_VIRTUAL",
+                }),
+            );
+            if let DataLayout::Chunked {
+                chunk_dimensions, ..
+            } = l
+            {
+                let rank = chunk_dimensions.len().saturating_sub(1);
+                lj.insert("dims".into(), json!(chunk_dimensions[..rank]));
+            }
+            cp.insert("layout".into(), J::Object(lj));
+        } else if let Err(e) = &info.layout {
+            self.problem(out, path, e)?;
+        }
+        if let Ok(Some(p)) = &info.filters {
+            let fl: Vec<J> = p
+                .filters
+                .iter()
+                .map(|f| {
+                    json!({"id": f.filter_id, "name": info::filter_name(f),
+                           "class": filter_class(f.filter_id), "parameters": f.client_data})
+                })
+                .collect();
+            cp.insert("filters".into(), J::Array(fl));
+        }
+        m.insert("creationProperties".into(), J::Object(cp));
+        if !self.opts.header_only
+            && let (Ok(dt), Ok(ds)) = (&info.dt, &info.ds)
+        {
+            // JSON values are built in memory, some 32+ bytes per element:
+            // hold them to the same budget as the raw data.
+            let n = crate::h5::num_elements(ds).unwrap_or(u64::MAX);
+            let read = if n.saturating_mul(JSON_BYTES_PER_ELEMENT) > self.h5.max_bytes {
+                Err(Error::new(format!(
+                    "{n} elements are too many to hold as JSON within --max-bytes {}",
+                    self.h5.max_bytes
+                ))
+                .with_kind(crate::h5::ErrorKind::Limit))
+            } else {
+                self.h5.read_dataset(path, dt, ds)
+            };
+            match read {
+                Ok(raw) => {
+                    let v = self.json_values(out, dt, ds, &raw)?;
+                    m.insert("value".into(), v);
+                }
+                Err(e) => {
+                    m.insert("value_error".into(), json!(e.to_string()));
+                    self.problem(out, path, &e)?;
+                }
+            }
+        }
+        m.insert("attributes".into(), self.json_attributes(out, path, h)?);
+        Ok(J::Object(m))
+    }
+
+    fn json_attributes(
+        &mut self,
+        out: &mut Out,
+        path: &str,
+        h: &ObjectHeader,
+    ) -> std::io::Result<J> {
+        let (attrs, errs) = match self.h5.attributes(h) {
+            Ok(x) => x,
+            Err(e) => {
+                self.problem(out, path, &e)?;
+                return Ok(J::Array(Vec::new()));
+            }
+        };
+        for e in errs {
+            self.problem(out, path, &Error::new(format!("attribute: {e}")))?;
+        }
+        let mut v = Vec::new();
+        for a in &attrs {
+            let mut m = Map::new();
+            m.insert("name".into(), json!(a.name));
+            m.insert("shape".into(), shape_json(&a.dataspace));
+            m.insert("type".into(), crate::dtype::json(&a.datatype));
+            {
+                match self.checked_attr(a) {
+                    Ok(()) => {
+                        let val = self.json_values(out, &a.datatype, &a.dataspace, &a.raw_data)?;
+                        m.insert("value".into(), val);
+                    }
+                    Err(e) => {
+                        m.insert("value_error".into(), json!(e.to_string()));
+                        self.problem(out, &format!("{path} attribute {}", a.name), &e)?;
+                    }
+                }
+            }
+            v.push(J::Object(m));
+        }
+        Ok(J::Array(v))
+    }
+
+    fn json_values(
+        &mut self,
+        out: &mut Out,
+        dt: &Datatype,
+        ds: &Dataspace,
+        raw: &[u8],
+    ) -> std::io::Result<J> {
+        let n = crate::h5::num_elements(ds).unwrap_or(0) as usize;
+        let dec = Decoder::new(self.h5);
+        let paths = |a: u64| self.paths().get(&a).cloned();
+        let mut flat = Vec::with_capacity(n);
+        let mut errors = 0usize;
+        for i in 0..n {
+            let v = dec.element(dt, raw, i);
+            if matches!(v, Value::Error(_)) {
+                errors += 1;
+            }
+            flat.push(value::to_json(&v, &paths));
+        }
+        if errors > 0 {
+            self.problems += 1;
+            writeln!(out.e, "h5rs dump: {errors} element(s) could not be decoded")?;
+        }
+        Ok(match ds.space_type {
+            DataspaceType::Null => J::Null,
+            DataspaceType::Scalar => flat.into_iter().next().unwrap_or(J::Null),
+            DataspaceType::Simple => nest(flat, &ds.dimensions),
+        })
+    }
+}
+
+/// Deepest group nesting the DDL output follows.
+const MAX_DDL_DEPTH: usize = 256;
+
+/// Memory budgeted per element for a dataset's value held as JSON.
+const JSON_BYTES_PER_ELEMENT: u64 = 64;
+
+/// `(i,j,k)` index of flat element `i` of an array with `dims`.
+fn index_text(mut i: u64, dims: &[u64]) -> String {
+    let mut idx = vec![0u64; dims.len()];
+    for (k, &d) in dims.iter().enumerate().rev() {
+        let d = d.max(1);
+        idx[k] = i % d;
+        i /= d;
+    }
+    idx.iter()
+        .map(|x| x.to_string())
+        .collect::<Vec<_>>()
+        .join(",")
+}
+
+/// Turn a flat row-major list into nested lists of shape `dims`.
+fn nest(flat: Vec<J>, dims: &[u64]) -> J {
+    if dims.len() <= 1 {
+        return J::Array(flat);
+    }
+    let inner: usize = dims[1..].iter().map(|&d| d as usize).product();
+    if inner == 0 {
+        return J::Array((0..dims[0]).map(|_| nest(Vec::new(), &dims[1..])).collect());
+    }
+    let mut it = flat.into_iter();
+    let mut outer = Vec::with_capacity(dims[0] as usize);
+    for _ in 0..dims[0] {
+        let part: Vec<J> = it.by_ref().take(inner).collect();
+        outer.push(nest(part, &dims[1..]));
+    }
+    J::Array(outer)
+}
+
+fn shape_json(ds: &Dataspace) -> J {
+    match ds.space_type {
+        DataspaceType::Null => json!({"class": "H5S_NULL"}),
+        DataspaceType::Scalar => json!({"class": "H5S_SCALAR"}),
+        DataspaceType::Simple => {
+            let mut m = Map::new();
+            m.insert("class".into(), json!("H5S_SIMPLE"));
+            m.insert("dims".into(), json!(ds.dimensions));
+            if let Some(max) = &ds.max_dimensions {
+                let mx: Vec<J> = max
+                    .iter()
+                    .map(|&d| {
+                        if d == u64::MAX {
+                            json!("H5S_UNLIMITED")
+                        } else {
+                            json!(d)
+                        }
+                    })
+                    .collect();
+                m.insert("maxdims".into(), J::Array(mx));
+            }
+            J::Object(m)
+        }
+    }
+}
+
+fn filter_class(id: u16) -> &'static str {
+    match id {
+        1 => "H5Z_FILTER_DEFLATE",
+        2 => "H5Z_FILTER_SHUFFLE",
+        3 => "H5Z_FILTER_FLETCHER32",
+        4 => "H5Z_FILTER_SZIP",
+        5 => "H5Z_FILTER_NBIT",
+        6 => "H5Z_FILTER_SCALEOFFSET",
+        _ => "H5Z_FILTER_USER",
+    }
+}
+
+/// Deterministic object id: kind prefix + header address (hdf5-json uses
+/// UUIDs; these are stable for a given file instead).
+pub fn obj_id(kind: Kind, addr: u64) -> String {
+    let p = match kind {
+        Kind::Dataset => 'd',
+        Kind::Datatype => 't',
+        _ => 'g',
+    };
+    format!("{p}-{addr:016x}")
+}

