(function () {
    const DB_NAME = 'ShortTrackOffline';
    const DB_VERSION = 1;
    let dbInstance = null;
    const cache = new Map();

    function open() {
        return new Promise((resolve, reject) => {
            if (dbInstance) { resolve(dbInstance); return; }
            const req = indexedDB.open(DB_NAME, DB_VERSION);
            req.onupgradeneeded = (e) => {
                const db = e.target.result;
                if (!db.objectStoreNames.contains('data')) {
                    db.createObjectStore('data', { keyPath: 'key' });
                }
            };
            req.onsuccess = (e) => { dbInstance = e.target.result; resolve(dbInstance); };
            req.onerror = (e) => reject(e.target.error);
        });
    }

    async function save(key, data) {
        cache.set(key, data);
        try {
            const db = await open();
            const tx = db.transaction('data', 'readwrite');
            tx.objectStore('data').put({ key, data, updatedAt: Date.now() });
            await new Promise((res, rej) => { tx.oncomplete = res; tx.onerror = rej; });
        } catch (e) { console.log('OfflineStore.save error:', e); }
    }

    async function get(key) {
        try {
            const db = await open();
            const tx = db.transaction('data', 'readonly');
            const req = tx.objectStore('data').get(key);
            return new Promise((resolve) => { req.onsuccess = () => resolve(req.result?.data || null); req.onerror = () => resolve(null); });
        } catch (e) { return null; }
    }

    function cacheGet(key) {
        return cache.get(key);
    }

    function cacheSet(key, data) {
        cache.set(key, data);
    }

    function cacheDelete(key) {
        cache.delete(key);
    }

    async function restoreCache() {
        try {
            const db = await open();
            const tx = db.transaction('data', 'readonly');
            const store = tx.objectStore('data');
            return new Promise((resolve) => {
                const req = store.openCursor();
                req.onsuccess = (e) => {
                    const cursor = e.target.result;
                    if (cursor) {
                        cache.set(cursor.value.key, cursor.value.data);
                        cursor.continue();
                    } else {
                        resolve();
                    }
                };
                req.onerror = () => resolve();
            });
        } catch (e) { return; }
    }

    async function remove(key) {
        cache.delete(key);
        try {
            const db = await open();
            const tx = db.transaction('data', 'readwrite');
            tx.objectStore('data').delete(key);
        } catch (e) {}
    }

    window.OfflineStore = {
        open,
        save,
        get,
        cacheGet,
        cacheSet,
        cacheDelete,
        restoreCache,
        remove,
        get cache() { return cache; },
    };
})();