import http from 'node:http';
import https from 'node:https';

const LOOPBACK_HOST = '127.0.0.1';
const LOOPBACK_NO_PROXY_ENTRIES = Object.freeze([
  LOOPBACK_HOST,
  'localhost',
  '::1',
]);
const DEFAULT_FALLBACK_PORTS = Object.freeze(
  Array.from({ length: 128 }, (_, index) => 62_000 + index),
);

export function withLoopbackNoProxy(environment) {
  const entries = [environment.NO_PROXY, environment.no_proxy]
    .flatMap((value) => (typeof value === 'string' ? value.split(',') : []))
    .map((value) => value.trim())
    .filter(Boolean);
  const noProxy = [...new Set([...entries, ...LOOPBACK_NO_PROXY_ENTRIES])].join(
    ',',
  );
  return { ...environment, NO_PROXY: noProxy, no_proxy: noProxy };
}

function proxyError(message) {
  const error = new Error(message);
  error.name = 'LlmTransientFaultProxyError';
  return error;
}

function parseUpstreamBaseUrl(value) {
  let parsed;
  try {
    parsed = value instanceof URL ? new URL(value.href) : new URL(value);
  } catch {
    throw proxyError('upstreamBaseUrl must be a valid HTTP(S) URL');
  }

  if (
    !['http:', 'https:'].includes(parsed.protocol) ||
    parsed.username ||
    parsed.password ||
    parsed.hash
  ) {
    throw proxyError(
      'upstreamBaseUrl must use HTTP(S) without userinfo or a fragment',
    );
  }

  return Object.freeze({
    protocol: parsed.protocol,
    hostname: parsed.hostname.replace(/^\[|\]$/gu, ''),
    port: parsed.port || undefined,
    hostHeader: parsed.host,
    basePathname: parsed.pathname.replace(/\/+$/gu, ''),
  });
}

function normalizeOptions(upstreamBaseUrlOrOptions, faultCount, extraOptions) {
  const options =
    typeof upstreamBaseUrlOrOptions === 'string' ||
    upstreamBaseUrlOrOptions instanceof URL
      ? {
          ...extraOptions,
          upstreamBaseUrl: upstreamBaseUrlOrOptions,
          faultCount,
        }
      : upstreamBaseUrlOrOptions;

  if (!options || typeof options !== 'object') {
    throw proxyError('proxy options are required');
  }

  const normalizedFaultCount = options.faultCount ?? 1;
  if (!Number.isSafeInteger(normalizedFaultCount) || normalizedFaultCount < 0) {
    throw proxyError('faultCount must be a non-negative safe integer');
  }
  if (
    options.holdAfterFault !== undefined &&
    typeof options.holdAfterFault !== 'boolean'
  ) {
    throw proxyError('holdAfterFault must be a boolean');
  }
  if (options.listen !== undefined && typeof options.listen !== 'function') {
    throw proxyError('listen must be a function');
  }
  if (
    options.shouldInjectFault !== undefined &&
    typeof options.shouldInjectFault !== 'function'
  ) {
    throw proxyError('shouldInjectFault must be a function');
  }

  const fallbackPorts = options.fallbackPorts ?? DEFAULT_FALLBACK_PORTS;
  if (
    !Array.isArray(fallbackPorts) ||
    fallbackPorts.length === 0 ||
    fallbackPorts.length > 512 ||
    fallbackPorts.some(
      (port) => !Number.isSafeInteger(port) || port < 49_152 || port > 65_535,
    )
  ) {
    throw proxyError('fallbackPorts must contain valid high ports');
  }

  return {
    upstream: parseUpstreamBaseUrl(options.upstreamBaseUrl),
    faultCount: normalizedFaultCount,
    holdAfterFault: options.holdAfterFault ?? false,
    fallbackPorts: [...new Set(fallbackPorts)],
    listen: options.listen ?? listenOnLoopback,
    shouldInjectFault: options.shouldInjectFault ?? null,
  };
}

function listenOnLoopback(server, { host, port }) {
  return new Promise((resolve, reject) => {
    const cleanup = () => {
      server.off('error', onError);
      server.off('listening', onListening);
    };
    const onError = (error) => {
      cleanup();
      reject(error);
    };
    const onListening = () => {
      cleanup();
      resolve();
    };

    server.once('error', onError);
    server.once('listening', onListening);
    try {
      server.listen({ host, port, exclusive: true });
    } catch (error) {
      cleanup();
      reject(error);
    }
  });
}

function errorCode(error) {
  return error && typeof error === 'object' && 'code' in error
    ? error.code
    : undefined;
}