crates/clawhdf5-tools/src/h5.rs

@@ -0,0 +1,718 @@
+//! The file model the tools share: an open file, its objects, their links
+//! and the messages that describe a dataset, all read through
+//! `clawhdf5-format` (and the `clawhdf5` facade for dataset values).
+//!
+//! Nothing here trusts the file: every size is checked before it is used,
+//! and every failure is an [`Error`] carrying the address it happened at.
+
+use std::cell::RefCell;
+use std::collections::HashMap;
+use std::path::{Path, PathBuf};
+use std::rc::Rc;
+
+use clawhdf5::File;
+use clawhdf5_format::attribute::{AttributeMessage, extract_attributes_tolerant};
+use clawhdf5_format::attribute_info::AttributeInfoMessage;
+use clawhdf5_format::btree_v2::{BTreeV2Header, collect_btree_v2_records};
+use clawhdf5_format::data_layout::DataLayout;
+use clawhdf5_format::dataspace::{Dataspace, DataspaceType};
+use clawhdf5_format::datatype::Datatype;
+use clawhdf5_format::error::FormatError;
+use clawhdf5_format::filter_pipeline::FilterPipeline;
+use clawhdf5_format::fractal_heap::FractalHeapHeader;
+use clawhdf5_format::global_heap::GlobalHeapCollection;
+use clawhdf5_format::group_v1;
+use clawhdf5_format::link_info::LinkInfoMessage;
+use clawhdf5_format::link_message::{LinkMessage, LinkTarget};
+use clawhdf5_format::message_type::MessageType;
+use clawhdf5_format::object_header::ObjectHeader;
+use clawhdf5_format::superblock::Superblock;
+use clawhdf5_format::symbol_table::SymbolTableMessage;
+
+/// Largest dataset or attribute (in bytes) the tools will decode by default.
+/// A corrupt dataspace can claim far more elements than the file holds; the
+/// limit keeps such a file from exhausting memory. `--max-bytes` changes it.
+pub const DEFAULT_MAX_BYTES: u64 = 1 << 30;
+
+/// Upper bound on the objects one traversal visits, so a crafted file with
+/// millions of links cannot make a walk run for ever.
+pub const MAX_OBJECTS: usize = 1_000_000;
+
+/// A tool error: what went wrong and, when known, the file address (relative
+/// to the superblock, as every HDF5 address is) of the structure involved.
+#[derive(Debug, Clone)]
+pub struct Error {
+    pub addr: Option<u64>,
+    pub msg: String,
+    pub kind: ErrorKind,
+}
+
+/// Whether an error says something is wrong with the file.
+#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+pub enum ErrorKind {
+    /// The file is damaged or not valid HDF5 (as far as clawhdf5 knows).
+    Corrupt,
+    /// Valid HDF5 that clawhdf5 cannot decode (a filter it does not
+    /// implement, external raw data files, ...).
+    Unsupported,
+    /// Refused by a limit of the tool (`--max-bytes`).
+    Limit,
+}
+
+impl Error {
+    pub fn new(msg: impl Into<String>) -> Self {
+        Self {
+            addr: None,
+            msg: msg.into(),
+            kind: ErrorKind::Corrupt,
+        }
+    }
+
+    pub fn at(addr: u64, msg: impl Into<String>) -> Self {
+        Self {
+            addr: Some(addr),
+            msg: msg.into(),
+            kind: ErrorKind::Corrupt,
+        }
+    }
+
+    pub fn with_kind(mut self, kind: ErrorKind) -> Self {
+        self.kind = kind;
+        self
+    }
+
+    /// The same error with `prefix: ` in front of its message.
+    pub fn context(mut self, prefix: &str) -> Self {
+        self.msg = format!("{prefix}: {}", self.msg);
+        self
+    }
+}
+
+fn format_error_kind(e: &FormatError) -> ErrorKind {
+    match e {
+        FormatError::UnsupportedFilter(_)
+        | FormatError::ExternalDataFilesUnsupported
+        | FormatError::ExternalLinkUnsupported { .. } => ErrorKind::Unsupported,
+        _ => ErrorKind::Corrupt,
+    }
+}
+
+impl std::fmt::Display for Error {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        match self.addr {
+            Some(a) => write!(f, "{} (at address {a:#x})", self.msg),
+            None => write!(f, "{}", self.msg),
+        }
+    }
+}
+
+impl From<FormatError> for Error {
+    fn from(e: FormatError) -> Self {
+        Error::new(e.to_string()).with_kind(format_error_kind(&e))
+    }
+}
+
+impl From<clawhdf5::Error> for Error {
+    fn from(e: clawhdf5::Error) -> Self {
+        let kind = match &e {
+            clawhdf5::Error::Format(f) => format_error_kind(f),
+            _ => ErrorKind::Corrupt,
+        };
+        Error::new(e.to_string()).with_kind(kind)
+    }
+}
+
+pub type Result<T> = std::result::Result<T, Error>;
+
+/// Attach an address to a format error.
+pub fn fe_at(addr: u64) -> impl Fn(FormatError) -> Error {
+    move |e| Error::at(addr, e.to_string())
+}
+
+/// What an object header describes.
+#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+pub enum Kind {
+    Group,
+    Dataset,
+    Datatype,
+    Unknown,
+}
+
+impl Kind {
+    pub fn of(h: &ObjectHeader) -> Kind {
+        let has = |t: MessageType| h.messages.iter().any(|m| m.msg_type == t);
+        if has(MessageType::DataLayout) {
+            Kind::Dataset
+        } else if has(MessageType::SymbolTable)
+            || has(MessageType::LinkInfo)
+            || has(MessageType::Link)
+            || has(MessageType::GroupInfo)
+        {
+            Kind::Group
+        } else if has(MessageType::Datatype) {
+            Kind::Datatype
+        } else {
+            Kind::Unknown
+        }
+    }
+
+    pub fn name(self) -> &'static str {
+        match self {
+            Kind::Group => "Group",
+            Kind::Dataset => "Dataset",
+            Kind::Datatype => "Type",
+            Kind::Unknown => "Unknown",
+        }
+    }
+}
+
+/// Where a link points.
+#[derive(Debug, Clone, PartialEq, Eq)]
+pub enum LinkKind {
+    Hard(u64),
+    Soft(String),
+    External {
+        file: String,
+        path: String,
+    },
+    /// A user-defined link class (type 65-255): its target means something
+    /// only to the application that registered the class.
+    UserDefined(u8),
+}
+
+#[derive(Debug, Clone)]
+pub struct Link {
+    pub name: String,
+    pub kind: LinkKind,
+}
+
+/// An open file.
+pub struct H5 {
+    pub path: PathBuf,
+    pub file: File,
+    pub max_bytes: u64,
+    heaps: RefCell<HashMap<u64, std::result::Result<Rc<GlobalHeapCollection>, String>>>,
+    /// Fractal heaps whose blocks were verified: `None` = sound.
+    verified_heaps: RefCell<HashMap<u64, Option<Error>>>,
+}
+
+impl H5 {
+    pub fn open(path: &Path) -> Result<H5> {
+        if !path.exists() {
+            return Err(Error::new(format!("{}: no such file", path.display())));
+        }
+        let file = File::open(path).map_err(|e| {
+            Error::new(format!(
+                "{}: not an HDF5 file this tool can open: {e}",
+                path.display()
+            ))
+        })?;
+        Ok(H5 {
+            path: path.to_path_buf(),
+            file,
+            max_bytes: DEFAULT_MAX_BYTES,
+            heaps: RefCell::new(HashMap::new()),
+            verified_heaps: RefCell::new(HashMap::new()),
+        })
+    }
+
+    /// The file's bytes from the superblock on: what every address indexes.
+    pub fn data(&self) -> &[u8] {
+        self.file.as_bytes()
+    }
+
+    pub fn sb(&self) -> &Superblock {
+        self.file.superblock()
+    }
+
+    pub fn os(&self) -> u8 {
+        self.sb().offset_size
+    }
+
+    pub fn ls(&self) -> u8 {
+        self.sb().length_size
+    }
+
+    pub fn root(&self) -> u64 {
+        self.sb().root_group_address
+    }
+
+    pub fn header(&self, addr: u64) -> Result<ObjectHeader> {
+        let off = usize::try_from(addr).map_err(|_| Error::at(addr, "address out of range"))?;
+        ObjectHeader::parse(self.data(), off, self.os(), self.ls())
+            .map_err(|e| Error::at(addr, format!("object header: {e}")))
+    }
+
+    /// The payload of the first message of type `t`, resolving a shared
+    /// message to the message it points at.
+    pub fn payload(&self, h: &ObjectHeader, t: MessageType) -> Result<Option<Vec<u8>>> {
+        match h.messages.iter().find(|m| m.msg_type == t) {
+            None => Ok(None),
+            Some(m) => {
+                clawhdf5_format::shared_message::message_data(self.data(), m, self.os(), self.ls())
+                    .map(|c| Some(c.into_owned()))
+                    .map_err(|e| Error::new(format!("{t:?} message: {e}")))
+            }
+        }
+    }
+
+    pub fn datatype(&self, h: &ObjectHeader) -> Result<Datatype> {
+        let b = self
+            .payload(h, MessageType::Datatype)?
+            .ok_or_else(|| Error::new("no datatype message"))?;
+        Datatype::parse(&b)
+            .map(|(d, _)| d)
+            .map_err(|e| Error::new(format!("datatype message: {e}")))
+    }
+
+    pub fn dataspace(&self, h: &ObjectHeader) -> Result<Dataspace> {
+        let b = self
+            .payload(h, MessageType::Dataspace)?
+            .ok_or_else(|| Error::new("no dataspace message"))?;
+        Dataspace::parse(&b, self.ls()).map_err(|e| Error::new(format!("dataspace message: {e}")))
+    }
+
+    pub fn layout(&self, h: &ObjectHeader) -> Result<DataLayout> {
+        let m = h
+            .messages
+            .iter()
+            .find(|m| m.msg_type == MessageType::DataLayout)
+            .ok_or_else(|| Error::new("no layout message"))?;
+        DataLayout::parse(&m.data, self.os(), self.ls())
+            .map_err(|e| Error::new(format!("layout message: {e}")))
+    }
+
+    pub fn filters(&self, h: &ObjectHeader) -> Result<Option<FilterPipeline>> {
+        match self.payload(h, MessageType::FilterPipeline)? {
+            None => Ok(None),
+            Some(b) => FilterPipeline::parse(&b)
+                .map(Some)
+                .map_err(|e| Error::new(format!("filter pipeline message: {e}"))),
+        }
+    }
+
+    /// Every attribute that can be read, plus one error per attribute that
+    /// cannot.
+    pub fn attributes(&self, h: &ObjectHeader) -> Result<(Vec<AttributeMessage>, Vec<String>)> {
+        if let Some(m) = h
+            .messages
+            .iter()
+            .find(|m| m.msg_type == MessageType::AttributeInfo)
+            && let Ok(ai) = AttributeInfoMessage::parse(&m.data, self.os())
+            && let Some(fh) = ai.fractal_heap_address
+        {
+            self.verified_heap(fh)
+                .map_err(|e| e.context("dense attribute storage"))?;
+        }
+        let (mut attrs, errs) = extract_attributes_tolerant(self.data(), h, self.os(), self.ls())
+            .map_err(|e| Error::new(format!("attributes: {e}")))?;
+        attrs.sort_by(|a, b| a.name.cmp(&b.name));
+        Ok((attrs, errs.iter().map(|e| e.to_string()).collect()))
+    }
+
+    /// Every link of the group whose header is `h`, sorted by name. An
+    /// object that is not a group has none.
+    pub fn links(&self, h: &ObjectHeader) -> Result<Vec<Link>> {
+        let os = self.os();
+        let ls = self.ls();
+        let data = self.data();
+        let mut out = Vec::new();
+        if let Some(m) = h
+            .messages
+            .iter()
+            .find(|m| m.msg_type == MessageType::SymbolTable)
+        {
+            let stm = SymbolTableMessage::parse(&m.data, os)
+                .map_err(|e| Error::new(format!("symbol table message: {e}")))?;
+            let entries = group_v1::resolve_v1_group_entries(data, &stm, os, ls)
+                .map_err(|e| Error::at(stm.btree_address, format!("symbol table: {e}")))?;
+            let has_soft = entries.iter().any(group_v1::is_v1_soft_link);
+            for e in entries {
+                if !group_v1::is_v1_soft_link(&e) {
+                    out.push(Link {
+                        name: e.name,
+                        kind: LinkKind::Hard(e.object_header_address),
+                    });
+                }
+            }
+            if has_soft {
+                let soft = group_v1::v1_soft_links(data, &stm, os, ls)
+                    .map_err(|e| Error::at(stm.btree_address, format!("soft links: {e}")))?;
+                for (name, target) in soft {
+                    out.push(Link {
+                        name,
+                        kind: LinkKind::Soft(target),
+                    });
+                }
+            }
+        } else {
+            let li = match h
+                .messages
+                .iter()
+                .find(|m| m.msg_type == MessageType::LinkInfo)
+            {
+                Some(m) => Some(
+                    LinkInfoMessage::parse(&m.data, os)
+                        .map_err(|e| Error::new(format!("link info message: {e}")))?,
+                ),
+                None => None,
+            };
+            for m in h
+                .messages
+                .iter()
+                .filter(|m| m.msg_type == MessageType::Link)
+            {
+                out.push(link_from_message(&m.data, os)?);
+            }
+            if let Some(li) = li
+                && let Some(fh) = li.fractal_heap_address
+            {
+                for bytes in self.dense_heap_objects(fh, li.btree_name_index_address, 5)? {
+                    out.push(link_from_message(&bytes, os)?);
+                }
+            }
+        }
+        out.sort_by(|a, b| a.name.cmp(&b.name));
+        Ok(out)
+    }
+
+    /// The objects a dense-storage B-tree (`btree`, of record type
+    /// `name_type`: 5 for links, 8 for attributes) indexes in the fractal
+    /// heap at `heap`.
+    pub fn dense_heap_objects(
+        &self,
+        heap: u64,
+        btree: Option<u64>,
+        name_type: u8,
+    ) -> Result<Vec<Vec<u8>>> {
+        self.verified_heap(heap)?;
+        let data = self.data();
+        let os = self.os();
+        let ls = self.ls();
+        let fh = FractalHeapHeader::parse(data, to_usize(heap)?, os, ls)
+            .map_err(|e| Error::at(heap, format!("fractal heap header: {e}")))?;
+        let bt = btree.ok_or_else(|| Error::at(heap, "dense storage without a name index"))?;
+        let hdr = BTreeV2Header::parse(data, to_usize(bt)?, os, ls)
+            .map_err(|e| Error::at(bt, format!("v2 B-tree header: {e}")))?;
+        let recs = collect_btree_v2_records(data, &hdr, os, ls)
+            .map_err(|e| Error::at(bt, format!("v2 B-tree: {e}")))?;
+        // Name-index records: hash(4) + heap ID; creation-order ones: order(8) + heap ID.
+        let skip = if hdr.tree_type == name_type { 4 } else { 8 };
+        let idlen = usize::from(fh.heap_id_length);
+        let mut out = Vec::with_capacity(recs.len());
+        for r in &recs {
+            let id = r
+                .data
+                .get(skip..skip + idlen)
+                .ok_or_else(|| Error::at(bt, "v2 B-tree record shorter than a heap ID"))?;
+            let obj = fh
+                .read_managed_object(data, id, os)
+                .map_err(|e| Error::at(heap, format!("fractal heap object: {e}")))?;
+            out.push(obj);
+        }
+        Ok(out)
+    }
+
+    /// Verify every block of the fractal heap at `addr` (once per heap):
+    /// the library reads heap objects without checking the blocks'
+    /// checksums, which libhdf5 does, so a damaged block would otherwise be
+    /// read as if it were sound.
+    pub fn verified_heap(&self, addr: u64) -> Result<()> {
+        if let Some(r) = self.verified_heaps.borrow().get(&addr) {
+            return r.clone().map_or(Ok(()), Err);
+        }
+        let report = crate::heap_blocks::verify(self, addr);
+        let n = report.problems.len();
+        let r = report.problems.into_iter().next().map(|mut e| {
+            if n > 1 {
+                e.msg = format!("{} (and {} more problems in this heap)", e.msg, n - 1);
+            }
+            e
+        });
+        self.verified_heaps.borrow_mut().insert(addr, r.clone());
+        r.map_or(Ok(()), Err)
+    }
+
+    /// The global heap object `idx` of the collection at `addr` (cached per
+    /// collection).
+    pub fn heap_object(&self, addr: u64, idx: u32) -> Result<Vec<u8>> {
+        let coll = {
+            let mut cache = self.heaps.borrow_mut();
+            cache
+                .entry(addr)
+                .or_insert_with(|| match usize::try_from(addr) {
+                    Ok(a) => GlobalHeapCollection::parse(self.data(), a, self.ls())
+                        .map(Rc::new)
+                        .map_err(|e| e.to_string()),
+                    Err(_) => Err("address out of range".into()),
+                })
+                .clone()
+                .map_err(|e| Error::at(addr, format!("global heap: {e}")))?
+        };
+        let idx16 = u16::try_from(idx)
+            .map_err(|_| Error::at(addr, format!("global heap object index {idx} out of range")))?;
+        coll.get_object(idx16)
+            .map(|o| o.data.clone())
+            .ok_or_else(|| Error::at(addr, format!("global heap has no object {idx}")))
+    }
+
+    /// The dataspace of the dataset at `path` with a virtual dataset's
+    /// extent resolved from its sources (as libhdf5 reports it) instead of
+    /// the stored one.
+    pub fn resolved_dataspace(&self, path: &str, h: &ObjectHeader) -> Result<Dataspace> {
+        let mut ds = self.dataspace(h)?;
+        if matches!(self.layout(h), Ok(DataLayout::Virtual { .. })) {
+            let d = self.file.dataset(path)?;
+            ds.dimensions = d.shape()?;
+            if ds.space_type == DataspaceType::Simple
+                && let Some(m) = &ds.max_dimensions
+                && m.len() != ds.dimensions.len()
+            {
+                ds.max_dimensions = None;
+            }
+        }
+        Ok(ds)
+    }
+
+    /// Read a dataset's values (raw, in file byte order) through the
+    /// `clawhdf5` facade, refusing one larger than `max_bytes`. `ds` must be
+    /// the [resolved](Self::resolved_dataspace) dataspace.
+    pub fn read_dataset(&self, path: &str, dt: &Datatype, ds: &Dataspace) -> Result<Vec<u8>> {
+        let need = byte_len(ds, dt)?;
+        if need > self.max_bytes {
+            return Err(Error::new(format!(
+                "dataset is {need} bytes, over the {} byte limit (--max-bytes)",
+                self.max_bytes
+            ))
+            .with_kind(ErrorKind::Limit));
+        }
+        let d = self.file.dataset(path)?;
+        let raw = d.read_selection(&clawhdf5::Selection::All)?;
+        if raw.len() as u64 != need {
+            return Err(Error::new(format!(
+                "read {} bytes, expected {need}",
+                raw.len()
+            )));
+        }
+        Ok(raw)
+    }
+
+    /// Walk every object reachable by hard links from the root, depth first
+    /// in name order, calling `visit` once per link (the root is visited
+    /// first with an empty link name). Each object is described once; later
+    /// hard links to it are reported with `first_path` set.
+    pub fn walk(&self, mut visit: impl FnMut(&WalkItem<'_>)) -> Result<()> {
+        self.walk_from(self.root(), "/", &mut visit)
+    }
+
+    pub fn walk_from(
+        &self,
+        start: u64,
+        start_path: &str,
+        visit: &mut dyn FnMut(&WalkItem<'_>),
+    ) -> Result<()> {
+        let mut seen: HashMap<u64, String> = HashMap::new();
+        let mut stack: Vec<(u64, String, Option<Link>, usize)> =
+            vec![(start, start_path.to_string(), None, 0)];
+        let mut count = 0usize;
+        while let Some((addr, path, link, depth)) = stack.pop() {
+            count += 1;
+            if count > MAX_OBJECTS {
+                return Err(Error::new(format!(
+                    "more than {MAX_OBJECTS} links; stopped walking"
+                )));
+            }
+            // Soft, external and user-defined links have no address.
+            if let Some(Link { kind, .. }) = &link
+                && !matches!(kind, LinkKind::Hard(_))
+            {
+                visit(&WalkItem {
+                    path: &path,
+                    link: link.as_ref(),
+                    addr: None,
+                    first_path: None,
+                    header: None,
+                    depth,
+                });
+                continue;
+            }
+            if let Some(first) = seen.get(&addr) {
+                visit(&WalkItem {
+                    path: &path,
+                    link: link.as_ref(),
+                    addr: Some(addr),
+                    first_path: Some(first),
+                    header: None,
+                    depth,
+                });
+                continue;
+            }
+            seen.insert(addr, path.clone());
+            let header = self.header(addr);
+            visit(&WalkItem {
+                path: &path,
+                link: link.as_ref(),
+                addr: Some(addr),
+                first_path: None,
+                header: Some(&header),
+                depth,
+            });
+            let Ok(h) = &header else { continue };
+            if Kind::of(h) != Kind::Group {
+                continue;
+            }
+            // Link errors are the visitor's to report (it sees the header).
+            let Ok(links) = self.links(h) else { continue };
+            let base = if path == "/" { "" } else { path.as_str() };
+            for l in links.into_iter().rev() {
+                let child = format!("{base}/{}", l.name);
+                let a = match l.kind {
+                    LinkKind::Hard(a) => a,
+                    _ => 0,
+                };
+                stack.push((a, child, Some(l), depth + 1));
+            }
+        }
+        Ok(())
+    }
+
+    /// Resolve an absolute object path to its header address, following
+    /// soft links like the library does.
+    pub fn resolve(&self, path: &str) -> Result<u64> {
+        let p = path.trim_matches('/');
+        if p.is_empty() {
+            return Ok(self.root());
+        }
+        clawhdf5_format::group_v2::resolve_path_any(self.data(), self.sb(), p)
+            .map_err(|e| Error::new(format!("{path}: {e}")))
+    }
+}
+
+/// An owned copy of a [`WalkItem`], from [`H5::walk_collect`].
+pub struct Visited {
+    pub path: String,
+    pub addr: Option<u64>,
+    pub first_path: Option<String>,
+    pub header: Option<Result<ObjectHeader>>,
+}
+
+impl H5 {
+    /// Every step of [`H5::walk`], collected, and the walk's own error.
+    pub fn walk_collect(&self) -> (Vec<Visited>, Result<()>) {
+        let mut items = Vec::new();
+        let r = self.walk(|it| {
+            items.push(Visited {
+                path: it.path.to_string(),
+                addr: it.addr,
+                first_path: it.first_path.map(str::to_string),
+                header: it.header.cloned(),
+            });
+        });
+        (items, r)
+    }
+}
+
+/// One step of [`H5::walk`].
+pub struct WalkItem<'a> {
+    pub path: &'a str,
+    /// The link that led here (`None` for the start object).
+    pub link: Option<&'a Link>,
+    /// Header address (`None` for a soft/external/user-defined link).
+    pub addr: Option<u64>,
+    /// Set when this object was already visited under another path.
+    pub first_path: Option<&'a str>,
+    /// The parsed header, for an object seen for the first time.
+    pub header: Option<&'a Result<ObjectHeader>>,
+    pub depth: usize,
+}
+
+fn link_from_message(data: &[u8], os: u8) -> Result<Link> {
+    match LinkMessage::parse(data, os) {
+        Ok(l) => Ok(Link {
+            name: l.name,
+            kind: match l.link_target {
+                LinkTarget::Hard {
+                    object_header_address,
+                } => LinkKind::Hard(object_header_address),
+                LinkTarget::Soft { target_path } => LinkKind::Soft(target_path),
+                LinkTarget::External {
+                    filename,
+                    object_path,
+                } => LinkKind::External {
+                    file: filename,
+                    path: object_path,
+                },
+            },
+        }),
+        Err(FormatError::InvalidLinkType(t)) if t >= 65 => Ok(Link {
+            name: user_defined_link_name(data).unwrap_or_else(|| "?".into()),
+            kind: LinkKind::UserDefined(t),
+        }),
+        Err(e) => Err(Error::new(format!("link message: {e}"))),
+    }
+}
+
+/// The name of a user-defined link, which `LinkMessage::parse` refuses.
+fn user_defined_link_name(d: &[u8]) -> Option<String> {
+    // version(1) flags(1) [type(1)] [corder(8)] [cset(1)] len(1|2|4|8) name
+    let flags = *d.get(1)?;
+    let mut p = 2usize;
+    if flags & 0x08 != 0 {
+        p += 1;
+    }
+    if flags & 0x04 != 0 {
+        p += 8;
+    }
+    if flags & 0x10 != 0 {
+        p += 1;
+    }
+    let w = 1usize << (flags & 0x03);
+    let mut n = 0usize;
+    for i in 0..w {
+        n |= usize::from(*d.get(p + i)?) << (8 * i);
+    }
+    p += w;
+    let name = d.get(p..p.checked_add(n)?)?;
+    Some(String::from_utf8_lossy(name).into_owned())
+}
+
+pub fn to_usize(a: u64) -> Result<usize> {
+    usize::try_from(a).map_err(|_| Error::at(a, "address out of range"))
+}
+
+/// Number of elements a dataspace holds (0 for a null dataspace).
+pub fn num_elements(ds: &Dataspace) -> Result<u64> {
+    match ds.space_type {
+        DataspaceType::Null => Ok(0),
+        DataspaceType::Scalar => Ok(1),
+        DataspaceType::Simple => ds
+            .dimensions
+            .iter()
+            .try_fold(1u64, |a, &d| a.checked_mul(d))
+            .ok_or_else(|| Error::new("dataspace element count overflows")),
+    }
+}
+
+/// Bytes needed for all of a dataspace's elements of type `dt`.
+pub fn byte_len(ds: &Dataspace, dt: &Datatype) -> Result<u64> {
+    num_elements(ds)?
+        .checked_mul(u64::from(dt.type_size()))
+        .ok_or_else(|| Error::new("dataset byte size overflows"))
+}
+
+/// Split a `FILE[/object/path]` argument the way h5ls does: the longest
+/// prefix that is an existing file is the file.
+pub fn split_file_arg(arg: &str) -> (String, Option<String>) {
+    if Path::new(arg).is_file() {
+        return (arg.to_string(), None);
+    }
+    let mut idx: Vec<usize> = arg.match_indices('/').map(|(i, _)| i).collect();
+    idx.reverse();
+    for i in idx {
+        let (f, rest) = arg.split_at(i);
+        if !f.is_empty() && Path::new(f).is_file() {
+            return (f.to_string(), Some(rest.to_string()));
+        }
+    }
+    (arg.to_string(), None)
+}

