const express = require('express');
const path = require('path');
const { GoogleAuth } = require('google-auth-library');

const app = express();
const port = process.env.PORT || 8080;
const BACKEND_URL = process.env.BACKEND_URL;

// Validate required environment variable at startup
if (!BACKEND_URL) {
  console.error('ERROR: BACKEND_URL environment variable is required');
  process.exit(1);
}

// Initialize Google Auth client (auto-detects credentials on Cloud Run)
const auth = new GoogleAuth();

app.use(express.json());

// Serve static React build
app.use(express.static(path.join(__dirname, 'dist')));

// Proxy endpoint - generates ID token and forwards to backend
app.post('/api/chat/stream', async (req, res) => {
  console.log('Proxy request received for /api/chat/stream');

  try {
    // Get ID token client with backend URL as audience
    const client = await auth.getIdTokenClient(BACKEND_URL);
    const headers = await client.getRequestHeaders();

    console.log('Generated ID token, forwarding to backend...');

    // Forward request to backend with auth
    const backendResponse = await fetch(`${BACKEND_URL}/chat/stream`, {
      method: 'POST',
      headers: {
        'Content-Type': 'application/json',
        ...headers,
      },
      body: JSON.stringify(req.body),
    });

    // Check if backend returned an error
    if (!backendResponse.ok) {
      const errorText = await backendResponse.text();
      console.error(`Backend error: ${backendResponse.status} - ${errorText}`);
      res.status(backendResponse.status).json({
        error: 'Backend error',
        status: backendResponse.status,
        message: errorText,
      });
      return;
    }

    // Set SSE headers and flush immediately
    res.setHeader('Content-Type', 'text/event-stream');
    res.setHeader('Cache-Control', 'no-cache');
    res.setHeader('Connection', 'keep-alive');
    res.flushHeaders?.();

    // Pipe raw SSE stream from backend to browser (no parsing)
    const reader = backendResponse.body.getReader();

    // Handle client disconnect
    req.on('close', () => {
      console.log('Client disconnected');
      reader.cancel();
    });

    // Stream loop
    while (true) {
      const { done, value } = await reader.read();
      if (done) {
        console.log('Stream complete');
        res.end();
        return;
      }
      res.write(value);
    }
  } catch (error) {
    console.error('Proxy error:', error);
    if (!res.headersSent) {
      res.status(500).json({ error: 'Proxy error', message: error.message });
    } else {
      res.end();
    }
  }
});

// Health check endpoint for Cloud Run
app.get('/health', (req, res) => {
  res.status(200).json({ status: 'healthy' });
});

// SPA fallback - all other routes serve index.html
app.get('*', (req, res) => {
  res.sendFile(path.join(__dirname, 'dist', 'index.html'));
});

// Listen on 0.0.0.0 for Cloud Run
app.listen(port, '0.0.0.0', () => {
  console.log(`Server listening on ${port}`);
  console.log(`Backend URL: ${BACKEND_URL}`);
});