package multicast

import (
	"bytes"
	"fmt"
	"log"
	"net"
	"net/netip"
	"time"
)

func Receiver(vpnNet netip.Prefix, selfVPNIP netip.Addr, ch chan<- Packet) {
	for {
		if err := receiver(vpnNet, selfVPNIP, ch); err != nil {
			log.Printf("[MCReader] %v", err)
		}
		time.Sleep(errorTimeout)
	}
}

func receiver(vpnNet netip.Prefix, selfVPNIP netip.Addr, ch chan<- Packet) error {
	selfIP := selfVPNIP.As4()[3]

	conn, err := net.ListenMulticastUDP("udp", nil, addr)
	if err != nil {
		return fmt.Errorf("bind: %w", err)
	}
	defer conn.Close()

	buf := make([]byte, SignedPacketSize+1) // +1 to detect oversized packets

	for {
		conn.SetReadDeadline(time.Now().Add(32 * time.Second))
		n, src, err := conn.ReadFromUDPAddrPort(buf)
		if err != nil {
			if ne, ok := err.(net.Error); ok && ne.Timeout() {
				continue
			}
			return fmt.Errorf("read: %w", err)
		}

		if n != SignedPacketSize {
			continue
		}

		packet := unmarshal(buf[:n])

		if packet.PeerIP == selfIP {
			continue
		}

		age := time.Since(time.Unix(packet.Timestamp, 0))
		if age > maxPacketAge || age < -maxPacketAge {
			continue
		}

		packet.Signed = bytes.Clone(packet.Signed)
		packet.Src = src.Addr().Unmap()
		ch <- packet
	}
}