package main

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

const (
	/*
		This is a nonsensical example.
	*/
	fragBits string = "0101010101000100" // [0101 0101 0100 0100], or 21828 (0x5544)
)

var (
	fragOffset     uint16 = 0x000d // 13
	fragFlagMask   uint16 = 0x07   // 0b0000111 to mask 3 bits
	fragOffsetMask uint16 = 0x1fff // 8191, a 13-bit mask
)

func ToFrag(flags, fo uint16) (frag uint16) {

	frag = (flags << fragOffset) | (fo & fragOffsetMask)

	return
}

func FromFrag(frag uint16) (flags, fo uint16) {

	flags = (frag >> fragOffset) & fragFlagMask
	fo = frag & fragOffsetMask

	return
}

func main() {

	var err error
	var u64 uint64
	var frag uint16
	var flags uint16
	var fo uint16

	// Given a fragmentation configuration of fragBits (see const at top)...
	if u64, err = strconv.ParseUint(fragBits, 2, 16); err != nil {
		log.Panicln(err)
	}
	frag = uint16(u64)
	// Prints:
	/*
		Frag is:		21828 (0x5544)
	*/
	fmt.Printf("Frag is:\t\t%d (%#04x)\n", frag, frag)

	flags, fo = FromFrag(frag)
	// Prints:
	/*
		Flags:			2 (0x0002)
		Fragmentation Offset:	5444 (0x1544)
	*/
	fmt.Printf(
		"Flags:\t\t\t%d (%#04x)\n"+
			"Fragmentation Offset:\t%d (%#04x)\n",
		flags, flags,
		fo, fo,
	)

	frag = ToFrag(flags, fo)
	// Prints:
	/*
		Confirmed Frag:		21828 (0x5544)
	*/
	fmt.Printf("Confirmed Frag:\t\t%d (%#02x)\n", frag, frag)
}