import dgram from 'node:dgram';
import { networkInterfaces } from 'node:os';

export const DISCOVERY_PORT = 3378;
export const DISCOVERY_MULTICAST = '239.255.42.99';
export const DISCOVERY_PREFIX = 'AGENTHUB|';

export function getLanIPv4(): string | undefined {
  for (const iface of Object.values(networkInterfaces())) {
    if (!iface) continue;
    for (const entry of iface) {
      if (entry.family === 'IPv4' && !entry.internal) {
        return entry.address;
      }
    }
  }
  return undefined;
}

export function resolveAdvertiseUrl(host: string, port: number): string {
  const advertiseHost = host === '0.0.0.0' || host === '::' ? getLanIPv4() ?? '127.0.0.1' : host;
  return `http://${advertiseHost}:${port}`;
}

export function startDiscoveryBroadcaster(serverUrl: string, options?: { port?: number; intervalMs?: number }) {
  const port = options?.port ?? DISCOVERY_PORT;
  const intervalMs = options?.intervalMs ?? 2000;
  const socket = dgram.createSocket('udp4');
  const message = Buffer.from(`${DISCOVERY_PREFIX}${serverUrl}`);

  socket.on('error', () => {
    // Discovery is best-effort; ignore network errors.
  });

  const send = () => {
    try {
      socket.send(message, 0, message.length, port, DISCOVERY_MULTICAST);
    } catch {
      try {
        socket.setBroadcast(true);
        socket.send(message, 0, message.length, port, '255.255.255.255');
      } catch {
        // Ignore send failures.
      }
    }
  };

  send();
  const interval = setInterval(send, intervalMs);

  return {
    socket,
    interval,
    stop: () => {
      clearInterval(interval);
      socket.close();
    },
  };
}

export function discoverServer(timeoutMs = 3000, port = DISCOVERY_PORT): Promise<string | undefined> {
  return new Promise((resolve) => {
    const socket = dgram.createSocket('udp4');
    let timer: NodeJS.Timeout;

    const cleanup = () => {
      try {
        socket.close();
      } catch {
        // Already closed.
      }
      clearTimeout(timer);
    };

    socket.on('message', (msg) => {
      const text = msg.toString();
      if (text.startsWith(DISCOVERY_PREFIX)) {
        cleanup();
        resolve(text.slice(DISCOVERY_PREFIX.length));
      }
    });

    socket.on('error', () => {
      cleanup();
      resolve(undefined);
    });

    socket.bind(port, () => {
      try {
        socket.addMembership(DISCOVERY_MULTICAST);
      } catch {
        // Multicast may be blocked; still try to receive broadcasts.
      }
    });

    timer = setTimeout(() => {
      cleanup();
      resolve(undefined);
    }, timeoutMs);
  });
}