blind_chess/packages/server/src/bot/casual-brain.ts

import { Game as EngineGame } from 'js-chess-engine';
import type { BoardView, Color, PieceType, PromotionType, Square } from '@blind-chess/shared';
import type {
  Brain,
  BrainAction,
  BrainInitArgs,
  BrainInput,
  CandidateMove,
} from './brain.js';

interface CasualOpts {
  seed?: number;
  /**
   * Engine difficulty for vanilla mode (1-5; 1 is weakest).
   * `js-chess-engine` level 1 plays at roughly beginner strength —
   * crushes random moves but loses to a careful human. Higher levels
   * raise both strength and per-move latency.
   */
  level?: 1 | 2 | 3 | 4 | 5;
}

const PIECE_VALUE: Record<PieceType, number> = {
  p: 1, n: 3, b: 3, r: 5, q: 9, k: 0,
};

export class CasualBrain implements Brain {
  private color: Color = 'w';
  private mode: 'blind' | 'vanilla' = 'blind';
  private level: 1 | 2 | 3 | 4 | 5;
  private rng: () => number;

  constructor(opts: CasualOpts = {}) {
    this.rng = mulberry32(opts.seed ?? Math.floor(Math.random() * 0xffffffff));
    this.level = opts.level ?? 2;
  }

  async init(args: BrainInitArgs): Promise<void> {
    this.color = args.color;
    this.mode = args.mode;
  }

  async decide(input: BrainInput): Promise<BrainAction> {
    if (input.drawOfferFromOpponent) {
      return { type: 'respond-draw', accept: this.acceptDraw(input.view) };
    }

    const filtered = this.excludeRejected(input.legalCandidates, input.attemptHistory);
    if (filtered.length === 0) {
      throw new Error('CasualBrain: zero candidates after exclusion');
    }

    // Vanilla mode: delegate to a real chess engine. The driver supplies
    // a FEN only in vanilla mode, so this branch is naturally gated.
    if (this.mode === 'vanilla' && input.fen) {
      const engineMove = this.engineMove(input.fen, filtered);
      if (engineMove) {
        return {
          type: 'commit',
          from: engineMove.from,
          to: engineMove.to,
          promotion: engineMove.promotion,
        };
      }
      // Fall through to heuristic if the engine produced something we
      // can't validate against the candidate list.
    }

    // Blind mode (or vanilla fallback): score-based heuristic.
    const choice = this.heuristicPick(filtered, input.view, input.ply);
    return {
      type: 'commit',
      from: choice.from,
      to: choice.to,
      promotion: choice.promotion,
    };
  }

  /**
   * Run js-chess-engine on the given FEN and return a candidate matching
   * its choice, or null if no match was found.
   */
  private engineMove(fen: string, candidates: CandidateMove[]): CandidateMove | null {
    let result: { move: Record<string, string> };
    try {
      const g = new EngineGame(fen);
      // randomness=30 picks among moves within 30 centipawns of best; this
      // breaks threefold-repetition draws when the bot is clearly winning
      // but doesn't see the conversion path.
      result = g.ai({ level: this.level, play: false, randomness: 30 }) as { move: Record<string, string> };
    } catch {
      return null;
    }
    const entry = Object.entries(result.move ?? {})[0];
    if (!entry) return null;
    const [fromUC, toUC] = entry;
    const from = (fromUC as string).toLowerCase() as Square;
    const to = (toUC as string).toLowerCase() as Square;
    // Find a candidate matching this from-to. If the move is a promotion,
    // js-chess-engine emits the destination square (e.g., {E7: 'E8'}) but
    // doesn't separately surface the promotion piece — default to queen.
    const matches = candidates.filter((c) => c.from === from && c.to === to);
    if (matches.length === 0) return null;
    const queen = matches.find((c) => c.promotion === 'q');
    if (queen) return queen;
    return matches[0]!;
  }

  /**
   * Score-based fallback used for blind mode and any vanilla case where
   * the engine's pick wasn't in the candidate list. Plays badly on purpose.
   */
  private heuristicPick(
    candidates: CandidateMove[],
    view: BoardView,
    ply: number,
  ): CandidateMove {
    const scored = candidates.map((c) => {
      let score = this.scoreMove(c, view, ply);
      if (c.promotion === 'q') score += 1000;
      else if (c.promotion === 'r') score += 500;
      else if (c.promotion === 'b') score += 100;
      else if (c.promotion === 'n') score += 50;
      return { move: c, score: score + this.rng() * 0.01 };
    });
    scored.sort((a, b) => b.score - a.score);
    return scored[0]!.move;
  }

  private excludeRejected(
    candidates: CandidateMove[],
    history: BrainInput['attemptHistory'],
  ): CandidateMove[] {
    if (history.length === 0) return candidates;
    const rejected = new Set(history.map((h) => moveKey(h.move)));
    return candidates.filter((c) => !rejected.has(moveKey(c)));
  }

  private scoreMove(move: CandidateMove, view: BoardView, ply: number): number {
    let score = 0;
    const destPiece = view.pieces[move.to];
    if (!destPiece) score += 50;

    const piece = view.pieces[move.from];
    if (!piece) return score;

    const ownStartingRank = this.color === 'w' ? '1' : '8';
    const ownPawnStartingRank = this.color === 'w' ? '2' : '7';

    if (ply < 16 && (piece.type === 'n' || piece.type === 'b')
        && move.from[1] === ownStartingRank) {
      score += 30;
    }

    if (piece.type === 'p' && move.from[1] === ownPawnStartingRank) {
      const file = move.from[0];
      if (file === 'd' || file === 'e') score += 25;
      else if (file === 'c' || file === 'f') score += 10;
    }

    const fromRank = parseInt(move.from[1]!, 10);
    const toRank = parseInt(move.to[1]!, 10);
    const advance = this.color === 'w' ? toRank - fromRank : fromRank - toRank;
    if (advance > 0) score += 15 * advance;

    if (move.from[1] === ownStartingRank && (piece.type === 'q' || piece.type === 'r')) {
      score -= 40;
    }

    return score;
  }

  private acceptDraw(view: BoardView): boolean {
    let own = 0;
    for (const sq of Object.keys(view.pieces) as Square[]) {
      const p = view.pieces[sq];
      if (p) own += PIECE_VALUE[p.type];
    }
    return own < 15;
  }
}

function moveKey(m: CandidateMove): string {
  return `${m.from}-${m.to}${m.promotion ?? ''}`;
}

function mulberry32(seed: number): () => number {
  let a = seed >>> 0;
  return function () {
    a = (a + 0x6d2b79f5) >>> 0;
    let t = a;
    t = Math.imul(t ^ (t >>> 15), t | 1);
    t ^= t + Math.imul(t ^ (t >>> 7), t | 61);
    return ((t ^ (t >>> 14)) >>> 0) / 4294967296;
  };
}