vendor/github.com/klauspost/compress/zstd/framedec.go - voltha-go - Gitiles

 // Copyright 2019+ Klaus Post. All rights reserved.
 // License information can be found in the LICENSE file.
 // Based on work by Yann Collet, released under BSD License.

 package zstd

 import (
 	"bytes"
 	"encoding/hex"
 	"errors"
 	"hash"
 	"io"
 	"sync"

 	"github.com/klauspost/compress/zstd/internal/xxhash"
 )

 type frameDec struct {
 	o      decoderOptions
 	crc    hash.Hash64
 	offset int64

 	WindowSize uint64

 	// maxWindowSize is the maximum windows size to support.
 	// should never be bigger than max-int.
 	maxWindowSize uint64

 	// In order queue of blocks being decoded.
 	decoding chan *blockDec

 	// Frame history passed between blocks
 	history history

 	rawInput byteBuffer

 	// Byte buffer that can be reused for small input blocks.
 	bBuf byteBuf

 	FrameContentSize uint64
 	frameDone        sync.WaitGroup

 	DictionaryID  *uint32
 	HasCheckSum   bool
 	SingleSegment bool

 	// asyncRunning indicates whether the async routine processes input on 'decoding'.
 	asyncRunningMu sync.Mutex
 	asyncRunning   bool
 }

 const (
 	// The minimum Window_Size is 1 KB.
 	MinWindowSize = 1 << 10
 	MaxWindowSize = 1 << 29
 )

 var (
 	frameMagic          = []byte{0x28, 0xb5, 0x2f, 0xfd}
 	skippableFrameMagic = []byte{0x2a, 0x4d, 0x18}
 )

 func newFrameDec(o decoderOptions) *frameDec {
 	d := frameDec{
 		o:             o,
 		maxWindowSize: MaxWindowSize,
 	}
 	if d.maxWindowSize > o.maxDecodedSize {
 		d.maxWindowSize = o.maxDecodedSize
 	}
 	return &d
 }

 // reset will read the frame header and prepare for block decoding.
 // If nothing can be read from the input, io.EOF will be returned.
 // Any other error indicated that the stream contained data, but
 // there was a problem.
 func (d *frameDec) reset(br byteBuffer) error {
 	d.HasCheckSum = false
 	d.WindowSize = 0
 	var b []byte
 	for {
 		b = br.readSmall(4)
 		if b == nil {
 			return io.EOF
 		}
 		if !bytes.Equal(b[1:4], skippableFrameMagic) || b[0]&0xf0 != 0x50 {
 			if debug {
 				println("Not skippable", hex.EncodeToString(b), hex.EncodeToString(skippableFrameMagic))
 			}
 			// Break if not skippable frame.
 			break
 		}
 		// Read size to skip
 		b = br.readSmall(4)
 		if b == nil {
 			println("Reading Frame Size EOF")
 			return io.ErrUnexpectedEOF
 		}
 		n := uint32(b[0]) | (uint32(b[1]) << 8) | (uint32(b[2]) << 16) | (uint32(b[3]) << 24)
 		println("Skipping frame with", n, "bytes.")
 		err := br.skipN(int(n))
 		if err != nil {
 			if debug {
 				println("Reading discarded frame", err)
 			}
 			return err
 		}
 	}
 	if !bytes.Equal(b, frameMagic) {
 		println("Got magic numbers: ", b, "want:", frameMagic)
 		return ErrMagicMismatch
 	}

 	// Read Frame_Header_Descriptor
 	fhd, err := br.readByte()
 	if err != nil {
 		println("Reading Frame_Header_Descriptor", err)
 		return err
 	}
 	d.SingleSegment = fhd&(1<<5) != 0

 	if fhd&(1<<3) != 0 {
 		return errors.New("reserved bit set on frame header")
 	}

