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// webhook.ts - High-Performance Messenger Webhook Handler
import express, { Request, Response } from 'express';
import crypto from 'crypto';
import { parseNepaliIntent } from 'nepali-messenger-nlp';

const app = express();
app.use(express.json({ verify: verifyMetaSignature }));

// 1. Authenticate Meta Webhook Signature
function verifyMetaSignature(req: Request, res: Response, buf: Buffer) {
  const signature = req.headers['x-hub-signature-256'] as string;
  if (!signature) throw new Error('Missing signature');
  
  const hmac = crypto.createHmac('sha256', process.env.META_APP_SECRET!);
  const digest = 'sha256=' + hmac.update(buf).digest('hex');
  if (signature !== digest) throw new Error('Invalid HMAC signature');
}

// 2. Process Customer Events with Sub-2s Execution
app.post('/webhook', async (req: Request, res: Response) => {
  res.status(200).send('EVENT_RECEIVED'); // Immediate 200 OK to Meta

  const event = req.body.entry?.[0]?.messaging?.[0];
  const senderId = event?.sender?.id;
  const messageText = event?.message?.text;

  if (senderId && messageText) {
    const intent = parseNepaliIntent(messageText); // <2ms Romanized NLP
    await dispatchSalesAgentResponse(senderId, intent);
  }
});