package main

import (
	`fmt`
	`log`
	"strconv"
)

const (
	vihlBits string = "01000101" // [0100 0101], or 69 (0x45)
	ipVer    uint8  = 0x04       // 4; unused in this program, but represented for completion's sake.
)

var (
	vihlLowerMask uint8 = 0x0f // 15
)

func ToVIHL(ver, ihl uint8) (vihl byte) {

	// In Go, a byte is exactly equal to a uint8
	// so type casting/conversion is unnecessary here.
	vihl = (ver << 4) | (ihl & vihlLowerMask)

	return
}

func FromVIHL(vihl byte) (ver, ihl uint8) {

	ver = vihl >> 4
	ihl = vihl & vihlLowerMask

	return
}

func main() {

	var err error
	var u64 uint64
	var vihl byte
	var ver uint8
	var ihl uint8

	// Given a V/IHL of vihlBits (see const at top)...
	if u64, err = strconv.ParseUint(vihlBits, 2, 8); err != nil {
		log.Panicln(err)
	}
	vihl = uint8(u64)
	// Prints:
	/*
		V/IHL is:		69 (0x45)
	*/
	fmt.Printf("V/IHL is:\t\t%d (%#02x)\n", vihl, vihl)

	ver, ihl = FromVIHL(vihl)
	// Prints:
	/*
		Version:		4 (0x04)
		IHL:			5 (0x05)
	*/
	fmt.Printf(
		"Version:\t\t%d (%#02x)\n"+
			"IHL:\t\t\t%d (%#02x)\n",
		ver, ver,
		ihl, ihl,
	)

	vihl = ToVIHL(ver, ihl)
	// Prints:
	/*
		Confirmed V/IHL:	69 (0x45)
	*/
	fmt.Printf("Confirmed V/IHL:\t%d (%#02x)\n", vihl, vihl)
}