async function bindServer(server, listen, fallbackPorts) {
  try {
    await listen(server, { host: LOOPBACK_HOST, port: 0 });
    return;
  } catch (error) {
    if (errorCode(error) !== 'EADDRINUSE') {
      throw proxyError('unable to bind transient fault proxy on loopback');
    }
  }

  for (const port of fallbackPorts) {
    try {
      await listen(server, { host: LOOPBACK_HOST, port });
      return;
    } catch (error) {
      if (errorCode(error) !== 'EADDRINUSE') {
        throw proxyError('unable to bind transient fault proxy on loopback');
      }
    }
  }

  throw proxyError('transient fault proxy fallback port pool is exhausted');
}

function isOriginFormPath(value) {
  return (
    typeof value === 'string' &&
    value.startsWith('/') &&
    !value.startsWith('//') &&
    !hasAsciiControlCharacter(value)
  );
}

function hasAsciiControlCharacter(value) {
  for (const character of value) {
    const codePoint = character.codePointAt(0);
    if (codePoint <= 0x1f || codePoint === 0x7f) return true;
  }
  return false;
}

function isWithinBasePath(value, basePathname) {
  let pathname;
  try {
    pathname = new URL(value, 'http://proxy.invalid').pathname;
  } catch {
    return false;
  }
  return (
    basePathname === '' ||
    pathname === basePathname ||
    pathname.startsWith(`${basePathname}/`)
  );
}

function resetSocket(socket) {
  if (!socket || socket.destroyed) return;
  try {
    if (typeof socket.resetAndDestroy === 'function') {
      socket.resetAndDestroy();
    } else {
      socket.destroy();
    }
  } catch {
    socket.destroy();
  }
}

function forwardHeaders(request, hostHeader) {
  const headers = Object.create(null);
  for (let index = 0; index < request.rawHeaders.length; index += 2) {
    const name = request.rawHeaders[index];
    const value = request.rawHeaders[index + 1];
    if (!name || value === undefined) continue;
    const lowerName = name.toLowerCase();
    if (lowerName === 'host' || lowerName === 'proxy-connection') continue;
    const existing = headers[lowerName];
    if (existing === undefined) {
      headers[lowerName] = value;
    } else if (Array.isArray(existing)) {
      existing.push(value);
    } else {
      headers[lowerName] = [existing, value];
    }
  }
  headers.host = hostHeader;
  return headers;
}

function failClosed(request, response, statusCode) {
  const body = statusCode === 405 ? 'method not allowed' : 'request rejected';
  request.resume();
  response.shouldKeepAlive = false;
  response.writeHead(statusCode, {
    connection: 'close',
    'content-length': Buffer.byteLength(body),
    'content-type': 'text/plain; charset=utf-8',
  });
  response.end(body);
}

function closeServer(server) {
  if (!server.listening) return Promise.resolve();
  return new Promise((resolve) => {
    server.close(() => resolve());
  });
}

/**
 * Starts a loopback-only proxy that resets the first configured POST requests.
 * The object form also accepts holdAfterFault, fallbackPorts, and a test listen strategy.
 */
export async function startLlmTransientFaultProxy(
  upstreamBaseUrlOrOptions,
  faultCount = 1,
  extraOptions = {},
) {
  const options = normalizeOptions(
    upstreamBaseUrlOrOptions,
    faultCount,
    extraOptions,
  );
  const downstreamSockets = new Set();
  const upstreamRequests = new Set();
  const upstreamSockets = new Set();
  const heldForwardingWaiters = new Set();
  const counterWaiters = new Set();
  const requestLog = [];

  let requestCount = 0;
  let faultInjectedCount = 0;
  let heldRequestCount = 0;
  let forwardedRequestCount = 0;
  let forwardingReleased = !options.holdAfterFault || options.faultCount === 0;
  let stopping = false;
  let stopped = false;
  let stopPromise;

  const stats = () =>
    Object.freeze({
      requestCount,
      faultInjectedCount,
      heldRequestCount,
      forwardedRequestCount,
      forwardingReleased,
      stopped,
    });
  const requestLogSnapshot = () =>
    Object.freeze(requestLog.map((entry) => Object.freeze({ ...entry })));

  const notifyCounterWaiters = (kind) => {
    for (const waiter of [...counterWaiters]) {
      if (waiter.kind !== kind) continue;
      clearTimeout(waiter.timer);
      counterWaiters.delete(waiter);
      waiter.resolve(stats());
    }
  };

