multistockfish

Multiple flavors of Stockfish Engine.

This plugin provides the following Stockfish engines:

• Stockfish 19, with a small embedded NNUE (~1MB)
• Stockfish 19, without embedded NNUE
• Fairy-Stockfish, for chess variants

Usage

Start an engine

An engine is a handle you create and dispose. Stockfish.create() starts one and completes when it is ready for commands.

import 'package:multistockfish/multistockfish.dart';

// defaults to StockfishFlavor.light
final stockfish = await Stockfish.create();

// state is a ValueListenable<StockfishState>
print(stockfish.state.value); // StockfishState.ready

// for latestNoNNUE, the NNUE file path is required
final latest = await Stockfish.create(
  flavor: StockfishFlavor.latestNoNNUE,
  nnuePath: '/path/to/nn-1a298aa575a0.nnue',
);

One engine per flavor

The handle is the flavor. At most one engine per StockfishFlavor can be live at a time: create() throws a StateError while another engine of the same flavor holds the slot, and dispose() frees it. Engines of different flavors are independent and can run side by side, each with its own stdin, stdout, state and diagnostics.

// An NNUE engine for analysis and a Fairy-Stockfish opponent, at the same time.
final analysis = await Stockfish.create(flavor: StockfishFlavor.light);
final opponent = await Stockfish.create(
  flavor: StockfishFlavor.variant,
  variant: 'crazyhouse',
);

// Refused: light is taken until `analysis` is disposed.
await Stockfish.create(flavor: StockfishFlavor.light); // throws StateError

An engine that ends releases its flavor's slot without waiting to be disposed, so a replacement can be created straight away. dispose() is still what you call to stop one that is still running.

UCI command

stockfish.stdin = 'isready';
stockfish.stdin = 'go movetime 3000';
stockfish.stdin = 'go infinite';
stockfish.stdin = 'stop';

Engine output is directed to a Stream<String>, add a listener to process results.

stockfish.stdout.listen((line) {
  // do something useful
  print(line);
});

create() only completes once the engine is ready, so a listener attached to stdout afterwards has already missed the banner and the UCI handshake. Pass onStdout to see those too — it is attached before the engine is spawned and receives every line for the engine's whole life.

final stockfish = await Stockfish.create(onStdout: console.add);

Quit / Hot reload

There are two active isolates per running Stockfish engine. That interferes with Flutter's hot reload feature so you need to dispose your engines before attempting to reload.

dispose() sends the UCI quit command, waits for the engine to exit and frees its flavor's slot. An engine that will not exit is given up on rather than waited for forever.

await stockfish.dispose();

// the stdout stream is closed, and the handle stays dead
print(stockfish.state.value); // StockfishState.disposed

Sending quit over stdin works too: the engine exits cleanly and the handle ends as disposed just the same. Only an engine that died badly ends as StockfishState.error.