//! `h5rs diff`: compare two files (or two objects) like h5diff.

use std::cell::OnceCell;
use std::collections::{BTreeMap, HashMap};
use std::rc::Rc;

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, Link, 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 (an OBJ that is itself a
soft link is compared as a link, as h5diff does, unless --follow-symlinks).

  -r, --report       list every differing element (position, values, difference)
  -q, --quiet        print nothing; only the exit status
  --follow-symlinks  compare the objects soft links lead to (and walk into
                     soft-linked groups) instead of the links' target paths;
                     two dangling links are the same
  -d, --delta D      numbers differ only when |a - b| > D
  -p, --relative R   numbers differ only when |a - b| / |a| > R (an R below
                     the f64 epsilon, 2.2e-16, compares exactly, as h5diff)
  -n, --count N      list at most N differing elements per object with -r
  -c, --compare      list objects that are not comparable (always done;
                     accepted for h5diff compatibility)
  --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,
    /// Compare the objects soft links lead to, not the links' targets.
    follow: bool,
}

/// 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,
        follow: false,
    };
    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,
            "--follow-symlinks" => opts.follow = 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>() {
                // As h5diff: a relative tolerance below the f64 epsilon
                // cannot be told from rounding, so it compares exactly.
                Some(r) if r >= 0.0 => {
                    opts.tol = if r < f64::EPSILON {
                        Tol::Exact
                    } else {
                        Tol::Relative(r)
                    }
                }
                _ => return args.usage_error(out, "--relative needs a number >= 0", USAGE),
            },
            // h5diff's -c: h5rs always lists them.
            "-c" | "--compare" => {}
            "-n" | "--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);
            }
            // h5diff's --count=N, --delta=D, --relative=R forms.
            s if s.starts_with("--") && s.contains('=') => {
                let (k, v) = s.split_once('=').unwrap_or((s, ""));
                if !matches!(k, "--count" | "--delta" | "--relative" | "--max-bytes") {
                    return args.usage_error(out, &format!("unknown option {a}"), USAGE);
                }
                args.push_front(k.to_string(), v.to_string());
            }
            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 start = match start(h5, obj, opts.follow) {
            Ok(x) => x,
            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 collected = match start {
            Start::Obj(addr) => collect(h5, addr, &base, opts.follow),
            Start::Link(e) => Ok(BTreeMap::from([(String::new(), e)])),
        };
        let entries = match collected {
            Ok(e) => e,
            Err(e) => {
                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
    })
}

/// What OBJ1/OBJ2 names.
enum Start {
    Obj(u64),
    /// A soft (not followed, or dangling), external or user-defined link.
    Link(Entry),
}

/// Resolve OBJ1/OBJ2. Soft links on the way to it are followed, as in any
/// HDF5 path; a soft link that *is* the named object is compared as a link
/// (its target path), as h5diff does, unless `follow`.
fn start(h5: &H5, obj: &str, follow: bool) -> crate::h5::Result<Start> {
    let p = obj.trim_matches('/');
    if !p.is_empty() {
        let (parent, name) = p.rsplit_once('/').unwrap_or(("", p));
        let link = h5
            .resolve(parent)
            .and_then(|a| h5.header(a))
            .and_then(|h| h5.links(&h))
            .ok()
            .and_then(|ls| ls.into_iter().find(|l| l.name == name));
        if let Some(l) = link {
            return Ok(match l.kind {
                LinkKind::Hard(a) => Start::Obj(a),
                LinkKind::Soft(t) if follow => match soft_target(h5, &format!("/{parent}"), &t) {
                    Ok(a) => Start::Obj(a),
                    Err(_) => Start::Link(Entry::Soft(t)),
                },
                LinkKind::Soft(t) => Start::Link(Entry::Soft(t)),
                LinkKind::External { file, path } => Start::Link(Entry::External(file, path)),
                LinkKind::UserDefined(t) => Start::Link(Entry::UserDefined(t)),
            });
        }
    }
    h5.resolve(obj).map(Start::Obj)
}

/// The object a soft link in group `parent` (an absolute path) leads to.
fn soft_target(h5: &H5, parent: &str, target: &str) -> crate::h5::Result<u64> {
    if target.starts_with('/') {
        h5.resolve(target)
    } else {
        h5.resolve(&format!("{}/{target}", parent.trim_end_matches('/')))
    }
}

/// One object as the path walk sees it: what its paths compare as, and
/// (for a group) its links.
struct Node {
    entry: Entry,
    links: Vec<Link>,
}

