emchess

import type { Personality } from "./personalities";

const ENGINE_URL = new URL(`${import.meta.env.BASE_URL}stockfish/stockfish-18-lite-single.js`, location.href).href;
const WASM_URL = new URL(`${import.meta.env.BASE_URL}stockfish/stockfish-18-lite-single.wasm`, location.href).href;

// A tiny worker that fetches the wasm as a plain binary and hands it to Stockfish
// through Module.wasmBinary. That path ignores the response's MIME type, so a server
// that serves .wasm as text/plain (a very common misconfig) can't break the engine.
const BOOTSTRAP = `self.onmessage=function(e){var d=e.data||{};if(!d.__boot)return;` +
  `fetch(d.wasmUrl).then(function(r){return r.arrayBuffer();}).then(function(buf){` +
  `self.Module={wasmBinary:buf,locateFile:function(p){return p.slice(-5)==='.wasm'?d.wasmUrl:p;}};` +
  `try{importScripts(d.jsUrl);postMessage('__ready');}catch(err){postMessage('__err '+err);}` +
  `}).catch(function(err){postMessage('__err '+err);});};`;

type Line = string;

export class Engine {
  private worker: Worker;
  private ready: Promise<void>;
  private listeners: ((line: Line) => void)[] = [];

  constructor() {
    const bootUrl = URL.createObjectURL(new Blob([BOOTSTRAP], { type: "text/javascript" }));
    this.worker = new Worker(bootUrl);
    this.worker.onmessage = (e) => {
      const line = typeof e.data === "string" ? e.data : String(e.data?.data ?? "");
      for (const fn of this.listeners) fn(line);
    };
    this.ready = this.boot();
  }

  private send(cmd: string) {
    this.worker.postMessage(cmd);
  }

  private waitFor(token: string, timeout = 0): Promise<void> {
    return new Promise((resolve, reject) => {
      let timer: ReturnType<typeof setTimeout> | undefined;
      const fn = (line: Line) => {
        if (line.includes(token)) {
          this.listeners = this.listeners.filter((l) => l !== fn);
          if (timer) clearTimeout(timer);
          resolve();
        }
      };
      this.listeners.push(fn);
      if (timeout > 0) {
        timer = setTimeout(() => {
          this.listeners = this.listeners.filter((l) => l !== fn);
          reject(new Error("engine timeout waiting for " + token));
        }, timeout);
      }
    });
  }

  private async boot() {
    this.worker.postMessage({ __boot: true, jsUrl: ENGINE_URL, wasmUrl: WASM_URL });
    await this.waitFor("__ready", 20000);
    this.send("uci");
    await this.waitFor("uciok", 20000);
    this.send("isready");
    await this.waitFor("readyok", 20000);
  }

  async configure(p: Personality) {
    try {
      await this.ready;
    } catch {
      return;
    }
    this.send("setoption name Skill Level value " + p.skill);
    this.send("setoption name MultiPV value " + Math.max(1, p.multipv));
    this.send("isready");
    await this.waitFor("readyok", 8000).catch(() => {});
  }

  // Returns a move in UCI form, never before the personality's think floor. Falls back
  // to a legal move if the engine is unavailable, so the game can never hang.
  async chooseMove(fen: string, p: Personality, legal: string[]): Promise<string | null> {
    const started = performance.now();
    const floor = p.think[0] + Math.random() * (p.think[1] - p.think[0]);

    let move: string | null;
    let alive = true;
    try {
      await this.ready;
    } catch {
      alive = false;
    }

    if (!alive) {
      move = randomOf(legal);
    } else if (legal.length && Math.random() < p.blunderChance) {
      move = randomOf(legal);
    } else {
      move = (await this.search(fen, p)) ?? randomOf(legal);
    }

    const elapsed = performance.now() - started;
    if (elapsed < floor) await sleep(floor - elapsed);
    return move;
  }

  private search(fen: string, p: Personality): Promise<string | null> {
    return new Promise((resolve) => {
      const candidates = new Map<number, string>();
      let done = false;
      const finish = (m: string | null) => {
        if (done) return;
        done = true;
        clearTimeout(timer);
        this.listeners = this.listeners.filter((l) => l !== fn);
        resolve(m);
      };
      const fn = (line: Line) => {
        if (line.startsWith("info")) {
          const pv = line.match(/multipv (\d+).*? pv (\w+)/);
          if (pv) candidates.set(Number(pv[1]), pv[2]);
        } else if (line.startsWith("bestmove")) {
          const best = line.split(" ")[1];
          finish(best === "(none)" ? null : pick(candidates, best, p.multipv));
        }
      };
      const timer = setTimeout(() => finish(null), p.movetime + 4000);
      this.listeners.push(fn);

      this.send("setoption name Skill Level value " + p.skill);
      this.send("setoption name MultiPV value " + Math.max(1, p.multipv));
      this.send("position fen " + fen);
      this.send("go movetime " + p.movetime);
    });
  }

  // Full-strength evaluation for the coach, from the side-to-move's point of view.
  async evaluate(fen: string, movetime = 500): Promise<{ cp: number | null; mate: number | null }> {
    try {
      await this.ready;
    } catch {
      return { cp: null, mate: null };
    }
    return new Promise((resolve) => {
      let cp: number | null = null;
      let mate: number | null = null;
      let done = false;
      const finish = () => {
        if (done) return;
        done = true;
        clearTimeout(timer);
        this.listeners = this.listeners.filter((l) => l !== fn);
        resolve({ cp, mate });
      };
      const fn = (line: Line) => {
        if (line.startsWith("info")) {
          const s = line.match(/score (cp|mate) (-?\d+)/);
          if (s) {
            if (s[1] === "cp") { cp = Number(s[2]); mate = null; }
            else { mate = Number(s[2]); cp = null; }
          }
        } else if (line.startsWith("bestmove")) {
          finish();
        }
      };
      const timer = setTimeout(finish, movetime + 4000);
      this.listeners.push(fn);
      this.send("setoption name Skill Level value 20");
      this.send("setoption name MultiPV value 1");
      this.send("position fen " + fen);
      this.send("go movetime " + movetime);
    });
  }

  destroy() {
    this.send("quit");
    this.worker.terminate();
  }
}

function pick(candidates: Map<number, string>, best: string, multipv: number): string {
  if (multipv <= 1 || candidates.size <= 1) return best;
  const weights = [0.62, 0.26, 0.09, 0.03];
  const roll = Math.random();
  let acc = 0;
  for (let i = 1; i <= Math.min(multipv, candidates.size); i++) {
    acc += weights[i - 1] ?? 0.02;
    if (roll < acc && candidates.has(i)) return candidates.get(i)!;
  }
  return best;
}

function randomOf(moves: string[]): string | null {
  return moves.length ? moves[Math.floor(Math.random() * moves.length)] : null;
}

function sleep(ms: number): Promise<void> {
  return new Promise((r) => setTimeout(r, ms));
}