/** Gateway proxy mode — pipes SSE from a remote gateway */

import type { Request, Response } from 'express';

export async function proxyToGateway(
  req: Request,
  res: Response,
  gatewayUrl: string,
): Promise<void> {
  const apiKey = process.env.GATEWAY_API_KEY || '';

  try {
    const upstream = await fetch(`${gatewayUrl}/llm/chat`, {
      method: 'POST',
      headers: {
        'Content-Type': 'application/json',
        'X-API-Key': apiKey,
      },
      body: JSON.stringify({ ...req.body, stream: true }),
    });

    if (!upstream.ok) {
      const text = await upstream.text();
      res.status(upstream.status).json({ error: text });
      return;
    }

    res.setHeader('Content-Type', 'text/event-stream');
    res.setHeader('Cache-Control', 'no-cache');
    res.setHeader('Connection', 'keep-alive');

    const reader = upstream.body?.getReader();
    if (!reader) {
      res.write(`data: ${JSON.stringify({ error: 'No response body' })}\n\n`);
      res.end();
      return;
    }

    const decoder = new TextDecoder();
    while (true) {
      const { done, value } = await reader.read();
      if (done) break;
      res.write(decoder.decode(value, { stream: true }));
    }
    res.end();
  } catch (error) {
    res.write(`data: ${JSON.stringify({ error: String(error) })}\n\n`);
    res.end();
  }
}