 	// Read Window_Descriptor
 	// https://github.com/facebook/zstd/blob/dev/doc/zstd_compression_format.md#window_descriptor
 	d.WindowSize = 0
 	if !d.SingleSegment {
 		wd, err := br.readByte()
 		if err != nil {
 			println("Reading Window_Descriptor", err)
 			return err
 		}
 		printf("raw: %x, mantissa: %d, exponent: %d\n", wd, wd&7, wd>>3)
 		windowLog := 10 + (wd >> 3)
 		windowBase := uint64(1) << windowLog
 		windowAdd := (windowBase / 8) * uint64(wd&0x7)
 		d.WindowSize = windowBase + windowAdd
 	}

 	// Read Dictionary_ID
 	// https://github.com/facebook/zstd/blob/dev/doc/zstd_compression_format.md#dictionary_id
 	d.DictionaryID = nil
 	if size := fhd & 3; size != 0 {
 		if size == 3 {
 			size = 4
 		}
 		b = br.readSmall(int(size))
 		if b == nil {
 			if debug {
 				println("Reading Dictionary_ID", io.ErrUnexpectedEOF)
 			}
 			return io.ErrUnexpectedEOF
 		}
 		var id uint32
 		switch size {
 		case 1:
 			id = uint32(b[0])
 		case 2:
 			id = uint32(b[0]) | (uint32(b[1]) << 8)
 		case 4:
 			id = uint32(b[0]) | (uint32(b[1]) << 8) | (uint32(b[2]) << 16) | (uint32(b[3]) << 24)
 		}
 		if debug {
 			println("Dict size", size, "ID:", id)
 		}
 		if id > 0 {
 			// ID 0 means "sorry, no dictionary anyway".
 			// https://github.com/facebook/zstd/blob/dev/doc/zstd_compression_format.md#dictionary-format
 			d.DictionaryID = &id
 		}
 	}

 	// Read Frame_Content_Size
 	// https://github.com/facebook/zstd/blob/dev/doc/zstd_compression_format.md#frame_content_size
 	var fcsSize int
 	v := fhd >> 6
 	switch v {
 	case 0:
 		if d.SingleSegment {
 			fcsSize = 1
 		}
 	default:
 		fcsSize = 1 << v
 	}
 	d.FrameContentSize = 0
 	if fcsSize > 0 {
 		b := br.readSmall(fcsSize)
 		if b == nil {
 			println("Reading Frame content", io.ErrUnexpectedEOF)
 			return io.ErrUnexpectedEOF
 		}
 		switch fcsSize {
 		case 1:
 			d.FrameContentSize = uint64(b[0])
 		case 2:
 			// When FCS_Field_Size is 2, the offset of 256 is added.
 			d.FrameContentSize = uint64(b[0]) | (uint64(b[1]) << 8) + 256
 		case 4:
 			d.FrameContentSize = uint64(b[0]) | (uint64(b[1]) << 8) | (uint64(b[2]) << 16) | (uint64(b[3]) << 24)
 		case 8:
 			d1 := uint32(b[0]) | (uint32(b[1]) << 8) | (uint32(b[2]) << 16) | (uint32(b[3]) << 24)
 			d2 := uint32(b[4]) | (uint32(b[5]) << 8) | (uint32(b[6]) << 16) | (uint32(b[7]) << 24)
 			d.FrameContentSize = uint64(d1) | (uint64(d2) << 32)
 		}
 		if debug {
 			println("field size bits:", v, "fcsSize:", fcsSize, "FrameContentSize:", d.FrameContentSize, hex.EncodeToString(b[:fcsSize]), "singleseg:", d.SingleSegment, "window:", d.WindowSize)
 		}
 	}
 	// Move this to shared.
 	d.HasCheckSum = fhd&(1<<2) != 0
 	if d.HasCheckSum {
 		if d.crc == nil {
 			d.crc = xxhash.New()
 		}
 		d.crc.Reset()
 	}

