package main
import (
	"encoding/binary"
	"fmt"
	restruct "gopkg.in/restruct.v1"
)
type MessageType uint8
const (
	MessageTypeOne MessageType = iota + 1
	MessageTypeTwo
	MessageTypeThree
)
// Message is the data to deserialize from the binary stream
type Message struct {
	Length     uint32      `struct:"uint32"` // message size in bytes (including length)
	Type       MessageType `struct:"uint8"`
	Version    uint8       `struct:"uint8:4"` // Just need 4 bits
	Subversion uint8       `struct:"uint8:2"` // just need 2 bits
	Optional   uint8       `struct:"uint8:1"` // just one bit --> '1' means next field is NOT present
	NodeName   string      ``
	ANumber    uint16      `struct:"uint16:10"` // just need 10 bits
}
// (length(4)+type(1)+(version(4bits)+Subversion(2bits)+Optional(1bit))) = 6 bytes
// need 32bit alignment
func main() {
	var inStream = []byte{0x08, // just 8 bytes needed
		0x01,       // messge type = MessageTypeOne
		0x4a,       // Version=0100 Subversion=10 Optional=1 ANumber = 0 (MSB bit)
		0x00, 0x60, // ANumber(000 0000 011) Padding = 0 0000 for 32 bits alignment
	}
	var msg Message
	err := restruct.Unpack(inStream, binary.BigEndian, &msg)
	if err != nil {
		panic(err)
	}
	fmt.Println(msg)
	// Expected:
	// msg.Length = 8
	// msg.Type = 1
	// msg.Version = 4
	// msg.Subversion = 2
	// msg.Optional = 1
	// msg.NodeName = ""
	// msg.ANumber = 3
}

func (m *Message) UnmarshalBinary(data []byte) error {
	m.Length = binary.BigEndian.Uint32(data[:4])
	if int(m.Length) > len(data) {
		return fmt.Errorf("not enough bytes")
	}
	m.Type = MessageType(data[4])
	m.Version = data[5] >> 4
	m.Subversion = data[5] >> 2 & 0x03
	m.Optional = data[5] >> 1 & 0x01
	// move the index for ANumber back if there's an optional string
	idx := 6
	if m.Optional == 0 {
		// remove the last two bytes for ANumber
		end := int(m.Length) - 2
		m.NodeName = string(data[6:end])
		idx = end
	}
	m.ANumber = uint16(data[idx]&0xc0)<<2 | uint16(data[idx]&0x3f<<2|data[idx+1]>>6)
	return nil

func (e *encoder) writeBits(f field, inBuf []byte) {
var inputLength uint8 = uint8(len(inBuf))
if f.BitSize == 0 {
// Having problems with complex64 type ... so we asume we want to read all
//f.BitSize = uint8(f.Type.Bits())
f.BitSize = 8 * inputLength
}
// destPos: Destination position ( in the result ) of the first bit in the first byte
var destPos uint8 = 8 - e.bitCounter
// originPos: Original position of the first bit in the first byte
var originPos uint8 = f.BitSize % 8
if originPos == 0 {
originPos = 8
}
// numBytes: number of complete bytes to hold the result
var numBytes uint8 = f.BitSize / 8
// numBits: number of remaining bits in the first non-complete byte of the result
var numBits uint8 = f.BitSize % 8
// number of positions we have to shift the bytes to get the result
var shift uint8
if originPos > destPos {
shift = originPos - destPos
} else {
shift = destPos - originPos
}
shift = shift % 8
var inputInitialIdx uint8 = inputLength - numBytes
if numBits > 0 {
inputInitialIdx = inputInitialIdx - 1
}
if originPos < destPos {
// shift left
carry := func(idx uint8) uint8 {
if (idx + 1) < inputLength {
return (inBuf[idx+1] >> (8 - shift))
}
return 0x00
}
mask := func(idx uint8) uint8 {
if idx == 0 {
return (0x01 << destPos) - 1
}
return 0xFF
}
var idx uint8 = 0
for inIdx := inputInitialIdx; inIdx < inputLength; inIdx++ {
e.buf[idx] |= ((inBuf[inIdx] << shift) | carry(inIdx)) & mask(idx)
idx++
}
} else {
// originPos >= destPos => shift right
var idx uint8 = 0
// carry : is a little bit tricky in this case because of the first case
// when idx == 0 and there is no carry at all
carry := func(idx uint8) uint8 {
if idx == 0 {
return 0x00
}
return (inBuf[idx-1] << (8 - shift))
}
mask := func(idx uint8) uint8 {
if idx == 0 {
return (0x01 << destPos) - 1
}
return 0xFF
}
inIdx := inputInitialIdx
for ; inIdx < inputLength; inIdx++ {
//note: Should the mask be done BEFORE the OR with carry?
e.buf[idx] |= ((inBuf[inIdx] >> shift) | carry(inIdx)) & mask(idx)
idx++
}
if ((e.bitCounter + f.BitSize) % 8) > 0 {
e.buf[idx] |= carry(inIdx)
}
}
//now we should update buffer and bitCounter
e.bitCounter = (e.bitCounter + f.BitSize) % 8
// move the head to the next non-complete byte used
headerUpdate := func() uint8 {
if (e.bitCounter == 0) && ((f.BitSize % 8) != 0) {
return (numBytes + 1)
}
return numBytes
}
e.buf = e.buf[headerUpdate():]
return
}