/**
 * Detect the local LAN IP (192.168.x.x, 10.x.x.x, 172.16-31.x.x)
 * using WebRTC ICE candidate gathering.
 *
 * Returns the first private IPv4 found, or undefined if none detected
 * (e.g. browser blocks mDNS candidates).
 */
export async function detectLocalIP(timeoutMs = 3000): Promise<string | undefined> {
  return new Promise((resolve) => {
    let resolved = false;
    const done = (ip?: string) => {
      if (resolved) return;
      resolved = true;
      pc.close();
      clearTimeout(timer);
      resolve(ip);
    };

    const timer = setTimeout(() => done(undefined), timeoutMs);

    const pc = new RTCPeerConnection({
      iceServers: [{ urls: "stun:stun.l.google.com:19302" }],
    });

    pc.createDataChannel("");

    pc.onicecandidate = (event) => {
      if (!event.candidate?.candidate) return;
      const match = event.candidate.candidate.match(
        /(?:srflx|host)\s+(\d{1,3}\.\d{1,3}\.\d{1,3}\.\d{1,3})\s/,
      );
      if (match) {
        const ip = match[1];
        if (isPrivateIPv4(ip)) {
          done(ip);
        }
      }
    };

    pc.createOffer()
      .then((offer) => pc.setLocalDescription(offer))
      .catch(() => done(undefined));
  });
}

function isPrivateIPv4(ip: string): boolean {
  return (
    ip.startsWith("192.168.") ||
    ip.startsWith("10.") ||
    /^172\.(1[6-9]|2\d|3[01])\./.test(ip)
  );
}