add cidr calc

[tomdoherty]

e.g.

package main

import (
	"bytes"
	"encoding/binary"
	"fmt"
	"net"
	"sort"
	"strconv"
	"strings"
)

func ip2int(ip net.IP) uint32 {
	if len(ip) == 16 {
		return binary.BigEndian.Uint32(ip[12:16])
	}
	return binary.BigEndian.Uint32(ip)
}

func int2ip(ip uint32) string {
	result := make(net.IP, 4)
	result[0] = byte(ip >> 24)
	result[1] = byte(ip >> 16)
	result[2] = byte(ip >> 8)
	result[3] = byte(ip)
	return result.String()
}

func ips2cidr(ipaddrs []string) string {
	realIPs := make([]net.IP, 0, len(ipaddrs))

	for _, ip := range ipaddrs {
		realIPs = append(realIPs, net.ParseIP(ip))
	}

	sort.Slice(realIPs, func(i, j int) bool {
		return bytes.Compare(realIPs[i], realIPs[j]) < 0
	})

	var and, xor uint32
	and = ip2int(realIPs[0]) & ip2int(realIPs[len(ipaddrs)-1])
	xor = ip2int(realIPs[0]) ^ ip2int(realIPs[len(ipaddrs)-1])

	IPv4 := int2ip(and)
	var mask string
	for _, o := range strings.Split(int2ip(xor), ".") {
		i, err := strconv.Atoi(o)
		if err != nil {
			panic(err)
		}

		asBin := strconv.FormatInt(int64(i), 2)
		out := fmt.Sprintf("%s%s", strings.Repeat("0", 8-len(asBin)), asBin)
		mask += out

	}
	cidr := strings.Index(mask, "1")

	return fmt.Sprintf("%s/%d", IPv4, cidr)
}

func main() {
	ipaddrs := []string{"192.168.1.1", "192.168.2.255", "192.168.3.255"}

	fmt.Println(ips2cidr(ipaddrs))
}