"use client";

/**
 * Auto-push for people who already turned sync on: a deck or homework write
 * schedules one background `syncNow("auto")` a moment later, so a deck frozen
 * on the laptop shows up on the phone without opening Settings. Debounced (a
 * burst of writes = one push), one run at a time, silent on failure — the data
 * stays dirty and Settings still offers the manual sync.
 *
 * Deliberately scoped to decks and homework: answers and chat messages change
 * constantly while studying, and pushing after every card would be noise.
 * A push that fails offline is retried when the browser comes back online.
 *
 * Imported dynamically from the stores (lib/storage, lib/homework) so the
 * sync module itself stays out of the first load and no import cycle forms.
 */

import { syncNow } from "./sync";
import { syncStore } from "./sync-state";

/** Long enough to swallow a burst (a save plus its follow-up writes). */
const DELAY_MS = 2500;

let timer: number | null = null;
let running = false;
let listening = false;

export function scheduleAutoSync(): void {
  if (typeof window === "undefined") return;
  const record = syncStore.getSnapshot();
  if (!record.enabled || !record.id) return;
  if (timer !== null) window.clearTimeout(timer);
  timer = window.setTimeout(() => {
    timer = null;
    void runAutoSync();
  }, DELAY_MS);
  listenForReconnect();
}

/** A push that failed while offline is retried when the connection returns. */
function listenForReconnect(): void {
  if (listening) return;
  listening = true;
  window.addEventListener("online", () => {
    const record = syncStore.getSnapshot();
    if (record.enabled && record.dirtyAt !== null) scheduleAutoSync();
  });
}

async function runAutoSync(): Promise<void> {
  if (running) return;
  if (typeof navigator !== "undefined" && navigator.onLine === false) return;
  const record = syncStore.getSnapshot();
  if (!record.enabled || record.dirtyAt === null) return;
  running = true;
  try {
    await syncNow("auto");
  } catch {
    /* stays dirty on purpose: Settings still shows the change and syncs it */
  } finally {
    running = false;
  }
}