OpenDiablo2/d2common/bitstream.go

package d2common

import (
	"log"

	"github.com/OpenDiablo2/OpenDiablo2/d2common/d2interface"
)

var _ d2interface.BitStream = &BitStream{} // Static check to confirm struct conforms to interface

const (
	maxBits     = 16
	bitsPerByte = 8
)

// BitStream is a utility class for reading groups of bits from a stream
type BitStream struct {
	data         []byte
	dataPosition int
	current      int
	bitCount     int
}

// CreateBitStream creates a new BitStream
func CreateBitStream(newData []byte) *BitStream {
	result := &BitStream{
		data:         newData,
		dataPosition: 0,
		current:      0,
		bitCount:     0,
	}

	return result
}

// ReadBits reads the specified number of bits and returns the value
func (v *BitStream) ReadBits(bitCount int) int {
	if bitCount > maxBits {
		log.Panic("Maximum BitCount is 16")
	}

	if !v.EnsureBits(bitCount) {
		return -1
	}

	result := v.current & (0xffff >> uint(maxBits-bitCount))
	v.WasteBits(bitCount)

	return result
}

// PeekByte returns the current byte without adjusting the position
func (v *BitStream) PeekByte() int {
	if !v.EnsureBits(bitsPerByte) {
		return -1
	}

	return v.current & 0xff
}

// EnsureBits ensures that the specified number of bits are available
func (v *BitStream) EnsureBits(bitCount int) bool {
	if bitCount <= v.bitCount {
		return true
	}

	if v.dataPosition >= len(v.data) {
		return false
	}

	nextValue := v.data[v.dataPosition]
	v.dataPosition++
	v.current |= int(nextValue) << uint(v.bitCount)
	v.bitCount += 8

	return true
}

// WasteBits dry-reads the specified number of bits
func (v *BitStream) WasteBits(bitCount int) {
	// noinspection GoRedundantConversion
	v.current >>= uint(bitCount)
	v.bitCount -= bitCount
}