/* =====================================================================
   LinguaClub service worker
   =====================================================================
   Strategy: cache-first for anything already visited, with a
   background network refresh each time (stale-while-revalidate), so
   pages open instantly once cached and quietly stay up to date.
   Nothing is precached by name here, since the site keeps growing with
   new tools and games; instead, every page a member actually opens
   gets cached automatically the first time, so it's available offline
   from then on. Bump CACHE_VERSION whenever site files change in a way
   that should force old cached copies to be replaced.
   ===================================================================== */

const CACHE_VERSION = 'linguaclub-v18';
const CORE_ASSETS = [
  'index.html',
  'manifest.json',
  'icons/icon-192.png',
  'icons/icon-512.png',
  'icons/apple-touch-icon.png'
];

self.addEventListener('install', (event) => {
  event.waitUntil(
    caches.open(CACHE_VERSION).then((cache) => cache.addAll(CORE_ASSETS)).catch(() => {
      /* offline during install, or a core asset is missing; not fatal */
    })
  );
  self.skipWaiting();
});

self.addEventListener('activate', (event) => {
  event.waitUntil(
    caches.keys().then((names) =>
      Promise.all(names.filter((n) => n !== CACHE_VERSION).map((n) => caches.delete(n)))
    )
  );
  self.clients.claim();
});

self.addEventListener('fetch', (event) => {
  const req = event.request;

  // Only handle GET requests on our own origin; let everything else
  // (fonts.googleapis.com, POSTs, etc.) go straight to the network.
  if (req.method !== 'GET' || new URL(req.url).origin !== self.location.origin) return;

  event.respondWith(
    caches.open(CACHE_VERSION).then(async (cache) => {
      const cached = await cache.match(req);
      const networkFetch = fetch(req)
        .then((res) => {
          if (res && res.status === 200) cache.put(req, res.clone());
          return res;
        })
        .catch(() => null);

      // Serve the cached copy immediately if we have one, refreshing
      // it in the background; otherwise wait for the network, and if
      // that fails too on a page navigation, fall back to the cached
      // home page rather than showing a browser error screen.
      if (cached) {
        networkFetch;
        return cached;
      }
      const fresh = await networkFetch;
      if (fresh) return fresh;
      if (req.mode === 'navigate') {
        const fallback = await cache.match('index.html');
        if (fallback) return fallback;
      }
      return new Response('Offline and not yet cached.', { status: 503, statusText: 'Offline' });
    })
  );
});