WASM ZSTD Decoder

Adaptation of the javascript shim from: https://github.com/donmccurdy/zstddec-wasm

The base64 converted wasm is placed into the C variable, you can't miss the long line. If you're interested in compiling the wasm yourself, see the instructions further down.

API:

import { ZSTDDecoder } from 'zstddec';

const decoder = new ZSTDDecoder();

await decoder.init();

// requires and returns Uint8Array
const decompressedArray = decoder.decode( compressedArray, uncompressedSize );

Example:

Include the js file in your html:

    <script src="zstddec-1.5.5.js"></script>

In your js, use code similar to this to use zstddec:

let zstdDecode = null;

function webAssemblyFail(e) {
    zstdDecode = null;
    console.log(e);
    console.error("Error loading zstddec, probable cause: webassembly not present or not working");
    zstd = false;
    // possibly display an error if your page requires zstddec
}

zstddec.decoder = new zstddec.ZSTDDecoder();
zstddec.promise = zstddec.decoder.init().catch(e => webAssemblyFail(e));
zstdDecode = zstddec.decoder.decode;

function arraybufferRequest() {
    let xhrOverride = new XMLHttpRequest();
    xhrOverride.responseType = 'arraybuffer';
    return xhrOverride;
}

function startPage() {
    let req = jQuery.ajax({
        url: "http://127.0.0.1/test.zstd", method: 'GET',
        xhr: arraybufferRequest,
        timeout: 15000,
    });

    req.done(function(data) {
        let arr = new Uint8Array(data);
        lastRequestSize = arr.byteLength;
        let res;
        try {
            res = zstdDecode( arr, 0 );
        } catch (e) {
            console.error(e);
            return;
        }
        // process the decompressed data, for example, make it a string and parse as json:
        let dstring = String.fromCharCode.apply(null, dresult);
        let djson = JSON.parse(dstring);
    });
}

zstddec.promise.then(function() {
    startPage();
});

Compiling WASM From Source:

Obtaining C Source:

wget https://github.com/facebook/zstd/releases/download/v1.5.5/zstd-1.5.5.tar.gz
tar xf zstd-1.5.5.tar.gz
cd zstd-1.5.5/build/single_file_libs
./combine.sh -r ../../lib -o zstddeclib.c zstddeclib-in.c
cp zstddeclib.c ../../..
cd ../../..

Apply Patch

patch -u zstddeclib.c -i patch.diff

• findDecompressedSize interface changed from unsigned long long to U32 for wasm / browser compatibility (uncompressed filesize limit 4294967294 bytes)
• The diff to the file created by combine.sh is also provided: patch.diff
• The modified file zstddeclib.c required for compilation is included in this directory

Compilation:

docker pull emscripten/emsdk:latest

docker run --rm -v $(pwd):/src -u $(id -u):$(id -g) emscripten/emsdk:latest emcc \
    zstddeclib.c -O3 \
    -Wl,--no-entry \
    -sEXPORTED_FUNCTIONS=_ZSTD_decompress,_ZSTD_findDecompressedSize,_ZSTD_isError,_malloc,_free \
    -sALLOW_MEMORY_GROWTH=1 \
    -sTOTAL_STACK=64kb \
    -sTOTAL_MEMORY=2Mb \
    -DNDEBUG \
    -o zstddec.wasm

TOTAL_MEMORY is the initial heap size which grows as necessary.

Convert to base64 for direct inclusion in js file:

base64 -w0 zstddec.wasm > zstddec.wasm.base64