av/codec/h264/h264dec/cabac.go

package h264dec

import (
	"bitbucket.org/ausocean/av/codec/h264/h264dec/bits"
	"github.com/pkg/errors"
)

const (
	NaCtxId            = 10000
	NA_SUFFIX          = -1
	MbAddrNotAvailable = 10000
)

// G.7.4.3.4 via G.7.3.3.4 via 7.3.2.13 for NalUnitType 20 or 21
// refLayerMbWidthC is equal to MbWidthC for the reference layer representation
func RefMbW(chromaFlag, refLayerMbWidthC int) int {
	if chromaFlag == 0 {
		return 16
	}
	return refLayerMbWidthC
}

// refLayerMbHeightC is equal to MbHeightC for the reference layer representation
func RefMbH(chromaFlag, refLayerMbHeightC int) int {
	if chromaFlag == 0 {
		return 16
	}
	return refLayerMbHeightC
}
func XOffset(xRefMin16, refMbW int) int {
	return (((xRefMin16 - 64) >> 8) << 4) - (refMbW >> 1)
}
func YOffset(yRefMin16, refMbH int) int {
	return (((yRefMin16 - 64) >> 8) << 4) - (refMbH >> 1)
}
func MbWidthC(sps *SPS) int {
	mbWidthC := 16 / SubWidthC(sps)
	if sps.ChromaFormatIDC == chromaMonochrome || sps.SeparateColorPlaneFlag {
		mbWidthC = 0
	}
	return mbWidthC
}
func MbHeightC(sps *SPS) int {
	mbHeightC := 16 / SubHeightC(sps)
	if sps.ChromaFormatIDC == chromaMonochrome || sps.SeparateColorPlaneFlag {
		mbHeightC = 0
	}
	return mbHeightC
}

// G.8.6.2.2.2
func Xr(x, xOffset, refMbW int) int {
	return (x + xOffset) % refMbW
}
func Yr(y, yOffset, refMbH int) int {
	return (y + yOffset) % refMbH
}

// G.8.6.2.2.2
func Xd(xr, refMbW int) int {
	if xr >= refMbW/2 {
		return xr - refMbW
	}
	return xr + 1
}
func Yd(yr, refMbH int) int {
	if yr >= refMbH/2 {
		return yr - refMbH
	}
	return yr + 1
}
func Ya(yd, refMbH, signYd int) int {
	return yd - (refMbH/2+1)*signYd
}

// 6.4.11.1
func MbAddr(xd, yd, predPartWidth int) {
	// TODO: Unfinished
	var n string
	if xd == -1 && yd == 0 {
		n = "A"
	}
	if xd == 0 && yd == -1 {
		n = "B"
	}
	if xd == predPartWidth && yd == -1 {
		n = "C"
	}
	if xd == -1 && yd == -1 {
		n = "D"
	}
	_ = n
}

func CondTermFlag(mbAddr, mbSkipFlag int) int {
	if mbAddr == MbAddrNotAvailable || mbSkipFlag == 1 {
		return 0
	}
	return 1
}

// s9.3.3 p 278: Returns the value of the syntax element
func (bin *Binarization) Decode(sliceContext *SliceContext, b *bits.BitReader, rbsp []byte) {}

// 9.3.3.1.1 : returns ctxIdxInc
func Decoder9_3_3_1_1_1(condTermFlagA, condTermFlagB int) int {
	return condTermFlagA + condTermFlagB
}

// 9-5
// 7-30 p 112
func SliceQPy(pps *PPS, header *SliceHeader) int {
	return 26 + pps.PicInitQpMinus26 + header.SliceQpDelta
}

// 9-5
func PreCtxState(m, n, sliceQPy int) int {
	// slicQPy-subY
	return Clip3(1, 126, ((m*Clip3(0, 51, sliceQPy))>>4)+n)
}

func Clip1y(x, bitDepthY int) int {
	return Clip3(0, (1<<uint(bitDepthY))-1, x)
}
func Clipc(x, bitDepthC int) int {
	return Clip3(0, (1<<uint(bitDepthC))-1, x)
}

