package main

import (
	"errors"
	"fmt"
	"log"
	"net"
	"net/http"
	"os"
	"time"
)

type conf struct {
	zone   string
	token  string
	server string
}

func main() {
	conf, err := readConfigVars()
	if err != nil {
		log.Fatal(err)
	}
	lastIPv6 := ""
	for {
		time.Sleep(1 * time.Second)
		if newIPv6, ok := checkForNewV6(lastIPv6); ok {
			lastIPv6 = newIPv6
			sendNewIP(conf)
		}
	}
}

// Checks if the newest IPv6 on the first Interface is not equal to the provided one
// Returns the new IPv6 if it changed
func checkForNewV6(lastIPv6 string) (string, bool) {
	interfaces, err := net.Interfaces()
	if err != nil {
		log.Println(err)
		return lastIPv6, false
	}
	currentIPv6 := ""
	for _, inter := range interfaces {
		addresses, err := inter.Addrs()
		if err != nil {
			log.Println(err)
			return lastIPv6, false
		}
		for _, addr := range addresses {
			if ip, ok := addr.(*net.IPNet); ok && !ip.IP.IsLoopback() {
				if ip.IP.To4() == nil {
					currentIPv6 = ip.IP.String()
				}
			}
		}
	}
	if currentIPv6 != lastIPv6 {
		return currentIPv6, true
	}
	return lastIPv6, false
}

// Updates the Prefix on the Server
func sendNewIP(config conf) error {
	_, err := http.Get(fmt.Sprintf("https://ipv6.dynv6.com/api/update?ipv6prefix=auto&token=%s&zone=%s", config.token, config.zone))
	if err != nil {
		return err
	}
	return nil
}

// Reads all ENVIRONMENT variables
func readConfigVars() (conf, error) {
	var zone, token, server string
	var present bool
	if zone, present = os.LookupEnv("ZONE"); !present {
		return conf{}, errors.New("No Zone Specified")
	}
	if token, present = os.LookupEnv("TOKEN"); !present {
		return conf{}, errors.New("No Token Specified")
	}
	if server, present = os.LookupEnv("SERVER"); !present {
		server = "ipv6.dynv6.com"
	}
	return conf{
		zone:   zone,
		token:  token,
		server: server,
	}, nil
}