package main

import (
	"encoding/binary"
	"log"
	"net"

	"github.com/cespare/xxhash/v2"
)

// TODO: write more convenient version
func addrMap(srcIp net.IP, dstCidr *net.IPNet) net.IP {
	addrlen := len(srcIp)
	if addrlen != len(dstCidr.IP) {
		log.Fatalf("addrMap(): src/dst size mismatch: %v vs %v", len(srcIp), len(dstCidr.IP))
	}

	var addr net.IP = make([]byte, addrlen)

	hsum := xxhash.Sum64(srcIp)
	bsum := binary.NativeEndian.AppendUint64([]byte{}, hsum)
	blen := len(bsum)
	if addrlen > blen {
		// extend bsum
		tmp := append(make([]byte, addrlen-blen), bsum...)
		bsum = tmp
	}
	if addrlen < blen {
		// trim bsum
		bsum = bsum[:addrlen]
	}

	// apply inverted mask to bsum and sum with addr
	for i := range addrlen / 4 {
		a := binary.NativeEndian.Uint32(dstCidr.IP[i*4:])
		b := binary.NativeEndian.Uint32(bsum[i*4:])
		m := binary.NativeEndian.Uint32(dstCidr.Mask[i*4:])
		a += (b & ^m)
		binary.NativeEndian.PutUint32(addr[i*4:], a)
	}

	return addr
}