// 5-5
func Clip3(x, y, z int) int {
	if z < x {
		return x
	}
	if z > y {
		return y
	}
	return z
}

type CABAC struct {
	PStateIdx int
	ValMPS    int
	Context   *SliceContext
}

// table 9-1
func initCabac(binarization *Binarization, context *SliceContext) *CABAC {
	var valMPS, pStateIdx int
	// TODO: When to use prefix, when to use suffix?
	ctxIdx := CtxIdx(
		binarization.binIdx,
		binarization.MaxBinIdxCtx.Prefix,
		binarization.CtxIdxOffset.Prefix)
	mn := MNVars[ctxIdx]

	preCtxState := PreCtxState(mn[0].M, mn[0].N, SliceQPy(context.PPS, context.Header))
	if preCtxState <= 63 {
		pStateIdx = 63 - preCtxState
		valMPS = 0
	} else {
		pStateIdx = preCtxState - 64
		valMPS = 1
	}
	_ = pStateIdx
	_ = valMPS
	// Initialization of context variables
	// Initialization of decoding engine
	return &CABAC{
		PStateIdx: pStateIdx,
		ValMPS:    valMPS,
		Context:   context,
	}
}

// Binarizations for macroblock types in slice types.
var (
	// binOfIMBTypes provides binarization strings for values of macroblock
	// type in I slices as defined in table 9-36 of the specifications.
	binOfIMBTypes = [numOfIMBTypes][]int{
		0:  {0},
		1:  {1, 0, 0, 0, 0, 0},
		2:  {1, 0, 0, 0, 0, 1},
		3:  {1, 0, 0, 0, 1, 0},
		4:  {1, 0, 0, 0, 1, 1},
		5:  {1, 0, 0, 1, 0, 0, 0},
		6:  {1, 0, 0, 1, 0, 0, 1},
		7:  {1, 0, 0, 1, 0, 1, 0},
		8:  {1, 0, 0, 1, 0, 1, 1},
		9:  {1, 0, 0, 1, 1, 0, 0},
		10: {1, 0, 0, 1, 1, 0, 1},
		11: {1, 0, 0, 1, 1, 1, 0},
		12: {1, 0, 0, 1, 1, 1, 1},
		13: {1, 0, 1, 0, 0, 0},
		14: {1, 0, 1, 0, 0, 1},
		15: {1, 0, 1, 0, 1, 0},
		16: {1, 0, 1, 0, 1, 1},
		17: {1, 0, 1, 1, 0, 0, 0},
		18: {1, 0, 1, 1, 0, 0, 1},
		19: {1, 0, 1, 1, 0, 1, 0},
		20: {1, 0, 1, 1, 0, 1, 1},
		21: {1, 0, 1, 1, 1, 0, 0},
		22: {1, 0, 1, 1, 1, 0, 1},
		23: {1, 0, 1, 1, 1, 1, 0},
		24: {1, 0, 1, 1, 1, 1, 1},
		25: {1, 1},
	}

	// binOfPOrSPMBTypes provides binarization strings for values of macroblock
	// type in P or SP slices as defined in table 9-37 of the specifications.
	// NB: binarization of macroblock types 5 to 30 is 1 and not included here.
	binOfPOrSPMBTypes = [5][]int{
		0: {0, 0, 0},
		1: {0, 1, 1},
		2: {0, 1, 0},
		3: {0, 0, 1},
		4: {},
	}

