blind_chess/packages/server/test/unit/bot/driver.test.ts

import { describe, it, expect, beforeEach, vi } from 'vitest';
import { Chess } from 'chess.js';
import { BotDriver } from '../../../src/bot/driver.js';
import type { Brain, BrainAction, BrainInput } from '../../../src/bot/brain.js';
import type { Game } from '../../../src/state.js';
import { RATE_LIMIT } from '../../../src/state.js';

function makeGame(opts: { mode?: 'blind' | 'vanilla'; fen?: string; status?: Game['status'] } = {}): Game {
  return {
    id: 'gabcd123',
    mode: opts.mode ?? 'blind',
    highlightingEnabled: false,
    status: opts.status ?? 'active',
    createdAt: Date.now(),
    chess: opts.fen ? new Chess(opts.fen) : new Chess(),
    moveHistory: [],
    announcements: [],
    players: {
      w: { token: 'w'.repeat(24), socket: null, joinedAt: 0,
            rateBucket: { tokens: RATE_LIMIT.capacity, last: 0 } },
      b: { token: 'b'.repeat(24), socket: null, joinedAt: 0,
            rateBucket: { tokens: RATE_LIMIT.capacity, last: 0 } },
    },
    armed: null,
    drawOffer: null,
    disconnectAt: {},
    aiOpponent: { color: 'b', brain: 'casual' },
  };
}

class StubBrain implements Brain {
  public decideCalls = 0;
  private script: BrainAction[] = [];
  init = vi.fn(async () => {});
  dispose = vi.fn(async () => {});
  decide = vi.fn(async (input: BrainInput): Promise<BrainAction> => {
    this.decideCalls++;
    if (this.script.length === 0) {
      // Default: trivial commit on the first legal candidate.
      if (input.legalCandidates.length === 0) throw new Error('no candidates');
      const c = input.legalCandidates[0]!;
      return { type: 'commit', from: c.from, to: c.to, promotion: c.promotion };
    }
    return this.script.shift()!;
  });

  enqueue(...actions: BrainAction[]) { this.script.push(...actions); }
}

describe('BotDriver', () => {
  let game: Game;
  let brain: StubBrain;
  let driver: BotDriver;

  beforeEach(async () => {
    game = makeGame();
    brain = new StubBrain();
    driver = new BotDriver({ game, brain, color: 'b' });
    await driver.init();
  });

  it('init() invokes brain.init with correct args', async () => {
    expect(brain.init).toHaveBeenCalledWith({
      color: 'b',
      mode: 'blind',
      gameId: 'gabcd123',
    });
  });

  it('onStateChange does nothing when not bot turn', async () => {
    // White to move (start). Bot is black.
    await driver.onStateChange();
    expect(brain.decide).not.toHaveBeenCalled();
  });

  it('onStateChange fires decide when it is bot turn', async () => {
    game.chess.move('e4');
    await driver.onStateChange();
    expect(brain.decide).toHaveBeenCalledTimes(1);
    expect(game.chess.turn()).toBe('w'); // turn advanced
  });

  it('mutex: second onStateChange while in-flight is a no-op', async () => {
    game.chess.move('e4');
    let release: () => void;
    const gate = new Promise<void>((r) => { release = r; });
    brain.decide.mockImplementationOnce(async (input) => {
      await gate;
      const c = input.legalCandidates[0]!;
      return { type: 'commit', from: c.from, to: c.to };
    });
    const p1 = driver.onStateChange();
    const p2 = driver.onStateChange();
    release!();
    await Promise.all([p1, p2]);
    expect(brain.decide).toHaveBeenCalledTimes(1);
  });

  it('retry on wont_help: pinned bishop scenario', async () => {
    // Black king h8, black bishop e7 pinned by white rook on e1.
    const fen = '4k2K/4b3/8/8/8/8/8/4R3 b - - 0 1';
    game = makeGame({ fen });
    brain = new StubBrain();
    driver = new BotDriver({ game, brain, color: 'b' });
    await driver.init();
    // First action: pinned-bishop move (FSM rejects with wont_help)
    // Second action: legal king move (black king at e8, not h8 which is white)
    brain.enqueue(
      { type: 'commit', from: 'e7', to: 'd6' },
      { type: 'commit', from: 'e8', to: 'f8' },
    );
    await driver.onStateChange();
    expect(brain.decide).toHaveBeenCalledTimes(2);
    expect(game.chess.turn()).toBe('w');
  });

  it('retry cap (5): after 5 wont_help, driver resigns the bot', async () => {
    const fen = '4k2K/4b3/8/8/8/8/8/4R3 b - - 0 1';
    game = makeGame({ fen });
    brain = new StubBrain();
    driver = new BotDriver({ game, brain, color: 'b' });
    await driver.init();
    for (let i = 0; i < 6; i++) {
      brain.enqueue({ type: 'commit', from: 'e7', to: 'd6' });
    }
    await driver.onStateChange();
    expect(game.status).toBe('finished');
    expect(game.endReason).toBe('resign');
    expect(game.winner).toBe('w');
    expect(brain.decide).toHaveBeenCalledTimes(5);
  });

  it('respond-draw: when drawOffer is from opponent, driver fires decide and dispatches', async () => {
    game.drawOffer = { from: 'w', at: Date.now() };
    brain.enqueue({ type: 'respond-draw', accept: true });
    await driver.onStateChange();
    expect(brain.decide).toHaveBeenCalledTimes(1);
    expect(game.status).toBe('finished');
    expect(game.endReason).toBe('draw_agreed');
  });

  it('dispose on game finished: subsequent onStateChange is a no-op', async () => {
    game.chess.move('e4');
    game.status = 'finished';
    await driver.onStateChange();
    expect(brain.decide).not.toHaveBeenCalled();
    expect(brain.dispose).toHaveBeenCalled();
  });

  it('bot move that delivers checkmate finalizes game.status', async () => {
    // FEN: '1k6/8/1K6/8/8/8/8/7Q w - - 0 1'
    // White king b6, white queen h1, black king b8.
    // Qh8# is mate: queen moves h1→h8, covers h8; white king b6 covers a7,b7,c7,a5,b5,c5.
    // Black king b8 escape squares (a7,b7,c7,a8,c8) are all covered. Verified with chess.js.
    const fen = '1k6/8/1K6/8/8/8/8/7Q w - - 0 1';
    game = makeGame({ fen });
    game.aiOpponent = { color: 'w', brain: 'casual' };
    brain = new StubBrain();
    driver = new BotDriver({ game, brain, color: 'w' });
    await driver.init();

    brain.enqueue({ type: 'commit', from: 'h1', to: 'h8' });
    await driver.onStateChange();

    expect(game.status).toBe('finished');
    expect(game.endReason).toBe('checkmate');
    expect(game.winner).toBe('w');
  });
});