package main

import (
	"fmt"
	"io"
	"net"

	"github.com/xtaci/kcp-go/v5"
)

func acceptSessions(ln *kcp.Listener) error {
	for {
		conn, err := ln.AcceptKCP()
		if err != nil {
			return err
		}
		go func() {
			defer conn.Close()
			io.Copy(conn, conn)
		}()
	}
}

func main() {
	serverConn, err := net.ListenPacket("udp", "127.0.0.1:0")
	if err != nil {
		panic(err)
	}
	defer serverConn.Close()
	ln, err := kcp.ServeConn(nil, 0, 0, serverConn)
	if err != nil {
		panic(err)
	}
	go acceptSessions(ln)

	thirdPartyAddr, err := net.ResolveUDPAddr("udp", "8.8.8.8:53")
	if err != nil {
		panic(err)
	}

	// Create a net.PacketConn to give to kcp.NewConn2.
	clientConn, err := net.ListenPacket("udp", ":0")
	if err != nil {
		panic(err)
	}

	// Use the net.PacketConn before giving it to KCP.
	n, err := clientConn.WriteTo([]byte("test\n"), thirdPartyAddr)
	fmt.Printf("UDP WriteTo -> %v, %v\n", n, err)

	fmt.Println()

	// Create a kcp.UDPSession, use it, and close it.
	client, err := kcp.NewConn2(serverConn.LocalAddr(), nil, 0, 0, clientConn)
	if err != nil {
		panic(err)
	}
	n, err = client.Write([]byte("hello\n"))
	fmt.Printf("KCP Write -> %v, %v\n", n, err)
	var buf [10]byte
	n, err = client.Read(buf[:])
	fmt.Printf("KCP Read -> %q, %v\n", buf[:n], err)
	err = client.Close()
	fmt.Printf("KCP Close -> %v\n", err)

	fmt.Println()

	// net.PacketConn should still work after closing KCP.
	n, err = clientConn.WriteTo([]byte("test\n"), thirdPartyAddr)
	fmt.Printf("UDP WriteTo -> %v, %v\n", n, err)
}

UDP WriteTo -> 5, <nil>

KCP Write -> 6, <nil>
KCP Read -> "hello\n", <nil>
KCP Close -> <nil>

UDP WriteTo -> 0, write udp [::]:53657->8.8.8.8:53: use of closed network connection

UDP WriteTo -> 5, <nil>

KCP Write -> 6, <nil>
KCP Read -> "hello\n", <nil>
KCP Close -> <nil>

UDP WriteTo -> 5, <nil>