 	if d.WindowSize == 0 && d.SingleSegment {
 		// We may not need window in this case.
 		d.WindowSize = d.FrameContentSize
 		if d.WindowSize < MinWindowSize {
 			d.WindowSize = MinWindowSize
 		}
 	}

 	if d.WindowSize > d.maxWindowSize {
 		printf("window size %d > max %d\n", d.WindowSize, d.maxWindowSize)
 		return ErrWindowSizeExceeded
 	}
 	// The minimum Window_Size is 1 KB.
 	if d.WindowSize < MinWindowSize {
 		println("got window size: ", d.WindowSize)
 		return ErrWindowSizeTooSmall
 	}
 	d.history.windowSize = int(d.WindowSize)
 	if d.o.lowMem && d.history.windowSize < maxBlockSize {
 		d.history.maxSize = d.history.windowSize * 2
 	} else {
 		d.history.maxSize = d.history.windowSize + maxBlockSize
 	}
 	// history contains input - maybe we do something
 	d.rawInput = br
 	return nil
 }

 // next will start decoding the next block from stream.
 func (d *frameDec) next(block *blockDec) error {
 	if debug {
 		printf("decoding new block %p:%p", block, block.data)
 	}
 	err := block.reset(d.rawInput, d.WindowSize)
 	if err != nil {
 		println("block error:", err)
 		// Signal the frame decoder we have a problem.
 		d.sendErr(block, err)
 		return err
 	}
 	block.input <- struct{}{}
 	if debug {
 		println("next block:", block)
 	}
 	d.asyncRunningMu.Lock()
 	defer d.asyncRunningMu.Unlock()
 	if !d.asyncRunning {
 		return nil
 	}
 	if block.Last {
 		// We indicate the frame is done by sending io.EOF
 		d.decoding <- block
 		return io.EOF
 	}
 	d.decoding <- block
 	return nil
 }

 // sendEOF will queue an error block on the frame.
 // This will cause the frame decoder to return when it encounters the block.
 // Returns true if the decoder was added.
 func (d *frameDec) sendErr(block *blockDec, err error) bool {
 	d.asyncRunningMu.Lock()
 	defer d.asyncRunningMu.Unlock()
 	if !d.asyncRunning {
 		return false
 	}

 	println("sending error", err.Error())
 	block.sendErr(err)
 	d.decoding <- block
 	return true
 }

 // checkCRC will check the checksum if the frame has one.
 // Will return ErrCRCMismatch if crc check failed, otherwise nil.
 func (d *frameDec) checkCRC() error {
 	if !d.HasCheckSum {
 		return nil
 	}
 	var tmp [4]byte
 	got := d.crc.Sum64()
 	// Flip to match file order.
 	tmp[0] = byte(got >> 0)
 	tmp[1] = byte(got >> 8)
 	tmp[2] = byte(got >> 16)
 	tmp[3] = byte(got >> 24)

 	// We can overwrite upper tmp now
 	want := d.rawInput.readSmall(4)
 	if want == nil {
 		println("CRC missing?")
 		return io.ErrUnexpectedEOF
 	}

 	if !bytes.Equal(tmp[:], want) {
 		if debug {
 			println("CRC Check Failed:", tmp[:], "!=", want)
 		}
 		return ErrCRCMismatch
 	}
 	if debug {
 		println("CRC ok", tmp[:])
 	}
 	return nil
 }

 func (d *frameDec) initAsync() {
 	if !d.o.lowMem && !d.SingleSegment {
 		// set max extra size history to 10MB.
 		d.history.maxSize = d.history.windowSize + maxBlockSize*5
 	}
 	// re-alloc if more than one extra block size.
 	if d.o.lowMem && cap(d.history.b) > d.history.maxSize+maxBlockSize {
 		d.history.b = make([]byte, 0, d.history.maxSize)
 	}
 	if cap(d.history.b) < d.history.maxSize {
 		d.history.b = make([]byte, 0, d.history.maxSize)
 	}
 	if cap(d.decoding) < d.o.concurrent {
 		d.decoding = make(chan *blockDec, d.o.concurrent)
 	}
 	if debug {
 		h := d.history
 		printf("history init. len: %d, cap: %d", len(h.b), cap(h.b))
 	}
 	d.asyncRunningMu.Lock()
 	d.asyncRunning = true
 	d.asyncRunningMu.Unlock()
 }

