blind_chess/packages/shared/src/geometric.ts

import type { Piece, Square } from './types.js';
import { fileIndex, rankIndex, squareAt } from './types.js';

type Dir = readonly [number, number];

const ROOK_DIRS: readonly Dir[] = [
  [1, 0], [-1, 0], [0, 1], [0, -1],
];

const BISHOP_DIRS: readonly Dir[] = [
  [1, 1], [1, -1], [-1, 1], [-1, -1],
];

const QUEEN_DIRS: readonly Dir[] = [...ROOK_DIRS, ...BISHOP_DIRS];

const KNIGHT_OFFSETS: readonly Dir[] = [
  [1, 2], [2, 1], [2, -1], [1, -2],
  [-1, -2], [-2, -1], [-2, 1], [-1, 2],
];

const KING_OFFSETS: readonly Dir[] = [
  [1, 0], [-1, 0], [0, 1], [0, -1],
  [1, 1], [1, -1], [-1, 1], [-1, -1],
];

function rays(from: Square, dirs: readonly Dir[], own: Set<Square>): Square[] {
  const out: Square[] = [];
  const f0 = fileIndex(from);
  const r0 = rankIndex(from);
  for (const [df, dr] of dirs) {
    let f = f0 + df;
    let r = r0 + dr;
    while (true) {
      const sq = squareAt(f, r);
      if (!sq) break;
      if (own.has(sq)) break;
      out.push(sq);
      f += df;
      r += dr;
    }
  }
  return out;
}

function jumps(from: Square, offsets: readonly Dir[], own: Set<Square>): Square[] {
  const out: Square[] = [];
  const f0 = fileIndex(from);
  const r0 = rankIndex(from);
  for (const [df, dr] of offsets) {
    const sq = squareAt(f0 + df, r0 + dr);
    if (sq && !own.has(sq)) out.push(sq);
  }
  return out;
}

function pawnGeometry(from: Square, color: 'w' | 'b', own: Set<Square>): Square[] {
  const out: Square[] = [];
  const f0 = fileIndex(from);
  const r0 = rankIndex(from);
  const dir = color === 'w' ? 1 : -1;
  const startRank = color === 'w' ? 1 : 6;

  const f1 = squareAt(f0, r0 + dir);
  if (f1 && !own.has(f1)) out.push(f1);

  if (r0 === startRank) {
    const f2 = squareAt(f0, r0 + 2 * dir);
    if (f1 && f2 && !own.has(f1) && !own.has(f2)) out.push(f2);
  }

  for (const df of [-1, 1]) {
    const cap = squareAt(f0 + df, r0 + dir);
    if (cap && !own.has(cap)) out.push(cap);
  }

  return out;
}

/**
 * Geometric (pseudo-legal-ish) moves for a piece.
 *
 * Reads ONLY: piece type/color, from-square, own-piece set.
 * Reads NOT: opponent piece positions, board history, anything else.
 *
 * The signature is the proof of zero opponent info leak. Castling is
 * intentionally excluded — castling legality depends on opponent state
 * (path through check, opponent pieces between king and rook).
 */
export function geometricMoves(
  piece: Piece,
  from: Square,
  ownSquares: Set<Square>,
): Square[] {
  switch (piece.type) {
    case 'n': return jumps(from, KNIGHT_OFFSETS, ownSquares);
    case 'k': return jumps(from, KING_OFFSETS, ownSquares);
    case 'b': return rays(from, BISHOP_DIRS, ownSquares);
    case 'r': return rays(from, ROOK_DIRS, ownSquares);
    case 'q': return rays(from, QUEEN_DIRS, ownSquares);
    case 'p': return pawnGeometry(from, piece.color, ownSquares);
  }
}