/*
 * UDP multicast discovery — streaming callback API.
 *
 * Announcement wire format (matches discovery.c):
 *   [u16 msg_type=0x0010][u32 payload_len]   <- 6-byte frame header
 *   [u8  protocol_version]
 *   [u16 site_id]
 *   [u16 tcp_port]
 *   [u16 function_flags]
 *   [u8  name_len]
 *   [bytes name...]
 */

import dgram from 'node:dgram';

const MULTICAST_GROUP = '224.0.0.251';
const DISCOVERY_PORT  = 5353;
const ANNOUNCE_TYPE   = 0x0010;
const HEADER_SIZE     = 6;
const ANN_FIXED_SIZE  = 8;

/*
 * Open a discovery socket, send an immediate announce to prompt replies,
 * and call on_peer for each newly-seen peer (deduplicated by addr+name).
 *
 * Returns a stop() function.  Call it when done (e.g. user closed the picker
 * or selected a node).  Safe to call multiple times.
 */
export function start_discovery(on_peer, on_error) {
	const sock = dgram.createSocket({ type: 'udp4', reuseAddr: true });
	const seen = new Set();
	let closed = false;

	sock.on('error', err => {
		console.error('discovery socket error:', err.message);
		if (on_error) { on_error(err); }
		stop();
	});

	sock.on('message', (msg, rinfo) => {
		if (msg.length < HEADER_SIZE) { return; }
		const msg_type    = msg.readUInt16LE(0);
		const payload_len = msg.readUInt32LE(2);
		if (msg_type !== ANNOUNCE_TYPE) { return; }
		if (msg.length < HEADER_SIZE + payload_len) { return; }
		if (payload_len < ANN_FIXED_SIZE) { return; }

		const p          = msg.slice(HEADER_SIZE);
		const site_id    = p.readUInt16LE(1);
		const tcp_port   = p.readUInt16LE(3);
		const func_flags = p.readUInt16LE(5);
		const name_len   = p.readUInt8(7);
		if (payload_len < ANN_FIXED_SIZE + name_len) { return; }
		const name       = p.toString('utf8', 8, 8 + name_len);

		const key = `${rinfo.address}:${name}`;
		if (seen.has(key)) { return; }
		seen.add(key);

		on_peer({ addr: rinfo.address, tcp_port, site_id, function_flags: func_flags, name });
	});

	sock.bind(DISCOVERY_PORT, () => {
		sock.addMembership(MULTICAST_GROUP);
		send_announce(sock);
	});

	function stop() {
		if (closed) { return; }
		closed = true;
		try { sock.close(); } catch {}
	}

	return stop;
}

function send_announce(sock) {
	const name        = Buffer.from('web-inspector', 'utf8');
	const payload_len = 8 + name.length;
	const buf         = Buffer.allocUnsafe(6 + payload_len);
	buf.writeUInt16LE(ANNOUNCE_TYPE, 0);
	buf.writeUInt32LE(payload_len, 2);
	buf.writeUInt8(1, 6);           /* protocol_version */
	buf.writeUInt16LE(0, 7);        /* site_id */
	buf.writeUInt16LE(0, 9);        /* tcp_port (0 = no server) */
	buf.writeUInt16LE(0, 11);       /* function_flags */
	buf.writeUInt8(name.length, 13);
	name.copy(buf, 14);
	sock.send(buf, 0, buf.length, DISCOVERY_PORT, MULTICAST_GROUP);
}