crates/clawhdf5-tools/src/heap_blocks.rs

@@ -0,0 +1,308 @@
+//! Walk every block of a fractal heap and verify it: signatures, the
+//! back-pointer to the heap header, each block's heap offset, and the
+//! checksums (always present on indirect blocks; on direct blocks when the
+//! heap header's flag says so). The library reads only the blocks an object
+//! lives in and does not verify block checksums, so `check` does it here.
+
+use std::collections::HashSet;
+
+use clawhdf5_format::checksum::jenkins_lookup3;
+use clawhdf5_format::fractal_heap::FractalHeapHeader;
+
+use crate::h5::{Error, H5};
+
+/// Heap header flag bit 1: direct blocks carry a checksum.
+const FLAG_CHECKSUM_DBLOCKS: u8 = 0x02;
+const MAX_DEPTH: u32 = 16;
+const MAX_BLOCKS: usize = 1 << 20;
+
+#[derive(Default, Debug)]
+pub struct HeapReport {
+    pub direct_blocks: usize,
+    pub indirect_blocks: usize,
+    /// Blocks whose checksum was verified.
+    pub checksums: usize,
+    pub problems: Vec<Error>,
+}
+
+struct Walk<'a> {
+    data: &'a [u8],
+    heap: u64,
+    fh: FractalHeapHeader,
+    checksum_dblocks: bool,
+    boff_bytes: usize,
+    os: usize,
+    ls: usize,
+    seen: HashSet<u64>,
+    r: HeapReport,
+}
+
+fn le(b: &[u8]) -> u64 {
+    b.iter()
+        .take(8)
+        .enumerate()
+        .fold(0u64, |a, (i, &x)| a | (u64::from(x) << (8 * i)))
+}
+
+fn undefined(v: u64, os: usize) -> bool {
+    if os >= 8 {
+        v == u64::MAX
+    } else {
+        v == (1u64 << (8 * os)) - 1
+    }
+}
+
+fn log2(v: u64) -> u32 {
+    63u32.saturating_sub(v.max(1).leading_zeros())
+}
+
+/// Verify the fractal heap whose header is at `heap`. The header itself is
+/// parsed (and its checksum verified) by the library; an error there is
+/// returned as the only problem.
+pub fn verify(h5: &H5, heap: u64) -> HeapReport {
+    let data = h5.data();
+    let Ok(off) = usize::try_from(heap) else {
+        return HeapReport {
+            problems: vec![Error::at(heap, "fractal heap address out of range")],
+            ..Default::default()
+        };
+    };
+    let fh = match FractalHeapHeader::parse(data, off, h5.os(), h5.ls()) {
+        Ok(f) => f,
+        Err(e) => {
+            return HeapReport {
+                problems: vec![Error::at(heap, format!("fractal heap header: {e}"))],
+                ..Default::default()
+            };
+        }
+    };
+    // Flags: signature(4) version(1) heap ID length(2) filter length(2) flags(1).
+    let flags = data.get(off + 9).copied().unwrap_or(0);
+    let mut w = Walk {
+        data,
+        heap,
+        checksum_dblocks: flags & FLAG_CHECKSUM_DBLOCKS != 0 && fh.filter_pipeline.is_none(),
+        boff_bytes: usize::from(fh.max_heap_size).div_ceil(8),
+        os: usize::from(h5.os()),
+        ls: usize::from(h5.ls()),
+        fh,
+        seen: HashSet::new(),
+        r: HeapReport::default(),
+    };
+    if w.fh.table_width == 0 || w.fh.starting_block_size == 0 || !w.fh.table_width.is_power_of_two()
+    {
+        w.problem(heap, "fractal heap header: invalid doubling table geometry");
+        return w.r;
+    }
+    let root = w.fh.root_block_address;
+    if !undefined(root, w.os) {
+        if w.fh.current_rows_in_root_indirect_block == 0 {
+            let size = w.fh.starting_block_size;
+            w.direct(root, size, 0);
+        } else {
+            let rows = w.fh.current_rows_in_root_indirect_block;
+            w.indirect(root, rows, 0, 0);
+        }
+    }
+    w.r
+}
+
+impl Walk<'_> {
+    fn problem(&mut self, addr: u64, msg: impl Into<String>) {
+        self.r.problems.push(Error::at(addr, msg));
+    }
+
+    fn row_size(&self, row: usize) -> Option<u64> {
+        let s = self.fh.starting_block_size;
+        if row <= 1 {
+            Some(s)
+        } else {
+            let sh = u32::try_from(row - 1).ok()?;
+            s.checked_mul(1u64.checked_shl(sh)?)
+        }
+    }
+
+    fn max_direct_rows(&self) -> usize {
+        let ratio = (self.fh.max_direct_block_size / self.fh.starting_block_size).max(1);
+        log2(ratio) as usize + 2
+    }
+
+    fn rows_for_size(&self, size: u64) -> u16 {
+        let first = log2(self.fh.starting_block_size) + log2(u64::from(self.fh.table_width));
+        (log2(size).saturating_sub(first) + 1) as u16
+    }
+
+    /// Common block prefix: signature, version, heap header address and
+    /// block offset. Returns the position after it, or `None` after
+    /// recording a problem.
+    fn prefix(&mut self, addr: u64, sig: &[u8; 4], what: &str, heap_offset: u64) -> Option<usize> {
+        if !self.seen.insert(addr) {
+            self.problem(
+                addr,
+                format!("fractal heap {what} block reached twice (cycle)"),
+            );
+            return None;
+        }
+        if self.seen.len() > MAX_BLOCKS {
+            self.problem(self.heap, "fractal heap has too many blocks; stopped");
+            return None;
+        }
+        let Ok(start) = usize::try_from(addr) else {
+            self.problem(
+                addr,
+                format!("fractal heap {what} block address out of range"),
+            );
+            return None;
+        };
+        let hdr_len = 5 + self.os + self.boff_bytes;
+        let Some(b) = start
+            .checked_add(hdr_len)
+            .and_then(|e| self.data.get(start..e))
+        else {
+            self.problem(
+                addr,
+                format!("fractal heap {what} block lies past the end of the file"),
+            );
+            return None;
+        };
+        if &b[..4] != sig {
+            self.problem(addr, format!("fractal heap {what} block: bad signature"));
+            return None;
+        }
+        if b[4] != 0 {
+            self.problem(addr, format!("fractal heap {what} block: version {}", b[4]));
+            return None;
+        }
+        let back = le(&b[5..5 + self.os]);
+        if back != self.heap {
+            self.problem(
+                addr,
+                format!(
+                    "fractal heap {what} block points at heap header {back:#x}, not {:#x}",
+                    self.heap
+                ),
+            );
+        }
+        let boff = le(&b[5 + self.os..hdr_len]);
+        if boff != heap_offset {
+            self.problem(
+                addr,
+                format!("fractal heap {what} block has heap offset {boff}, expected {heap_offset}"),
+            );
+        }
+        Some(start + hdr_len)
+    }
+
+    fn direct(&mut self, addr: u64, size: u64, heap_offset: u64) {
+        let Some(pos) = self.prefix(addr, b"FHDB", "direct", heap_offset) else {
+            return;
+        };
+        self.r.direct_blocks += 1;
+        if self.fh.filter_pipeline.is_some() {
+            return; // stored filtered: its size on disk is not the block size
+        }
+        let start = pos - (5 + self.os + self.boff_bytes);
+        let Some(end) = usize::try_from(size)
+            .ok()
+            .and_then(|s| start.checked_add(s))
+        else {
+            self.problem(addr, "fractal heap direct block size out of range");
+            return;
+        };
+        let Some(block) = self.data.get(start..end) else {
+            self.problem(
+                addr,
+                "fractal heap direct block extends past the end of the file",
+            );
+            return;
+        };
+        if self.checksum_dblocks {
+            let Some(stored) = block.get(pos - start..pos - start + 4) else {
+                self.problem(addr, "fractal heap direct block too small for its checksum");
+                return;
+            };
+            let stored = u32::from_le_bytes([stored[0], stored[1], stored[2], stored[3]]);
+            let mut copy = block.to_vec();
+            copy[pos - start..pos - start + 4].fill(0);
+            let computed = jenkins_lookup3(&copy);
+            self.r.checksums += 1;
+            if computed != stored {
+                self.problem(
+                    addr,
+                    format!(
+                        "fractal heap direct block: checksum mismatch: stored {stored:#010x}, computed {computed:#010x}"
+                    ),
+                );
+            }
+        }
+    }
+
+    fn indirect(&mut self, addr: u64, nrows: u16, heap_offset: u64, depth: u32) {
+        if depth > MAX_DEPTH {
+            self.problem(addr, "fractal heap indirect blocks nested too deeply");
+            return;
+        }
+        let Some(mut pos) = self.prefix(addr, b"FHIB", "indirect", heap_offset) else {
+            return;
+        };
+        self.r.indirect_blocks += 1;
+        let start = pos - (5 + self.os + self.boff_bytes);
+        let width = usize::from(self.fh.table_width);
+        let filtered = self.fh.filter_pipeline.is_some();
+        let direct_rows = self.max_direct_rows();
+        let mut children: Vec<(u64, bool, u64, u64)> = Vec::new(); // addr, direct, size/rows, offset
+        let mut off = heap_offset;
+        for row in 0..usize::from(nrows) {
+            let Some(rs) = self.row_size(row) else {
+                self.problem(addr, "fractal heap row size overflows");
+                return;
+            };
+            let direct = row < direct_rows;
+            for _ in 0..width {
+                let Some(b) = self.data.get(pos..pos + self.os) else {
+                    self.problem(
+                        addr,
+                        "fractal heap indirect block extends past the end of the file",
+                    );
+                    return;
+                };
+                let child = le(b);
+                pos += self.os;
+                if direct && filtered {
+                    pos += self.ls + 4;
+                }
+                if !undefined(child, self.os) {
+                    children.push((child, direct, rs, off));
+                }
+                off = off.saturating_add(rs);
+            }
+        }
+        let Some(stored) = self.data.get(pos..pos + 4) else {
+            self.problem(
+                addr,
+                "fractal heap indirect block extends past the end of the file",
+            );
+            return;
+        };
+        let stored = u32::from_le_bytes([stored[0], stored[1], stored[2], stored[3]]);
+        let computed = jenkins_lookup3(&self.data[start..pos]);
+        self.r.checksums += 1;
+        if computed != stored {
+            self.problem(
+                addr,
+                format!(
+                    "fractal heap indirect block: checksum mismatch: stored {stored:#010x}, computed {computed:#010x}"
+                ),
+            );
+            return; // its child pointers cannot be trusted
+        }
+        for (child, direct, size, off) in children {
+            if direct {
+                self.direct(child, size, off);
+            } else {
+                let rows = self.rows_for_size(size);
+                self.indirect(child, rows, off, depth + 1);
+            }
+        }
+    }
+}

