import { useSyncExternalStore } from "react";

const listeners = new Set<() => void>();
let lastTick = Date.now();
let intervalId: ReturnType<typeof setInterval> | null = null;

function ensureClock() {
  if (intervalId != null || typeof window === "undefined") return;
  intervalId = setInterval(() => {
    lastTick = Date.now();
    for (const fn of listeners) fn();
  }, 30_000);
}

function subscribe(fn: () => void) {
  ensureClock();
  listeners.add(fn);
  return () => {
    listeners.delete(fn);
    if (listeners.size === 0 && intervalId != null) {
      clearInterval(intervalId);
      intervalId = null;
    }
  };
}

function snapshot() {
  return lastTick;
}

function serverSnapshot() {
  return 0;
}

/** Returns a timestamp that ticks once every 30 seconds — safe for "is expired"
 *  badges and relative-time labels without making the render impure. */
export function useNow(): number {
  return useSyncExternalStore(subscribe, snapshot, serverSnapshot);
}