const crypto = require('crypto');
const express = require('express');
const app = express();

// Middleware to parse JSON bodies
app.use(express.json());

// Utility function to validate signature
function validateSignature(payload, signature, privateKey) {
  const hmac = crypto.createHmac('sha256', privateKey);
  const expectedSignature = hmac.update(JSON.stringify(payload)).digest('hex');
  return crypto.timingSafeEqual(
    Buffer.from(signature),
    Buffer.from(expectedSignature)
  );
}

// Database model for processed webhooks (example using Mongoose)
const ProcessedWebhook = mongoose.model('ProcessedWebhook', {
  transaction_id: { type: String, unique: true },
  processed_at: { type: Date, default: Date.now }
});

// Webhook handler
app.post('/webhook', async (req, res) => {
  try {
    const signature = req.headers['opm-signature'];
    const payload = req.body;
    const privateKey = process.env.OPM_PRIVATE_KEY;

    // 1. Validate signature
    if (!validateSignature(payload, signature, privateKey)) {
      console.error('Invalid signature for transaction:', payload.transaction_id);
      return res.status(400).send('Invalid signature');
    }

    // 2. Check for duplicate webhook
    const existingWebhook = await ProcessedWebhook.findOne({
      transaction_id: payload.transaction_id
    });

    if (existingWebhook) {
      console.log('Duplicate webhook received:', payload.transaction_id);
      return res.status(200).send('Webhook already processed');
    }

    // 3. Process the webhook asynchronously
    res.status(200).send('Webhook received');

    // 4. Business logic
    await processWebhookData(payload);

    // 5. Mark webhook as processed
    await ProcessedWebhook.create({
      transaction_id: payload.transaction_id
    });

  } catch (error) {
    console.error('Webhook processing error:', error);
    // Still return 200 to acknowledge receipt
    if (!res.headersSent) {
      res.status(200).send('Webhook received with errors');
    }
    
    // Store failed webhook for retry
    await storeFailedWebhook(payload, error);
  }
});

// Business logic processing
async function processWebhookData(payload) {
  const {
    transaction_id,
    reference,
    meta_data,
    status,
    amount,
    buyer_email,
    buyer_name
  } = payload;

  // Update order status
  await Orders.findOneAndUpdate(
    { reference },
    {
      payment_status: status,
      payment_confirmed: true,
      transaction_id
    }
  );

  // Process meta data
  if (meta_data?.order_id) {
    await fulfillOrder(meta_data.order_id);
  }

  // Send confirmation email
  await sendPaymentConfirmation({
    email: buyer_email,
    name: buyer_name,
    amount,
    reference
  });

  // Additional business logic...
}

// Failed webhook storage
async function storeFailedWebhook(payload, error) {
  await FailedWebhooks.create({
    transaction_id: payload.transaction_id,
    payload: payload,
    error: error.message,
    created_at: new Date(),
    retry_count: 0
  });
}

// Start server
const PORT = process.env.PORT || 3000;
app.listen(PORT, () => {
  console.log(`Webhook server listening on port ${PORT}`);
});