package main

import (
	"encoding/binary"
	"encoding/hex"
	"fmt"
	"log"
	"net"
	"os"
	"os/exec"
	"sync"

	"baalvpn/shared"

	"github.com/songgao/water"
)

const IFACE_NAME = "baalvpn"

type Peer struct {
	RealIP     string
	InternalIP string
}

var (
	peers      = map[string]*Peer{}
	peersMutex sync.Mutex
	nextIP     = 3
	iface      *water.Interface
)

func main() {
	if os.Geteuid() != 0 {
		panic("root permissions needed")
	}

	config := water.Config{DeviceType: water.TAP}
	config.Name = IFACE_NAME

	var err error
	iface, err = water.New(config)
	if err != nil {
		panic(err)
	}

	if err := exec.Command("ip", "addr", "add", "10.1.0.2/24", "dev", IFACE_NAME).Run(); err != nil {
		panic(err)
	}
	if err := exec.Command("ip", "link", "set", "dev", IFACE_NAME, "up").Run(); err != nil {
		panic(err)
	}

	receivePackets()
}

func receivePackets() {
	log.Println("Listening for packets...")

	listenAddr, err := net.ResolveUDPAddr("udp", ":38000")
	if err != nil {
		panic(err)
	}
	listener, err := net.ListenUDP("udp", listenAddr)
	if err != nil {
		panic(err)
	}
	defer listener.Close()

	for {
		buffer := make([]byte, 50000)
		n, addr, err := listener.ReadFromUDP(buffer)
		if err != nil {
			log.Println(err)
			continue
		}
		if n == 0 {
			continue
		}

		go receivePacket(listener, buffer[:n], addr)
	}
}

func receivePacket(conn *net.UDPConn, pkt []byte, addr *net.UDPAddr) {
	if len(pkt) < 4 {
		log.Println("packet too short")
		return
	}
	version := binary.LittleEndian.Uint16(pkt[0:])
	if version != shared.PROTO_VERSION {
		log.Println("mismatched packet version")
		return
	}

	pktType := binary.LittleEndian.Uint16(pkt[2:])

	peerIP := addr.IP.String()

	switch pktType {
	case shared.REQ_REGISTER:
		peersMutex.Lock()
		defer peersMutex.Unlock()

		internalIP := fmt.Sprintf("10.1.0.%d", nextIP)

		if peer, ok := peers[addr.IP.String()]; ok {
			internalIP = peer.InternalIP
		} else {
			peers[addr.IP.String()] = &Peer{
				RealIP:     peerIP,
				InternalIP: internalIP,
			}
			nextIP++
		}

		resp := shared.BuildPkt(shared.RESP_REGISTER, net.ParseIP(internalIP).To4())
		if _, err := conn.WriteToUDP(resp, addr); err != nil {
			log.Println(err)
		}
	case shared.REQ_DATA:
		fmt.Println("received packet from " + peerIP + ":")
		fmt.Println(hex.EncodeToString(pkt[4:]))
	default:
		log.Println("unknown packet type")
	}
}