fertig-classic-games/src/games/chess/ChessAI.js

// Chess AI — alpha-beta minimax with piece-square tables and a Nerts-style
// 1..5 skill model.
//   • depth   — search depth in plies (capped for browser responsiveness)
//   • blunder — chance to ignore the best move and play a random legal one
//   • noise   — random value added to root move scores (flattens decisions)
//   • delay   — "thinking" pause (ms range) before moving, for natural pacing
// Captures are searched first so alpha-beta prunes hard at the higher depths.

import {
  getLegalMoves, applyMoveRaw, isKingAttacked, SIZE,
} from './ChessLogic.js';

const SKILL_PROFILES = {
  1: { depth: 1, blunder: 0.45, noise: 90, delay: [900, 1500] },
  2: { depth: 2, blunder: 0.30, noise: 55, delay: [800, 1300] },
  3: { depth: 2, blunder: 0.15, noise: 30, delay: [700, 1100] },
  4: { depth: 3, blunder: 0.05, noise: 12, delay: [550, 950] },
  5: { depth: 4, blunder: 0.00, noise: 0,  delay: [450, 850] },
};

const VALUE = { p: 100, n: 320, b: 330, r: 500, q: 900, k: 0 };
const MATE = 1000000;

const PST = {
  p: [
    [0, 0, 0, 0, 0, 0, 0, 0],
    [50, 50, 50, 50, 50, 50, 50, 50],
    [10, 10, 20, 30, 30, 20, 10, 10],
    [5, 5, 10, 25, 25, 10, 5, 5],
    [0, 0, 0, 20, 20, 0, 0, 0],
    [5, -5, -10, 0, 0, -10, -5, 5],
    [5, 10, 10, -20, -20, 10, 10, 5],
    [0, 0, 0, 0, 0, 0, 0, 0],
  ],
  n: [
    [-50, -40, -30, -30, -30, -30, -40, -50],
    [-40, -20, 0, 0, 0, 0, -20, -40],
    [-30, 0, 10, 15, 15, 10, 0, -30],
    [-30, 5, 15, 20, 20, 15, 5, -30],
    [-30, 0, 15, 20, 20, 15, 0, -30],
    [-30, 5, 10, 15, 15, 10, 5, -30],
    [-40, -20, 0, 5, 5, 0, -20, -40],
    [-50, -40, -30, -30, -30, -30, -40, -50],
  ],
  b: [
    [-20, -10, -10, -10, -10, -10, -10, -20],
    [-10, 0, 0, 0, 0, 0, 0, -10],
    [-10, 0, 5, 10, 10, 5, 0, -10],
    [-10, 5, 5, 10, 10, 5, 5, -10],
    [-10, 0, 10, 10, 10, 10, 0, -10],
    [-10, 10, 10, 10, 10, 10, 10, -10],
    [-10, 5, 0, 0, 0, 0, 5, -10],
    [-20, -10, -10, -10, -10, -10, -10, -20],
  ],
  r: [
    [0, 0, 0, 0, 0, 0, 0, 0],
    [5, 10, 10, 10, 10, 10, 10, 5],
    [-5, 0, 0, 0, 0, 0, 0, -5],
    [-5, 0, 0, 0, 0, 0, 0, -5],
    [-5, 0, 0, 0, 0, 0, 0, -5],
    [-5, 0, 0, 0, 0, 0, 0, -5],
    [-5, 0, 0, 0, 0, 0, 0, -5],
    [0, 0, 0, 5, 5, 0, 0, 0],
  ],
  q: [
    [-20, -10, -10, -5, -5, -10, -10, -20],
    [-10, 0, 0, 0, 0, 0, 0, -10],
    [-10, 0, 5, 5, 5, 5, 0, -10],
    [-5, 0, 5, 5, 5, 5, 0, -5],
    [0, 0, 5, 5, 5, 5, 0, -5],
    [-10, 5, 5, 5, 5, 5, 0, -10],
    [-10, 0, 5, 0, 0, 0, 0, -10],
    [-20, -10, -10, -5, -5, -10, -10, -20],
  ],
  k: [
    [-30, -40, -40, -50, -50, -40, -40, -30],
    [-30, -40, -40, -50, -50, -40, -40, -30],
    [-30, -40, -40, -50, -50, -40, -40, -30],
    [-30, -40, -40, -50, -50, -40, -40, -30],
    [-20, -30, -30, -40, -40, -30, -30, -20],
    [-10, -20, -20, -20, -20, -20, -20, -10],
    [20, 20, 0, 0, 0, 0, 20, 20],
    [20, 30, 10, 0, 0, 10, 30, 20],
  ],
};

