Deduplicate an list of CIDR subnets

uniquenet.go

// Command uniquenet takes a file containing a list of CIDR-notated subnets and returns a de-duplicated list.
// Deduplication is more than just literal duplicates, it also takes into consideration if a subnet is fully represented
// by another larger subnet in the list.
//
// Usage: uniquenet [-6] <File path>
// By default, uniquenet only returns a list of IPv4 subnets. Pass -6 to switch to IPv6 only.
// File path must be a text file where each line contains only a CIDR address, that is a network address '/' prefix
// length.
package main

import (
	"bufio"
	"fmt"
	"net"
	"os"
	"strings"
)

func main() {
	if len(os.Args) < 2 {
		fmt.Fprintln(os.Stderr, fmt.Sprintf("Usage: %s [-6] <File path>", os.Args[0]))
		os.Exit(1)
	}

	var filePath string
	var includeV4 = true
	var includeV6 = false

	for _, arg := range os.Args[1:] {
		if arg == "-6" {
			includeV4 = false
			includeV6 = true
		} else {
			filePath = arg
		}
	}

	f, err := os.Open(filePath)
	if err != nil {
		fmt.Fprintln(os.Stderr, err.Error())
		os.Exit(1)
	}
	defer f.Close()

	subnets := []net.IPNet{}

	scanner := bufio.NewScanner(f)
	for scanner.Scan() {
		line := scanner.Text()
		_, net, err := net.ParseCIDR(line)
		if err != nil {
			fmt.Fprintln(os.Stderr, fmt.Sprintf("Invalid line '%s': %s", line, err.Error()))
			os.Exit(1)
		}

		isV6 := strings.Contains(line, ":")
		if (!isV6 && includeV4) || (isV6 && includeV6) {
			subnets = append(subnets, *net)
		}
	}

	netMap := map[string]int{}

	for _, subnet := range subnets {
		netStr := subnet.String()
		for _, otherSubnet := range subnets {
			otherNetStr := otherSubnet.String()

			if netStr == otherNetStr {
				continue
			}

			if otherSubnet.Contains(subnet.IP) {
				v := netMap[netStr]
				v++
				netMap[netStr] = v
			}
		}
	}

	for i := len(subnets) - 1; i >= 0; i-- {
		netStr := subnets[i].String()
		if netMap[netStr] > 0 {
			subnets = append(subnets[:i], subnets[i+1:]...)
		}
	}

	for _, subnet := range subnets {
		fmt.Println(subnet.String())
	}
}