joke-app/packages/games/src/mahjong/bot.ts

/**
 * 마작 computer player (cautious beginner). Wins whenever it may (tsumo / ron), declares riichi as soon as a closed
 * hand is tenpai, and otherwise discards to lower its shanten, keeping the discard with the most useful draws
 * (ukeire); among equals honors / terminals go first and dora stay. Against a riichi with a far-off hand it throws
 * tiles the riichi player already discarded. Calls only pon of 역패, or pon / chi that speeds up a hand that already
 * has a yaku (역패 meld or 쿠이탕). Between hands it presses ready. Everything comes from its own view.
 */
import type { BotStrategy } from '../bots/types';
import { answerRequests, best, pick, playable } from '../bots/policy';
import { doraFromIndicator, isDragon, isYaochu, kindFromCode } from './tiles';
import type { MahjongAction, MahjongView } from './types';

const kindOfCode = (code: string): number => kindFromCode(code.slice(0, 2))!;
const counts = (codes: readonly string[]): number[] => {
  const c = new Array<number>(34).fill(0);
  for (const x of codes) c[kindOfCode(x)]!++;
  return c;
};

/**
 * Per suit (or the honors), the best number of partial shapes for each (sets, pair) split: `t[p * 5 + m]` = most
 * taatsu with `m` sets and `p` pairs-as-head (−1 = impossible). Cached by the suit's counts, so shanten is cheap.
 */
const SUIT_CACHE = new Map<string, Int8Array>();
function suitTable(cs: number[], seq: boolean): Int8Array {
  const key = (seq ? 's' : 'z') + cs.join('');
  let out = SUIT_CACHE.get(key);
  if (out) return out;
  const t = new Int8Array(10).fill(-1);
  const c = [...cs];
  const n = c.length;
  const go = (i: number, m: number, tt: number, p: number): void => {
    while (i < n && c[i] === 0) i++;
    if (i === n) {
      const j = p * 5 + Math.min(m, 4);
      if (tt > t[j]!) t[j] = Math.min(tt, 8);
      return;
    }
    if (c[i]! >= 3) {
      c[i]! -= 3;
      go(i, m + 1, tt, p);
      c[i]! += 3;
    }
    if (seq && i <= n - 3 && c[i + 1]! > 0 && c[i + 2]! > 0) {
      c[i]!--, c[i + 1]!--, c[i + 2]!--;
      go(i, m + 1, tt, p);
      c[i]!++, c[i + 1]!++, c[i + 2]!++;
    }
    if (c[i]! >= 2) {
      c[i]! -= 2;
      if (!p) go(i, m, tt, 1);
      go(i, m, tt + 1, p);
      c[i]! += 2;
    }
    if (seq && i <= n - 2 && c[i + 1]! > 0) {
      c[i]!--, c[i + 1]!--;
      go(i, m, tt + 1, p);
      c[i]!++, c[i + 1]!++;
    }
    if (seq && i <= n - 3 && c[i + 2]! > 0) {
      c[i]!--, c[i + 2]!--;
      go(i, m, tt + 1, p);
      c[i]!++, c[i + 2]!++;
    }
    c[i]!--;
    go(i + 1, m, tt, p);
    c[i]!++;
  };
  go(0, 0, 0, 0);
  out = t;
  SUIT_CACHE.set(key, out);
  return out;
}

/** Standard-form shanten (4 sets + pair, `melds` already called); -1 = complete. */
function shantenStd(c: number[], melds: number): number {
  // Combine the four tables: best taatsu for every (pair, sets) total.
  let acc = new Int8Array(10).fill(-1);
  acc[0] = 0;
  for (const tab of [suitTable(c.slice(0, 9), true), suitTable(c.slice(9, 18), true), suitTable(c.slice(18, 27), true), suitTable(c.slice(27), false)]) {
    const next = new Int8Array(10).fill(-1);
    for (let a = 0; a < 10; a++) {
      if (acc[a]! < 0) continue;
      for (let b = 0; b < 10; b++) {
        if (tab[b]! < 0) continue;
        const p = Math.floor(a / 5) + Math.floor(b / 5);
        if (p > 1) continue;
        const j = p * 5 + Math.min((a % 5) + (b % 5), 4);
        next[j] = Math.max(next[j]!, acc[a]! + tab[b]!);
      }
    }
    acc = next;
  }
  let best = 8;
  for (let j = 0; j < 10; j++) {
    if (acc[j]! < 0) continue;
    const sets = Math.min((j % 5) + melds, 4);
    best = Math.min(best, 8 - 2 * sets - Math.min(acc[j]!, 4 - sets) - Math.floor(j / 5));
  }
  return best;
}