  const waitForCounter = (kind, timeoutMs) => {
    if (!Number.isSafeInteger(timeoutMs) || timeoutMs <= 0) {
      return Promise.reject(proxyError('timeoutMs must be a positive integer'));
    }
    const current = kind === 'fault' ? faultInjectedCount : heldRequestCount;
    if (current > 0) return Promise.resolve(stats());
    if (stopping || stopped) {
      return Promise.reject(
        proxyError('proxy stopped before the wait completed'),
      );
    }

    return new Promise((resolve, reject) => {
      const waiter = { kind, resolve, reject, timer: undefined };
      waiter.timer = setTimeout(() => {
        counterWaiters.delete(waiter);
        reject(proxyError(`timed out waiting for proxy ${kind}`));
      }, timeoutMs);
      waiter.timer.unref?.();
      counterWaiters.add(waiter);
    });
  };

  const releaseHeldForwarding = (shouldForward) => {
    for (const waiter of [...heldForwardingWaiters]) {
      heldForwardingWaiters.delete(waiter);
      waiter.complete(shouldForward);
    }
  };

  const waitForForwardingRelease = (request, response) => {
    if (forwardingReleased) return Promise.resolve(true);
    if (stopping) return Promise.resolve(false);

    return new Promise((resolve) => {
      const onAborted = () => waiter.complete(false);
      const onClosed = () => waiter.complete(false);
      const waiter = {
        complete: (shouldForward) => {
          heldForwardingWaiters.delete(waiter);
          request.off('aborted', onAborted);
          response.off('close', onClosed);
          resolve(shouldForward);
        },
      };
      request.once('aborted', onAborted);
      response.once('close', onClosed);
      heldForwardingWaiters.add(waiter);
      if (forwardingReleased) waiter.complete(true);
      if (stopping) waiter.complete(false);
    });
  };

  const sendBadGateway = (request, response) => {
    if (stopping || response.destroyed) return;
    if (response.headersSent) {
      response.destroy();
      return;
    }
    failClosed(request, response, 502);
  };

  const forwardRequest = (request, response, requestMetadata) => {
    requestMetadata.forwardingStartedAtMs = Date.now();
    const transport = options.upstream.protocol === 'https:' ? https : http;
    let upstreamRequest;
    try {
      upstreamRequest = transport.request({
        protocol: options.upstream.protocol,
        hostname: options.upstream.hostname,
        port: options.upstream.port,
        method: request.method,
        path: request.url,
        headers: forwardHeaders(request, options.upstream.hostHeader),
        agent: false,
        setHost: false,
      });
    } catch {
      sendBadGateway(request, response);
      return;
    }

    forwardedRequestCount += 1;
    upstreamRequests.add(upstreamRequest);
    upstreamRequest.once('close', () =>
      upstreamRequests.delete(upstreamRequest),
    );
    upstreamRequest.on('socket', (socket) => {
      upstreamSockets.add(socket);
      socket.once('close', () => upstreamSockets.delete(socket));
      socket.on('error', () => {});
    });

    let upstreamResponse;
    const stopUpstream = () => {
      upstreamResponse?.destroy();
      upstreamRequest.destroy();
    };
    request.once('aborted', stopUpstream);
    request.once('error', stopUpstream);
    response.once('error', stopUpstream);
    response.once('close', () => {
      if (!response.writableEnded) stopUpstream();
    });

    upstreamRequest.once('response', (receivedResponse) => {
      upstreamResponse = receivedResponse;
      receivedResponse.once('error', () => {
        if (!response.destroyed) response.destroy();
      });
      receivedResponse.once('aborted', () => {
        if (!response.destroyed) response.destroy();
      });

      if (stopping || response.destroyed) {
        receivedResponse.destroy();
        return;
      }
      try {
        if (receivedResponse.statusMessage) {
          response.writeHead(
            receivedResponse.statusCode ?? 502,
            receivedResponse.statusMessage,
            receivedResponse.rawHeaders,
          );
        } else {
          response.writeHead(
            receivedResponse.statusCode ?? 502,
            receivedResponse.rawHeaders,
          );
        }
      } catch {
        receivedResponse.destroy();
        sendBadGateway(request, response);
        return;
      }
      receivedResponse.pipe(response);
    });
    upstreamRequest.once('error', () => {
      sendBadGateway(request, response);
    });
    request.pipe(upstreamRequest);
  };

  const handleRequest = async (request, response, expectsContinue) => {
    requestCount += 1;
    const requestMetadata = {
      sequence: requestCount,
      acceptedAtMs: Date.now(),
      faultInjectedAtMs: null,
      heldAtMs: null,
      forwardingStartedAtMs: null,
    };
    requestLog.push(requestMetadata);
    request.on('error', () => {});
    response.on('error', () => {});