/// Every path below the object at `start`, relative to it (`""` is `start`
/// itself). Each hard link is its own path, so an object linked under two
/// names is compared under both, with everything below it: two files that
/// hold the same values are equal whether one shares an object between
/// names and the other stores copies. A hard link back to an ancestor (a
/// cycle) is recorded, not descended into. With `follow`, a soft link is
/// walked like a hard link to its target (a dangling one stays a link);
/// `base` is the absolute path of `start`, for relative link targets.
fn collect(
    h5: &H5,
    start: u64,
    base: &str,
    follow: bool,
) -> crate::h5::Result<BTreeMap<String, Entry>> {
    let mut cache: HashMap<u64, Rc<Node>> = HashMap::new();
    let mut node = |addr: u64| -> Rc<Node> {
        cache
            .entry(addr)
            .or_insert_with(|| {
                Rc::new(match h5.header(addr) {
                    Err(e) => Node {
                        entry: Entry::Broken(e.to_string()),
                        links: Vec::new(),
                    },
                    Ok(h) => match Kind::of(&h) {
                        Kind::Group => match h5.links(&h) {
                            Ok(links) => Node {
                                entry: Entry::Obj(addr, Kind::Group),
                                links,
                            },
                            Err(e) => Node {
                                entry: Entry::Broken(format!("links: {e}")),
                                links: Vec::new(),
                            },
                        },
                        k => Node {
                            entry: Entry::Obj(addr, k),
                            links: Vec::new(),
                        },
                    },
                })
            })
            .clone()
    };
    let mut entries = BTreeMap::new();
    // (address, relative path, addresses of the groups above it)
    let mut stack: Vec<(u64, String, Rc<Vec<u64>>)> =
        vec![(start, String::new(), Rc::new(Vec::new()))];
    while let Some((addr, path, above)) = stack.pop() {
        if entries.len() >= crate::h5::MAX_OBJECTS {
            return Err(crate::h5::Error::new(format!(
                "more than {} paths; stopped walking",
                crate::h5::MAX_OBJECTS
            )));
        }
        let n = node(addr);
        entries.insert(path.clone(), n.entry.clone());
        if n.links.is_empty() || above.contains(&addr) {
            continue;
        }
        let mut chain = (*above).clone();
        chain.push(addr);
        let chain = Rc::new(chain);
        for l in &n.links {
            let child = format!("{path}/{}", l.name);
            match &l.kind {
                LinkKind::Hard(a) => stack.push((*a, child, chain.clone())),
                LinkKind::Soft(t) if follow => match soft_target(h5, &format!("{base}{path}"), t) {
                    Ok(a) => stack.push((a, child, chain.clone())),
                    Err(_) => {
                        entries.insert(child, Entry::Soft(t.clone()));
                    }
                },
                LinkKind::Soft(t) => {
                    entries.insert(child, Entry::Soft(t.clone()));
                }
                LinkKind::External { file, path } => {
                    entries.insert(child, Entry::External(file.clone(), path.clone()));
                }
                LinkKind::UserDefined(t) => {
                    entries.insert(child, Entry::UserDefined(*t));
                }
            }
        }
    }
    Ok(entries)
}

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)) => {
                // Followed links that are left are dangling on both sides,
                // which h5diff counts as the same.
                if x != y && !self.opts.follow {
                    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 (int_of(&va), int_of(&vb), number(&va), number(&vb)) {
                    (Some(x), Some(y), ..) => x.abs_diff(y).to_string(),
                    (_, _, 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 {
        // Integers in integer arithmetic: through f64 they lose precision
        // above 2^53, and values that differ would compare equal.
        if let (Some(p), Some(q)) = (int_of(x), int_of(y)) {
            return int_close(p, q, self.opts.tol);
        }
        if let (Some(p), Some(q)) = (number(x), number(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,
    }
}

/// `close` for integers, exactly: the difference is taken in i128, so no
/// precision is lost at any 64-bit magnitude.
fn int_close(a: i128, b: i128, tol: Tol) -> bool {
    if a == b {
        return true;
    }
    let d = a.abs_diff(b);
    match tol {
        Tol::Exact => false,
        // d is whole, so d <= t exactly when d <= floor(t) (saturating).
        Tol::Delta(t) => d <= t.floor() as u128,
        // d >= 1 here, so the quotient is never rounded to 0.
        Tol::Relative(r) => a != 0 && d as f64 / a.unsigned_abs() as f64 <= 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 integer_tolerances_are_exact_above_2_pow_53() {
        let big = 1i128 << 60;
        assert!(!int_close(big, big + 1, Tol::Delta(0.0)));
        assert!(!int_close(big, big + 1, Tol::Delta(0.5)));
        assert!(int_close(big, big + 1, Tol::Delta(1.0)));
        assert!(!int_close(big, big + 200, Tol::Delta(199.9)));
        assert!(!int_close(big, big + 1, Tol::Relative(0.0)));
        assert!(!int_close(big, big + 1, Tol::Relative(1e-19)));
        assert!(int_close(big, big + 1, Tol::Relative(1e-18)));
        let umax = i128::from(u64::MAX);
        assert!(!int_close(umax, umax - 1, Tol::Delta(0.0)));
        assert!(int_close(
            i128::from(i64::MIN),
            umax,
            Tol::Delta(f64::INFINITY)
        ));
        assert!(!int_close(0, 1, Tol::Relative(1e9)));
    }

    #[test]
    fn positions() {
        assert_eq!(index(5, &[3, 4]), "1 1");
        assert_eq!(index(0, &[1]), "0");
    }
}