 // startDecoder will start decoding blocks and write them to the writer.
 // The decoder will stop as soon as an error occurs or at end of frame.
 // When the frame has finished decoding the *bufio.Reader
 // containing the remaining input will be sent on frameDec.frameDone.
 func (d *frameDec) startDecoder(output chan decodeOutput) {
 	written := int64(0)

 	defer func() {
 		d.asyncRunningMu.Lock()
 		d.asyncRunning = false
 		d.asyncRunningMu.Unlock()

 		// Drain the currently decoding.
 		d.history.error = true
 	flushdone:
 		for {
 			select {
 			case b := <-d.decoding:
 				b.history <- &d.history
 				output <- <-b.result
 			default:
 				break flushdone
 			}
 		}
 		println("frame decoder done, signalling done")
 		d.frameDone.Done()
 	}()
 	// Get decoder for first block.
 	block := <-d.decoding
 	block.history <- &d.history
 	for {
 		var next *blockDec
 		// Get result
 		r := <-block.result
 		if r.err != nil {
 			println("Result contained error", r.err)
 			output <- r
 			return
 		}
 		if debug {
 			println("got result, from ", d.offset, "to", d.offset+int64(len(r.b)))
 			d.offset += int64(len(r.b))
 		}
 		if !block.Last {
 			// Send history to next block
 			select {
 			case next = <-d.decoding:
 				if debug {
 					println("Sending ", len(d.history.b), "bytes as history")
 				}
 				next.history <- &d.history
 			default:
 				// Wait until we have sent the block, so
 				// other decoders can potentially get the decoder.
 				next = nil
 			}
 		}

 		// Add checksum, async to decoding.
 		if d.HasCheckSum {
 			n, err := d.crc.Write(r.b)
 			if err != nil {
 				r.err = err
 				if n != len(r.b) {
 					r.err = io.ErrShortWrite
 				}
 				output <- r
 				return
 			}
 		}
 		written += int64(len(r.b))
 		if d.SingleSegment && uint64(written) > d.FrameContentSize {
 			println("runDecoder: single segment and", uint64(written), ">", d.FrameContentSize)
 			r.err = ErrFrameSizeExceeded
 			output <- r
 			return
 		}
 		if block.Last {
 			r.err = d.checkCRC()
 			output <- r
 			return
 		}
 		output <- r
 		if next == nil {
 			// There was no decoder available, we wait for one now that we have sent to the writer.
 			if debug {
 				println("Sending ", len(d.history.b), " bytes as history")
 			}
 			next = <-d.decoding
 			next.history <- &d.history
 		}
 		block = next
 	}
 }

 // runDecoder will create a sync decoder that will decode a block of data.
 func (d *frameDec) runDecoder(dst []byte, dec *blockDec) ([]byte, error) {
 	saved := d.history.b

 	// We use the history for output to avoid copying it.
 	d.history.b = dst
 	// Store input length, so we only check new data.
 	crcStart := len(dst)
 	var err error
 	for {
 		err = dec.reset(d.rawInput, d.WindowSize)
 		if err != nil {
 			break
 		}
 		if debug {
 			println("next block:", dec)
 		}
 		err = dec.decodeBuf(&d.history)
 		if err != nil || dec.Last {
 			break
 		}
 		if uint64(len(d.history.b)) > d.o.maxDecodedSize {
 			err = ErrDecoderSizeExceeded
 			break
 		}
 		if d.SingleSegment && uint64(len(d.history.b)) > d.o.maxDecodedSize {
 			println("runDecoder: single segment and", uint64(len(d.history.b)), ">", d.o.maxDecodedSize)
 			err = ErrFrameSizeExceeded
 			break
 		}
 	}
 	dst = d.history.b
 	if err == nil {
 		if d.HasCheckSum {
 			var n int
 			n, err = d.crc.Write(dst[crcStart:])
 			if err == nil {
 				if n != len(dst)-crcStart {
 					err = io.ErrShortWrite
 				} else {
 					err = d.checkCRC()
 				}
 			}
 		}
 	}
 	d.history.b = saved
 	return dst, err
 }
	// Copyright 2019+ Klaus Post. All rights reserved.
	// License information can be found in the LICENSE file.
	// Based on work by Yann Collet, released under BSD License.

