firebase.js

// ========================================
// Firebase Configuration and Initialization
// ========================================

let firebaseApp = null;
let auth = null;
let db = null;
let currentUser = null;

// Wait for Firebase modules to load
window.addEventListener('load', async () => {
    try {
        // Give time for Firebase SDK to load
        await new Promise(resolve => setTimeout(resolve, 100));
        
        // Check if Firebase is already configured
        if (typeof getFirebaseConfig === 'function') {
            const firebaseConfig = getFirebaseConfig();
            if (firebaseConfig) {
                try {
                    await initializeFirebase(firebaseConfig);
                } catch (error) {
                    console.error('Firebase initialization error:', error);
                }
            }
        }
    } catch (error) {
        console.error('Error in Firebase load event:', error);
    }
});

// ========================================
// Configuration Management
// ========================================

function getFirebaseConfig() {
    const config = localStorage.getItem('firebaseConfig');
    return config ? JSON.parse(config) : null;
}

function saveFirebaseConfig(config) {
    localStorage.setItem('firebaseConfig', JSON.stringify(config));
}

function clearFirebaseConfig() {
    localStorage.removeItem('firebaseConfig');
}

// ========================================
// Firebase Initialization
// ========================================

async function initializeFirebase(config) {
    try {
        // Use globally available Firebase objects (loaded from CDN)
        if (typeof firebase !== 'undefined' && firebase.initializeApp) {
            // Initialize Firebase
            firebaseApp = firebase.initializeApp(config);
            auth = firebase.auth();
            db = firebase.firestore();
            
            // Set persistence
            await auth.setPersistence(firebase.auth.Auth.Persistence.LOCAL);
            
            // Enable offline persistence
            try {
                db.enablePersistence()
                    .then(() => console.log('Offline persistence enabled'))
                    .catch((err) => {
                        if (err.code === 'failed-precondition') {
                            console.warn('Multiple tabs open, persistence can only be enabled in one tab at a time.');
                        } else if (err.code === 'unimplemented') {
                            console.warn('The current browser does not support offline persistence');
                        }
                    });
            } catch (err) {
                console.warn('Persistence setup error:', err);
            }
            
            // Set up auth state listener
            auth.onAuthStateChanged(handleAuthStateChange);
            
            updateSyncIndicator('synced');
            showToast('Connected to Firebase', 'success');
            
            return true;
        } else {
            console.error('Firebase SDK not available');
            updateSyncIndicator('error');
            showToast('Firebase SDK not loaded', 'error');
            return false;
        }
    } catch (error) {
        console.error('Firebase initialization error:', error);
        updateSyncIndicator('error');
        showToast('Firebase connection failed: ' + error.message, 'error');
        return false;
    }
}

// ========================================
// Auth State Handler
// ========================================

function handleAuthStateChange(user) {
    currentUser = user;
    if (user) {
        // User is signed in
        console.log('User signed in:', user.email);
        // Load user decks
        if (typeof loadUserDecks === 'function') {
            loadUserDecks();
        }
    } else {
        // User is signed out
        console.log('User signed out');
        // Show welcome screen or continue as guest
        const hasSeenWelcome = localStorage.getItem('hasSeenWelcome');
        if (!hasSeenWelcome) {
            if (typeof showWelcomeScreen === 'function') {
                showWelcomeScreen();
            }
        } else {
            if (typeof continueAsGuest === 'function') {
                continueAsGuest();
            }
        }
    }
}

// ========================================
// Export Functions
// ========================================

window.getFirebaseConfig = getFirebaseConfig;
window.saveFirebaseConfig = saveFirebaseConfig;
window.clearFirebaseConfig = clearFirebaseConfig;
window.initializeFirebase = initializeFirebase;
window.handleAuthStateChange = handleAuthStateChange;
window.getCurrentUser = () => currentUser;