crates/clawhdf5-tools/src/info.rs

@@ -0,0 +1,245 @@
+//! Dataset facts shared by `ls`, `dump`, `stat` and `check`: shape text,
+//! layout, filters and storage.
+
+use clawhdf5_format::chunked_read::{ChunkInfo, list_chunks};
+use clawhdf5_format::data_layout::DataLayout;
+use clawhdf5_format::dataspace::{Dataspace, DataspaceType};
+use clawhdf5_format::datatype::Datatype;
+use clawhdf5_format::filter_pipeline::{FilterDescription, FilterPipeline};
+use clawhdf5_format::message_type::MessageType;
+use clawhdf5_format::object_header::ObjectHeader;
+
+use crate::h5::{Error, H5, Result};
+
+/// h5ls's `{10/Inf, 20}` shape text. `always_max` prints `cur/max` for every
+/// dimension (h5ls -v).
+pub fn shape_text(ds: &Dataspace, always_max: bool) -> String {
+    match ds.space_type {
+        DataspaceType::Null => "{NULL}".into(),
+        DataspaceType::Scalar => "{SCALAR}".into(),
+        DataspaceType::Simple => {
+            let dims: Vec<String> = ds
+                .dimensions
+                .iter()
+                .enumerate()
+                .map(|(i, &d)| {
+                    let m = ds.max_dimensions.as_ref().and_then(|m| m.get(i).copied());
+                    match m {
+                        Some(u64::MAX) => format!("{d}/Inf"),
+                        Some(m) if m != d || always_max => format!("{d}/{m}"),
+                        None if always_max => format!("{d}/{d}"),
+                        _ => d.to_string(),
+                    }
+                })
+                .collect();
+            format!("{{{}}}", dims.join(", "))
+        }
+    }
+}
+
+/// h5dump's `SIMPLE { ( 3, 4 ) / ( 3, H5S_UNLIMITED ) }`.
+pub fn dataspace_ddl(ds: &Dataspace) -> String {
+    match ds.space_type {
+        DataspaceType::Null => "NULL".into(),
+        DataspaceType::Scalar => "SCALAR".into(),
+        DataspaceType::Simple => {
+            let cur: Vec<String> = ds.dimensions.iter().map(|d| d.to_string()).collect();
+            let max: Vec<String> = ds
+                .dimensions
+                .iter()
+                .enumerate()
+                .map(
+                    |(i, &d)| match ds.max_dimensions.as_ref().and_then(|m| m.get(i).copied()) {
+                        Some(u64::MAX) => "H5S_UNLIMITED".into(),
+                        Some(m) => m.to_string(),
+                        None => d.to_string(),
+                    },
+                )
+                .collect();
+            format!(
+                "SIMPLE {{ ( {} ) / ( {} ) }}",
+                cur.join(", "),
+                max.join(", ")
+            )
+        }
+    }
+}
+
+pub fn filter_name(f: &FilterDescription) -> String {
+    let known = match f.filter_id {
+        1 => "deflate",
+        2 => "shuffle",
+        3 => "fletcher32",
+        4 => "szip",
+        5 => "nbit",
+        6 => "scaleoffset",
+        307 => "bzip2",
+        32000 => "lzf",
+        32001 => "blosc",
+        32004 => "lz4",
+        32008 => "bitshuffle",
+        32013 => "zfp",
+        32015 => "zstd",
+        32026 => "blosc2",
+        _ => "",
+    };
+    if !known.is_empty() {
+        return known.into();
+    }
+    match &f.name {
+        Some(n) if !n.is_empty() => n.clone(),
+        _ => "user-defined".into(),
+    }
+}
+
+/// `deflate-1 OPT {4}` as h5ls prints a filter.
+pub fn filter_text(f: &FilterDescription) -> String {
+    let opt = if f.flags & 1 != 0 { " OPT" } else { "" };
+    let cd = if f.client_data.is_empty() {
+        String::new()
+    } else {
+        format!(
+            " {{{}}}",
+            f.client_data
+                .iter()
+                .map(|c| c.to_string())
+                .collect::<Vec<_>>()
+                .join(", ")
+        )
+    };
+    format!("{}-{}{opt}{cd}", filter_name(f), f.filter_id)
+}
+
+pub fn layout_name(l: &DataLayout) -> &'static str {
+    match l {
+        DataLayout::Compact { .. } => "compact",
+        DataLayout::Contiguous { .. } => "contiguous",
+        DataLayout::Chunked { .. } => "chunked",
+        DataLayout::Virtual { .. } => "virtual",
+    }
+}
+
+/// Chunk index kind of a chunked layout.
+pub fn chunk_index_name(l: &DataLayout) -> &'static str {
+    match l {
+        DataLayout::Chunked {
+            version,
+            chunk_index_type,
+            ..
+        } => {
+            if *version < 4 {
+                return "v1 B-tree";
+            }
+            match chunk_index_type {
+                Some(1) => "single chunk",
+                Some(2) => "implicit",
+                Some(3) => "fixed array",
+                Some(4) => "extensible array",
+                Some(5) => "v2 B-tree",
+                _ => "unknown",
+            }
+        }
+        _ => "",
+    }
+}
+
+/// Everything about one dataset that can be learned without reading its
+/// values.
+pub struct DsInfo {
+    pub dt: Result<Datatype>,
+    pub ds: Result<Dataspace>,
+    pub layout: Result<DataLayout>,
+    pub filters: Result<Option<FilterPipeline>>,
+    pub external: bool,
+}
+
+impl DsInfo {
+    pub fn read(h5: &H5, path: &str, h: &ObjectHeader) -> DsInfo {
+        DsInfo {
+            dt: h5.datatype(h),
+            ds: h5.resolved_dataspace(path, h),
+            layout: h5.layout(h),
+            filters: h5.filters(h),
+            external: h
+                .messages
+                .iter()
+                .any(|m| m.msg_type == MessageType::ExternalDataFiles),
+        }
+    }
+
+    pub fn logical_bytes(&self) -> Option<u64> {
+        let (Ok(dt), Ok(ds)) = (&self.dt, &self.ds) else {
+            return None;
+        };
+        crate::h5::byte_len(ds, dt).ok()
+    }
+}
+
+/// Every allocated chunk (empty when none are). Errors only for a corrupt
+/// chunk index.
+pub fn chunks(
+    h5: &H5,
+    layout: &DataLayout,
+    ds: &Dataspace,
+    dt: &Datatype,
+) -> Result<Vec<ChunkInfo>> {
+    let DataLayout::Chunked { btree_address, .. } = layout else {
+        return Ok(Vec::new());
+    };
+    let Some(addr) = *btree_address else {
+        return Ok(Vec::new());
+    };
+    list_chunks(
+        h5.data(),
+        layout,
+        ds,
+        dt.type_size() as usize,
+        h5.os(),
+        h5.ls(),
+    )
+    .map(|(c, _)| c)
+    .map_err(|e| {
+        Error::at(
+            addr,
+            format!("chunk index ({}): {e}", chunk_index_name(layout)),
+        )
+    })
+}
+
+/// Bytes of raw data the dataset has allocated in the file (what libhdf5's
+/// `H5Dget_storage_size` reports).
+pub fn allocated_bytes(h5: &H5, info: &DsInfo) -> Result<u64> {
+    let layout = info.layout.as_ref().map_err(Clone::clone)?;
+    Ok(match layout {
+        DataLayout::Compact { data } => data.len() as u64,
+        DataLayout::Contiguous { address, size } => {
+            if address.is_some() {
+                *size
+            } else {
+                0
+            }
+        }
+        DataLayout::Chunked { .. } => {
+            let dt = info.dt.as_ref().map_err(Clone::clone)?;
+            let ds = info.ds.as_ref().map_err(Clone::clone)?;
+            chunks(h5, layout, ds, dt)?
+                .iter()
+                .map(|c| u64::from(c.chunk_size))
+                .sum()
+        }
+        DataLayout::Virtual { .. } => 0,
+    })
+}
+
+/// Number of hard links to an object, as its header records it.
+pub fn link_count(h: &ObjectHeader) -> u64 {
+    if let Some(rc) = h.reference_count {
+        return u64::from(rc);
+    }
+    h.messages
+        .iter()
+        .find(|m| m.msg_type == MessageType::ObjectReferenceCount)
+        .and_then(|m| m.data.get(1..5))
+        .map(|b| u64::from(u32::from_le_bytes([b[0], b[1], b[2], b[3]])))
+        .unwrap_or(1)
+}

crates/clawhdf5-tools/src/lib.rs

@@ -0,0 +1,63 @@
+//! `h5rs`: HDF5 command-line tools built on clawhdf5 alone, no libhdf5.
+//!
+//! The binary's subcommands (`ls`, `dump`, `stat`, `diff`, `check`) live in
+//! the modules below; [`run`] dispatches to them. See the crate README for
+//! the command reference.
+
+pub mod check;
+pub mod cli;
+pub mod diff;
+pub mod dtype;
+pub mod dump;
+pub mod h5;
+pub mod heap_blocks;
+pub mod info;
+pub mod ls;
+pub mod stat;
+pub mod value;
+
+use cli::{Args, Out};
+
+pub const USAGE: &str = "\
+h5rs: HDF5 tools in pure Rust (clawhdf5, no libhdf5)
+
+usage: h5rs <command> [options] ...
+
+commands:
+  ls     list objects (like h5ls)
+  dump   print a file's structure and values as DDL or JSON (like h5dump)
+  stat   object, layout, filter and storage statistics (like h5stat)
+  diff   compare two files (like h5diff)
+  check  validate a file's structure and checksums
+
+Run `h5rs <command> --help` for a command's options.";
+
+/// Run `h5rs` with `argv` (without the program name). Returns the exit
+/// status.
+pub fn run(argv: Vec<String>, out: &mut Out) -> std::io::Result<i32> {
+    let mut it = argv.into_iter();
+    let Some(cmd) = it.next() else {
+        writeln!(out.e, "{USAGE}")?;
+        return Ok(2);
+    };
+    let rest: Vec<String> = it.collect();
+    match cmd.as_str() {
+        "ls" => ls::run(&mut Args::new("ls", rest), out),
+        "dump" => dump::run(&mut Args::new("dump", rest), out),
+        "stat" => stat::run(&mut Args::new("stat", rest), out),
+        "diff" => diff::run(&mut Args::new("diff", rest), out),
+        "check" => check::run(&mut Args::new("check", rest), out),
+        "-h" | "--help" | "help" => {
+            writeln!(out.o, "{USAGE}")?;
+            Ok(0)
+        }
+        "-V" | "--version" => {
+            writeln!(out.o, "h5rs {}", env!("CARGO_PKG_VERSION"))?;
+            Ok(0)
+        }
+        other => {
+            writeln!(out.e, "h5rs: unknown command {other:?}\n\n{USAGE}")?;
+            Ok(2)
+        }
+    }
+}

crates/clawhdf5-tools/src/ls.rs

@@ -0,0 +1,385 @@
+//! `h5rs ls`: list a file's objects like h5ls.
+
+use std::collections::HashMap;
+
+use clawhdf5_format::attribute::AttributeMessage;
+use clawhdf5_format::dataspace::DataspaceType;
+use clawhdf5_format::object_header::ObjectHeader;
+
+use crate::cli::{Args, Out};
+use crate::h5::{H5, Kind, Link, LinkKind, split_file_arg};
+use crate::info::{self, DsInfo};
+use crate::value::{self, Decoder};
+
+pub const USAGE: &str = "\
+usage: h5rs ls [-r] [-v] [--max-bytes N] FILE[/OBJECT]
+
+List the objects in FILE (or in the group OBJECT), one per line, like h5ls:
+name, kind, shape and, for datasets, the datatype.
+
+  -r, --recursive   list every object below, with full paths
+  -v, --verbose     also print address, link count, layout, chunking,
+                    storage, filters, datatype and attributes
+  --max-bytes N     largest attribute value decoded for -v (default 1 GiB)
+
+Exit status: 0 listed, 1 some object could not be described, 2 error.";
+
+struct Ls<'a> {
+    h5: &'a H5,
+    verbose: bool,
+    kinds: HashMap<u64, Kind>,
+    problems: usize,
+}
+
+pub fn run(args: &mut Args, out: &mut Out) -> std::io::Result<i32> {
+    let mut recursive = false;
+    let mut verbose = false;
+    let mut max_bytes = None;
+    let mut target = None;
+    while let Some(a) = args.next() {
+        match a.as_str() {
+            "-r" | "--recursive" => recursive = true,
+            "-v" | "--verbose" => verbose = true,
+            "-rv" | "-vr" => {
+                recursive = true;
+                verbose = true;
+            }
+            "--max-bytes" => match args.number() {
+                Some(n) => max_bytes = Some(n),
+                None => return args.usage_error(out, "--max-bytes needs a number", USAGE),
+            },
+            "-h" | "--help" => {
+                writeln!(out.o, "{USAGE}")?;
+                return Ok(0);
+            }
+            s if s.starts_with('-') && s.len() > 1 => {
+                return args.usage_error(out, &format!("unknown option {a}"), USAGE);
+            }
+            _ if target.is_none() => target = Some(a),
+            _ => return args.usage_error(out, &format!("unexpected argument {a}"), USAGE),
+        }
+    }
+    let Some(target) = target else {
+        return args.usage_error(out, "missing FILE", USAGE);
+    };
+    let (file, obj) = split_file_arg(&target);
+    let mut h5 = match H5::open(std::path::Path::new(&file)) {
+        Ok(h) => h,
+        Err(e) => {
+            writeln!(out.e, "h5rs ls: {e}")?;
+            return Ok(2);
+        }
+    };
+    if let Some(m) = max_bytes {
+        h5.max_bytes = m;
+    }
+    let mut ls = Ls {
+        h5: &h5,
+        verbose,
+        kinds: HashMap::new(),
+        problems: 0,
+    };
+    let obj_path = obj.unwrap_or_else(|| "/".into());
+    let addr = match h5.resolve(&obj_path) {
+        Ok(a) => a,
+        Err(e) => {
+            writeln!(out.e, "h5rs ls: {obj_path}: not found: {e}")?;
+            return Ok(2);
+        }
+    };
+    let header = match h5.header(addr) {
+        Ok(h) => h,
+        Err(e) => {
+            writeln!(out.e, "h5rs ls: {obj_path}: {e}")?;
+            return Ok(2);
+        }
+    };
+    let is_root = obj_path.trim_matches('/').is_empty();
+    if Kind::of(&header) != Kind::Group && !is_root {
+        // h5ls names a single object by its base name, or with -r by its
+        // path as given (without the leading slash).
+        let name = if recursive {
+            obj_path.trim_start_matches('/').to_string()
+        } else {
+            obj_path.rsplit('/').next().unwrap_or(&obj_path).to_string()
+        };
+        ls.object_line(out, &name, &obj_path, addr, &Ok(header))?;
+    } else if recursive {
+        let start = if is_root { "/" } else { "" };
+        let mut io_err = None;
+        let res = h5.walk_from(addr, start, &mut |item| {
+            if io_err.is_some() || (item.link.is_none() && !is_root) {
+                return; // listing a group's contents, not the group
+            }
+            let path = if item.path.is_empty() { "/" } else { item.path };
+            let full = if is_root {
+                path.to_string()
+            } else {
+                format!("{}{path}", obj_path.trim_end_matches('/'))
+            };
+            if let Err(e) = ls.item(
+                out,
+                path,
+                &full,
+                item.link,
+                item.addr,
+                item.first_path,
+                item.header,
+            ) {
+                io_err = Some(e);
+            }
+        });
+        if let Some(e) = io_err {
+            return Err(e);
+        }
+        if let Err(e) = res {
+            writeln!(out.e, "h5rs ls: {e}")?;
+            ls.problems += 1;
+        }
+    } else {
+        match h5.links(&header) {
+            Ok(links) => {
+                for l in &links {
+                    let (a, hdr) = match l.kind {
+                        LinkKind::Hard(a) => (Some(a), Some(h5.header(a))),
+                        _ => (None, None),
+                    };
+                    let full = format!("{}/{}", obj_path.trim_end_matches('/'), l.name);
+                    ls.item(out, &l.name, &full, Some(l), a, None, hdr.as_ref())?;
+                }
+            }
+            Err(e) => {
+                writeln!(out.e, "h5rs ls: {obj_path}: cannot list group: {e}")?;
+                ls.problems += 1;
+            }
+        }
+    }
+    Ok(if ls.problems > 0 { 1 } else { 0 })
+}
+
+impl Ls<'_> {
+    #[allow(clippy::too_many_arguments)]
+    fn item(
+        &mut self,
+        out: &mut Out,
+        name: &str,
+        full: &str,
+        link: Option<&Link>,
+        addr: Option<u64>,
+        first: Option<&str>,
+        header: Option<&crate::h5::Result<ObjectHeader>>,
+    ) -> std::io::Result<()> {
+        if let Some(l) = link {
+            match &l.kind {
+                LinkKind::Soft(t) => return writeln!(out.o, "{name:<24} Soft Link {{{t}}}"),
+                LinkKind::External { file, path } => {
+                    return writeln!(out.o, "{name:<24} External Link {{{file}/{path}}}");
+                }
+                LinkKind::UserDefined(t) => {
+                    return writeln!(out.o, "{name:<24} User-defined link (type {t})");
+                }
+                LinkKind::Hard(_) => {}
+            }
+        }
+        let Some(addr) = addr else { return Ok(()) };
+        if let Some(first) = first {
+            let k = self.kinds.get(&addr).copied().unwrap_or(Kind::Unknown);
+            return writeln!(out.o, "{name:<24} {}, same as {first}", k.name());
+        }
+        match header {
+            Some(h) => self.object_line(out, name, full, addr, h),
+            None => Ok(()),
+        }
+    }
+
+    fn object_line(
+        &mut self,
+        out: &mut Out,
+        name: &str,
+        full: &str,
+        addr: u64,
+        header: &crate::h5::Result<ObjectHeader>,
+    ) -> std::io::Result<()> {
+        let h = match header {
+            Ok(h) => h,
+            Err(e) => {
+                self.problems += 1;
+                writeln!(out.o, "{name:<24} ** error **")?;
+                return writeln!(out.e, "h5rs ls: {name}: {e}");
+            }
+        };
+        let kind = Kind::of(h);
+        self.kinds.insert(addr, kind);
+        match kind {
+            Kind::Dataset => {
+                let info = DsInfo::read(self.h5, full, h);
+                let shape = match &info.ds {
+                    Ok(ds) => info::shape_text(ds, self.verbose),
+                    Err(_) => "{?}".into(),
+                };
+                if self.verbose {
+                    writeln!(out.o, "{name:<24} Dataset {shape}")?;
+                } else {
+                    let t = match &info.dt {
+                        Ok(dt) => crate::dtype::short(dt),
+                        Err(_) => "?".into(),
+                    };
+                    writeln!(out.o, "{name:<24} Dataset {shape} {t}")?;
+                }
+                for e in [
+                    info.dt.as_ref().err(),
+                    info.ds.as_ref().err(),
+                    info.layout.as_ref().err(),
+                    info.filters.as_ref().err(),
+                ]
+                .into_iter()
+                .flatten()
+                {
+                    self.problems += 1;
+                    writeln!(out.e, "h5rs ls: {name}: {e}")?;
+                }
+                if self.verbose {
+                    self.dataset_details(out, addr, h, &info)?;
+                }
+            }
+            k => {
+                writeln!(out.o, "{name:<24} {}", k.name())?;
+                if self.verbose {
+                    writeln!(out.o, "    Address:   {addr}")?;
+                    writeln!(out.o, "    Links:     {}", info::link_count(h))?;
+                    if k == Kind::Datatype
+                        && let Ok(dt) = self.h5.datatype(h)
+                    {
+                        writeln!(out.o, "    Type:      {}", crate::dtype::long(&dt))?;
+                    }
+                    self.attributes(out, name, h)?;
+                }
+            }
+        }
+        Ok(())
+    }
+
+    fn dataset_details(
+        &mut self,
+        out: &mut Out,
+        addr: u64,
+        h: &ObjectHeader,
+        info: &DsInfo,
+    ) -> std::io::Result<()> {
+        writeln!(out.o, "    Address:   {addr}")?;
+        writeln!(out.o, "    Links:     {}", info::link_count(h))?;
+        if let Ok(l) = &info.layout {
+            let idx = info::chunk_index_name(l);
+            if idx.is_empty() {
+                writeln!(out.o, "    Layout:    {}", info::layout_name(l))?;
+            } else {
+                writeln!(
+                    out.o,
+                    "    Layout:    {} ({idx} index)",
+                    info::layout_name(l)
+                )?;
+            }
+            if let clawhdf5_format::data_layout::DataLayout::Chunked {
+                chunk_dimensions, ..
+            } = l
+            {
+                let rank = chunk_dimensions.len().saturating_sub(1);
+                let dims = &chunk_dimensions[..rank];
+                let esize = info
+                    .dt
+                    .as_ref()
+                    .map(|d| u64::from(d.type_size()))
+                    .unwrap_or(0);
+                let bytes = dims
+                    .iter()
+                    .try_fold(esize, |a, &d| a.checked_mul(u64::from(d)))
+                    .map(|b| b.to_string())
+                    .unwrap_or_else(|| "?".into());
+                writeln!(
+                    out.o,
+                    "    Chunks:    {{{}}} {bytes} bytes",
+                    dims.iter()
+                        .map(|d| d.to_string())
+                        .collect::<Vec<_>>()
+                        .join(", ")
+                )?;
+            }
+        }
+        if info.external {
+            writeln!(out.o, "    External:  raw data in external files")?;
+        }
+        let logical = info.logical_bytes();
+        match (logical, info::allocated_bytes(self.h5, info)) {
+            (Some(l), Ok(a)) => {
+                if a > 0 {
+                    writeln!(
+                        out.o,
+                        "    Storage:   {l} logical bytes, {a} allocated bytes, {:.2}% utilization",
+                        l as f64 * 100.0 / a as f64
+                    )?;
+                } else {
+                    writeln!(out.o, "    Storage:   {l} logical bytes, 0 allocated bytes")?;
+                }
+            }
+            (_, Err(e)) => {
+                self.problems += 1;
+                writeln!(out.e, "h5rs ls: {e}")?;
+            }
+            _ => {}
+        }
+        if let Ok(Some(p)) = &info.filters {
+            for (i, f) in p.filters.iter().enumerate() {
+                writeln!(out.o, "    Filter-{i}:  {}", info::filter_text(f))?;
+            }
+        }
+        if let Ok(dt) = &info.dt {
+            writeln!(out.o, "    Type:      {}", crate::dtype::long(dt))?;
+        }
+        self.attributes(out, "", h)
+    }
+
+    fn attributes(&mut self, out: &mut Out, name: &str, h: &ObjectHeader) -> std::io::Result<()> {
+        let (attrs, errs) = match self.h5.attributes(h) {
+            Ok(x) => x,
+            Err(e) => {
+                self.problems += 1;
+                return writeln!(out.e, "h5rs ls: {name}: {e}");
+            }
+        };
+        for e in errs {
+            self.problems += 1;
+            writeln!(out.e, "h5rs ls: {name}: attribute: {e}")?;
+        }
+        for a in &attrs {
+            self.attribute(out, a)?;
+        }
+        Ok(())
+    }
+
+    fn attribute(&mut self, out: &mut Out, a: &AttributeMessage) -> std::io::Result<()> {
+        let shape = match a.dataspace.space_type {
+            DataspaceType::Scalar => "scalar".to_string(),
+            DataspaceType::Null => "null".to_string(),
+            DataspaceType::Simple => info::shape_text(&a.dataspace, false),
+        };
+        writeln!(out.o, "    Attribute: {} {shape}", a.name)?;
+        writeln!(
+            out.o,
+            "        Type:      {}",
+            crate::dtype::long(&a.datatype)
+        )?;
+        let n = crate::h5::num_elements(&a.dataspace).unwrap_or(0);
+        if n == 0 {
+            return Ok(());
+        }
+        let dec = Decoder::new(self.h5);
+        let shown = n.min(8) as usize;
+        let mut vals = Vec::with_capacity(shown);
+        for i in 0..shown {
+            let v = dec.element(&a.datatype, &a.raw_data, i);
+            vals.push(value::text(&v, &|_| None));
+        }
+        let more = if n > 8 { ", ..." } else { "" };
+        writeln!(out.o, "        Data:      {}{more}", vals.join(", "))
+    }
+}