	package zstd

	import (
	"bytes"
	"encoding/hex"
	"errors"
	"hash"
	"io"
	"sync"

	"github.com/klauspost/compress/zstd/internal/xxhash"
	)

	type frameDec struct {
	o decoderOptions
	crc hash.Hash64
	offset int64

	WindowSize uint64

	// maxWindowSize is the maximum windows size to support.
	// should never be bigger than max-int.
	maxWindowSize uint64

	// In order queue of blocks being decoded.
	decoding chan *blockDec

	// Frame history passed between blocks
	history history

	rawInput byteBuffer

	// Byte buffer that can be reused for small input blocks.
	bBuf byteBuf

	FrameContentSize uint64
	frameDone sync.WaitGroup

	DictionaryID *uint32
	HasCheckSum bool
	SingleSegment bool

	// asyncRunning indicates whether the async routine processes input on 'decoding'.
	asyncRunningMu sync.Mutex
	asyncRunning bool
	}

	const (
	// The minimum Window_Size is 1 KB.
	MinWindowSize = 1 << 10
	MaxWindowSize = 1 << 29
	)

	var (
	frameMagic = []byte{0x28, 0xb5, 0x2f, 0xfd}
	skippableFrameMagic = []byte{0x2a, 0x4d, 0x18}
	)

	func newFrameDec(o decoderOptions) *frameDec {
	d := frameDec{
	o: o,
	maxWindowSize: MaxWindowSize,
	}
	if d.maxWindowSize > o.maxDecodedSize {
	d.maxWindowSize = o.maxDecodedSize
	}
	return &d
	}

	// reset will read the frame header and prepare for block decoding.
	// If nothing can be read from the input, io.EOF will be returned.
	// Any other error indicated that the stream contained data, but
	// there was a problem.
	func (d *frameDec) reset(br byteBuffer) error {
	d.HasCheckSum = false
	d.WindowSize = 0
	var b []byte
	for {
	b = br.readSmall(4)
	if b == nil {
	return io.EOF
	}
	if !bytes.Equal(b[1:4], skippableFrameMagic) \|\| b[0]&0xf0 != 0x50 {
	if debug {
	println("Not skippable", hex.EncodeToString(b), hex.EncodeToString(skippableFrameMagic))
	}
	// Break if not skippable frame.
	break
	}
	// Read size to skip
	b = br.readSmall(4)
	if b == nil {
	println("Reading Frame Size EOF")
	return io.ErrUnexpectedEOF
	}
	n := uint32(b[0]) \| (uint32(b[1]) << 8) \| (uint32(b[2]) << 16) \| (uint32(b[3]) << 24)
	println("Skipping frame with", n, "bytes.")
	err := br.skipN(int(n))
	if err != nil {
	if debug {
	println("Reading discarded frame", err)
	}
	return err
	}
	}
	if !bytes.Equal(b, frameMagic) {
	println("Got magic numbers: ", b, "want:", frameMagic)
	return ErrMagicMismatch
	}

