blind_chess/packages/server/test/unit/bot/casual-brain.test.ts

import { describe, it, expect } from 'vitest';
import { CasualBrain } from '../../../src/bot/casual-brain.js';
import type { BrainInput, CandidateMove } from '../../../src/bot/brain.js';
import type { BoardView } from '@blind-chess/shared';

function makeInput(overrides: Partial<BrainInput> = {}): BrainInput {
  const view: BoardView = {
    pieces: { e2: { color: 'w', type: 'p' } },
    toMove: 'w',
    inCheck: false,
  };
  return {
    view,
    newAnnouncements: [],
    legalCandidates: [{ from: 'e2', to: 'e4' }],
    attemptHistory: [],
    drawOfferFromOpponent: false,
    ply: 0,
    ...overrides,
  };
}

describe('CasualBrain', () => {
  it('init() resolves', async () => {
    const brain = new CasualBrain({ seed: 1 });
    await brain.init({ color: 'w', mode: 'blind', gameId: 'casualab' });
  });

  it('single candidate -> picks it', async () => {
    const brain = new CasualBrain({ seed: 1 });
    await brain.init({ color: 'w', mode: 'blind', gameId: 'g1' });
    const action = await brain.decide(makeInput());
    expect(action.type).toBe('commit');
    if (action.type === 'commit') {
      expect(action.from).toBe('e2');
      expect(action.to).toBe('e4');
    }
  });

  it('zero candidates -> throws', async () => {
    const brain = new CasualBrain({ seed: 1 });
    await brain.init({ color: 'w', mode: 'blind', gameId: 'g1' });
    await expect(brain.decide(makeInput({ legalCandidates: [] }))).rejects.toThrow();
  });

  it('attemptHistory excludes the rejected move', async () => {
    const brain = new CasualBrain({ seed: 1 });
    await brain.init({ color: 'w', mode: 'blind', gameId: 'g1' });
    const input = makeInput({
      legalCandidates: [
        { from: 'e2', to: 'e4' },
        { from: 'd2', to: 'd4' },
      ],
      attemptHistory: [{ move: { from: 'e2', to: 'e4' }, rejection: 'wont_help' }],
    });
    const action = await brain.decide(input);
    expect(action.type).toBe('commit');
    if (action.type === 'commit') {
      expect(action.from).toBe('d2');
      expect(action.to).toBe('d4');
    }
  });

  it('promotion: when multiple candidates differ only by promotion, picks queen', async () => {
    const brain = new CasualBrain({ seed: 1 });
    await brain.init({ color: 'w', mode: 'blind', gameId: 'g1' });
    const candidates: CandidateMove[] = [
      { from: 'a7', to: 'a8', promotion: 'q' },
      { from: 'a7', to: 'a8', promotion: 'r' },
      { from: 'a7', to: 'a8', promotion: 'b' },
      { from: 'a7', to: 'a8', promotion: 'n' },
    ];
    const action = await brain.decide(makeInput({ legalCandidates: candidates }));
    if (action.type === 'commit') expect(action.promotion).toBe('q');
  });

  it('draw offer at material parity -> accept', async () => {
    const brain = new CasualBrain({ seed: 1 });
    await brain.init({ color: 'w', mode: 'blind', gameId: 'g1' });
    // White has 1 king + 1 rook = 5 material; Casual heuristic accepts when own material < 15.
    const view: BoardView = {
      pieces: {
        e1: { color: 'w', type: 'k' },
        a1: { color: 'w', type: 'r' },
      },
      toMove: 'w', inCheck: false,
    };
    const action = await brain.decide({
      view, newAnnouncements: [], legalCandidates: [{ from: 'e1', to: 'e2' }],
      attemptHistory: [], drawOfferFromOpponent: true, ply: 30,
    });
    expect(action.type).toBe('respond-draw');
    if (action.type === 'respond-draw') expect(action.accept).toBe(true);
  });

  it('never voluntarily offers resign', async () => {
    const brain = new CasualBrain({ seed: 1 });
    await brain.init({ color: 'w', mode: 'blind', gameId: 'g1' });
    for (let i = 0; i < 50; i++) {
      const action = await brain.decide(makeInput({ ply: i }));
      expect(action.type).not.toBe('resign');
    }
  });

  it('seeded determinism: same seed + same input -> same move', async () => {
    const candidates: CandidateMove[] = [
      { from: 'e2', to: 'e4' },
      { from: 'd2', to: 'd4' },
      { from: 'g1', to: 'f3' },
    ];
    const a = new CasualBrain({ seed: 42 });
    await a.init({ color: 'w', mode: 'blind', gameId: 'g1' });
    const b = new CasualBrain({ seed: 42 });
    await b.init({ color: 'w', mode: 'blind', gameId: 'g1' });
    const aAct = await a.decide(makeInput({ legalCandidates: candidates }));
    const bAct = await b.decide(makeInput({ legalCandidates: candidates }));
    expect(aAct).toEqual(bAct);
  });

  it('opening moves favor center pawns over flank pawns (e/d > a/h)', async () => {
    const candidates: CandidateMove[] = [
      { from: 'a2', to: 'a3' },
      { from: 'h2', to: 'h3' },
      { from: 'e2', to: 'e4' },
      { from: 'd2', to: 'd4' },
    ];
    // Many seeds → assert e2 or d2 wins majority. The score gap (center pawn
    // scores ~25 + 15*2 = 55 over flank pawn ~15*1 = 15) is well over the
    // 0.01 random tiebreak, so center should win nearly always.
    let centerHits = 0;
    for (let s = 0; s < 20; s++) {
      const b = new CasualBrain({ seed: s });
      await b.init({ color: 'w', mode: 'blind', gameId: 'g1' });
      const a = await b.decide(makeInput({ legalCandidates: candidates, ply: 0 }));
      if (a.type === 'commit' && (a.from === 'e2' || a.from === 'd2')) centerHits++;
    }
    expect(centerHits).toBeGreaterThan(15);
  });
});