crates/clawhdf5-tools/src/main.rs

@@ -0,0 +1,58 @@
+//! The `h5rs` binary. A panic anywhere is a bug: it is caught, reported as
+//! an internal error and turned into exit status 3, never a crash.
+
+use std::io::Write;
+use std::panic::{self, AssertUnwindSafe};
+
+use clawhdf5_tools::cli::Out;
+
+/// Exit status for an internal error (a caught panic).
+const INTERNAL_ERROR: i32 = 3;
+
+fn main() {
+    panic::set_hook(Box::new(|info| {
+        let msg = if let Some(s) = info.payload().downcast_ref::<&str>() {
+            (*s).to_string()
+        } else if let Some(s) = info.payload().downcast_ref::<String>() {
+            s.clone()
+        } else {
+            "unknown panic".into()
+        };
+        let at = info
+            .location()
+            .map(|l| format!(" at {}:{}", l.file(), l.line()))
+            .unwrap_or_default();
+        eprintln!("h5rs: internal error (this is a bug; please report it): {msg}{at}");
+    }));
+    let argv: Vec<String> = std::env::args().skip(1).collect();
+    let stdout = std::io::stdout();
+    let mut o = std::io::BufWriter::new(stdout.lock());
+    let mut e = std::io::stderr();
+    let result = panic::catch_unwind(AssertUnwindSafe(|| {
+        let mut out = Out {
+            o: &mut o,
+            e: &mut e,
+        };
+        clawhdf5_tools::run(argv, &mut out)
+    }));
+    let code = match result {
+        Ok(Ok(code)) => match o.flush() {
+            Ok(()) => code,
+            Err(err) if err.kind() == std::io::ErrorKind::BrokenPipe => code,
+            Err(err) => {
+                eprintln!("h5rs: {err}");
+                2
+            }
+        },
+        Ok(Err(err)) if err.kind() == std::io::ErrorKind::BrokenPipe => 0,
+        Ok(Err(err)) => {
+            eprintln!("h5rs: {err}");
+            2
+        }
+        Err(_) => {
+            let _ = o.flush();
+            INTERNAL_ERROR
+        }
+    };
+    std::process::exit(code);
+}

crates/clawhdf5-tools/src/stat.rs

@@ -0,0 +1,305 @@
+//! `h5rs stat`: object, layout, filter and storage statistics, laid out like
+//! h5stat's report.
+
+use std::collections::BTreeMap;
+
+use clawhdf5_format::data_layout::DataLayout;
+use clawhdf5_format::dataspace::DataspaceType;
+
+use crate::cli::{Args, Out};
+use crate::h5::{H5, Kind, LinkKind};
+use crate::info::{self, DsInfo};
+
+pub const USAGE: &str = "\
+usage: h5rs stat FILE
+
+Print statistics for FILE, in the layout of h5stat's report: object counts,
+links, dataset ranks, layouts, filters, attribute counts, raw-data size and
+a file-space summary. Metadata space is not broken down by structure (h5stat
+does); it is reported as one figure together with free space.
+
+Exit status: 0 printed, 1 some object could not be read (counted as
+\"unreadable\"), 2 error.";
+
+#[derive(Default)]
+struct Stats {
+    groups: u64,
+    datasets: u64,
+    datatypes: u64,
+    other: u64,
+    unreadable: u64,
+    links: u64,
+    max_links_to_object: u64,
+    max_objects_in_group: u64,
+    group_sizes: BTreeMap<usize, u64>,
+    ranks: BTreeMap<usize, u64>,
+    max_1d: u64,
+    layout: [u64; 4],
+    external: u64,
+    no_filter: u64,
+    filter_counts: BTreeMap<&'static str, u64>,
+    raw_data: u64,
+    raw_errors: u64,
+    attr_objects: u64,
+    max_attrs: u64,
+    attr_counts: BTreeMap<usize, u64>,
+}
+
+const FILTER_KINDS: [&str; 8] = [
+    "GZIP",
+    "SHUFFLE",
+    "FLETCHER32",
+    "SZIP",
+    "NBIT",
+    "SCALEOFFSET",
+    "USER-DEFINED",
+    "",
+];
+
+fn filter_kind(id: u16) -> &'static str {
+    match id {
+        1 => "GZIP",
+        2 => "SHUFFLE",
+        3 => "FLETCHER32",
+        4 => "SZIP",
+        5 => "NBIT",
+        6 => "SCALEOFFSET",
+        _ => "USER-DEFINED",
+    }
+}
+
+pub fn run(args: &mut Args, out: &mut Out) -> std::io::Result<i32> {
+    let mut file = None;
+    while let Some(a) = args.next() {
+        match a.as_str() {
+            "-h" | "--help" => {
+                writeln!(out.o, "{USAGE}")?;
+                return Ok(0);
+            }
+            s if s.starts_with('-') && s.len() > 1 => {
+                return args.usage_error(out, &format!("unknown option {a}"), USAGE);
+            }
+            _ if file.is_none() => file = Some(a),
+            _ => return args.usage_error(out, &format!("unexpected argument {a}"), USAGE),
+        }
+    }
+    let Some(file) = file else {
+        return args.usage_error(out, "missing FILE", USAGE);
+    };
+    let h5 = match H5::open(std::path::Path::new(&file)) {
+        Ok(h) => h,
+        Err(e) => {
+            writeln!(out.e, "h5rs stat: {e}")?;
+            return Ok(2);
+        }
+    };
+    let mut s = Stats::default();
+    let mut errors: Vec<String> = Vec::new();
+    let walk = h5.walk(|it| {
+        if let Some(l) = it.link
+            && !matches!(l.kind, LinkKind::Hard(_))
+        {
+            s.links += 1;
+            return;
+        }
+        if it.first_path.is_some() {
+            return;
+        }
+        let Some(Ok(h)) = it.header else {
+            if let Some(Err(e)) = it.header {
+                s.unreadable += 1;
+                errors.push(format!("{}: {e}", it.path));
+            }
+            return;
+        };
+        s.max_links_to_object = s.max_links_to_object.max(info::link_count(h));
+        match h5.attributes(h) {
+            Ok((a, errs)) => {
+                let n = a.len() + errs.len();
+                if n > 0 {
+                    s.attr_objects += 1;
+                    s.max_attrs = s.max_attrs.max(n as u64);
+                    *s.attr_counts.entry(n).or_default() += 1;
+                }
+                for e in errs {
+                    errors.push(format!("{}: attribute: {e}", it.path));
+                }
+            }
+            Err(e) => errors.push(format!("{}: {e}", it.path)),
+        }
+        match Kind::of(h) {
+            Kind::Group => {
+                s.groups += 1;
+                match h5.links(h) {
+                    Ok(l) => {
+                        s.max_objects_in_group = s.max_objects_in_group.max(l.len() as u64);
+                        *s.group_sizes.entry(l.len()).or_default() += 1;
+                    }
+                    Err(e) => {
+                        s.unreadable += 1;
+                        errors.push(format!("{}: {e}", it.path));
+                    }
+                }
+            }
+            Kind::Datatype => s.datatypes += 1,
+            Kind::Unknown => {
+                // The root group of a file with no links at all has no
+                // group messages; count it as a group.
+                if it.link.is_none() {
+                    s.groups += 1;
+                    *s.group_sizes.entry(0).or_default() += 1;
+                } else {
+                    s.other += 1;
+                }
+            }
+            Kind::Dataset => {
+                s.datasets += 1;
+                let info = DsInfo::read(&h5, it.path, h);
+                if let Ok(ds) = &info.ds {
+                    let rank = match ds.space_type {
+                        DataspaceType::Simple => ds.dimensions.len(),
+                        _ => 0,
+                    };
+                    *s.ranks.entry(rank).or_default() += 1;
+                    if rank == 1 {
+                        s.max_1d = s.max_1d.max(ds.dimensions[0]);
+                    }
+                }
+                match &info.layout {
+                    Ok(l) => {
+                        let i = match l {
+                            DataLayout::Compact { .. } => 0,
+                            DataLayout::Contiguous { .. } => 1,
+                            DataLayout::Chunked { .. } => 2,
+                            DataLayout::Virtual { .. } => 3,
+                        };
+                        s.layout[i] += 1;
+                    }
+                    Err(e) => errors.push(format!("{}: {e}", it.path)),
+                }
+                if info.external {
+                    s.external += 1;
+                }
+                match &info.filters {
+                    Ok(Some(p)) if !p.filters.is_empty() => {
+                        let mut kinds: Vec<&'static str> =
+                            p.filters.iter().map(|f| filter_kind(f.filter_id)).collect();
+                        kinds.sort_unstable();
+                        kinds.dedup();
+                        for k in kinds {
+                            *s.filter_counts.entry(k).or_default() += 1;
+                        }
+                    }
+                    Ok(_) => s.no_filter += 1,
+                    Err(e) => errors.push(format!("{}: {e}", it.path)),
+                }
+                match info::allocated_bytes(&h5, &info) {
+                    Ok(b) => s.raw_data = s.raw_data.saturating_add(b),
+                    Err(e) => {
+                        s.raw_errors += 1;
+                        errors.push(format!("{}: storage size: {e}", it.path));
+                    }
+                }
+            }
+        }
+    });
+    if let Err(e) = walk {
+        errors.push(e.to_string());
+    }
+    report(&h5, &file, &s, out)?;
+    for e in &errors {
+        writeln!(out.e, "h5rs stat: {e}")?;
+    }
+    Ok(if errors.is_empty() { 0 } else { 1 })
+}
+
+fn report(h5: &H5, file: &str, s: &Stats, out: &mut Out) -> std::io::Result<()> {
+    let o = &mut out.o;
+    writeln!(o, "Filename: {file}")?;
+    writeln!(o, "File information")?;
+    writeln!(o, "\t# of unique groups: {}", s.groups)?;
+    writeln!(o, "\t# of unique datasets: {}", s.datasets)?;
+    writeln!(o, "\t# of unique named datatypes: {}", s.datatypes)?;
+    writeln!(o, "\t# of unique links: {}", s.links)?;
+    writeln!(o, "\t# of unique other: {}", s.other)?;
+    if s.unreadable > 0 {
+        writeln!(o, "\t# of unreadable objects: {}", s.unreadable)?;
+    }
+    writeln!(o, "\tMax. # of links to object: {}", s.max_links_to_object)?;
+    writeln!(
+        o,
+        "\tMax. # of objects in group: {}",
+        s.max_objects_in_group
+    )?;
+    let sb = h5.sb();
+    writeln!(o, "Superblock:")?;
+    writeln!(o, "\tVersion: {}", sb.version)?;
+    writeln!(o, "\tSize of offsets: {} bytes", sb.offset_size)?;
+    writeln!(o, "\tSize of lengths: {} bytes", sb.length_size)?;
+    writeln!(o, "\tUser block: {} bytes", h5.file.user_block_size())?;
+    writeln!(o, "Group bins:")?;
+    for (n, c) in &s.group_sizes {
+        writeln!(o, "\t# of groups with {n} link(s): {c}")?;
+    }
+    writeln!(o, "\tTotal # of groups: {}", s.groups)?;
+    writeln!(o, "Dataset dimension information:")?;
+    writeln!(
+        o,
+        "\tMax. rank of datasets: {}",
+        s.ranks.keys().next_back().copied().unwrap_or(0)
+    )?;
+    writeln!(o, "\tDataset ranks:")?;
+    for (r, c) in &s.ranks {
+        writeln!(o, "\t\t# of dataset with rank {r}: {c}")?;
+    }
+    writeln!(o, "1-D Dataset information:")?;
+    writeln!(o, "\tMax. dimension size of 1-D datasets: {}", s.max_1d)?;
+    writeln!(o, "Dataset storage information:")?;
+    writeln!(o, "\tTotal raw data size: {}", s.raw_data)?;
+    if s.raw_errors > 0 {
+        writeln!(
+            o,
+            "\tDatasets whose storage size could not be read: {}",
+            s.raw_errors
+        )?;
+    }
+    writeln!(o, "Dataset layout information:")?;
+    for (i, n) in ["COMPACT", "CONTIG", "CHUNKED", "VIRTUAL"]
+        .iter()
+        .enumerate()
+    {
+        writeln!(o, "\tDataset layout counts[{n}]: {}", s.layout[i])?;
+    }
+    writeln!(o, "\tDatasets with external raw data: {}", s.external)?;
+    writeln!(o, "Dataset filters information:")?;
+    writeln!(o, "\tNumber of datasets with:")?;
+    writeln!(o, "\t\tNO filter: {}", s.no_filter)?;
+    for k in FILTER_KINDS.iter().filter(|k| !k.is_empty()) {
+        writeln!(
+            o,
+            "\t\t{k} filter: {}",
+            s.filter_counts.get(k).copied().unwrap_or(0)
+        )?;
+    }
+    writeln!(o, "Attribute information:")?;
+    for (n, c) in &s.attr_counts {
+        writeln!(o, "\t# of objects with {n} attribute(s): {c}")?;
+    }
+    writeln!(
+        o,
+        "\tTotal # of objects with attributes: {}",
+        s.attr_objects
+    )?;
+    writeln!(o, "\tMax. # of attributes to objects: {}", s.max_attrs)?;
+    let total = std::fs::metadata(&h5.path).map(|m| m.len()).unwrap_or(0);
+    let ub = h5.file.user_block_size();
+    writeln!(o, "Summary of file space information:")?;
+    writeln!(o, "  User block: {ub} bytes")?;
+    writeln!(o, "  Raw data: {} bytes", s.raw_data)?;
+    writeln!(
+        o,
+        "  Metadata and free space: {} bytes",
+        total.saturating_sub(s.raw_data).saturating_sub(ub)
+    )?;
+    writeln!(o, "Total space: {total} bytes")
+}