	// Read Frame_Header_Descriptor
	fhd, err := br.readByte()
	if err != nil {
	println("Reading Frame_Header_Descriptor", err)
	return err
	}
	d.SingleSegment = fhd&(1<<5) != 0

	if fhd&(1<<3) != 0 {
	return errors.New("reserved bit set on frame header")
	}

	// Read Window_Descriptor
	// https://github.com/facebook/zstd/blob/dev/doc/zstd_compression_format.md#window_descriptor
	d.WindowSize = 0
	if !d.SingleSegment {
	wd, err := br.readByte()
	if err != nil {
	println("Reading Window_Descriptor", err)
	return err
	}
	printf("raw: %x, mantissa: %d, exponent: %d\n", wd, wd&7, wd>>3)
	windowLog := 10 + (wd >> 3)
	windowBase := uint64(1) << windowLog
	windowAdd := (windowBase / 8) * uint64(wd&0x7)
	d.WindowSize = windowBase + windowAdd
	}

	// Read Dictionary_ID
	// https://github.com/facebook/zstd/blob/dev/doc/zstd_compression_format.md#dictionary_id
	d.DictionaryID = nil
	if size := fhd & 3; size != 0 {
	if size == 3 {
	size = 4
	}
	b = br.readSmall(int(size))
	if b == nil {
	if debug {
	println("Reading Dictionary_ID", io.ErrUnexpectedEOF)
	}
	return io.ErrUnexpectedEOF
	}
	var id uint32
	switch size {
	case 1:
	id = uint32(b[0])
	case 2:
	id = uint32(b[0]) \| (uint32(b[1]) << 8)
	case 4:
	id = uint32(b[0]) \| (uint32(b[1]) << 8) \| (uint32(b[2]) << 16) \| (uint32(b[3]) << 24)
	}
	if debug {
	println("Dict size", size, "ID:", id)
	}
	if id > 0 {
	// ID 0 means "sorry, no dictionary anyway".
	// https://github.com/facebook/zstd/blob/dev/doc/zstd_compression_format.md#dictionary-format
	d.DictionaryID = &id
	}
	}

	// Read Frame_Content_Size
	// https://github.com/facebook/zstd/blob/dev/doc/zstd_compression_format.md#frame_content_size
	var fcsSize int
	v := fhd >> 6
	switch v {
	case 0:
	if d.SingleSegment {
	fcsSize = 1
	}
	default:
	fcsSize = 1 << v
	}
	d.FrameContentSize = 0
	if fcsSize > 0 {
	b := br.readSmall(fcsSize)
	if b == nil {
	println("Reading Frame content", io.ErrUnexpectedEOF)
	return io.ErrUnexpectedEOF
	}
	switch fcsSize {
	case 1:
	d.FrameContentSize = uint64(b[0])
	case 2:
	// When FCS_Field_Size is 2, the offset of 256 is added.
	d.FrameContentSize = uint64(b[0]) \| (uint64(b[1]) << 8) + 256
	case 4:
	d.FrameContentSize = uint64(b[0]) \| (uint64(b[1]) << 8) \| (uint64(b[2]) << 16) \| (uint64(b[3]) << 24)
	case 8:
	d1 := uint32(b[0]) \| (uint32(b[1]) << 8) \| (uint32(b[2]) << 16) \| (uint32(b[3]) << 24)
	d2 := uint32(b[4]) \| (uint32(b[5]) << 8) \| (uint32(b[6]) << 16) \| (uint32(b[7]) << 24)
	d.FrameContentSize = uint64(d1) \| (uint64(d2) << 32)
	}
	if debug {
	println("field size bits:", v, "fcsSize:", fcsSize, "FrameContentSize:", d.FrameContentSize, hex.EncodeToString(b[:fcsSize]), "singleseg:", d.SingleSegment, "window:", d.WindowSize)
	}
	}
	// Move this to shared.
	d.HasCheckSum = fhd&(1<<2) != 0
	if d.HasCheckSum {
	if d.crc == nil {
	d.crc = xxhash.New()
	}
	d.crc.Reset()
	}

