package main

import (
	"context"
	"fmt"
	"log"
	"net"
	"os/exec"
	"strings"
	"time"
)

const (
	resolverTimeout = time.Millisecond * time.Duration(8000)
)

func getResolver(nameserver string) *net.Resolver {
	return &net.Resolver{
		// use go's built-in dns resolver
		PreferGo: true,
		// specify alternate dialer for go's dns resolver
		Dial: func(ctx context.Context, network, address string) (net.Conn, error) {
			d := net.Dialer{
				Timeout: resolverTimeout,
			}
			return d.DialContext(ctx, network, nameserver)
		},
	}
}

func requiresRRUpdate(record string, addr net.IP, resolver *net.Resolver) bool {
	// TODO: the context could be replaced in order to allow cancelling the dns
	// query if the user disconnects prematurely
	addrs, err := resolver.LookupIP(context.Background(), "ip", record)
	if err != nil {
		log.Printf("dns lookup failed: %s", err)
		// enforce update, it's better than not trying at all
		return true
	}

	// check if the current ip matches
	for _, ip := range addrs {
		if ip.Equal(addr) {
			// the ip seems to be still up-to-date -> no update required
			return false
		}
	}
	return true
}

func updateRR(entry *RRConfig, addr net.IP) error {
	query := generateQuery(entry, addr)
	return executeQuery(query)
}

func generateQuery(entry *RRConfig, addr net.IP) string {
	var q strings.Builder

	fmt.Fprintf(&q, "key %s:%s %s\n", entry.Tsigalgo, entry.Tsigid, entry.Tsigkey)
	fmt.Fprintf(&q, "server %s\n", entry.Nameserver)
	fmt.Fprintf(&q, "zone %s\n", entry.Zonename)
	// TODO: check if addr is ipv4 or ipv6 (-> update A or AAAA)
	fmt.Fprintf(&q, "delete %s. A\n", entry.Recordname)
	fmt.Fprintf(&q, "add %s. %d A %s\n", entry.Recordname, entry.Ttl, addr.String())
	fmt.Fprintf(&q, "send\n")

	return q.String()
}

func executeQuery(query string) error {
	cmd := exec.Command("knsupdate", "-v")
	cmd.Stdin = strings.NewReader(query)
	output, err := cmd.CombinedOutput()
	// TODO: is outputting stdout+stderr on failure even necessary?
	if err != nil {
		log.Printf("executeQuery failed: %s", output)
	}

	return err
}