/** Shanten over standard form and 칠대자 (closed only). */
function shanten(c: number[], melds: number): number {
  let s = shantenStd(c, melds);
  if (melds === 0) {
    const pairs = c.filter((x) => x >= 2).length;
    const kinds = c.filter((x) => x >= 1).length;
    s = Math.min(s, 6 - pairs + Math.max(0, 7 - kinds));
  }
  return s;
}

/** Kinds worth drawing next to the hand (same honor, or a suit tile within 2 of one I hold). */
function nearKinds(c: number[]): number[] {
  const out: number[] = [];
  for (let k = 0; k < 34; k++) {
    if (k >= 27) {
      if (c[k]! > 0) out.push(k);
    } else {
      const lo = k - (k % 9);
      const from = Math.max(lo, k - 2);
      const to = Math.min(lo + 8, k + 2);
      if (c.slice(from, to + 1).some((x) => x > 0)) out.push(k);
    }
  }
  return out;
}

/** Shanten and number of unseen tiles that would lower it, for a 13-tile (or 13 − 3·melds) hand. */
function evaluate(c: number[], melds: number, seen: number[]): { sh: number; uke: number } {
  const sh = shanten(c, melds);
  let uke = 0;
  for (const k of nearKinds(c)) {
    const left = 4 - seen[k]!;
    if (left <= 0) continue;
    c[k]!++;
    if (shanten(c, melds) < sh) uke += left;
    c[k]!--;
  }
  return { sh, uke };
}

export const mahjongBot: BotStrategy<MahjongView, MahjongAction> = ({ view, legal, random }) => {
  const req = answerRequests(legal);
  if (req) return req;
  const has = (t: string) => legal.find((a) => a.type === t);
  const ready = has('ready');
  if (ready) return ready;
  const my = view.my;
  if (!my) return pick(playable(legal), random);
  const ron = legal.find((a) => a.type === 'claim' && a.call === 'ron');
  if (ron) return ron;
  const tsumo = has('tsumo');
  if (tsumo) return tsumo;
  const kita = has('kita');
  if (kita) return kita;
  const kyuushu = has('kyuushu');
  if (kyuushu) return kyuushu;

  const me = view.seats[my.seat]!;
  const melds = me.melds.length;
  const sanma = view.mode === 'sanma';
  const handCodes = [...my.hand, ...(my.drawn ? [my.drawn] : [])];
  const c = counts(handCodes);
  // Tiles I can see: my hand, every discard still on the river, every meld, kita and dora indicators.
  const seen = [...c];
  for (const s of view.seats) {
    for (const d of s.discards) if (!d.called) seen[kindOfCode(d.tile)]!++;
    if (s.seat !== my.seat) for (const m of s.melds) for (const t of m.tiles) seen[kindOfCode(t)]!++;
    for (const t of s.kita) seen[kindOfCode(t)]!++;
  }
  for (const m of me.melds) for (const t of m.tiles) seen[kindOfCode(t)]!++;
  for (const t of view.doraIndicators) seen[kindOfCode(t)]!++;
  const dora = new Set(view.doraIndicators.map((t) => doraFromIndicator(kindOfCode(t), sanma)));
  const yakuhai = (k: number) => isDragon(k) || k === 27 + me.wind || k === 27 + view.roundWind;

