function noop() {}

export interface Deferred<T = void> extends Promise<T> {
  resolve(value: T | PromiseLike<T>): void;
  reject(reason?: unknown): void;
}

export function deferred<T = void>() {
  const { promise, resolve, reject } = Promise.withResolvers<T>();
  const p = promise as Deferred<T>;
  return (p.resolve = resolve), (p.reject = reject), p;
}

export function notifier<T = void>() {
  let p: Deferred<T> | null = null;

  return {
    listen(): Promise<T> {
      return (p ??= deferred());
    },

    notify(value: T) {
      p &&= (p.resolve(value), null);
    },
  };
}

export interface Channel<T> {
  send: Sender<T>;
  recv: Receiver<T>;
}

export interface Sender<T> {
  (value: T): void;
  close(reason?: unknown): void;
}

export interface Receiver<T> {
  (): Promise<T | null> | T | null;
  try(): T | null;
  close(reason?: unknown): void;
}

export function channel<T = void>(): Channel<T> {
  const q = new Map<number, Entry>();
  let head = 0;
  let tail = 0;
  let open = true;
  let err: unknown = null;

  type Entry =
    | { has: true; value: T }
    | { has: false; value: Deferred<T | null> };

  function send(value: T) {
    if (open) {
      const i = head++;
      let n = q.get(i);
      if (!n) q.set(i, (n = { has: true, value }));
      else if (!n.has) q.delete(i), n.value.resolve(value);
      else n.value = value;
    } else {
      if (err !== null) throw err;
      else return;
    }
  }

  function recv() {
    if (open) {
      const i = tail++;
      let n = q.get(i);
      if (!n) return q.set(i, (n = { has: false, value: deferred() })), n.value;
      else if (!n.has) return n.value;
      else return q.delete(i), n.value;
    } else {
      if (err !== null) throw err;
      else return null;
    }
  }

  function try_recv() {
    if (open) {
      const n = q.get(tail);
      if (n?.has) return q.delete(tail++), n.value;
      else return null;
    } else {
      return null;
    }
  }

  recv.try = try_recv;
  send.close = recv.close = function close(reason?: unknown) {
    if (!open) return;
    else (open = false), (err = reason ?? null);
    for (const p of q.values()) {
      if (p.has) continue;
      else if (err !== null) p.value.reject(err);
      else p.value.resolve(null);
    }
    q.clear();
  };

  return { send, recv };
}

channel.sender = function sender<T>(
  f: (recv: Receiver<T>) => void | PromiseLike<void>
): Sender<T> {
  const { send, recv } = channel<T>();
  Promise.resolve(f(recv)).then(noop).then(recv.close, recv.close);
  return send;
};

channel.receiver = function receiver<T>(
  f: (send: Sender<T>) => void | PromiseLike<void>
): Receiver<T> {
  const { send, recv } = channel<T>();
  Promise.resolve(f(send)).then(noop).then(send.close, send.close);
  return recv;
};

export function semaphore(count = 1) {
  const { send: signal, recv: wait } = channel();
  let n = count;

  async function acquire() {
    if (--n < 0) await wait();
    return release;
  }

  function release() {
    if (n++ < 0) signal();
  }

  function reset(new_count = count) {
    n = count = new_count;
  }

  acquire.release = release;
  acquire.reset = reset;
  release[Symbol.dispose] = release;
  release satisfies Disposable;

  return acquire;
}

export interface PoolToken<T> extends Iterable<T, void, void> {
  readonly value: T;
  release(): void;
}

export function pool<T>(size: number, f: () => T | PromiseLike<T>) {
  const lock = semaphore(size);
  const all = new Set<T>();
  const free: T[] = [];
  const Token = class implements PoolToken<T> {
    #value;
    #open = true;

    get value() {
      if (this.#open) return this.#value;
      else throw new TypeError(`borrowed value is already released`);
    }

    constructor(value: T) {
      this.#value = value;
    }

    *[Symbol.iterator]() {
      yield this.value;
    }

    release() {
      this.#open &&=
        (lock.release(), all.has(this.#value) && free.push(this.#value), false);
    }

    [Symbol.dispose]() {
      this.release();
    }
  };

  async function acquire(): Promise<PoolToken<T>> {
    await lock();
    try {
      let value;
      if (free.length !== 0) value = free.pop() as T;
      else value = await f();
      return all.add(value), new Token(value);
    } catch (e) {
      throw (lock.release(), e);
    }
  }

  function forget(value: T) {
    if (all.delete(value)) {
      const idx = free.indexOf(value);
      if (idx !== -1) free.splice(idx, 1);
    }
  }

  acquire.forget = forget;
  return acquire;
}