class ParallelRunner {
  private funQueue: (() => Promise<any>)[] = [];
  private funRunning: number = 0;

  constructor(private _parallelFunctions: number) {}

  public get parallelFunctions(): number {
    return this._parallelFunctions;
  }

  public set parallelFunctions(v: number) {
    this._parallelFunctions = v;
    this.runNext();
  }

  private addFunction(f: () => Promise<any>): void {
    if (this.funRunning >= this._parallelFunctions) {
      this.funQueue.push(f);
    } else {
      this.funRunning += 1;
      f();
    }
  }

  private runNext(): void {
    while (this.funRunning < this._parallelFunctions) {
      const f = this.funQueue.shift();
      if (f === undefined) {
        return;
      }
      this.funRunning += 1;
      f();
    }
  }

  run<T extends readonly unknown[], R>(
    f: (...args: [...T]) => Promise<R>,
    ...args: T
  ): Promise<R> {
    return new Promise((res, rej) => {
      let fun = async () => {
        try {
          res(await f(...args));
        } catch (e: any) {
          rej(e);
        } finally {
          this.runNext();
        }
      };
      this.addFunction(fun);
    });
  }
}