package dnsxl

import (
	"net"
	"strconv"
	"strings"
)

// reverseName converts an IPv4 or IPv6 address into the reversed name
// notation as described in RFC 1034 (for IPv4) or RFC 3596 (for IPv6).
func reverseName(ip net.IP) string {
	const hexdig = "0123456789abcdef"
	var b strings.Builder
	if ip4 := ip.To4(); ip4 != nil {
		b.WriteString(strconv.Itoa(int(ip4[3])))
		b.WriteByte('.')
		b.WriteString(strconv.Itoa(int(ip4[2])))
		b.WriteByte('.')
		b.WriteString(strconv.Itoa(int(ip4[1])))
		b.WriteByte('.')
		b.WriteString(strconv.Itoa(int(ip4[0])))
	} else {
		for i := 15; i >= 0; i-- {
			x := ip[i]
			b.WriteByte(hexdig[x&15])
			b.WriteByte('.')
			b.WriteByte(hexdig[x>>4])
			if i > 0 {
				b.WriteByte('.')
			}
		}
	}
	return b.String()
}