crates/clawhdf5-tools/src/value.rs

@@ -0,0 +1,483 @@
+//! Decoding one element of any datatype into a [`Value`], and printing it.
+//!
+//! Decoding never panics: a short buffer, an unknown byte order or a
+//! dangling heap reference becomes [`Value::Error`].
+
+use clawhdf5_format::datatype::{Datatype, DatatypeByteOrder, ReferenceType, StringPadding};
+use serde_json::Value as J;
+
+use crate::dtype;
+use crate::h5::H5;
+
+#[derive(Debug, Clone, PartialEq)]
+pub enum Value {
+    Int(i128),
+    /// A float and the width (in bits) it was stored with, so it prints at
+    /// its own precision.
+    Float(f64, u8),
+    Str(String),
+    /// Opaque, bitfield, time and oversized integers.
+    Bytes(Vec<u8>),
+    /// An enum member (name, when the value matches one) and its value.
+    Enum(Option<String>, i128),
+    Compound(Vec<(String, Value)>),
+    Array(Vec<Value>),
+    /// A variable-length sequence.
+    Seq(Vec<Value>),
+    /// A reference: the referenced object's address (`None` = null).
+    Ref(Option<u64>),
+    /// A region or attribute reference, kept as its bytes.
+    OtherRef(Vec<u8>),
+    Error(String),
+}
+
+pub fn hex(b: &[u8]) -> String {
+    let mut s = String::from("0x");
+    for x in b {
+        s.push_str(&format!("{x:02x}"));
+    }
+    s
+}
+
+/// Element bytes as an unsigned integer (at most 16 bytes).
+fn bits(b: &[u8], order: &DatatypeByteOrder) -> Option<u128> {
+    if b.len() > 16 {
+        return None;
+    }
+    let mut v = 0u128;
+    match order {
+        DatatypeByteOrder::LittleEndian => {
+            for (i, x) in b.iter().enumerate() {
+                v |= u128::from(*x) << (8 * i);
+            }
+        }
+        DatatypeByteOrder::BigEndian => {
+            for x in b {
+                v = (v << 8) | u128::from(*x);
+            }
+        }
+        DatatypeByteOrder::Vax => return None,
+    }
+    Some(v)
+}
+
+/// The value of an integer (fixed-point) element, honouring its bit offset
+/// and precision. `None` when it cannot be represented (over 16 bytes) or
+/// `dt` is not an integer.
+pub fn decode_int(dt: &Datatype, b: &[u8]) -> Option<i128> {
+    let Datatype::FixedPoint {
+        size,
+        byte_order,
+        signed,
+        bit_offset,
+        bit_precision,
+    } = dt
+    else {
+        return None;
+    };
+    let size = usize::try_from(*size).ok()?;
+    let v = bits(b.get(..size)?, byte_order)?;
+    let off = u32::from(*bit_offset);
+    let prec = u32::from(*bit_precision).min(128);
+    if off >= 128 || prec == 0 {
+        return Some(0);
+    }
+    let mut x = v >> off;
+    if prec < 128 {
+        x &= (1u128 << prec) - 1;
+    }
+    if *signed && prec < 128 && (x >> (prec - 1)) & 1 == 1 {
+        x |= !0u128 << prec;
+        return Some(x as i128);
+    }
+    if !*signed && prec == 128 && x > i128::MAX as u128 {
+        return None;
+    }
+    Some(x as i128)
+}
+
+fn decode_float(dt: &Datatype, b: &[u8]) -> Value {
+    let Datatype::FloatingPoint {
+        size, byte_order, ..
+    } = dt
+    else {
+        return Value::Error("not a float".into());
+    };
+    let Ok(n) = usize::try_from(*size) else {
+        return Value::Error("float size".into());
+    };
+    let Some(b) = b.get(..n) else {
+        return Value::Error("short element".into());
+    };
+    if dtype::is_ieee(dt)
+        && let Some(v) = bits(b, byte_order)
+    {
+        return match n {
+            2 => Value::Float(
+                f64::from(clawhdf5_format::float16::f16_bits_to_f32(v as u16)),
+                16,
+            ),
+            4 => Value::Float(f64::from(f32::from_bits(v as u32)), 32),
+            _ => Value::Float(f64::from_bits(v as u64), 64),
+        };
+    }
+    // Non-IEEE layouts (N-Bit floats, VAX order): the library converts.
+    match clawhdf5_format::data_read::read_as_f64(b, dt) {
+        Ok(v) if v.len() == 1 => Value::Float(v[0], (n * 8).min(64) as u8),
+        Ok(_) => Value::Error("float conversion".into()),
+        Err(e) => Value::Error(e.to_string()),
+    }
+}
+
+fn trim_string(b: &[u8], pad: Option<&StringPadding>) -> String {
+    let cut = b.iter().position(|&c| c == 0).unwrap_or(b.len());
+    let mut s = &b[..cut];
+    if matches!(pad, Some(StringPadding::SpacePad)) {
+        while let [rest @ .., b' '] = s {
+            s = rest;
+        }
+    }
+    String::from_utf8_lossy(s).into_owned()
+}
+
+/// Little-endian unsigned integer of `b` (up to 8 bytes).
+fn le(b: &[u8]) -> u64 {
+    b.iter()
+        .take(8)
+        .enumerate()
+        .fold(0u64, |a, (i, &x)| a | (u64::from(x) << (8 * i)))
+}
+
+/// Decodes elements of one file.
+pub struct Decoder<'a> {
+    pub h5: &'a H5,
+}
+
+impl<'a> Decoder<'a> {
+    pub fn new(h5: &'a H5) -> Self {
+        Self { h5 }
+    }
+
+    /// Decode element `i` of `raw`, an array of `dt` elements.
+    pub fn element(&self, dt: &Datatype, raw: &[u8], i: usize) -> Value {
+        let size = dt.type_size() as usize;
+        match i
+            .checked_mul(size)
+            .and_then(|s| raw.get(s..s.checked_add(size)?))
+        {
+            Some(b) => self.decode(dt, b, 0),
+            None => Value::Error("element out of range".into()),
+        }
+    }
+
+    pub fn decode(&self, dt: &Datatype, b: &[u8], depth: u32) -> Value {
+        if depth > 32 {
+            return Value::Error("datatype nesting too deep".into());
+        }
+        let size = dt.type_size() as usize;
+        let Some(b) = b.get(..size) else {
+            return Value::Error("short element".into());
+        };
+        match dt {
+            Datatype::FixedPoint { .. } => match decode_int(dt, b) {
+                Some(v) => Value::Int(v),
+                None => Value::Bytes(b.to_vec()),
+            },
+            Datatype::FloatingPoint { .. } => decode_float(dt, b),
+            Datatype::Time { .. } | Datatype::BitField { .. } | Datatype::Opaque { .. } => {
+                Value::Bytes(b.to_vec())
+            }
+            Datatype::String { padding, .. } => Value::Str(trim_string(b, Some(padding))),
+            Datatype::Compound { members, .. } => {
+                let mut out = Vec::with_capacity(members.len());
+                for m in members {
+                    let off = usize::try_from(m.byte_offset).unwrap_or(usize::MAX);
+                    let v = match b.get(off..) {
+                        Some(mb) => self.decode(&m.datatype, mb, depth + 1),
+                        None => Value::Error("member out of bounds".into()),
+                    };
+                    out.push((m.name.clone(), v));
+                }
+                Value::Compound(out)
+            }
+            Datatype::Reference { ref_type, .. } => match ref_type {
+                ReferenceType::Object | ReferenceType::Object2 => {
+                    match clawhdf5_format::data_read::read_object_references(b, dt, self.h5.os()) {
+                        Ok(r) if r.len() == 1 => {
+                            let a = r[0].address;
+                            let undef = a == u64::MAX
+                                || (self.h5.os() < 8 && a == (1u64 << (8 * self.h5.os())) - 1);
+                            Value::Ref(if undef || a == 0 { None } else { Some(a) })
+                        }
+                        Ok(_) => Value::Error("reference".into()),
+                        Err(e) => Value::Error(e.to_string()),
+                    }
+                }
+                _ => Value::OtherRef(b.to_vec()),
+            },
+            Datatype::Enumeration {
+                base_type, members, ..
+            } => {
+                let Some(v) = decode_int(base_type, b) else {
+                    return Value::Bytes(b.to_vec());
+                };
+                let bs = base_type.type_size() as usize;
+                let name = members
+                    .iter()
+                    .find(|m| m.value.get(..bs) == b.get(..bs))
+                    .map(|m| m.name.clone());
+                Value::Enum(name, v)
+            }
+            Datatype::Array {
+                base_type,
+                dimensions,
+            } => {
+                let n = dimensions
+                    .iter()
+                    .try_fold(1usize, |a, &d| a.checked_mul(d as usize));
+                let bs = base_type.type_size() as usize;
+                let Some(n) = n.filter(|n| n.checked_mul(bs).is_some_and(|t| t <= b.len())) else {
+                    return Value::Error("array larger than its element".into());
+                };
+                let mut out = Vec::with_capacity(n);
+                for k in 0..n {
+                    out.push(self.decode(base_type, &b[k * bs..], depth + 1));
+                }
+                Value::Array(out)
+            }
+            Datatype::VariableLength {
+                is_string,
+                padding,
+                base_type,
+                ..
+            } => self.decode_vlen(*is_string, padding.as_ref(), base_type, b, depth),
+        }
+    }
+
+    fn decode_vlen(
+        &self,
+        is_string: bool,
+        padding: Option<&StringPadding>,
+        base: &Datatype,
+        b: &[u8],
+        depth: u32,
+    ) -> Value {
+        let os = usize::from(self.h5.os());
+        let (Some(lenb), Some(addrb), Some(idxb)) =
+            (b.get(..4), b.get(4..4 + os), b.get(4 + os..8 + os))
+        else {
+            return Value::Error("short VL element".into());
+        };
+        let len = le(lenb) as usize;
+        let addr = le(addrb);
+        let idx = le(idxb) as u32;
+        let undef = if os >= 8 {
+            u64::MAX
+        } else {
+            (1u64 << (8 * os)) - 1
+        };
+        let obj = if len == 0 || addr == 0 || addr == undef {
+            Vec::new()
+        } else {
+            match self.h5.heap_object(addr, idx) {
+                Ok(o) => o,
+                Err(e) => return Value::Error(e.to_string()),
+            }
+        };
+        if is_string {
+            let l = len.min(obj.len());
+            return Value::Str(trim_string(&obj[..l], padding));
+        }
+        let bs = base.type_size() as usize;
+        if bs == 0 {
+            return Value::Error("VL base type of size 0".into());
+        }
+        match len.checked_mul(bs) {
+            Some(need) if need <= obj.len() => {}
+            _ => return Value::Error("VL sequence longer than its heap object".into()),
+        }
+        let mut out = Vec::with_capacity(len);
+        for k in 0..len {
+            out.push(self.decode(base, &obj[k * bs..], depth + 1));
+        }
+        Value::Seq(out)
+    }
+}
+
+/// Format a float like C's `%g` would at full round-trip precision: plain
+/// digits for moderate magnitudes, an exponent otherwise.
+pub fn fmt_float(v: f64, width: u8) -> String {
+    if v.is_nan() {
+        return "NaN".into();
+    }
+    if v.is_infinite() {
+        return if v > 0.0 { "Inf".into() } else { "-Inf".into() };
+    }
+    let a = v.abs();
+    let plain = a == 0.0 || (1e-5..1e16).contains(&a);
+    match (width, plain) {
+        (16 | 32, true) => format!("{}", v as f32),
+        (16 | 32, false) => format!("{:e}", v as f32),
+        (_, true) => format!("{v}"),
+        (_, false) => format!("{v:e}"),
+    }
+}
+
+fn escape(s: &str) -> String {
+    let mut o = String::with_capacity(s.len() + 2);
+    for c in s.chars() {
+        match c {
+            '"' => o.push_str("\\\""),
+            '\\' => o.push_str("\\\\"),
+            '\n' => o.push_str("\\n"),
+            '\r' => o.push_str("\\r"),
+            '\t' => o.push_str("\\t"),
+            c if (c as u32) < 0x20 => o.push_str(&format!("\\{:03o}", c as u32)),
+            c => o.push(c),
+        }
+    }
+    o
+}
+
+/// A fixed-length string element's bytes quoted as h5dump prints a
+/// null-padded string: every byte, NULs as `\000`.
+pub fn quote_bytes(b: &[u8]) -> String {
+    format!("\"{}\"", escape(&String::from_utf8_lossy(b)))
+}
+
+/// Text form, as in an h5dump DATA block.
+pub fn text(v: &Value, h5paths: &dyn Fn(u64) -> Option<String>) -> String {
+    match v {
+        Value::Int(i) => i.to_string(),
+        Value::Float(f, w) => fmt_float(*f, *w),
+        Value::Str(s) => format!("\"{}\"", escape(s)),
+        Value::Bytes(b) => hex(b),
+        Value::Enum(Some(n), _) => n.clone(),
+        Value::Enum(None, i) => i.to_string(),
+        Value::Compound(ms) => format!(
+            "{{ {} }}",
+            ms.iter()
+                .map(|(_, v)| text(v, h5paths))
+                .collect::<Vec<_>>()
+                .join(", ")
+        ),
+        Value::Array(vs) => format!(
+            "[ {} ]",
+            vs.iter()
+                .map(|v| text(v, h5paths))
+                .collect::<Vec<_>>()
+                .join(", ")
+        ),
+        Value::Seq(vs) => format!(
+            "({})",
+            vs.iter()
+                .map(|v| text(v, h5paths))
+                .collect::<Vec<_>>()
+                .join(", ")
+        ),
+        Value::Ref(None) => "NULL".into(),
+        Value::Ref(Some(a)) => match h5paths(*a) {
+            Some(p) => format!("\"{p}\""),
+            None => format!("{a:#x}"),
+        },
+        Value::OtherRef(b) => hex(b),
+        Value::Error(e) => format!("<error: {e}>"),
+    }
+}
+
+/// hdf5-json value form.
+pub fn to_json(v: &Value, h5paths: &dyn Fn(u64) -> Option<String>) -> J {
+    match v {
+        Value::Int(i) => {
+            if let Ok(x) = i64::try_from(*i) {
+                J::from(x)
+            } else if let Ok(x) = u64::try_from(*i) {
+                J::from(x)
+            } else {
+                J::from(i.to_string())
+            }
+        }
+        Value::Float(f, _) => {
+            if f.is_finite() {
+                serde_json::Number::from_f64(*f)
+                    .map(J::Number)
+                    .unwrap_or(J::Null)
+            } else if f.is_nan() {
+                J::from("NaN")
+            } else if *f > 0.0 {
+                J::from("Infinity")
+            } else {
+                J::from("-Infinity")
+            }
+        }
+        Value::Str(s) => J::from(s.as_str()),
+        Value::Bytes(b) | Value::OtherRef(b) => J::from(hex(b)),
+        Value::Enum(_, i) => to_json(&Value::Int(*i), h5paths),
+        Value::Compound(ms) => J::Array(ms.iter().map(|(_, v)| to_json(v, h5paths)).collect()),
+        Value::Array(vs) | Value::Seq(vs) => {
+            J::Array(vs.iter().map(|v| to_json(v, h5paths)).collect())
+        }
+        Value::Ref(None) => J::Null,
+        Value::Ref(Some(a)) => J::from(h5paths(*a).unwrap_or_else(|| format!("{a:#x}"))),
+        Value::Error(e) => serde_json::json!({ "error": e }),
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    fn int(size: u32, signed: bool, order: DatatypeByteOrder, off: u16, prec: u16) -> Datatype {
+        Datatype::FixedPoint {
+            size,
+            byte_order: order,
+            signed,
+            bit_offset: off,
+            bit_precision: prec,
+        }
+    }
+
+    #[test]
+    fn integers_decode_with_order_offset_and_sign() {
+        let le = DatatypeByteOrder::LittleEndian;
+        let be = DatatypeByteOrder::BigEndian;
+        assert_eq!(
+            decode_int(&int(2, true, le.clone(), 0, 16), &[0xff, 0xff]),
+            Some(-1)
+        );
+        assert_eq!(
+            decode_int(&int(2, false, le, 0, 16), &[0xff, 0xff]),
+            Some(65535)
+        );
+        assert_eq!(
+            decode_int(&int(2, true, be.clone(), 0, 16), &[0x80, 0x00]),
+            Some(-32768)
+        );
+        // 17-bit signed field at offset 4: -5
+        let stored = (((-5i32) as u32) & 0x1_FFFF) << 4;
+        assert_eq!(
+            decode_int(&int(4, true, be, 4, 17), &stored.to_be_bytes()),
+            Some(-5)
+        );
+        assert_eq!(
+            decode_int(&int(4, true, DatatypeByteOrder::Vax, 0, 32), &[0; 4]),
+            None
+        );
+        assert_eq!(
+            decode_int(
+                &int(4, true, DatatypeByteOrder::LittleEndian, 0, 32),
+                &[0; 2]
+            ),
+            None
+        );
+    }
+
+    #[test]
+    fn floats_print_at_their_own_precision() {
+        assert_eq!(fmt_float(f64::from(0.1f32), 32), "0.1");
+        assert_eq!(fmt_float(0.1, 64), "0.1");
+        assert_eq!(fmt_float(1e20, 64), "1e20");
+        assert_eq!(fmt_float(2.0, 64), "2");
+        assert_eq!(fmt_float(f64::NAN, 64), "NaN");
+    }
+}

