/**
 * The timers of `backgroundTimers.ts`: a worker's keep their pace in a hidden tab, where the page's
 * own are held to once a second. `{ id, ms, repeat }` starts one, `{ id, ms: 0 }` ends it; each time
 * it fires, its id comes back.
 */
const timers = new Map<number, ReturnType<typeof setTimeout>>();

self.onmessage = (event: MessageEvent<{ id: number; ms: number; repeat: boolean }>) => {
  const { id, ms, repeat } = event.data;
  const running = timers.get(id);
  if (running !== undefined) {
    clearTimeout(running);
    clearInterval(running);
    timers.delete(id);
  }
  if (ms <= 0) return;
  if (repeat)
    timers.set(
      id,
      setInterval(() => self.postMessage(id), ms),
    );
  else
    timers.set(
      id,
      setTimeout(() => {
        timers.delete(id);
        self.postMessage(id);
      }, ms),
    );
};