// simple counting semaphore, for blocking async ops
// cribbed mostly from https://github.com/ComFreek/async-playground

export class Semaphore {
  #counter = 0;
  #resolvers: ((taken: boolean) => void)[] = [];

  constructor(count = 0) {
    this.#counter = count;
  }

  take(signal?: AbortSignal) {
    return new Promise<boolean>((resolve) => {
      if (signal?.aborted) return resolve(false);

      // if there's resources available, use them

      this.#counter--;
      if (this.#counter >= 0) return resolve(true);

      // otherwise add to pending
      // and explicitly remove the resolver from the list on abort

      this.#resolvers.push(resolve);
      signal?.addEventListener("abort", () => {
        const index = this.#resolvers.indexOf(resolve);
        if (index >= 0) {
          this.#resolvers.splice(index, 1);
          this.#counter++;
        }

        resolve(false);
      });
    });
  }

  poll() {
    if (this.#counter <= 0) return false;

    this.#counter--;
    return true;
  }

  free() {
    this.#counter++;

    if (this.#resolvers.length > 0) {
      const resolver = this.#resolvers.shift();
      resolver && queueMicrotask(() => resolver(true));
    }
  }
}