crates/clawhdf5-tools/tests/gen_files.py

@@ -0,0 +1,115 @@
+"""Write the HDF5 files the h5rs interop tests run on.
+
+usage: gen_files.py OUTDIR
+
+Writes OUTDIR/{earliest,latest}.h5 (the same content with the oldest and the
+newest file-format structures: symbol tables and v1 B-trees vs. v2 object
+headers, fractal heaps, v2 B-trees and the chunk indexes of HDF5 1.10+), the
+pairs the diff tests compare, and a file with a user block. Prints one JSON
+object with the values h5py reads back, for the dump tests.
+"""
+
+import json
+import os
+import sys
+
+import h5py
+import numpy as np
+
+out = sys.argv[1]
+
+
+def content(f, dense):
+    f.attrs["title"] = "h5rs test"
+    f.attrs["version"] = np.int64(3)
+    f.attrs["scale"] = np.array([0.5, 1.5], dtype="f4")
+    f["contig"] = np.arange(12, dtype="f8").reshape(3, 4)
+    dcpl = h5py.h5p.create(h5py.h5p.DATASET_CREATE)
+    dcpl.set_layout(h5py.h5d.COMPACT)
+    space = h5py.h5s.create_simple((4,))
+    h5py.h5d.create(f.id, b"compact", h5py.h5t.STD_I16LE, space, dcpl).write(
+        h5py.h5s.ALL, h5py.h5s.ALL, np.arange(4, dtype="<i2")
+    )
+    g = f.create_group("grp")
+    g.attrs["units"] = "m"
+    g.create_dataset(
+        "gz", data=np.arange(1000, dtype="i4"), chunks=(100,), compression="gzip",
+        shuffle=True, fletcher32=True,
+    )
+    g.create_dataset("ext1", data=np.arange(50, dtype="u2"), chunks=(8,), maxshape=(None,))
+    g.create_dataset(
+        "ext2", data=np.arange(60, dtype="f4").reshape(6, 10), chunks=(4, 4), maxshape=(None, None)
+    )
+    g.create_dataset("fixed", data=np.arange(64, dtype="i8").reshape(8, 8), chunks=(3, 3))
+    g.create_dataset("single", data=np.arange(10, dtype="i4"), chunks=(10,))
+    g.create_dataset("sparse", shape=(100,), dtype="i4", chunks=(10,), fillvalue=-1)
+    g["sparse"][20:30] = 7
+    sub = g.create_group("sub")
+    sub["scalar"] = np.float32(2.5)
+    sub["empty"] = h5py.Empty("f8")
+    f["strings"] = np.array([b"ab", b"cde"])
+    f["vlstr"] = np.array(["x", "yy", "zzz"], dtype=h5py.string_dtype())
+    f["cmp"] = np.array([(1, 2.5), (3, 4.5)], dtype=[("a", "i2"), ("b", ">f4")])
+    f.create_dataset(
+        "enum", data=np.array([0, 1, 1], dtype="u1"),
+        dtype=h5py.enum_dtype({"RED": 0, "GREEN": 1}, basetype="u1"),
+    )
+    f["be"] = np.arange(5, dtype=">i4")
+    f["arr"] = np.array([([1, 2, 3],), ([4, 5, 6],)], dtype=[("v", "3i4")])
+    f["named_t"] = np.dtype("i8")
+    f["soft"] = h5py.SoftLink("/contig")
+    f["dangling"] = h5py.SoftLink("/nowhere")
+    f["external"] = h5py.ExternalLink("other.h5", "/x")
+    f["hard2"] = g["sub"]
+    many = f.create_group("many")
+    for i in range(12 if dense else 4):
+        many[f"d{i:02}"] = np.int32(i)
+        many.attrs[f"a{i:02}"] = i
+
+
+values = {}
+# "latest" under HDF5 2.0 writes datatype messages that libhdf5 1.14 tools
+# cannot read, so the newest format is taken as 1.14's.
+for libver in ("earliest", "latest"):
+    path = os.path.join(out, f"{libver}.h5")
+    bounds = ("earliest", "v114") if libver == "earliest" else ("v114", "v114")
+    with h5py.File(path, "w", libver=bounds) as f:
+        content(f, True)
+    with h5py.File(path, "r") as f:
+        vals = {}
+
+        def grab(name, obj):
+            if isinstance(obj, h5py.Dataset) and obj.dtype.kind in "iuf" and obj.shape is not None:
+                vals["/" + name] = obj[()].tolist()
+
+        f.visititems(grab)
+        values[libver] = vals
+
+# diff pairs
+def small(path, data=None, extra=False, attr=False, shape=(3, 4), dtype="f8"):
+    with h5py.File(os.path.join(out, path), "w") as f:
+        d = np.arange(12, dtype=dtype).reshape(shape) if data is None else data
+        f["d"] = d
+        f["g/x"] = np.arange(3)
+        if extra:
+            f["only_here"] = 1
+        if attr:
+            f["d"].attrs["u"] = 1
+
+
+base = np.arange(12, dtype="f8").reshape(3, 4)
+small("base.h5")
+small("same.h5")
+changed = base.copy()
+changed[0, 2] += 0.001
+changed[2, 3] += 0.001
+small("changed.h5", data=changed)
+small("extra.h5", extra=True)
+small("attr.h5", attr=True)
+small("reshaped.h5", shape=(4, 3))
+small("int.h5", dtype="i4")
+
+with h5py.File(os.path.join(out, "userblock.h5"), "w", userblock_size=1024) as f:
+    f["d"] = np.arange(10)
+
+print(json.dumps(values))

