# HDF5 viewer in the browser

A single page that opens an HDF5 or NetCDF-4 file entirely in the browser
with `clawhdf5-wasm` (clawhdf5's reader compiled to WebAssembly): drop a
file or give a URL, browse its groups, and look at a dataset's type, shape,
attributes and values (a 50 x 12 window at a time, read as a hyperslab,
with the leading dimensions of a 3-D+ dataset held at chosen indices). A
local file never leaves the page. A file given by URL is not downloaded:
only the byte ranges each view needs are fetched (HTTP range requests),
and the header shows the requests and bytes that has cost so far.

## Build and open

```bash
rustup target add wasm32-unknown-unknown
cargo install wasm-bindgen-cli --version 0.2.129   # must equal the crate version; build.sh checks
bash examples/wasm-viewer/build.sh                  # writes examples/wasm-viewer/pkg/ (not committed)
python3 -m http.server -d examples/wasm-viewer 8000 # wasm cannot load from file://
```

Then open <http://localhost:8000/>. The URL box, or
`?file=<url>&path=<object>`, opens a file by URL (same origin, or a server
sending CORS headers, see Limits) and selects an object in it, e.g.
`?file=data/run1.h5&path=/results/energy`. Python's `http.server` does not
answer range requests, so a file served by it is downloaded whole (the
header says so); `test/serve.py` does:

```bash
python3 examples/wasm-viewer/test/serve.py --root data=/path/to/files --root =examples/wasm-viewer
# prints its port; open http://127.0.0.1:<port>/?file=data/run1.h5
```

## JavaScript API

```js
import init, { open, openUrl } from "./pkg/clawhdf5_wasm.js";
await init();
const f = open(new Uint8Array(await blob.arrayBuffer()));
f.list("/");                          // [{ name, kind: "group" | "dataset" }], groups first
f.info("/grid");                      // { shape, maxshape, dtype, elementShape }
f.attrs("/grid");                     // [{ name, value, dtype }]
f.read("/grid");                      // { shape, dtype, data }
f.readHyperslab("/grid", [0, 0], [10, 5], [2, 1]);  // start, count, stride?, block?
f.free();

// A file on a web server, read by HTTP range requests as needed. The same
// methods, each returning a promise.
const r = await openUrl("https://example.org/run1.h5", { blockSize: 1 << 20 });
await r.list("/");
await r.readHyperslab("/grid", [0, 0], [10, 5]);  // fetches only the chunks it touches
r.stats();    // { lazy, size, requests, bytesFetched, cachedBytes, passes }
r.free();
```

`openUrl(url, opts)` options, all optional: `blockSize` (bytes per block
fetched, 512 B to 64 MiB, default 1 MiB), `cacheSize` (bytes of blocks kept
between calls, default 64 MiB), `fallback` (`"download"`, the default,
reads the whole file when the server ignores `Range`, up to `maxDownload`
bytes, default 512 MiB; `"error"` refuses such a server), `headers` and
`credentials` (passed to every request), `parallel` (requests in flight,
default 6), `fetch` (a `fetch`-compatible function to use).

How it works: the reader is synchronous and a page cannot block on the
network, so each call runs as a *pass* over the blocks fetched so far. A
pass that needs a block not yet fetched is abandoned, the missing blocks
are fetched (in parallel, adjacent blocks in one request), and the pass is
run again, until one completes (`docs/design/range-reads.md`, M4). Opening
costs one request (the first block, which also gives the file's size);
listing a group whose metadata is in blocks already fetched costs none;
reading a chunked dataset costs a round trip for its chunk index (a few
for a deep one) and one batch of requests for its chunks. Every answer is
checked — a `206` with exactly the bytes asked for, from the same file
(ETag or Last-Modified, and length) — or the call fails.

`data` is the typed array of the stored width (`Float64Array`,
`Float32Array` also for `f16`, `Int8Array` ... `BigInt64Array`,
`BigUint64Array`), or an array of strings for fixed- and variable-length
strings and enumerations (h5py booleans read as `"TRUE"`/`"FALSE"`). Array
datatypes are flattened, their dimensions appended to `shape`. Anything
else throws an `Error` naming the type.