function profileFor(skill) {
  return SKILL_PROFILES[Math.max(1, Math.min(5, skill | 0))] ?? SKILL_PROFILES[3];
}

export function nextThinkDelay(skill) {
  const [lo, hi] = profileFor(skill).delay;
  return lo + Math.random() * (hi - lo);
}

// Static evaluation from `aiColor`'s perspective (positive = good for AI).
function evaluate(board, aiColor) {
  let score = 0; // white's perspective
  for (let r = 0; r < SIZE; r++) {
    for (let c = 0; c < SIZE; c++) {
      const p = board[r][c];
      if (!p) continue;
      const table = PST[p.type];
      const pst = p.color === 'white' ? table[r][c] : table[SIZE - 1 - r][c];
      const v = VALUE[p.type] + pst;
      score += p.color === 'white' ? v : -v;
    }
  }
  return aiColor === 'white' ? score : -score;
}

// Captures first (most-valuable-victim heuristic) for better pruning.
function orderMoves(board, moves) {
  return moves
    .map((m) => {
      let s = 0;
      if (m.capture) s += 10 * VALUE[board[m.capture[0]][m.capture[1]].type] - VALUE[m.piece];
      if (m.promotion) s += VALUE[m.promotion];
      return { m, s };
    })
    .sort((a, b) => b.s - a.s)
    .map((x) => x.m);
}

function search(state, depth, alpha, beta, aiColor) {
  const moves = getLegalMoves(state);
  if (moves.length === 0) {
    if (isKingAttacked(state.board, state.turn)) {
      // Side to move is checkmated. Prefer faster mates via the depth bonus.
      return state.turn === aiColor ? -(MATE + depth) : (MATE + depth);
    }
    return 0; // stalemate
  }
  if (depth <= 0) return evaluate(state.board, aiColor);

  const ordered = orderMoves(state.board, moves);
  if (state.turn === aiColor) {
    let value = -Infinity;
    for (const m of ordered) {
      value = Math.max(value, search(applyMoveRaw(state, m), depth - 1, alpha, beta, aiColor));
      alpha = Math.max(alpha, value);
      if (alpha >= beta) break;
    }
    return value;
  }
  let value = Infinity;
  for (const m of ordered) {
    value = Math.min(value, search(applyMoveRaw(state, m), depth - 1, alpha, beta, aiColor));
    beta = Math.min(beta, value);
    if (beta <= alpha) break;
  }
  return value;
}

// Return the chosen move object for `aiColor`, or null if none.
export function chooseMove(state, aiColor, skill) {
  const prof = profileFor(skill);
  const moves = getLegalMoves(state);
  if (moves.length === 0) return null;
  if (Math.random() < prof.blunder) {
    return moves[Math.floor(Math.random() * moves.length)];
  }

  const ordered = orderMoves(state.board, moves);
  let best = null;
  let bestScore = -Infinity;
  for (const m of ordered) {
    const val = search(applyMoveRaw(state, m), prof.depth - 1, -Infinity, Infinity, aiColor)
      + (Math.random() * 2 - 1) * prof.noise;
    if (val > bestScore) { bestScore = val; best = m; }
  }
  return best ?? moves[0];
}