// Local network discovery — minimal helper used by the legacy code-pairing probe.
//
// `getLocalSubnet` returns the /24 prefix (e.g. "192.168.1") that the
// browser is on, derived from a host-relative WebRTC ICE candidate. The
// extension uses this to scan the LAN for the peer's pairing-port
// WebSocket when a legacy addressless code has to be resolved. Legacy v2.14
// discovery/pair probes have been removed — see code-pairing.ts for the
// v3 pairing-port scan and sync-transport.ts for the rate-limited
// Phase-2 subnet probe.

// Returns e.g. "192.168.1" or null if unavailable.
export async function getLocalSubnet(): Promise<string | null> {
  return new Promise((resolve) => {
    try {
      const pc = new RTCPeerConnection({ iceServers: [] });
      pc.createDataChannel('');
      pc.onicecandidate = (e) => {
        if (!e.candidate) { pc.close(); resolve(null); return; }
        const m = e.candidate.candidate.match(/(\d+\.\d+\.\d+)\.\d+/);
        if (m) { pc.close(); resolve(m[1]); }
      };
      pc.createOffer().then(o => pc.setLocalDescription(o)).catch(() => { pc.close(); resolve(null); });
      setTimeout(() => { try { pc.close(); } catch { /* ignore */ } resolve(null); }, 2000);
    } catch {
      resolve(null);
    }
  });
}