Semaphore.ts

export type SemaphoreFunction = (done: () => void) => void;

/**
 * Based on https://www.derpturkey.com/simple-semaphore-in-nodejs/
 */
export default class Semaphore {
  readonly max: number;
  private _fns: Array<SemaphoreFunction>;
  private _active: number;

  constructor(max: number = 1) {
    this.max = max;
    this._fns = [];
    this._active = 0;
  }

  get remaining() {
    return this._fns.length;
  }

  get active() {
    return this._active;
  }

  take(fn: SemaphoreFunction) {
    this._fns.push(fn);
    this._try();
  }

  private _done() {
    this._active -= 1;
    this._try();
  }

  private _try(): boolean {
    if (this._active === this.max || this._fns.length === 0) {
      return false;
    }
    const fn = this._fns.shift();
    this._active += 1;
    fn?.(this._done.bind(this));
    return true;
  }
}