blind_chess/packages/server/src/bot/candidates.ts

import {
  geometricMoves,
  type Color,
  type Piece,
  type PieceType,
  type PromotionType,
  type Square,
} from '@blind-chess/shared';
import type { Game } from '../state.js';
import { ownSquares } from '../view.js';
import type { CandidateMove } from './brain.js';

const PROMOTION_TYPES: PromotionType[] = ['q', 'r', 'b', 'n'];

export function legalCandidates(game: Game, color: Color): CandidateMove[] {
  if (game.mode === 'vanilla') return vanillaCandidates(game, color);
  return blindCandidates(game, color);
}

function vanillaCandidates(game: Game, color: Color): CandidateMove[] {
  // chess.js only returns moves for the side to move via `.moves()`. To get a
  // hypothetical move list for the other color we'd need to rotate — but the
  // bot driver only invokes legalCandidates when it's the bot's turn, so this
  // is fine in practice. Tests for "wrong color" use blind mode.
  if (game.chess.turn() !== color) return [];

  const moves = game.chess.moves({ verbose: true }) as Array<{
    from: Square; to: Square; promotion?: PromotionType;
  }>;
  return moves.map((m) => ({ from: m.from, to: m.to, promotion: m.promotion }));
}

function blindCandidates(game: Game, color: Color): CandidateMove[] {
  const own = ownSquares(game, color);
  const board = game.chess.board();
  const out: CandidateMove[] = [];

  for (const row of board) {
    if (!row) continue;
    for (const cell of row) {
      if (!cell) continue;
      if (cell.color !== color) continue;
      const piece: Piece = { color: cell.color, type: cell.type as PieceType };
      const from = cell.square as Square;
      const tos = geometricMoves(piece, from, own);
      for (const to of tos) {
        if (isPromotionSquare(piece, to)) {
          for (const promo of PROMOTION_TYPES) {
            out.push({ from, to, promotion: promo });
          }
        } else {
          out.push({ from, to });
        }
      }
    }
  }
  return out;
}

function isPromotionSquare(piece: Piece, to: Square): boolean {
  if (piece.type !== 'p') return false;
  const rank = to[1];
  return (piece.color === 'w' && rank === '8') || (piece.color === 'b' && rank === '1');
}