import { useSyncExternalStore } from "react";

const MINUTE = 60_000;
const stores = new Map();

function getStore(refreshInterval) {
  const existing = stores.get(refreshInterval);
  if (existing) return existing;

  const listeners = new Set();
  let now = null; // null until mounted, so SSR and hydration render the same thing
  let timer = null;

  const store = {
    getSnapshot: () => now,
    subscribe: (onStoreChange) => {
      listeners.add(onStoreChange);

      if (!timer) {
        now = Date.now();
        timer = setInterval(() => {
          now = Date.now();
          listeners.forEach((listener) => listener());
        }, refreshInterval);
      }

      return () => {
        listeners.delete(onStoreChange);
        if (!listeners.size) {
          clearInterval(timer);
          timer = null;
        }
      };
    },
  };

  stores.set(refreshInterval, store);
  return store;
}

// Returns null on the server and during hydration, then the current time.
export default function useCurrentTime(refreshInterval = MINUTE) {
  const { subscribe, getSnapshot } = getStore(refreshInterval);
  return useSyncExternalStore(subscribe, getSnapshot, () => null);
}