"use client";

import { useEffect } from "react";

import { localeHeader } from "@/lib/i18n";
import { buildReminderMessage } from "@/lib/reminder";
import { authHeaders } from "@/lib/settings";

/**
 * Keeps the reminder text the server holds in step with the notes.
 *
 * The reminder is written in the browser (lib/reminder.ts) because the notes
 * never leave it, and the due cards move all day: a nudge that still says
 * "3 cards are due" on a morning with nothing due is worse than no nudge. This
 * refreshes it when the app opens and whenever the tab is visible again,
 * throttled to twice a day. It does nothing at all when this browser is not
 * subscribed, has no push support, or the server has reminders switched off.
 */

const REFRESH_KEY = "freezmo.push.refresh.v1";
const REFRESH_MS = 6 * 60 * 60 * 1000;

function fresh(): boolean {
  try {
    const at = Number(window.localStorage.getItem(REFRESH_KEY) ?? 0);
    return Number.isFinite(at) && Date.now() - at < REFRESH_MS;
  } catch {
    return false;
  }
}

function remember(): void {
  try {
    window.localStorage.setItem(REFRESH_KEY, String(Date.now()));
  } catch {
    /* private mode */
  }
}

export function PushReminderSync() {
  useEffect(() => {
    if (typeof navigator === "undefined" || !("serviceWorker" in navigator) || !("PushManager" in window)) return;
    if (fresh()) return;
    let done = false;
    const sync = async () => {
      if (done) return;
      try {
        const registration = await navigator.serviceWorker.ready;
        const subscription = await registration.pushManager.getSubscription();
        if (!subscription) return; // reminders are off for this browser
        const message = await buildReminderMessage();
        const response = await fetch("/api/push", {
          method: "POST",
          headers: { "Content-Type": "application/json", ...authHeaders(), ...localeHeader() },
          body: JSON.stringify({ ...subscription.toJSON(), message }),
        });
        if (response.ok) {
          done = true;
          remember();
        }
      } catch {
        /* the stored reminder stays as it is */
      }
    };
    void sync();
    const onVisible = () => {
      if (document.visibilityState === "visible") void sync();
    };
    document.addEventListener("visibilitychange", onVisible);
    return () => document.removeEventListener("visibilitychange", onVisible);
  }, []);

  return null;
}