import type { DuplicateGame } from './game';
import type { GhostMarker, BoardId, Color, Square } from './types';
import { PLAYERS, PLAYER_BOARDS, PLAYER_COLOR } from './boards';

/** Squares occupied by a piece of `color` on `board`. */
function colorSquares(game: DuplicateGame, board: BoardId, color: Color): Set<Square> {
  const set = new Set<Square>();
  for (const row of game.boards[board].board()) {
    for (const cell of row) {
      if (cell && cell.color === color) set.add(cell.square);
    }
  }
  return set;
}

/**
 * Ghosts across all four players. A player's non-ghost pieces always occupy
 * identical squares on both their boards (they move in lockstep), so a piece is
 * a ghost iff the player's other board has no same-colour piece on that square.
 */
export function ghosts(game: DuplicateGame): GhostMarker[] {
  const markers: GhostMarker[] = [];
  for (const player of PLAYERS) {
    const [a, b] = PLAYER_BOARDS[player];
    const color = PLAYER_COLOR[player];
    const sqA = colorSquares(game, a, color);
    const sqB = colorSquares(game, b, color);
    for (const sq of sqA) if (!sqB.has(sq)) markers.push({ board: a, square: sq });
    for (const sq of sqB) if (!sqA.has(sq)) markers.push({ board: b, square: sq });
  }
  return markers;
}