export class Timer {
  private intervalHdl?: NodeJS.Timer;
  private subscribers: {
    [id: number]: {
      lastTick: number;
      seconds: number;
      tick: () => void;
    };
  } = {};

  private static INSTANCE?: Timer;
  public static get instance(): Timer {
    if (!Timer.INSTANCE) {
      Timer.INSTANCE = new Timer();
    }
    return Timer.INSTANCE;
  }

  private constructor() {}

  public start() {
    if (this.intervalHdl) {
      return;
    }
    this.intervalHdl = setInterval(this.loop.bind(this), 1000);
  }

  private loop() {
    const now = new Date();
    Object.values(this.subscribers).forEach(sub => {
      if (now.getTime() >= sub.lastTick + sub.seconds * 1000) {
        sub.lastTick = now.getTime();
        sub.tick();
      }
    });
  }

  public subscribe(seconds: number, tick: () => void) {
    const lastTick = new Date().getTime();
    const id =
      Object.keys(this.subscribers)
        .map(k => Number(k))
        .reduce((r, id) => Math.max(r, id), 0) + 1;
    this.subscribers[id] = { lastTick, seconds, tick };
    return id;
  }

  public unsubscribe(id: number) {
    if (typeof this.subscribers[id] === 'function') {
      delete this.subscribers[id];
    }
  }

  public stop() {
    if (!this.intervalHdl) return;
    clearInterval(this.intervalHdl);
    this.intervalHdl = undefined;
  }
}