stockfish.wasm

WebAssembly port of the strong chess engine Stockfish. See it in action on https://lichess.org/analysis.

Requirements

Uses the latest WebAssembly threading proposal.

Chrome

• Since 79: Can allocate additional memory. The default allocation suffices for up to 2 threads and 16 MB hash.
• Enabled by default since 74 (desktop only)
• Origin Trial from 70 to 75
• Flag chrome://flags/#enable-webassembly-threads since 70

Firefox

• Since Firefox 72: Structured cloning can no longer be enabled with flags.

• Since Firefox 71: Requires javascript.options.shared_memory and dom.postMessage.sharedArrayBuffer.withCOOP_COEP to be enabled in about:flags and these HTTP headers to be set on the top level response:

  Cross-Origin-Embedder-Policy: require-corp
  Cross-Origin-Opener-Policy: same-origin
  

• Firefox 68 to 70: Requires javascript.options.shared_memory to be enabled in about:flags

Other browsers

No support.

See stockfish.js for a more portable but single-threaded version.

Feature detection

function wasmThreadsSupported() {
  // WebAssembly 1.0
  var source = Uint8Array.of(0x0, 0x61, 0x73, 0x6d, 0x01, 0x00, 0x00, 0x00);
  if (typeof WebAssembly !== 'object' || !WebAssembly.validate(source)) return false;

  // SharedArrayBuffer
  if (typeof SharedArrayBuffer !== 'function') return false;

  // Atomics
  if (typeof Atomics !== 'object') return false;

  // Shared memory
  var mem = new WebAssembly.Memory({shared: true, initial: 8, maximum: 16});
  if (!(mem.buffer instanceof SharedArrayBuffer)) return false;

  // Structured cloning
  try {
    // You have to make sure nobody cares about these messages!
    window.postMessage(mem, '*');
  } catch (e) {
    return false;
  }

  // Growable shared memory (optional)
  try {
    mem.grow(8);
  } catch (e) {
    return false;
  }

  return true;
}

Current limitations

• Hashtable: 1024 MB.
• Threads: 16.
• Can hang when UCI protocol is misused. (Do not send invalid commands or positions. While the engine is searching, do not change options or start additional searches).
• No Syzygy tablebase support.

Building

Assuming em++ (>= 1.39.6) is available:

npm run-script prepare

Usage

Requires stockfish.js, stockfish.wasm and stockfish.worker.js (total size ~1.5M, ~200K gzipped) to be served from the same directory.

<script src="stockfish.js"></script>
<script>
var sf = Stockfish();
sf.addMessageListener(function (line) {
  console.log(line);
});

sf.postMessage('uci');
</script>

Optional: Callbacks assigned to sf['onRuntimeInitialized'] and sf['onAbort'] will be called for successful/failed initialization.

License

Thanks to the Stockfish team for sharing the engine under the GPL3.