crates/clawhdf5-tools/tests/h5rs_interop.rs

@@ -0,0 +1,569 @@
+//! `h5rs` against libhdf5's own tools, on files h5py writes
+//! (`tests/gen_files.py`): `ls` against h5ls, `stat` against h5stat, `dump`
+//! against h5dump, `diff` exit codes against h5diff, `dump --json` values
+//! against h5py, and `check` on valid and deliberately corrupted files.
+//!
+//! Needs python3 with h5py (`CLAWHDF5_PYTHON`) and, for the comparisons,
+//! the libhdf5 command-line tools (h5ls, h5stat, h5dump, h5diff) on `PATH`.
+//! Each test skips when what it needs is missing, unless
+//! `CLAWHDF5_REQUIRE_INTEROP=1`, which turns a skip into a failure.
+
+use std::collections::BTreeMap;
+use std::path::{Path, PathBuf};
+use std::process::{Command, Output};
+
+use clawhdf5_format::checksum::jenkins_lookup3;
+
+fn python() -> String {
+    std::env::var("CLAWHDF5_PYTHON").unwrap_or_else(|_| "python3".to_string())
+}
+
+fn interop_required() -> bool {
+    std::env::var("CLAWHDF5_REQUIRE_INTEROP").is_ok_and(|v| v == "1")
+}
+
+fn python_available() -> bool {
+    Command::new(python())
+        .args(["-c", "import h5py, numpy"])
+        .output()
+        .map(|o| o.status.success())
+        .unwrap_or(false)
+}
+
+fn tool_available(name: &str) -> bool {
+    Command::new(name)
+        .arg("--version")
+        .output()
+        .map(|o| o.status.success())
+        .unwrap_or(false)
+}
+
+/// Skip (return true) when `what` is unavailable, or fail when interop is
+/// required.
+fn missing(ok: bool, what: &str) -> bool {
+    if ok {
+        return false;
+    }
+    assert!(
+        !interop_required(),
+        "CLAWHDF5_REQUIRE_INTEROP=1 but {what} is not available"
+    );
+    eprintln!("SKIP: {what} not available");
+    true
+}
+
+fn h5rs(args: &[&str]) -> Output {
+    Command::new(env!("CARGO_BIN_EXE_h5rs"))
+        .args(args)
+        .output()
+        .expect("run h5rs")
+}
+
+fn run(tool: &str, args: &[&str]) -> Output {
+    Command::new(tool).args(args).output().expect("run tool")
+}
+
+fn stdout(o: &Output) -> String {
+    String::from_utf8_lossy(&o.stdout).into_owned()
+}
+
+/// The generated files and what h5py read from them.
+struct Files {
+    dir: tempfile::TempDir,
+    values: serde_json::Value,
+}
+
+impl Files {
+    fn path(&self, name: &str) -> PathBuf {
+        self.dir.path().join(name)
+    }
+
+    fn p(&self, name: &str) -> String {
+        self.path(name).to_string_lossy().into_owned()
+    }
+}
+
+fn generate() -> Option<Files> {
+    if missing(python_available(), "python3 with h5py") {
+        return None;
+    }
+    let dir = tempfile::tempdir().unwrap();
+    let script = Path::new(env!("CARGO_MANIFEST_DIR")).join("tests/gen_files.py");
+    let out = Command::new(python())
+        .arg(&script)
+        .arg(dir.path())
+        .output()
+        .expect("run gen_files.py");
+    assert!(
+        out.status.success(),
+        "gen_files.py failed:\n{}",
+        String::from_utf8_lossy(&out.stderr)
+    );
+    let values = serde_json::from_slice(&out.stdout).expect("gen_files.py output");
+    Some(Files { dir, values })
+}
+
+const LIBVERS: [&str; 2] = ["earliest.h5", "latest.h5"];
+
+// ---------------------------------------------------------------------------
+// ls
+// ---------------------------------------------------------------------------
+
+/// Every h5ls line must be the start of the matching h5rs line (h5rs adds
+/// the datatype after a dataset's shape).
+fn assert_ls_like(ours: &str, reference: &str, what: &str) {
+    let ours: Vec<&str> = ours.lines().collect();
+    let refs: Vec<&str> = reference.lines().collect();
+    assert_eq!(
+        ours.len(),
+        refs.len(),
+        "{what}: line count\nh5rs:\n{}\nh5ls:\n{}",
+        ours.join("\n"),
+        refs.join("\n")
+    );
+    for (o, r) in ours.iter().zip(&refs) {
+        let r = r.trim_end();
+        assert!(
+            o.starts_with(r) && (o.len() == r.len() || r.contains("Dataset {")),
+            "{what}:\n  h5rs: {o}\n  h5ls: {r}"
+        );
+    }
+}
+
+#[test]
+fn ls_matches_h5ls() {
+    let Some(f) = generate() else { return };
+    if missing(tool_available("h5ls"), "h5ls") {
+        return;
+    }
+    for name in LIBVERS.iter().chain(&["userblock.h5"]) {
+        let p = f.p(name);
+        for (args, what) in [(vec!["-r"], "recursive"), (vec![], "root")] {
+            let mut a: Vec<&str> = args.clone();
+            a.push(&p);
+            let ours = h5rs(&[&["ls"], a.as_slice()].concat());
+            assert!(ours.status.success(), "{name} {what}: {:?}", ours);
+            let reference = run("h5ls", &a);
+            assert_ls_like(
+                &stdout(&ours),
+                &stdout(&reference),
+                &format!("{name} {what}"),
+            );
+        }
+    }
+    // A group, and a dataset, named after the file.
+    for obj in ["/grp", "/grp/sub", "/contig", "/grp/ext2"] {
+        let arg = format!("{}{obj}", f.p("latest.h5"));
+        let ours = h5rs(&["ls", &arg]);
+        let reference = run("h5ls", &[&arg]);
+        assert_ls_like(&stdout(&ours), &stdout(&reference), &arg);
+        let ours = h5rs(&["ls", "-r", &arg]);
+        let reference = run("h5ls", &["-r", &arg]);
+        assert_ls_like(&stdout(&ours), &stdout(&reference), &format!("-r {arg}"));
+    }
+}
+
+#[test]
+fn ls_verbose_describes_layout_filters_and_attributes() {
+    let Some(f) = generate() else { return };
+    let o = h5rs(&["ls", "-r", "-v", &f.p("latest.h5")]);
+    assert!(o.status.success());
+    let s = stdout(&o);
+    for want in [
+        "Layout:    chunked (extensible array index)",
+        "Layout:    chunked (v2 B-tree index)",
+        "Layout:    chunked (fixed array index)",
+        "Layout:    chunked (single chunk index)",
+        "Filter-0:  shuffle-2",
+        "Filter-1:  deflate-1",
+        "Filter-2:  fletcher32-3",
+        "Chunks:    {100} 400 bytes",
+        "Attribute: units scalar",
+        "Data:      \"m\"",
+    ] {
+        assert!(s.contains(want), "missing {want:?} in\n{s}");
+    }
+    let o = h5rs(&["ls", "-v", &format!("{}/grp", f.p("earliest.h5"))]);
+    assert!(stdout(&o).contains("Layout:    chunked (v1 B-tree index)"));
+}
+
+// ---------------------------------------------------------------------------
+// stat
+// ---------------------------------------------------------------------------
+
+/// `key: value` lines of an h5stat-style report.
+fn stat_facts(s: &str) -> BTreeMap<String, String> {
+    s.lines()
+        .filter_map(|l| {
+            let (k, v) = l.rsplit_once(':')?;
+            Some((k.trim().to_string(), v.trim().to_string()))
+        })
+        .collect()
+}
+
+#[test]
+fn stat_matches_h5stat() {
+    let Some(f) = generate() else { return };
+    if missing(tool_available("h5stat"), "h5stat") {
+        return;
+    }
+    let keys = [
+        "# of unique groups",
+        "# of unique datasets",
+        "# of unique named datatypes",
+        "# of unique links",
+        "# of unique other",
+        "Max. # of links to object",
+        "Max. # of objects in group",
+        "Max. rank of datasets",
+        "Max. dimension size of 1-D datasets",
+        "Total raw data size",
+        "Dataset layout counts[COMPACT]",
+        "Dataset layout counts[CONTIG]",
+        "Dataset layout counts[CHUNKED]",
+        "Dataset layout counts[VIRTUAL]",
+        "NO filter",
+        "GZIP filter",
+        "SHUFFLE filter",
+        "FLETCHER32 filter",
+        "SZIP filter",
+        "NBIT filter",
+        "SCALEOFFSET filter",
+        "USER-DEFINED filter",
+        "Max. # of attributes to objects",
+        "Total space",
+    ];
+    for name in LIBVERS.iter().chain(&["userblock.h5", "base.h5"]) {
+        let p = f.p(name);
+        let ours = h5rs(&["stat", &p]);
+        assert!(ours.status.success(), "{name}: {ours:?}");
+        let reference = run("h5stat", &[&p]);
+        assert!(reference.status.success(), "h5stat {name} failed");
+        let (o, r) = (stat_facts(&stdout(&ours)), stat_facts(&stdout(&reference)));
+        for k in keys {
+            // h5stat leaves out the attribute summary when nothing has one.
+            let Some(want) = r.get(k) else { continue };
+            assert_eq!(o.get(k), Some(want), "{name}: {k}");
+        }
+    }
+}
+
+// ---------------------------------------------------------------------------
+// dump
+// ---------------------------------------------------------------------------
+
+#[test]
+fn dump_matches_h5dump() {
+    let Some(f) = generate() else { return };
+    if missing(tool_available("h5dump"), "h5dump") {
+        return;
+    }
+    for name in LIBVERS.iter().chain(&["userblock.h5", "changed.h5"]) {
+        let p = f.p(name);
+        for args in [vec![], vec!["-A"]] {
+            let mut a = args.clone();
+            a.push(p.as_str());
+            let ours = h5rs(&[&["dump"], a.as_slice()].concat());
+            assert!(ours.status.success(), "{name} {args:?}: {ours:?}");
+            let reference = run("h5dump", &a);
+            // h5dump names the file as given; h5rs by its file name.
+            let r = stdout(&reference).replacen(&p, name, 1);
+            assert_eq!(stdout(&ours), r, "{name} {args:?}");
+        }
+    }
+    let p = f.p("latest.h5");
+    let ours = h5rs(&["dump", "-d", "/grp/ext2", &p]);
+    let reference = run("h5dump", &["-d", "/grp/ext2", &p]);
+    assert_eq!(
+        stdout(&ours),
+        stdout(&reference).replacen(&p, "latest.h5", 1)
+    );
+}
+
+#[test]
+fn dump_json_values_match_h5py() {
+    let Some(f) = generate() else { return };
+    for name in LIBVERS {
+        let o = h5rs(&["dump", "--json", &f.p(name)]);
+        assert!(o.status.success(), "{name}: {o:?}");
+        let doc: serde_json::Value = serde_json::from_slice(&o.stdout).unwrap();
+        assert_eq!(doc["apiVersion"], "1.1.1");
+        let root = doc["root"].as_str().unwrap();
+        assert!(doc["groups"][root].is_object());
+        let mut by_path = BTreeMap::new();
+        for (_, d) in doc["datasets"].as_object().unwrap() {
+            for a in d["alias"].as_array().unwrap() {
+                by_path.insert(a.as_str().unwrap().to_string(), d.clone());
+            }
+        }
+        let want = f.values[name.trim_end_matches(".h5")].as_object().unwrap();
+        assert!(want.len() > 10);
+        for (path, v) in want {
+            let d = by_path
+                .get(path)
+                .unwrap_or_else(|| panic!("{name}: no {path}"));
+            let got = &d["value"];
+            // JSON numbers: compare as f64 (h5py's ints and floats both
+            // round-trip exactly through JSON here).
+            assert_eq!(flatten(got), flatten(v), "{name}: {path}");
+        }
+        let links = doc["groups"][root]["links"].as_array().unwrap();
+        let soft = links.iter().find(|l| l["title"] == "soft").unwrap();
+        assert_eq!(soft["class"], "H5L_TYPE_SOFT");
+        assert_eq!(soft["h5path"], "/contig");
+        let ext = links.iter().find(|l| l["title"] == "external").unwrap();
+        assert_eq!(ext["class"], "H5L_TYPE_EXTERNAL");
+        assert_eq!(ext["file"], "other.h5");
+    }
+}
+
+fn flatten(v: &serde_json::Value) -> Vec<f64> {
+    match v {
+        serde_json::Value::Array(a) => a.iter().flat_map(flatten).collect(),
+        serde_json::Value::Number(n) => vec![n.as_f64().unwrap()],
+        other => panic!("not numeric: {other}"),
+    }
+}
+
+// ---------------------------------------------------------------------------
+// diff
+// ---------------------------------------------------------------------------
+
+fn code(o: &Output) -> i32 {
+    o.status.code().expect("exit code")
+}
+
+#[test]
+fn diff_exit_codes_match_h5diff() {
+    let Some(f) = generate() else { return };
+    if missing(tool_available("h5diff"), "h5diff") {
+        return;
+    }
+    let (b, same, changed, extra, attr) = (
+        f.p("base.h5"),
+        f.p("same.h5"),
+        f.p("changed.h5"),
+        f.p("extra.h5"),
+        f.p("attr.h5"),
+    );
+    let missing_file = f.p("does-not-exist.h5");
+    let cases: Vec<(Vec<&str>, i32)> = vec![
+        (vec![&b, &same], 0),
+        (vec![&b, &b], 0),
+        (vec![&b, &changed], 1),
+        (vec![&b, &changed, "/d"], 1),
+        (vec![&b, &changed, "/g"], 0),
+        (vec!["-d", "0.01", &b, &changed], 0),
+        (vec!["-d", "0.0001", &b, &changed], 1),
+        (vec!["-p", "0.01", &b, &changed], 0),
+        (vec!["-p", "1e-9", &b, &changed], 1),
+        (vec![&b, &extra], 1),
+        (vec![&b, &attr], 1),
+        (vec![&b, &attr, "/g"], 0),
+        (vec![&b, &same, "/nope"], 2),
+        (vec![&b, &missing_file], 2),
+        (vec![&b, &same, "/d", "/d"], 0),
+    ];
+    for (args, want) in cases {
+        let theirs = code(&run("h5diff", &args));
+        assert_eq!(theirs, want, "h5diff {args:?} (test expectation)");
+        let o = h5rs(&[&["diff"], args.as_slice()].concat());
+        assert_eq!(code(&o), want, "h5rs diff {args:?}: {}", stdout(&o));
+        let q = h5rs(&[&["diff", "-q"], args.as_slice()].concat());
+        assert_eq!(code(&q), want, "h5rs diff -q {args:?}");
+        assert!(q.stdout.is_empty(), "-q printed output");
+    }
+    // The report lists the differing positions.
+    let o = h5rs(&["diff", "-r", &b, &changed, "/d"]);
+    let s = stdout(&o);
+    assert!(s.contains("[ 0 2 ]") && s.contains("[ 2 3 ]"), "{s}");
+    assert!(s.contains("2 difference(s) found"), "{s}");
+}
+
+/// Where h5rs deliberately differs from h5diff: objects that cannot be
+/// compared (another shape, another datatype class) are a difference.
+#[test]
+fn diff_counts_incomparable_objects_as_different() {
+    let Some(f) = generate() else { return };
+    let b = f.p("base.h5");
+    for other in ["reshaped.h5", "int.h5"] {
+        let o = h5rs(&["diff", &b, &f.p(other)]);
+        assert_eq!(code(&o), 1, "{other}: {}", stdout(&o));
+        assert!(stdout(&o).contains("Not comparable"), "{other}");
+    }
+}
+
+// ---------------------------------------------------------------------------
+// check
+// ---------------------------------------------------------------------------
+
+#[test]
+fn check_accepts_valid_files() {
+    let Some(f) = generate() else { return };
+    for name in LIBVERS.iter().chain(&[
+        "userblock.h5",
+        "base.h5",
+        "changed.h5",
+        "extra.h5",
+        "attr.h5",
+    ]) {
+        let o = h5rs(&["check", "--data", &f.p(name)]);
+        assert_eq!(code(&o), 0, "{name}:\n{}", stdout(&o));
+        assert!(stdout(&o).contains("no problems found"));
+    }
+    // The newest format's checksummed structures were all verified.
+    let s = stdout(&h5rs(&["check", &f.p("latest.h5")]));
+    assert!(s.contains("superblock 1, v2 object headers 33,"), "{s}");
+    assert!(s.contains("chunk indexes 5"), "{s}");
+}
+
+/// Signatures of checksummed structures, and where each one's checksum
+/// is: found by trying every end offset (the checksum is the Jenkins
+/// lookup3 hash of the bytes before it). A fractal heap direct block's
+/// checksum is instead stored near its start, computed over the whole block
+/// with the field zeroed.
+fn checksummed_structures(data: &[u8]) -> Vec<(&'static str, usize, usize)> {
+    const SIGS: [&str; 13] = [
+        "OHDR", "OCHK", "BTHD", "BTIN", "BTLF", "FRHP", "FHIB", "EAHD", "EAIB", "EASB", "EADB",
+        "FAHD", "FADB",
+    ];
+    let mut found = Vec::new();
+    for start in 0..data.len().saturating_sub(4) {
+        let sig = &data[start..start + 4];
+        if let Some(name) = SIGS.iter().find(|s| s.as_bytes() == sig) {
+            let limit = data.len().min(start + 65536);
+            for end in start + 8..limit.saturating_sub(4) {
+                let stored = u32::from_le_bytes(data[end..end + 4].try_into().unwrap());
+                if jenkins_lookup3(&data[start..end]) == stored {
+                    found.push((*name, start, end));
+                    break;
+                }
+            }
+        } else if sig == b"FHDB" {
+            // version(1) heap address(8) block offset(1..8) checksum(4)
+            'found: for boff in 1..=8usize {
+                let at = start + 5 + 8 + boff;
+                let Some(stored) = data.get(at..at + 4) else {
+                    break;
+                };
+                let stored = u32::from_le_bytes(stored.try_into().unwrap());
+                for size in [512usize, 1024, 2048, 4096, 8192, 16384, 32768, 65536] {
+                    let Some(block) = data.get(start..start + size) else {
+                        break;
+                    };
+                    let mut b = block.to_vec();
+                    b[at - start..at - start + 4].fill(0);
+                    if jenkins_lookup3(&b) == stored {
+                        found.push(("FHDB", start, at));
+                        break 'found;
+                    }
+                }
+            }
+        }
+    }
+    found
+}
+
+#[test]
+fn check_flags_every_corrupted_checksum() {
+    let Some(f) = generate() else { return };
+    let path = f.path("latest.h5");
+    let data = std::fs::read(&path).unwrap();
+    let structures = checksummed_structures(&data);
+    let kinds: std::collections::BTreeSet<&str> = structures.iter().map(|s| s.0).collect();
+    for want in [
+        "OHDR", "OCHK", "BTHD", "BTLF", "FRHP", "FHDB", "EAHD", "EAIB", "EADB", "FAHD", "FADB",
+    ] {
+        assert!(kinds.contains(want), "test file has no {want}: {kinds:?}");
+    }
+    let bad = f.path("bad.h5");
+    for (name, start, at) in structures {
+        let mut d = data.clone();
+        d[at] ^= 0x01;
+        std::fs::write(&bad, &d).unwrap();
+        let o = h5rs(&["check", &bad.to_string_lossy()]);
+        let s = stdout(&o);
+        assert_eq!(
+            code(&o),
+            1,
+            "{name} at {start:#x}: checksum flip not flagged\n{s}"
+        );
+        assert!(
+            s.lines()
+                .any(|l| l.starts_with("problem: 0x") && l.contains("checksum")),
+            "{name} at {start:#x}: no checksum problem reported\n{s}"
+        );
+    }
+}
+
+#[test]
+fn check_flags_a_truncated_file() {
+    let Some(f) = generate() else { return };
+    let data = std::fs::read(f.path("latest.h5")).unwrap();
+    let bad = f.path("truncated.h5");
+    std::fs::write(&bad, &data[..data.len() * 3 / 4]).unwrap();
+    let o = h5rs(&["check", &bad.to_string_lossy()]);
+    assert_eq!(code(&o), 1);
+    assert!(stdout(&o).contains("file is truncated"), "{}", stdout(&o));
+}
+
+/// A v1 B-tree chunk index has no checksum; its keys are checked against
+/// the dataset: a chunk offset that is not a multiple of the chunk size.
+#[test]
+fn check_flags_a_misaligned_chunk() {
+    let Some(f) = generate() else { return };
+    let path = f.path("earliest.h5");
+    let mut data = std::fs::read(&path).unwrap();
+    // The leaf chunk B-tree of /grp/gz (10 deflated chunks of 100 i32, so
+    // each stored in under 400 bytes): "TREE" type(1)=1 level(1)=0
+    // entries(2) left(8) right(8), then key, child, key, child, ... where a
+    // rank-1 chunk key is size(4) mask(4) offsets(2 x 8) and a child is 8.
+    // Key 1 starts at 24 + 24 + 8 = 56; its dimension-0 offset (100) at 64.
+    let mut patched = false;
+    for start in 0..data.len() - 72 {
+        if &data[start..start + 4] == b"TREE" && data[start + 4] == 1 && data[start + 5] == 0 {
+            let entries = u16::from_le_bytes(data[start + 6..start + 8].try_into().unwrap());
+            let size0 = u32::from_le_bytes(data[start + 24..start + 28].try_into().unwrap());
+            let off1 = u64::from_le_bytes(data[start + 64..start + 72].try_into().unwrap());
+            if entries == 10 && size0 < 400 && off1 == 100 {
+                data[start + 64..start + 72].copy_from_slice(&103u64.to_le_bytes());
+                patched = true;
+                break;
+            }
+        }
+    }
+    assert!(patched, "no chunk B-tree found for /grp/gz");
+    let bad = f.path("misaligned.h5");
+    std::fs::write(&bad, &data).unwrap();
+    let o = h5rs(&["check", &bad.to_string_lossy()]);
+    let s = stdout(&o);
+    assert_eq!(code(&o), 1, "{s}");
+    assert!(
+        s.contains("/grp/gz: chunk at [103, 0] offset 103 in dimension 0 is not a multiple of the chunk size 100"),
+        "{s}"
+    );
+}
+
+#[test]
+fn every_subcommand_rejects_a_non_hdf5_file_cleanly() {
+    let dir = tempfile::tempdir().unwrap();
+    let p = dir.path().join("junk.h5");
+    std::fs::write(&p, b"this is not an HDF5 file at all").unwrap();
+    let p = p.to_string_lossy().into_owned();
+    for args in [
+        vec!["ls", p.as_str()],
+        vec!["dump", p.as_str()],
+        vec!["stat", p.as_str()],
+        vec!["diff", p.as_str(), p.as_str()],
+    ] {
+        let o = h5rs(&args);
+        assert_eq!(code(&o), 2, "{args:?}");
+        assert!(String::from_utf8_lossy(&o.stderr).contains("not an HDF5 file"));
+    }
+    let o = h5rs(&["check", &p]);
+    assert_eq!(code(&o), 1);
+    assert!(stdout(&o).contains("no HDF5 signature"));
+    assert_eq!(code(&h5rs(&["nonsense"])), 2);
+    assert_eq!(code(&h5rs(&["ls"])), 2);
+    assert_eq!(code(&h5rs(&["--help"])), 0);
+}