	// binOfBMBTypes provides binarization strings for values of macroblock
	// type in B slice as defined in table 9-37 of the specifications.
	// NB: binarization of macroblock types 23 to 48 is 111101 and is not
	// included here.
	binOfBMBTypes = [23][]int{
		0:  {0},
		1:  {1, 0, 0},
		2:  {1, 0, 1},
		3:  {1, 1, 0, 0, 0, 0},
		4:  {1, 1, 0, 0, 0, 1},
		5:  {1, 1, 0, 0, 1, 0},
		6:  {1, 1, 0, 0, 1, 1},
		7:  {1, 1, 0, 1, 0, 0},
		8:  {1, 1, 0, 1, 0, 1},
		9:  {1, 1, 0, 1, 1, 0},
		10: {1, 1, 0, 1, 1, 1},
		11: {1, 1, 1, 1, 1, 0},
		12: {1, 1, 1, 0, 0, 0, 0},
		13: {1, 1, 1, 0, 0, 0, 1},
		14: {1, 1, 1, 0, 0, 1, 0},
		15: {1, 1, 1, 0, 0, 1, 1},
		16: {1, 1, 1, 0, 1, 0, 0},
		17: {1, 1, 1, 0, 1, 0, 1},
		18: {1, 1, 1, 0, 1, 1, 0},
		19: {1, 1, 1, 0, 1, 1, 1},
		20: {1, 1, 1, 1, 0, 0, 0},
		21: {1, 1, 1, 1, 0, 0, 1},
		22: {1, 1, 1, 1, 1, 1},
	}
)

// Binarizations for sub-macroblock types in slice types.
var (
	// binOfPorSPSubMBTypes provides binarization strings for values of sub-macroblock
	// type in P or SP slices as defined in table 9-38 of the specifications.
	binOfPOrSPSubMBTypes = [4][]int{
		0: {1},
		1: {0, 0},
		2: {0, 1, 1},
		3: {0, 1, 0},
	}

	// binOfBSubMBTypes provides binarization strings for values of sub-macroblock
	// type in B slices as defined in table 9-38 of the specifications.
	binOfBSubMBTypes = [numOfBSubMBTypes][]int{
		0:  {1},
		1:  {1, 0, 0},
		2:  {1, 0, 1},
		3:  {1, 1, 0, 0, 0},
		4:  {1, 1, 0, 0, 1},
		5:  {1, 1, 0, 1, 0},
		6:  {1, 1, 0, 1, 1},
		7:  {1, 1, 1, 0, 0, 0},
		8:  {1, 1, 1, 0, 0, 1},
		9:  {1, 1, 1, 0, 1, 0},
		10: {1, 1, 1, 0, 1, 1},
		11: {1, 1, 1, 1, 0},
		12: {1, 1, 1, 1, 1},
	}
)

// Errors used by mbTypeBinarization.
var (
	errBadMbType      = errors.New("macroblock type outside of valid range")
	errBadMbSliceType = errors.New("bad slice type for macroblock")
)

// mbTypeBinarization returns the macroblock type binarization for the given
// macroblock type value and slice type using the process defined in section
// 9.3.2.5 of the specifications.
func mbTypeBinarization(v, slice int) ([]int, error) {
	switch slice {
	case sliceTypeI:
		if v < minIMbType || v > maxIMbType {
			return nil, errBadMbType
		}
		return binOfIMBTypes[v], nil

	case sliceTypeSI:
		if v < minSIMbType || v > maxSIMbType {
			return nil, errBadMbType
		}
		if v == sliceTypeSI {
			return []int{0}, nil
		}
		return append([]int{1}, binOfIMBTypes[v-1]...), nil

	case sliceTypeP, sliceTypeSP:
		if v < minPOrSPMbType || v > maxPOrSPMbType || v == P8x8ref0 {
			return nil, errBadMbType
		}
		if v < 5 {
			return binOfPOrSPMBTypes[v], nil
		}
		return append([]int{1}, binOfIMBTypes[v-5]...), nil

	case sliceTypeB:
		if v < minBMbType || v > maxBMbType {
			return nil, errBadMbType
		}
		if v < 23 {
			return binOfBMBTypes[v], nil
		}
		return append([]int{1, 1, 1, 1, 0, 1}, binOfIMBTypes[v-23]...), nil

	default:
		return nil, errBadMbSliceType
	}
}

// Error used by subMbTypeBinarization.
var errBadSubMbSliceType = errors.New("bad slice type for sub-macroblock")