  // ---- claim window (ron handled above): pon of 역패, or a call that speeds up a hand that already has a yaku.
  if (my.claim) {
    const pass = has('pass') ?? { type: 'pass' as const };
    if (my.riichi) return pass;
    const k = kindOfCode(my.claim.tile);
    const now = shanten(c, melds);
    const openYakuhai = me.melds.some((m) => m.type !== 'chi' && yakuhai(kindOfCode(m.tiles[0]!)));
    const simplesOnly = (ks: number[]) => ks.every((x) => !isYaochu(x));
    let bestCall: MahjongAction | null = null;
    let bestSh = now;
    for (const a of legal) {
      if (a.type !== 'claim' || (a.call !== 'pon' && a.call !== 'chi')) continue;
      const used = (a.tiles ?? []).map(kindOfCode);
      const rest = [...c];
      for (const u of used) rest[u]!--;
      // After the call one tile must go: the best shanten among those discards.
      let after = 9;
      for (let d = 0; d < 34; d++) {
        if (!rest[d]) continue;
        rest[d]!--;
        after = Math.min(after, shanten(rest, melds + 1));
        rest[d]!++;
      }
      const meldKinds = me.melds.flatMap((m) => m.tiles.map(kindOfCode));
      const handYaochu = rest.reduce((n, x, i) => n + (isYaochu(i) ? x : 0), 0);
      const tanyaoOk = view.options.kuitan && simplesOnly([k, ...used, ...meldKinds]) && handYaochu <= 1;
      const isYakuhaiPon = a.call === 'pon' && yakuhai(k);
      const ok = isYakuhaiPon ? after <= now : (openYakuhai || tanyaoOk) && after < now;
      if (ok && (after < bestSh || (after === bestSh && !bestCall))) {
        bestCall = a;
        bestSh = after;
      }
    }
    return bestCall ?? pass;
  }

  // ---- my turn: closed kan when it doesn't hurt (not in riichi: keep it simple), then the discard.
  const discards = legal.filter((a): a is Extract<MahjongAction, { type: 'discard' }> => a.type === 'discard');
  if (!discards.length) return pick(playable(legal), random);
  const riichiFoes = view.seats.filter((s) => s.seat !== my.seat && s.riichi);
  const genbutsu = riichiFoes.map((s) => new Set(s.discards.map((d) => kindOfCode(d.tile))));
  const scored = new Map<string, { sh: number; uke: number }>();
  const evalDiscard = (code: string) => {
    let e = scored.get(code.slice(0, 2));
    if (!e) {
      const k = kindOfCode(code);
      c[k]!--;
      e = evaluate(c, melds, seen);
      c[k]!++;
      scored.set(code.slice(0, 2), e);
    }
    return e;
  };
  if (!my.riichi) {
    const minSh = Math.min(...discards.map((a) => evalDiscard(a.tile).sh));
    for (const a of legal) {
      if (a.type !== 'ankan' && a.type !== 'kakan') continue;
      const k = kindOfCode(a.tile);
      const rest = [...c];
      rest[k] = a.type === 'ankan' ? 0 : rest[k]! - 1;
      if (shanten(rest, melds + (a.type === 'ankan' ? 1 : 0)) <= minSh) return a;
    }
  }
  const score = (a: Extract<MahjongAction, { type: 'discard' }>): number => {
    const k = kindOfCode(a.tile);
    const { sh, uke } = evalDiscard(a.tile);
    let v = -sh * 1000 + uke * 10;
    if (a.riichi) v += uke > 0 ? 500 : -5000; // riichi whenever tenpai on a live wait
    if (k >= 27) v += yakuhai(k) && c[k]! >= 2 ? -3 : yakuhai(k) ? 2 : 4;
    else if (isYaochu(k)) v += 1;
    if (dora.has(k)) v -= 6;
    if (a.tile.endsWith('r')) v -= 6;
    // Defense: far from tenpai against a riichi → genbutsu first.
    const safe = genbutsu.filter((g) => g.has(k)).length;
    if (riichiFoes.length) v += sh >= 2 ? safe * 3000 : safe * 5;
    return v;
  };
  return best(discards, score, random);
};