package main

import (
	"flag"
	"log"
	"net"
)

var (
	me   = flag.String("me", "", "my local address")
	peer = flag.String("peer", "", "peer address")
)

func mustResolveUDPAddr(s string) *net.UDPAddr {
	a, err := net.ResolveUDPAddr("udp4", s)
	if err != nil {
		log.Panicf("resolve addr:%s err:%v", s, err)
	}
	return a
}

func mustListenUDP(a *net.UDPAddr) *net.UDPConn {
	c, err := net.ListenUDP("udp4", a)
	if err != nil {
		log.Panicf("listenudp addr:%v err:%v", a, err)
	}
	return c
}

func main() {
	flag.Parse()
	c := mustListenUDP(mustResolveUDPAddr(*me))
	if *peer != "" {
		go func(a *net.UDPAddr) {
			b := []byte("abc")
			n, err := c.WriteToUDP(b, a)
			if err != nil || n != len(b) {
				log.Panicf("writetoudp addr:%v err:%v n:%d", a, err, n)
			}
		}(mustResolveUDPAddr(*peer))
	}

	b := make([]byte, 10)
	_, _, err := c.ReadFromUDP(b)
	if err != nil {
		log.Panicf("readfromudp err:%v", err)
	}
}