fertig-classic-games/public/src/games/mahjong/MahjongAI.js

// Mahjong — heuristic opponent. No Phaser, no timers, no state mutation.
// Discards minimise shanten with an ukeire (useful-draw count) tiebreak at
// skill 3+; skills 1-2 use a cheap tile-isolation heuristic instead and claim
// every pung/chow they are offered. Higher skills subtract tiles they can see
// from ukeire counts and lean toward safe discards once an opponent shows a
// big exposed hand. Wins are always taken.

import { isSuited, isHonor, rankOf } from './MahjongData.js';
import { counts34, shanten, claimOptionsFor } from './MahjongLogic.js';

const SKILL_PROFILES = {
  1: { noise: 3.0, claimAlways: true,  seenAware: false, defense: 0.0, delay: [750, 1300] },
  2: { noise: 2.0, claimAlways: true,  seenAware: false, defense: 0.0, delay: [700, 1200] },
  3: { noise: 1.0, claimAlways: false, seenAware: false, defense: 0.3, delay: [620, 1050] },
  4: { noise: 0.4, claimAlways: false, seenAware: true,  defense: 0.7, delay: [540, 950] },
  5: { noise: 0.0, claimAlways: false, seenAware: true,  defense: 1.0, delay: [460, 850] },
};
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);
}

// Tiles visible to everyone: all rivers, exposed melds and bonus rows. (We
// also count concealed kongs — a minor peek that keeps the bookkeeping flat.)
function seenCounts(state) {
  const seen = new Array(34).fill(0);
  for (const river of state.discards) for (const k of river) seen[k]++;
  for (const p of state.players) {
    for (const m of p.melds) for (const k of m.kinds) seen[k]++;
  }
  return seen;
}

// Cheap usefulness of holding `kind` — for the skill 1-2 discard heuristic.
function usefulness(counts, kind) {
  let u = (counts[kind] - 1) * 3;
  if (isHonor(kind)) return u + (counts[kind] >= 2 ? 3 : 0);
  const r = rankOf(kind);
  if (r >= 2 && counts[kind - 1] > 0) u += 2;
  if (r <= 8 && counts[kind + 1] > 0) u += 2;
  if (r >= 3 && counts[kind - 2] > 0) u += 1;
  if (r <= 7 && counts[kind + 2] > 0) u += 1;
  if (r === 1 || r === 9) u -= 1;
  return u;
}

// Draws that could improve a hand: pairing an existing tile or extending a
// suited shape. A brand-new isolated kind never lowers standard shanten.
function improverKinds(counts) {
  const out = new Set();
  for (let k = 0; k < 34; k++) {
    if (counts[k] === 0) continue;
    out.add(k);
    if (!isSuited(k)) continue;
    const r = rankOf(k);
    for (const d of [-2, -1, 1, 2]) {
      const t = k + d;
      if (r + d >= 1 && r + d <= 9) out.add(t);
    }
  }
  return [...out];
}

// Pick a discard for the player on turn (hand holds 14 − 3·melds tiles).
export function chooseDiscard(state, seat, skill = 3) {
  const prof = profileFor(skill);
  const p = state.players[seat];
  const counts = counts34(p.hand);
  const kinds = [];
  for (let k = 0; k < 34; k++) if (counts[k] > 0) kinds.push(k);

  if (prof.claimAlways) { // skills 1-2: discard the least useful tile
    let best = kinds[0], bestScore = Infinity;
    for (const k of kinds) {
      const s = usefulness(counts, k) + Math.random() * prof.noise * 2;
      if (s < bestScore) { bestScore = s; best = k; }
    }
    return best;
  }

  const m = p.melds.length;
  let minSh = Infinity;
  const perKind = new Map();
  for (const k of kinds) {
    counts[k]--;
    const s = shanten(counts, m);
    counts[k]++;
    perKind.set(k, s);
    if (s < minSh) minSh = s;
  }
  const candidates = kinds.filter((k) => perKind.get(k) === minSh);

  const seen = prof.seenAware ? seenCounts(state) : null;
  const threat = prof.defense > 0 && minSh >= 2
    && state.players.some((q) => q.seat !== seat && q.melds.length >= 3);

