package peer

import (
	"encoding/binary"
	"fmt"
	"log"
	"net"
	"time"
)

const beaconLen = 35 // 1 VPN IP byte + 32 WG pubkey + 2 WG listen port

func RunMCWriter(g Globals) {
	conn, err := net.ListenMulticastUDP("udp", nil, multicastAddr)
	if err != nil {
		log.Fatalf("[MCWriter] bind: %v", err)
	}

	for range time.Tick(broadcastInterval) {
		beacon, err := buildBeacon(g)
		if err != nil {
			log.Printf("[MCWriter] build beacon: %v", err)
			continue
		}
		log.Printf("[MCWriter] Broadcasting on %v...", multicastAddr)
		if _, err := conn.WriteToUDP(beacon, multicastAddr); err != nil {
			log.Printf("[MCWriter] write: %v", err)
		}
	}
}

func buildBeacon(g Globals) ([]byte, error) {
	dev, err := g.WGClient.Device(g.WGDevName)
	if err != nil {
		return nil, fmt.Errorf("get WG device: %w", err)
	}
	beacon := make([]byte, beaconLen)
	beacon[0] = g.LocalPeerIP
	pubKey := g.WGPrivKey.PublicKey()
	copy(beacon[1:33], pubKey[:])
	binary.BigEndian.PutUint16(beacon[33:35], uint16(dev.ListenPort))
	return beacon, nil
}