// subMbTypeBinarization returns the binarization of a sub-macroblock type
// given the slice in which it is in using the process defined in section
// 9.3.2.5 of the specifications.
func subMbTypeBinarization(v, slice int) ([]int, error) {
	switch slice {
	case sliceTypeP, sliceTypeSP:
		if v < minPOrSPSubMbType || v > maxPOrSPSubMbType {
			return nil, errBadMbType
		}
		return binOfPOrSPSubMBTypes[v], nil

	case sliceTypeB:
		if v < minBSubMbType || v > maxBSubMbType {
			return nil, errBadMbType
		}
		return binOfBSubMBTypes[v], nil

	default:
		return nil, errBadSubMbSliceType
	}
}

// Table 9-34
type MaxBinIdxCtx struct {
	// When false, Prefix is the MaxBinIdxCtx
	IsPrefixSuffix bool
	Prefix, Suffix int
}
type CtxIdxOffset struct {
	// When false, Prefix is the MaxBinIdxCtx
	IsPrefixSuffix bool
	Prefix, Suffix int
}

// Table 9-34
type Binarization struct {
	SyntaxElement string
	BinarizationType
	MaxBinIdxCtx
	CtxIdxOffset
	UseDecodeBypass int
	// TODO: Why are these private but others aren't?
	binIdx    int
	binString []int
}
type BinarizationType struct {
	PrefixSuffix   bool
	FixedLength    bool
	Unary          bool
	TruncatedUnary bool
	CMax           bool
	// 9.3.2.3
	UEGk      bool
	CMaxValue int
}