	if d.WindowSize == 0 && d.SingleSegment {
	// We may not need window in this case.
	d.WindowSize = d.FrameContentSize
	if d.WindowSize < MinWindowSize {
	d.WindowSize = MinWindowSize
	}
	}

	if d.WindowSize > d.maxWindowSize {
	printf("window size %d > max %d\n", d.WindowSize, d.maxWindowSize)
	return ErrWindowSizeExceeded
	}
	// The minimum Window_Size is 1 KB.
	if d.WindowSize < MinWindowSize {
	println("got window size: ", d.WindowSize)
	return ErrWindowSizeTooSmall
	}
	d.history.windowSize = int(d.WindowSize)
	if d.o.lowMem && d.history.windowSize < maxBlockSize {
	d.history.maxSize = d.history.windowSize * 2
	} else {
	d.history.maxSize = d.history.windowSize + maxBlockSize
	}
	// history contains input - maybe we do something
	d.rawInput = br
	return nil
	}

	// next will start decoding the next block from stream.
	func (d frameDec) next(block blockDec) error {
	if debug {
	printf("decoding new block %p:%p", block, block.data)
	}
	err := block.reset(d.rawInput, d.WindowSize)
	if err != nil {
	println("block error:", err)
	// Signal the frame decoder we have a problem.
	d.sendErr(block, err)
	return err
	}
	block.input <- struct{}{}
	if debug {
	println("next block:", block)
	}
	d.asyncRunningMu.Lock()
	defer d.asyncRunningMu.Unlock()
	if !d.asyncRunning {
	return nil
	}
	if block.Last {
	// We indicate the frame is done by sending io.EOF
	d.decoding <- block
	return io.EOF
	}
	d.decoding <- block
	return nil
	}

	// sendEOF will queue an error block on the frame.
	// This will cause the frame decoder to return when it encounters the block.
	// Returns true if the decoder was added.
	func (d frameDec) sendErr(block blockDec, err error) bool {
	d.asyncRunningMu.Lock()
	defer d.asyncRunningMu.Unlock()
	if !d.asyncRunning {
	return false
	}

	println("sending error", err.Error())
	block.sendErr(err)
	d.decoding <- block
	return true
	}

	// checkCRC will check the checksum if the frame has one.
	// Will return ErrCRCMismatch if crc check failed, otherwise nil.
	func (d *frameDec) checkCRC() error {
	if !d.HasCheckSum {
	return nil
	}
	var tmp [4]byte
	got := d.crc.Sum64()
	// Flip to match file order.
	tmp[0] = byte(got >> 0)
	tmp[1] = byte(got >> 8)
	tmp[2] = byte(got >> 16)
	tmp[3] = byte(got >> 24)

	// We can overwrite upper tmp now
	want := d.rawInput.readSmall(4)
	if want == nil {
	println("CRC missing?")
	return io.ErrUnexpectedEOF
	}

	if !bytes.Equal(tmp[:], want) {
	if debug {
	println("CRC Check Failed:", tmp[:], "!=", want)
	}
	return ErrCRCMismatch
	}
	if debug {
	println("CRC ok", tmp[:])
	}
	return nil
	}

	func (d *frameDec) initAsync() {
	if !d.o.lowMem && !d.SingleSegment {
	// set max extra size history to 10MB.
	d.history.maxSize = d.history.windowSize + maxBlockSize*5
	}
	// re-alloc if more than one extra block size.
	if d.o.lowMem && cap(d.history.b) > d.history.maxSize+maxBlockSize {
	d.history.b = make([]byte, 0, d.history.maxSize)
	}
	if cap(d.history.b) < d.history.maxSize {
	d.history.b = make([]byte, 0, d.history.maxSize)
	}
	if cap(d.decoding) < d.o.concurrent {
	d.decoding = make(chan *blockDec, d.o.concurrent)
	}
	if debug {
	h := d.history
	printf("history init. len: %d, cap: %d", len(h.b), cap(h.b))
	}
	d.asyncRunningMu.Lock()
	d.asyncRunning = true
	d.asyncRunningMu.Unlock()
	}

