import { Router } from 'express';
import { AppError } from '../lib/errors.js';

const HOP_BY_HOP_HEADERS = new Set([
  'connection',
  'content-length',
  'host',
  'keep-alive',
  'proxy-authenticate',
  'proxy-authorization',
  'te',
  'trailer',
  'transfer-encoding',
  'upgrade',
]);

const DIRECT_CORE_ALIASES = [
  { methods: new Set(['POST']), pattern: /^\/upload-file$/, targetPath: (pathname) => `/core${pathname}` },
  { methods: new Set(['POST']), pattern: /^\/create-signed-url$/, targetPath: (pathname) => `/core${pathname}` },
  { methods: new Set(['POST']), pattern: /^\/invoke-llm$/, targetPath: (pathname) => `/core${pathname}` },
  { methods: new Set(['POST']), pattern: /^\/rapid-orders\/transcribe$/, targetPath: (pathname) => `/core${pathname}` },
  { methods: new Set(['POST']), pattern: /^\/rapid-orders\/parse$/, targetPath: (pathname) => `/core${pathname}` },
  { methods: new Set(['POST']), pattern: /^\/rapid-orders\/process$/, targetPath: (pathname) => `/core${pathname}` },
  { methods: new Set(['POST']), pattern: /^\/staff\/profile\/photo$/, targetPath: (pathname) => `/core${pathname}` },
  {
    methods: new Set(['POST', 'PUT']),
    pattern: /^\/staff\/profile\/documents\/([^/]+)\/upload$/,
    targetPath: (_pathname, match) => `/core/staff/documents/${match[1]}/upload`,
  },
  {
    methods: new Set(['POST', 'PUT']),
    pattern: /^\/staff\/profile\/attire\/([^/]+)\/upload$/,
    targetPath: (_pathname, match) => `/core/staff/attire/${match[1]}/upload`,
  },
  {
    methods: new Set(['POST']),
    pattern: /^\/staff\/profile\/certificates$/,
    targetPath: () => '/core/staff/certificates/upload',
  },
  {
    methods: new Set(['DELETE']),
    pattern: /^\/staff\/profile\/certificates\/([^/]+)$/,
    targetPath: (_pathname, match) => `/core/staff/certificates/${match[1]}`,
  },
  { methods: new Set(['POST']), pattern: /^\/staff\/documents\/([^/]+)\/upload$/, targetPath: (pathname) => `/core${pathname}` },
  { methods: new Set(['POST']), pattern: /^\/staff\/attire\/([^/]+)\/upload$/, targetPath: (pathname) => `/core${pathname}` },
  { methods: new Set(['POST']), pattern: /^\/staff\/certificates\/upload$/, targetPath: (pathname) => `/core${pathname}` },
  { methods: new Set(['DELETE']), pattern: /^\/staff\/certificates\/([^/]+)$/, targetPath: (pathname) => `/core${pathname}` },
  { methods: new Set(['POST']), pattern: /^\/verifications$/, targetPath: (pathname) => `/core${pathname}` },
  { methods: new Set(['GET']), pattern: /^\/verifications\/([^/]+)$/, targetPath: (pathname) => `/core${pathname}` },
  { methods: new Set(['POST']), pattern: /^\/verifications\/([^/]+)\/review$/, targetPath: (pathname) => `/core${pathname}` },
  { methods: new Set(['POST']), pattern: /^\/verifications\/([^/]+)\/retry$/, targetPath: (pathname) => `/core${pathname}` },
];

function resolveTarget(pathname, method) {
  const upperMethod = method.toUpperCase();

  if (pathname.startsWith('/core')) {
    return {
      baseUrl: process.env.CORE_API_BASE_URL,
      upstreamPath: pathname,
    };
  }

  if (pathname.startsWith('/commands')) {
    return {
      baseUrl: process.env.COMMAND_API_BASE_URL,
      upstreamPath: pathname,
    };
  }

  if (pathname.startsWith('/query')) {
    return {
      baseUrl: process.env.QUERY_API_BASE_URL,
      upstreamPath: pathname,
    };
  }

  for (const alias of DIRECT_CORE_ALIASES) {
    if (!alias.methods.has(upperMethod)) continue;
    const match = pathname.match(alias.pattern);
    if (!match) continue;
    return {
      baseUrl: process.env.CORE_API_BASE_URL,
      upstreamPath: alias.targetPath(pathname, match),
    };
  }

  if ((upperMethod === 'GET' || upperMethod === 'HEAD') && (pathname.startsWith('/client') || pathname.startsWith('/staff'))) {
    return {
      baseUrl: process.env.QUERY_API_BASE_URL,
      upstreamPath: `/query${pathname}`,
    };
  }

  if (['POST', 'PUT', 'PATCH', 'DELETE'].includes(upperMethod) && (pathname.startsWith('/client') || pathname.startsWith('/staff'))) {
    return {
      baseUrl: process.env.COMMAND_API_BASE_URL,
      upstreamPath: `/commands${pathname}`,
    };
  }

  return null;
}

function copyHeaders(source, target) {
  for (const [key, value] of source.entries()) {
    if (HOP_BY_HOP_HEADERS.has(key.toLowerCase())) continue;
    target.setHeader(key, value);
  }
}

async function forwardRequest(req, res, next, fetchImpl) {
  try {
    const requestUrl = new URL(req.originalUrl, 'http://localhost');
    const target = resolveTarget(requestUrl.pathname, req.method);
    if (!target?.baseUrl) {
      throw new AppError('NOT_FOUND', `No upstream configured for ${requestUrl.pathname}`, 404);
    }

    const url = new URL(`${target.upstreamPath}${requestUrl.search}`, target.baseUrl);
    const headers = new Headers();
    for (const [key, value] of Object.entries(req.headers)) {
      if (value == null || HOP_BY_HOP_HEADERS.has(key.toLowerCase())) continue;
      if (Array.isArray(value)) {
        for (const item of value) headers.append(key, item);
      } else {
        headers.set(key, value);
      }
    }
    headers.set('x-request-id', req.requestId);

    const upstream = await fetchImpl(url, {
      method: req.method,
      headers,
      body: req.method === 'GET' || req.method === 'HEAD' ? undefined : req,
      duplex: req.method === 'GET' || req.method === 'HEAD' ? undefined : 'half',
    });

    copyHeaders(upstream.headers, res);
    res.status(upstream.status);

    const buffer = Buffer.from(await upstream.arrayBuffer());
    return res.send(buffer);
  } catch (error) {
    return next(error);
  }
}

export function createProxyRouter(options = {}) {
  const router = Router();
  const fetchImpl = options.fetchImpl || fetch;

  router.use((req, res, next) => forwardRequest(req, res, next, fetchImpl));

  return router;
}