// 9.3.2.5
func NewBinarization(syntaxElement string, data *SliceData) *Binarization {
	sliceTypeName := data.SliceTypeName
	logger.Printf("debug: binarization of %s in sliceType %s\n", syntaxElement, sliceTypeName)
	binarization := &Binarization{SyntaxElement: syntaxElement}
	switch syntaxElement {
	case "CodedBlockPattern":
		binarization.BinarizationType = BinarizationType{PrefixSuffix: true}
		binarization.MaxBinIdxCtx = MaxBinIdxCtx{IsPrefixSuffix: true, Prefix: 3, Suffix: 1}
		binarization.CtxIdxOffset = CtxIdxOffset{IsPrefixSuffix: true, Prefix: 73, Suffix: 77}
	case "IntraChromaPredMode":
		binarization.BinarizationType = BinarizationType{
			TruncatedUnary: true, CMax: true, CMaxValue: 3}
		binarization.MaxBinIdxCtx = MaxBinIdxCtx{Prefix: 1}
		binarization.CtxIdxOffset = CtxIdxOffset{Prefix: 64}
	case "MbQpDelta":
		binarization.BinarizationType = BinarizationType{}
		binarization.MaxBinIdxCtx = MaxBinIdxCtx{Prefix: 2}
		binarization.CtxIdxOffset = CtxIdxOffset{Prefix: 60}
	case "MvdLnEnd0":
		binarization.UseDecodeBypass = 1
		binarization.BinarizationType = BinarizationType{UEGk: true}
		binarization.MaxBinIdxCtx = MaxBinIdxCtx{IsPrefixSuffix: true, Prefix: 4, Suffix: NA_SUFFIX}
		binarization.CtxIdxOffset = CtxIdxOffset{
			IsPrefixSuffix: true,
			Prefix:         40,
			Suffix:         NA_SUFFIX,
		}
	case "MvdLnEnd1":
		binarization.UseDecodeBypass = 1
		binarization.BinarizationType = BinarizationType{UEGk: true}
		binarization.MaxBinIdxCtx = MaxBinIdxCtx{
			IsPrefixSuffix: true,
			Prefix:         4,
			Suffix:         NA_SUFFIX,
		}
		binarization.CtxIdxOffset = CtxIdxOffset{
			IsPrefixSuffix: true,
			Prefix:         47,
			Suffix:         NA_SUFFIX,
		}
		// 9.3.2.5
	case "MbType":
		logger.Printf("debug: \tMbType is %s\n", data.MbTypeName)
		switch sliceTypeName {
		case "SI":
			binarization.BinarizationType = BinarizationType{PrefixSuffix: true}
			binarization.MaxBinIdxCtx = MaxBinIdxCtx{IsPrefixSuffix: true, Prefix: 0, Suffix: 6}
			binarization.CtxIdxOffset = CtxIdxOffset{IsPrefixSuffix: true, Prefix: 0, Suffix: 3}
		case "I":
			binarization.BinarizationType = BinarizationType{}
			binarization.MaxBinIdxCtx = MaxBinIdxCtx{Prefix: 6}
			binarization.CtxIdxOffset = CtxIdxOffset{Prefix: 3}
		case "SP":
			fallthrough
		case "P":
			binarization.BinarizationType = BinarizationType{PrefixSuffix: true}
			binarization.MaxBinIdxCtx = MaxBinIdxCtx{IsPrefixSuffix: true, Prefix: 2, Suffix: 5}
			binarization.CtxIdxOffset = CtxIdxOffset{IsPrefixSuffix: true, Prefix: 14, Suffix: 17}
		}
	case "MbFieldDecodingFlag":
		binarization.BinarizationType = BinarizationType{
			FixedLength: true, CMax: true, CMaxValue: 1}
		binarization.MaxBinIdxCtx = MaxBinIdxCtx{Prefix: 0}
		binarization.CtxIdxOffset = CtxIdxOffset{Prefix: 70}
	case "PrevIntra4x4PredModeFlag":
		fallthrough
	case "PrevIntra8x8PredModeFlag":
		binarization.BinarizationType = BinarizationType{FixedLength: true, CMax: true, CMaxValue: 1}
		binarization.MaxBinIdxCtx = MaxBinIdxCtx{Prefix: 0}
		binarization.CtxIdxOffset = CtxIdxOffset{Prefix: 68}
	case "RefIdxL0":
		fallthrough
	case "RefIdxL1":
		binarization.BinarizationType = BinarizationType{Unary: true}
		binarization.MaxBinIdxCtx = MaxBinIdxCtx{Prefix: 2}
		binarization.CtxIdxOffset = CtxIdxOffset{Prefix: 54}
	case "RemIntra4x4PredMode":
		fallthrough
	case "RemIntra8x8PredMode":
		binarization.BinarizationType = BinarizationType{FixedLength: true, CMax: true, CMaxValue: 7}
		binarization.MaxBinIdxCtx = MaxBinIdxCtx{Prefix: 0}
		binarization.CtxIdxOffset = CtxIdxOffset{Prefix: 69}
	case "TransformSize8x8Flag":
		binarization.BinarizationType = BinarizationType{FixedLength: true, CMax: true, CMaxValue: 1}
		binarization.MaxBinIdxCtx = MaxBinIdxCtx{Prefix: 0}
		binarization.CtxIdxOffset = CtxIdxOffset{Prefix: 399}
	}
	return binarization
}
func (b *Binarization) IsBinStringMatch(bits []int) bool {
	for i, _b := range bits {
		if b.binString[i] != _b {
			return false
		}
	}
	return len(b.binString) == len(bits)
}

// 9.3.1.2: output is codIRange and codIOffset
func initDecodingEngine(bitReader *bits.BitReader) (int, int, error) {
	logger.Printf("debug: initializing arithmetic decoding engine\n")
	codIRange := 510
	codIOffset, err := bitReader.ReadBits(9)
	if err != nil {
		return 0, 0, errors.Wrap(err, "could not read codIOffset")
	}
	logger.Printf("debug: codIRange: %d :: codIOffsset: %d\n", codIRange, codIOffset)
	return codIRange, int(codIOffset), nil
}

