/**
 * @file throttle.ts
 * @description Leading + trailing throttle for live-refresh callbacks. Unlike a
 * trailing debounce, a steady stream of WebSocket events can never postpone the
 * refresh forever: it fires at once, then at most once per `ms`, and once more
 * after the burst so the last change is never lost.
 * @author Nguyễn Ngọc Trí Vĩ <vinnt@smartgift.vn>
 */

export interface Throttled {
  (): void;
  cancel(): void;
}

export function throttle(fn: () => void, ms: number): Throttled {
  let last = 0;
  let timer: ReturnType<typeof setTimeout> | undefined;
  const run = () => {
    timer = undefined;
    last = Date.now();
    fn();
  };
  const t = (() => {
    if (timer) return;
    const wait = last + ms - Date.now();
    if (wait <= 0) run();
    else timer = setTimeout(run, wait);
  }) as Throttled;
  t.cancel = () => {
    clearTimeout(timer);
    timer = undefined;
  };
  return t;
}