'use strict';

const http = require('http');
const https = require('https');

const PORT = Number(process.env.PORT) || 3000;
const PROXY_TIMEOUT_MS = Number(process.env.PROXY_TIMEOUT_MS) || 30000;
const VM_DOCS_BASE_URL = process.env.VM_DOCS_BASE_URL;

if (!VM_DOCS_BASE_URL) {
  throw new Error('VM_DOCS_BASE_URL environment variable is required');
}

const UPSTREAM = new URL(VM_DOCS_BASE_URL);
if (UPSTREAM.protocol !== 'http:' && UPSTREAM.protocol !== 'https:') {
  throw new Error('VM_DOCS_BASE_URL must start with http:// or https://');
}

const UPSTREAM_MODULE = UPSTREAM.protocol === 'https:' ? https : http;
const UPSTREAM_BASE_PATH = UPSTREAM.pathname.replace(/\/+$/, '');
const UPSTREAM_PORT = UPSTREAM.port || (UPSTREAM.protocol === 'https:' ? 443 : 80);

const STAND_RE = /^[A-Za-z0-9._-]+$/;
const FORWARD_HEADERS = ['content-type', 'content-length', 'cache-control', 'content-encoding', 'location'];

function respond(res, status, body) {
  if (!res.headersSent) {
    res.writeHead(status, {
      'Content-Type': 'text/plain; charset=utf-8',
      'Content-Length': Buffer.byteLength(body),
    });
  }
  res.end(body);
}

function proxyRequest(req, res, targetPath) {
  const options = {
    protocol: UPSTREAM.protocol,
    hostname: UPSTREAM.hostname,
    port: UPSTREAM_PORT,
    method: req.method,
    path: targetPath,
    timeout: PROXY_TIMEOUT_MS,
  };

  const upstreamReq = UPSTREAM_MODULE.request(options, (upstreamRes) => {
    const headers = {};
    for (const name of FORWARD_HEADERS) {
      if (upstreamRes.headers[name] !== undefined) {
        headers[name] = upstreamRes.headers[name];
      }
    }
    res.writeHead(upstreamRes.statusCode || 502, headers);
    upstreamRes.pipe(res);
  });

  upstreamReq.on('timeout', () => {
    upstreamReq.destroy(new Error('Upstream timeout'));
  });

  upstreamReq.on('error', () => {
    if (res.headersSent) {
      res.destroy();
    } else {
      respond(res, 502, 'Bad Gateway');
    }
  });

  req.on('aborted', () => upstreamReq.destroy());

  upstreamReq.end();
}

const server = http.createServer((req, res) => {
  if (req.method !== 'GET' && req.method !== 'HEAD') {
    respond(res, 405, 'Method Not Allowed');
    return;
  }

  const rawUrl = req.url || '/';
  const qIndex = rawUrl.indexOf('?');
  const rawPath = qIndex === -1 ? rawUrl : rawUrl.slice(0, qIndex);
  const rawQuery = qIndex === -1 ? '' : rawUrl.slice(qIndex);

  if (rawPath === '/health') {
    respond(res, 200, 'ok');
    return;
  }

  let decoded;
  try {
    decoded = decodeURIComponent(rawPath);
  } catch (_) {
    respond(res, 400, 'Bad Request');
    return;
  }

  if (decoded.split('/').some((seg) => seg === '..')) {
    respond(res, 400, 'Bad Request');
    return;
  }

  let targetPath;
  if (decoded === '/') {
    targetPath = UPSTREAM_BASE_PATH + '/' + rawQuery;
  } else {
    const stand = decoded.split('/').filter(Boolean)[0];
    if (!stand || !STAND_RE.test(stand)) {
      respond(res, 400, 'Bad Request');
      return;
    }
    targetPath = UPSTREAM_BASE_PATH + rawPath + rawQuery;
  }

  proxyRequest(req, res, targetPath);
});

if (require.main === module) {
  server.listen(PORT, () => {
    console.log(`[docs-proxy] listening on port ${PORT}, upstream: ${VM_DOCS_BASE_URL}`);
  });
}

module.exports = { server };