  let best = candidates[0], bestScore = -Infinity;
  for (const k of candidates) {
    counts[k]--;
    let ukeire = 0;
    for (const t of improverKinds(counts)) {
      counts[t]++;
      if (shanten(counts, m) < minSh) {
        let left = 4 - counts[t] + 1;
        if (seen) left -= seen[t];
        if (left > 0) ukeire += left;
      }
      counts[t]--;
    }
    counts[k]++;
    let score = ukeire + Math.random() * prof.noise * 2;
    // someone looks close to winning — favour tiles already seen on the table
    if (threat && seen && seen[k] >= 1) score += 4 * prof.defense;
    if (score > bestScore) { bestScore = score; best = k; }
  }
  return best;
}

// React to another player's discard. `options` comes from claimOptionsFor.
// Returns a claim ({ type, tiles? }) or null to pass.
export function chooseClaim(state, seat, options, skill = 3) {
  if (!options) return null;
  if (options.win) return { type: 'win' };
  const prof = profileFor(skill);
  const p = state.players[seat];
  const kind = state.lastDiscard.kind;
  const m = p.melds.length;
  const counts = counts34(p.hand);

  if (prof.claimAlways) {
    if (options.kong) return { type: 'kong' };
    if (options.pung) return { type: 'pung' };
    if (options.chows.length) return { type: 'chow', tiles: options.chows[0] };
    return null;
  }

  const s0 = shanten(counts, m);
  if (options.kong) { // free replacement draw — accept when not a step back
    counts[kind] -= 3;
    const s = shanten(counts, m + 1);
    counts[kind] += 3;
    if (s <= s0) return { type: 'kong' };
  }
  if (options.pung) {
    counts[kind] -= 2;
    const s = shanten(counts, m + 1);
    counts[kind] += 2;
    if (s < s0) return { type: 'pung' };
  }
  for (const tiles of options.chows) {
    counts[tiles[0]]--; counts[tiles[1]]--;
    const s = shanten(counts, m + 1);
    counts[tiles[0]]++; counts[tiles[1]]++;
    if (s < s0) return { type: 'chow', tiles };
  }
  return null;
}

// Decide on a self-drawn win or a kong declaration while awaiting discard.
// `actions` comes from selfActions. Returns { type: 'win' },
// { type: 'kong', spec } or null to just discard.
export function chooseSelfAction(state, seat, actions, skill = 3) {
  if (actions.canWin) return { type: 'win' };
  const prof = profileFor(skill);
  if (!actions.concealedKongs.length && !actions.addedKongs.length) return null;
  if (prof.claimAlways) {
    if (actions.concealedKongs.length) return { type: 'kong', spec: { type: 'concealed', kind: actions.concealedKongs[0] } };
    return { type: 'kong', spec: { type: 'added', kind: actions.addedKongs[0] } };
  }

  const p = state.players[seat];
  const m = p.melds.length;
  const counts = counts34(p.hand);
  let bestDiscardSh = Infinity;
  for (let k = 0; k < 34; k++) {
    if (counts[k] === 0) continue;
    counts[k]--;
    const s = shanten(counts, m);
    counts[k]++;
    if (s < bestDiscardSh) bestDiscardSh = s;
  }
  for (const k of actions.concealedKongs) {
    counts[k] -= 4;
    const s = shanten(counts, m + 1);
    counts[k] += 4;
    if (s <= bestDiscardSh) return { type: 'kong', spec: { type: 'concealed', kind: k } };
  }
  for (const k of actions.addedKongs) {
    counts[k]--;
    const s = shanten(counts, m); // pung→kong: still one meld, one tile fewer
    counts[k]++;
    if (s <= bestDiscardSh) return { type: 'kong', spec: { type: 'added', kind: k } };
  }
  return null;
}

// Collect claim intents from every AI seat for the current discard. The
// scene merges these with the human's choice before calling resolveClaims.
export function collectAIClaims(state, humanSeat = 0) {
  const intents = [];
  for (let seat = 0; seat < 4; seat++) {
    if (seat === humanSeat) continue;
    const p = state.players[seat];
    if (!p.isAI) continue;
    const options = claimOptionsFor(state, seat);
    if (!options) continue;
    const claim = chooseClaim(state, seat, options, p.skill);
    if (claim) intents.push({ seat, claim });
  }
  return intents;
}