// SPDX-License-Identifier: GPL-3.0-or-later
//
// Gamengager - Stockfish 연결 코드 (Stockfish UCI connector)
// Copyright (C) 2026 Gamengager
//
// This program is free software: you can redistribute it and/or modify it under the terms of the
// GNU General Public License as published by the Free Software Foundation, either version 3 of the
// License, or (at your option) any later version. This program is distributed in the hope that it
// will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
// FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for more details:
// https://www.gnu.org/licenses/gpl-3.0.html
//
// 원본 그대로 배포하는 Stockfish(별도 파일, Web Worker)와 표준 UCI 텍스트 명령만 주고받는다.

export const ENGINE_URL = "/engines/stockfish/stockfish-19-lite-single.js";

export interface EngineLevel {
  skill: number;
  movetimeMs: number;
}

const INIT_TIMEOUT_MS = 20_000;

export class StockfishEngine {
  private worker: Worker | null = null;
  private listeners = new Set<(line: string) => void>();
  private ready: Promise<void> | null = null;
  private currentSkill = -1;

  /** 엔진을 불러와 UCI 준비가 끝날 때까지 기다린다. 이미 불렀으면 그대로 돌려준다. */
  load(): Promise<void> {
    if (this.ready) return this.ready;
    this.ready = new Promise<void>((resolve, reject) => {
      let worker: Worker;
      try {
        worker = new Worker(ENGINE_URL);
      } catch (e) {
        reject(e);
        return;
      }
      this.worker = worker;
      const timer = setTimeout(() => fail(new Error("engine init timeout")), INIT_TIMEOUT_MS);
      const fail = (err: unknown) => {
        clearTimeout(timer);
        this.dispose();
        reject(err);
      };
      worker.onmessage = (e: MessageEvent<string>) => {
        const line = typeof e.data === "string" ? e.data : String(e.data);
        for (const fn of this.listeners) fn(line);
      };
      worker.onerror = (e) => fail(e);
      this.waitFor("uciok")
        .then(() => {
          this.send("setoption name UCI_LimitStrength value false");
          this.send("isready");
          return this.waitFor("readyok");
        })
        .then(() => {
          clearTimeout(timer);
          resolve();
        }, fail);
      this.send("uci");
    });
    // 실패하면 다음 load()에서 다시 시도할 수 있게 한다
    this.ready.catch(() => {
      this.ready = null;
    });
    return this.ready;
  }

  /** FEN 국면에서 최선 수를 UCI 표기(e2e4, e7e8q)로 돌려준다. 둘 수 없으면 null. */
  async bestMove(fen: string, level: EngineLevel): Promise<string | null> {
    await this.load();
    // 멈춘 이전 계산의 bestmove가 새 계산의 답으로 섞이지 않도록 먼저 끝나기를 기다린다
    if (this.searching) await this.searching;
    if (level.skill !== this.currentSkill) {
      this.send(`setoption name Skill Level value ${level.skill}`);
      this.currentSkill = level.skill;
    }
    this.send("ucinewgame");
    this.send(`position fen ${fen}`);
    const line = this.waitFor("bestmove");
    this.searching = line;
    this.send(`go movetime ${level.movetimeMs}`);
    const result = await line;
    if (this.searching === line) this.searching = null;
    const move = result.split(/\s+/)[1];
    return !move || move === "(none)" ? null : move;
  }

  private searching: Promise<string> | null = null;

  /** 계산 중인 수를 멈춘다 (결과는 버린다). */
  stop(): void {
    this.send("stop");
  }

  dispose(): void {
    this.worker?.terminate();
    this.worker = null;
    this.listeners.clear();
    this.ready = null;
    this.searching = null;
    this.currentSkill = -1;
  }

  private send(command: string): void {
    this.worker?.postMessage(command);
  }

  /** 주어진 단어로 시작하는 줄이 올 때까지 기다린다. */
  private waitFor(prefix: string): Promise<string> {
    return new Promise((resolve) => {
      const fn = (line: string) => {
        if (line.startsWith(prefix)) {
          this.listeners.delete(fn);
          resolve(line);
        }
      };
      this.listeners.add(fn);
    });
  }
}