go function to generate all the reverse DNS delegations needed to cover a netip.Prefix

PrefixDelegs.go

package main

import (
	"fmt"
	"net/netip"
	"strconv"
	"strings"
)

func PrefixDelegs(p netip.Prefix) []string {
	if !p.IsValid() {
		return nil
	}

	prefixLen := p.Bits()
	addr := p.Masked().Addr()

	var (
		baseZone     string
		bitsPerLabel int
		labelRadix   int
		labels       []string
	)

	if addr.Is6() {
		baseZone = "ip6.arpa"
		bitsPerLabel = 4
		labelRadix = 16
		labels = strings.Split(strings.ReplaceAll(addr.StringExpanded(), ":", ""), "")
	} else {
		baseZone = "in-addr.arpa"
		bitsPerLabel = 8
		labelRadix = 10
		labels = strings.Split(addr.String(), ".")
	}

	numFixedLabels, remFixedBits := prefixLen/bitsPerLabel, prefixLen%bitsPerLabel
	for _, l := range labels[:numFixedLabels] {
		baseZone = l + "." + baseZone
	}
	// baseZone is now the closest enclosing zone for the prefix

	if remFixedBits == 0 {
		// Prefix length aligns with label boundary, return single zone
		return []string{baseZone}
	}

	// Multiple zones are needed to cover prefix, start with current first non-fixed label
	// up to maximum value able to be represented by remaining non-fixed bits
	lastLabel, _ := strconv.ParseUint(labels[numFixedLabels], labelRadix, bitsPerLabel)
	lastLabelEnd := lastLabel + 1<<(bitsPerLabel-remFixedBits) - 1

	var zones []string
	for ; lastLabel <= lastLabelEnd; lastLabel++ {
		zones = append(zones, strconv.FormatUint(lastLabel, labelRadix)+"."+baseZone)
	}

	return zones
}

func main() {
	fmt.Printf("%+v\n", PrefixDelegs(netip.MustParsePrefix("2001:db8:8000::/33")))
}