## Limits

- Read-only. `open(bytes)` holds the whole file in memory.
- `openUrl`: a cross-origin server must allow CORS and expose
  `Content-Range` (`Access-Control-Expose-Headers: Content-Range`) or answer
  `HEAD` with `Content-Length`; if the page cannot see `ETag` or
  `Last-Modified` either, a file replaced on the server is detected only by
  a change of length. A call keeps what it reads until it finishes, so a
  whole read of a large dataset needs its stored bytes in memory; read
  windows of large datasets. More in `docs/known-issues.md`.
- Compound, reference, opaque and variable-length-sequence datasets are
  refused with an error. Attributes of those types are listed with
  `value: null` and their `dtype`.
- No Zstd or SZIP filters (they link C): such a dataset fails with
  `unsupported filter`. Deflate, shuffle, Fletcher-32, LZ4, N-Bit and
  scale-offset are read (within the limits in `docs/known-issues.md`).
- Virtual datasets whose sources are in other files, and external links,
  cannot be followed: there is no file system.

## Tests

`test/run.sh` builds the package, writes `fixture.h5` (h5py) and
`fixture.nc` (netCDF4) with `test/make_fixture.py`, and `big.h5`, a 200 MB
h5py file (`WASM_BIG_MB` sets its size, 0 leaves it out; it goes under
`TMPDIR`), serves them with `test/serve.py` (range requests, a request
counter, and `/norange/...` for a server without range support), then:

- runs `test/test.mjs` under Node: every dataset (whole and a strided
  hyperslab), listing and attribute is compared with what libhdf5 reads
  back, error paths are checked, and so are the page's DOM-free helpers
  (`viewer-lib.js`). Then the same checks on the files opened by URL (1 MiB
  and 512 B blocks), calls in flight at once, the request budget of
  `big.h5` (listing it and reading small things of it must take at most 8
  requests and under 5% of the file), the download fallback and its
  limit, and the errors: HTTP status, a file that changes, a server that
  sends the wrong bytes or stops honouring `Range`.
  `CLAWHDF5_WASM_CORPUS=DIR` also compares every HDF5 file under `DIR` (up
  to 16 MiB) read by URL with the same file read from bytes;
- runs `test/browser.sh`: loads the page in headless Chromium with
  `?file=fix/fixture.h5&path=...` for eight objects and checks the rendered
  tree, types, shapes, attribute and value cells, the request counter, and
  the error shown for an unsupported type; then a server without range
  support, and `big.h5` (a small dataset and a window of the large one,
  with a single-digit percentage of the file fetched). Skipped when no
  Chromium is found (`CHROME` names one; a Playwright download under
  `~/.cache/ms-playwright` is picked up). Drag-and-drop, the file picker
  and the URL box are not driven by it; they share `setFile()` with the
  `?file=` path.

The same expectations are checked natively, without Node, by
`crates/clawhdf5-wasm/tests/h5py_interop.rs`, and the lazy reader against
the in-memory one by `tests/lazy.rs` (with `CLAWHDF5_WASM_CORPUS`, over the
corpus too), which is what CI runs (the CI container has no Node or
browser).

## Size

Measured 2026-09-26 on tank (rustc 1.98.1, wasm-bindgen 0.2.129, gzip 1.14,
`gzip -9 -n`), after `bash examples/wasm-viewer/build.sh`. The package is
larger now and the table has not been re-measured: the reader has grown
since, and `openUrl` (2026-09-27) made the facade's range-read path
reachable from JavaScript and added promise glue and `remote.js`.

| | raw | gzip -9 |
|---|---:|---:|
| `pkg/clawhdf5_wasm_bg.wasm` (profile `wasm-release`, opt-level `s`) | 627,501 B | 191,639 B |
| `pkg/clawhdf5_wasm.js` (wasm-bindgen glue) | 21,826 B | 4,487 B |
| same wasm at opt-level `z` | 693,068 B | 192,550 B |
| same wasm at opt-level `3` | 544,035 B | 198,803 B |
| h5wasm 0.10.3: wasm embedded in `dist/esm/hdf5_util.js` | 3,544,184 B | 907,096 B |
| h5wasm 0.10.3: `dist/esm/hdf5_util.js` as shipped | 4,150,134 B | 986,699 B |

h5wasm figures: `npm pack h5wasm@0.10.3` (npm reports
`dist.unpackedSize` 14,731,385 B for the whole package), wasm extracted from
the `binaryDecode` literal in `hdf5_util.js`. h5wasm is the whole of libhdf5
(writing, every datatype, plugins), so this compares download size, not
equal functionality. No `wasm-opt` pass was applied (binaryen is not
installed on tank). opt-level `s` is used because it is the smallest
compressed.