// 9.3.3.2: output is value of the bin
func NewArithmeticDecoding(context *SliceContext, binarization *Binarization, ctxIdx, codIRange, codIOffset int) (ArithmeticDecoding, error) {
	a := ArithmeticDecoding{Context: context, Binarization: binarization}
	logger.Printf("debug: decoding bypass %d, for ctx %d\n", binarization.UseDecodeBypass, ctxIdx)
	// TODO: Implement
	if binarization.UseDecodeBypass == 1 {
		// TODO: 9.3.3.2.3 : DecodeBypass()
		var err error
		codIOffset, a.BinVal, err = a.DecodeBypass(context.Slice.Data, codIRange, codIOffset)
		if err != nil {
			return ArithmeticDecoding{}, errors.Wrap(err, "error from DecodeBypass getting codIOffset and BinVal")
		}

	} else if binarization.UseDecodeBypass == 0 && ctxIdx == 276 {
		// TODO: 9.3.3.2.4 : DecodeTerminate()
	} else {
		// TODO: 9.3.3.2.1 : DecodeDecision()
	}
	a.BinVal = -1
	return a, nil
}

// 9.3.3.2.3
// Invoked when bypassFlag is equal to 1
func (a ArithmeticDecoding) DecodeBypass(sliceData *SliceData, codIRange, codIOffset int) (int, int, error) {
	// Decoded value binVal
	codIOffset = codIOffset << uint(1)
	// TODO: Concurrency check
	// TODO: Possibly should be codIOffset | ReadOneBit
	shift, err := sliceData.BitReader.ReadBits(1)
	if err != nil {
		return 0, 0, errors.Wrap(err, "coult not read shift bit from sliceData.")
	}
	codIOffset = codIOffset << uint(shift)
	if codIOffset >= codIRange {
		a.BinVal = 1
		codIOffset -= codIRange
	} else {
		a.BinVal = 0
	}
	return codIOffset, a.BinVal, nil
}

// 9.3.3.2.4
// Decodes endOfSliceFlag and I_PCM
// Returns codIRange, codIOffSet, decoded value of binVal
func (a ArithmeticDecoding) DecodeTerminate(sliceData *SliceData, codIRange, codIOffset int) (int, int, int, error) {
	codIRange -= 2
	if codIOffset >= codIRange {
		a.BinVal = 1
		// Terminate CABAC decoding, last bit inserted into codIOffset is = 1
		// this is now also the rbspStopOneBit
		// TODO: How is this denoting termination?
		return codIRange, codIOffset, a.BinVal, nil
	}
	a.BinVal = 0
	var err error
	codIRange, codIOffset, err = a.RenormD(sliceData, codIRange, codIOffset)
	if err != nil {
		return 0, 0, 0, errors.Wrap(err, "error from RenormD")
	}
	return codIRange, codIOffset, a.BinVal, nil
}

// 9.3.3.2.2 Renormalization process of ADecEngine
// Returns codIRange, codIOffset
func (a ArithmeticDecoding) RenormD(sliceData *SliceData, codIRange, codIOffset int) (int, int, error) {
	if codIRange >= 256 {
		return codIRange, codIOffset, nil
	}
	codIRange = codIRange << uint(1)
	codIOffset = codIOffset << uint(1)
	bit, err := sliceData.BitReader.ReadBits(1)
	if err != nil {
		return 0, 0, errors.Wrap(err, "could not read bit from sliceData")
	}
	codIOffset = codIOffset | int(bit)
	return a.RenormD(sliceData, codIRange, codIOffset)
}

type ArithmeticDecoding struct {
	Context      *SliceContext
	Binarization *Binarization
	BinVal       int
}

// 9.3.3.2.1
// returns: binVal, updated codIRange, updated codIOffset
func (a ArithmeticDecoding) BinaryDecision(ctxIdx, codIRange, codIOffset int) (int, int, int, error) {
	var binVal int
	cabac := initCabac(a.Binarization, a.Context)
	// Derivce codIRangeLPS
	qCodIRangeIdx := (codIRange >> 6) & 3
	pStateIdx := cabac.PStateIdx
	codIRangeLPS, err := retCodIRangeLPS(pStateIdx, qCodIRangeIdx)
	if err != nil {
		return 0, 0, 0, errors.Wrap(err, "could not get codIRangeLPS from retCodIRangeLPS")
	}

	codIRange = codIRange - codIRangeLPS
	if codIOffset >= codIRange {
		binVal = 1 - cabac.ValMPS
		codIOffset -= codIRange
		codIRange = codIRangeLPS
	} else {
		binVal = cabac.ValMPS
	}

