buildx/vendor/github.com/apparentlymart/go-cidr/cidr/wrangling.go

package cidr

import (
	"fmt"
	"math/big"
	"net"
)

func ipToInt(ip net.IP) (*big.Int, int) {
	val := &big.Int{}
	val.SetBytes([]byte(ip))
	if len(ip) == net.IPv4len {
		return val, 32
	} else if len(ip) == net.IPv6len {
		return val, 128
	} else {
		panic(fmt.Errorf("Unsupported address length %d", len(ip)))
	}
}

func intToIP(ipInt *big.Int, bits int) net.IP {
	ipBytes := ipInt.Bytes()
	ret := make([]byte, bits/8)
	// Pack our IP bytes into the end of the return array,
	// since big.Int.Bytes() removes front zero padding.
	for i := 1; i <= len(ipBytes); i++ {
		ret[len(ret)-i] = ipBytes[len(ipBytes)-i]
	}
	return net.IP(ret)
}

func insertNumIntoIP(ip net.IP, bigNum *big.Int, prefixLen int) net.IP {
	ipInt, totalBits := ipToInt(ip)
	bigNum.Lsh(bigNum, uint(totalBits-prefixLen))
	ipInt.Or(ipInt, bigNum)
	return intToIP(ipInt, totalBits)
}