    if (request.method !== 'POST') {
      failClosed(request, response, 405);
      return;
    }
    if (
      !isOriginFormPath(request.url) ||
      !isWithinBasePath(request.url, options.upstream.basePathname)
    ) {
      failClosed(request, response, 400);
      return;
    }

    let shouldInjectFault = faultInjectedCount < options.faultCount;
    if (shouldInjectFault && options.shouldInjectFault) {
      const decision = await options.shouldInjectFault(
        Object.freeze({
          sequence: requestMetadata.sequence,
          acceptedAtMs: requestMetadata.acceptedAtMs,
        }),
      );
      if (typeof decision !== 'boolean') {
        throw proxyError('shouldInjectFault must resolve to a boolean');
      }
      shouldInjectFault = decision;
    }

    if (stopping || request.destroyed || response.destroyed) {
      resetSocket(request.socket);
      return;
    }

    if (
      options.holdAfterFault &&
      faultInjectedCount > 0 &&
      !forwardingReleased
    ) {
      heldRequestCount += 1;
      requestMetadata.heldAtMs = Date.now();
      notifyCounterWaiters('held');
      const shouldForward = await waitForForwardingRelease(request, response);
      if (!shouldForward) {
        resetSocket(request.socket);
        return;
      }
    }

    if (shouldInjectFault && faultInjectedCount < options.faultCount) {
      faultInjectedCount += 1;
      requestMetadata.faultInjectedAtMs = Date.now();
      notifyCounterWaiters('fault');
      resetSocket(request.socket);
      return;
    }

    if (stopping || request.destroyed || response.destroyed) {
      resetSocket(request.socket);
      return;
    }
    if (expectsContinue) response.writeContinue();
    forwardRequest(request, response, requestMetadata);
  };

  const dispatchRequest = (request, response, expectsContinue = false) => {
    void handleRequest(request, response, expectsContinue).catch(() => {
      sendBadGateway(request, response);
    });
  };

  const server = http.createServer();
  server.on('request', (request, response) => {
    dispatchRequest(request, response);
  });
  server.on('checkContinue', (request, response) => {
    dispatchRequest(request, response, true);
  });
  server.on('checkExpectation', (request, response) => {
    failClosed(request, response, 400);
  });
  server.on('connect', (_request, socket) => resetSocket(socket));
  server.on('upgrade', (_request, socket) => resetSocket(socket));
  server.on('clientError', (_error, socket) => resetSocket(socket));
  server.on('connection', (socket) => {
    downstreamSockets.add(socket);
    socket.once('close', () => downstreamSockets.delete(socket));
    socket.on('error', () => {});
  });

  try {
    await bindServer(server, options.listen, options.fallbackPorts);
    const address = server.address();
    if (
      !address ||
      typeof address === 'string' ||
      address.address !== LOOPBACK_HOST
    ) {
      throw proxyError('transient fault proxy did not bind to loopback');
    }
  } catch (error) {
    for (const socket of downstreamSockets) socket.destroy();
    await closeServer(server);
    if (error?.name === 'LlmTransientFaultProxyError') throw error;
    throw proxyError('unable to start transient fault proxy');
  }

  server.on('error', () => {});
  const address = server.address();
  const port = address.port;
  const url = `http://${LOOPBACK_HOST}:${port}`;
  const baseUrl = `${url}${options.upstream.basePathname}`;

  const releaseForwarding = () => {
    if (stopping || forwardingReleased) return stats();
    forwardingReleased = true;
    releaseHeldForwarding(true);
    return stats();
  };

  const stop = () => {
    if (stopPromise) return stopPromise;
    stopPromise = (async () => {
      stopping = true;
      releaseHeldForwarding(false);
      for (const waiter of [...counterWaiters]) {
        clearTimeout(waiter.timer);
        counterWaiters.delete(waiter);
        waiter.reject(proxyError('proxy stopped before the wait completed'));
      }

      const closePromise = closeServer(server);
      for (const request of [...upstreamRequests]) request.destroy();
      for (const socket of [...upstreamSockets]) socket.destroy();
      for (const socket of [...downstreamSockets]) socket.destroy();
      server.closeAllConnections?.();
      await closePromise;
      stopped = true;
    })();
    return stopPromise;
  };

  return Object.freeze({
    url,
    baseUrl,
    port,
    get stats() {
      return stats();
    },
    getStats: stats,
    getRequestLog: requestLogSnapshot,
    waitForFault(timeoutMs = 5_000) {
      return waitForCounter('fault', timeoutMs);
    },
    waitForHeldRequest(timeoutMs = 5_000) {
      return waitForCounter('held', timeoutMs);
    },
    releaseForwarding,
    stop,
  });
}