	// TODO: Do StateTransition and then RenormD happen here? See: 9.3.3.2.1
	return binVal, codIRange, codIOffset, nil
}

// 9.3.3.2.1.1
// Returns: pStateIdx, valMPS
func (c *CABAC) StateTransitionProcess(binVal int) {
	if binVal == c.ValMPS {
		c.PStateIdx = stateTransxTab[c.PStateIdx].TransIdxMPS
	} else {
		if c.PStateIdx == 0 {
			c.ValMPS = 1 - c.ValMPS
		}
		c.PStateIdx = stateTransxTab[c.PStateIdx].TransIdxLPS
	}
}

var ctxIdxLookup = map[int]map[int]int{
	3:  {0: NaCtxId, 1: 276, 2: 3, 3: 4, 4: NaCtxId, 5: NaCtxId},
	14: {0: 0, 1: 1, 2: NaCtxId},
	17: {0: 0, 1: 276, 2: 1, 3: 2, 4: NaCtxId},
	27: {0: NaCtxId, 1: 3, 2: NaCtxId},
	32: {0: 0, 1: 276, 2: 1, 3: 2, 4: NaCtxId},
	36: {2: NaCtxId, 3: 3, 4: 3, 5: 3},
	40: {0: NaCtxId},
	47: {0: NaCtxId, 1: 3, 2: 4, 3: 5},
	54: {0: NaCtxId, 1: 4},
	64: {0: NaCtxId, 1: 3, 2: 3},
	69: {0: 0, 1: 0, 2: 0},
	77: {0: NaCtxId, 1: NaCtxId},
}

// 9.3.3.1
// Returns ctxIdx
func CtxIdx(binIdx, maxBinIdxCtx, ctxIdxOffset int) int {
	ctxIdx := NaCtxId
	// table 9-39
	c, ok := ctxIdxLookup[ctxIdxOffset]
	if ok {
		v, ok := c[binIdx]
		if ok {
			return v
		}
	}

	switch ctxIdxOffset {
	case 0:
		if binIdx != 0 {
			return NaCtxId
		}
		// 9.3.3.1.1.3
	case 3:
		return 7
	case 11:
		if binIdx != 0 {
			return NaCtxId
		}

		// 9.3.3.1.1.3
	case 14:
		if binIdx > 2 {
			return NaCtxId
		}
	case 17:
		return 3
	case 21:
		if binIdx < 3 {
			ctxIdx = binIdx
		} else {
			return NaCtxId
		}
	case 24:
		// 9.3.3.1.1.1
	case 27:
		return 5
	case 32:
		return 3
	case 36:
		if binIdx == 0 || binIdx == 1 {
			ctxIdx = binIdx
		}
	case 40:
		fallthrough
	case 47:
		return 6
	case 54:
		if binIdx > 1 {
			ctxIdx = 5
		}
	case 60:
		if binIdx == 0 {
			// 9.3.3.1.1.5
		}
		if binIdx == 1 {
			ctxIdx = 2
		}
		if binIdx > 1 {
			ctxIdx = 3
		}
	case 64:
		return NaCtxId
	case 68:
		if binIdx != 0 {
			return NaCtxId
		}
		ctxIdx = 0
	case 69:
		return NaCtxId
	case 70:
		if binIdx != 0 {
			return NaCtxId
		}
		// 9.3.3.1.1.2
	case 77:
		return NaCtxId
	case 276:
		if binIdx != 0 {
			return NaCtxId
		}
		ctxIdx = 0
	case 399:
		if binIdx != 0 {
			return NaCtxId
		}
		// 9.3.3.1.1.10
	}

	return ctxIdx
}