/**
 * server.js
 * SmartStudent Backend API Main Server
 * This file is the entry point for the SmartStudent Express application.
 * It sets up the server, database connection, middleware, and
 * schedules background tasks.
 */
const express = require('express');
const http = require('http'); // Required for session store setup
const cors = require('cors');
const helmet = require('helmet');
const morgan = require('morgan');
const dotenv = require('dotenv');
const { EventEmitter } = require('events'); // Event bus for inter-module communication
const cron = require('node-cron'); // Cron job for scheduled tasks
const { DateTime } = require('luxon'); // Luxon for date and time handling

// --- Database & Session Store Dependencies ---
const pgp = require('pg-promise')();
const session = require('express-session');
const pgSession = require('connect-pg-simple')(session);

// --- Local Dependencies ---
const logger = require('./utils/logger'); // Winston logger
const buildRouter = require('./routes'); // Your route definitions

// --- Load Environment Variables ---
dotenv.config();

// --- Main Application Setup ---
const app = express();
const eventBus = new EventEmitter(); // Event bus for system-wide events

// --- Database Connection ---
const dbConfig = {
    connectionString: process.env.DATABASE_URL,
    ssl: { rejectUnauthorized: false }
};

if (!dbConfig.connectionString) {
    logger.error('❌ DATABASE_URL environment variable is not set. Exiting.');
    process.exit(1);
}

const pool = new pgp.Pool(dbConfig);
const db = pgp(dbConfig);

// Test database connection on startup
db.connect()
    .then(obj => {
        obj.done(); // success, release connection
        logger.info('✅ PostgreSQL connected successfully.');
    })
    .catch(error => {
        logger.error('❌ Database connection error:', error.message);
        process.exit(1); // Exit if DB connection fails
    });

// --- Middleware Setup ---
app.use(cors({
    origin: process.env.FRONTEND_URL || '*', // Use env var for production
    credentials: true
}));
app.use(helmet()); // Secure Express apps by setting various HTTP headers
app.use(morgan('tiny', { stream: { write: message => logger.info(message.trim()) } })); // Logging with Winston
app.use(express.json()); // Body parser for JSON
app.use(express.urlencoded({ extended: false })); // Body parser for URL-encoded data


// --- Session Configuration ---
if (!process.env.SESSION_SECRET) {
    logger.error('❌ SESSION_SECRET environment variable is not set. Exiting.');
    process.exit(1);
}

const sessionStore = new pgSession({
    pool: pool, // Use the pg-promise pool
    tableName: 'user_sessions'
});

app.use(session({
    store: sessionStore,
    secret: process.env.SESSION_SECRET,
    resave: false,
    saveUninitialized: false,
    cookie: {
        secure: process.env.NODE_ENV === 'production', // Use secure cookies in production
        httpOnly: true,
        maxAge: 1000 * 60 * 60 * 24 * 7 // 7 days
    }
}));

cron.schedule('*/5 * * * *', () => { // Runs every 5 minutes
    logger.info('⏰ Running scheduled reminder check...');
    const now = DateTime.now().toJSDate();
    // In a real application, you would query the database here for tasks/reminders
    // that are due soon and send out notifications using webpush or Twilio.
    logger.info(`Checking for upcoming reminders at ${now}`);
});


// --- Mount the API Router ---
// Pass the database pool and event bus to the router
app.use('/', buildRouter(db, eventBus));


// --- Error Handling for API Calls ---
app.use((req, res) => {
    res.status(404).json({ error: 'Not Found', message: `The requested path '${req.path}' does not exist.` });
});

app.use((err, req, res, next) => {
    logger.error('Unhandled Error:', err.stack);
    res.status(500).json({ error: 'Internal Server Error', message: 'An unexpected error occurred.' });
});


// --- Unhandled Rejection and Uncaught Exception Handlers ---
// These ensure the process doesn't crash on unhandled async errors
process.on('unhandledRejection', (reason, promise) => {
    logger.error('❌ Unhandled Rejection at:', promise, 'reason:', reason);
    // Application might be in an unstable state, so exit gracefully
    server.close(() => process.exit(1));
});

process.on('uncaughtException', err => {
    logger.error('❌ Uncaught Exception:', err.stack);
    // Application might be in an unstable state, so exit gracefully
    server.close(() => process.exit(1));
});


// --- Start the Server ---
const PORT = process.env.PORT || 4000;
const server = http.createServer(app);

server.listen(PORT, () => {
    logger.info(`✅ Server is running on port ${PORT} in ${process.env.NODE_ENV || 'development'} mode.`);