	// startDecoder will start decoding blocks and write them to the writer.
	// The decoder will stop as soon as an error occurs or at end of frame.
	// When the frame has finished decoding the *bufio.Reader
	// containing the remaining input will be sent on frameDec.frameDone.
	func (d *frameDec) startDecoder(output chan decodeOutput) {
	written := int64(0)

	defer func() {
	d.asyncRunningMu.Lock()
	d.asyncRunning = false
	d.asyncRunningMu.Unlock()

	// Drain the currently decoding.
	d.history.error = true
	flushdone:
	for {
	select {
	case b := <-d.decoding:
	b.history <- &d.history
	output <- <-b.result
	default:
	break flushdone
	}
	}
	println("frame decoder done, signalling done")
	d.frameDone.Done()
	}()
	// Get decoder for first block.
	block := <-d.decoding
	block.history <- &d.history
	for {
	var next *blockDec
	// Get result
	r := <-block.result
	if r.err != nil {
	println("Result contained error", r.err)
	output <- r
	return
	}
	if debug {
	println("got result, from ", d.offset, "to", d.offset+int64(len(r.b)))
	d.offset += int64(len(r.b))
	}
	if !block.Last {
	// Send history to next block
	select {
	case next = <-d.decoding:
	if debug {
	println("Sending ", len(d.history.b), "bytes as history")
	}
	next.history <- &d.history
	default:
	// Wait until we have sent the block, so
	// other decoders can potentially get the decoder.
	next = nil
	}
	}

	// Add checksum, async to decoding.
	if d.HasCheckSum {
	n, err := d.crc.Write(r.b)
	if err != nil {
	r.err = err
	if n != len(r.b) {
	r.err = io.ErrShortWrite
	}
	output <- r
	return
	}
	}
	written += int64(len(r.b))
	if d.SingleSegment && uint64(written) > d.FrameContentSize {
	println("runDecoder: single segment and", uint64(written), ">", d.FrameContentSize)
	r.err = ErrFrameSizeExceeded
	output <- r
	return
	}
	if block.Last {
	r.err = d.checkCRC()
	output <- r
	return
	}
	output <- r
	if next == nil {
	// There was no decoder available, we wait for one now that we have sent to the writer.
	if debug {
	println("Sending ", len(d.history.b), " bytes as history")
	}
	next = <-d.decoding
	next.history <- &d.history
	}
	block = next
	}
	}

	// runDecoder will create a sync decoder that will decode a block of data.
	func (d frameDec) runDecoder(dst []byte, dec blockDec) ([]byte, error) {
	saved := d.history.b

	// We use the history for output to avoid copying it.
	d.history.b = dst
	// Store input length, so we only check new data.
	crcStart := len(dst)
	var err error
	for {
	err = dec.reset(d.rawInput, d.WindowSize)
	if err != nil {
	break
	}
	if debug {
	println("next block:", dec)
	}
	err = dec.decodeBuf(&d.history)
	if err != nil \|\| dec.Last {
	break
	}
	if uint64(len(d.history.b)) > d.o.maxDecodedSize {
	err = ErrDecoderSizeExceeded
	break
	}
	if d.SingleSegment && uint64(len(d.history.b)) > d.o.maxDecodedSize {
	println("runDecoder: single segment and", uint64(len(d.history.b)), ">", d.o.maxDecodedSize)
	err = ErrFrameSizeExceeded
	break
	}
	}
	dst = d.history.b
	if err == nil {
	if d.HasCheckSum {
	var n int
	n, err = d.crc.Write(dst[crcStart:])
	if err == nil {
	if n != len(dst)-crcStart {
	err = io.ErrShortWrite
	} else {
	err = d.checkCRC()
	}
	}
	}
	}
	d.history.b = saved
	return dst, err
	}