src/compress/flate/inflate.go - go - Git at Google

 // Copyright 2009 The Go Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.

 // Package flate implements the DEFLATE compressed data format, described in
 // RFC 1951.  The gzip and zlib packages implement access to DEFLATE-based file
 // formats.
 package flate

 import (
 	"bufio"
 	"io"
 	"math/bits"
 	"strconv"
 	"sync"
 )

 const (
 	maxCodeLen = 16 // max length of Huffman code
 	// The next three numbers come from the RFC section 3.2.7, with the
 	// additional proviso in section 3.2.5 which implies that distance codes
 	// 30 and 31 should never occur in compressed data.
 	maxNumLit  = 286
 	maxNumDist = 30
 	numCodes   = 19 // number of codes in Huffman meta-code
 )

 // Initialize the fixedHuffmanDecoder only once upon first use.
 var fixedOnce sync.Once
 var fixedHuffmanDecoder huffmanDecoder

 // A CorruptInputError reports the presence of corrupt input at a given offset.
 type CorruptInputError int64

 func (e CorruptInputError) Error() string {
 	return "flate: corrupt input before offset " + strconv.FormatInt(int64(e), 10)
 }

 // An InternalError reports an error in the flate code itself.
 type InternalError string

 func (e InternalError) Error() string { return "flate: internal error: " + string(e) }

 // A ReadError reports an error encountered while reading input.
 //
 // Deprecated: No longer returned.
 type ReadError struct {
 	Offset int64 // byte offset where error occurred
 	Err    error // error returned by underlying Read
 }

 func (e *ReadError) Error() string {
 	return "flate: read error at offset " + strconv.FormatInt(e.Offset, 10) + ": " + e.Err.Error()
 }

 // A WriteError reports an error encountered while writing output.
 //
 // Deprecated: No longer returned.
 type WriteError struct {
 	Offset int64 // byte offset where error occurred
 	Err    error // error returned by underlying Write
 }

 func (e *WriteError) Error() string {
 	return "flate: write error at offset " + strconv.FormatInt(e.Offset, 10) + ": " + e.Err.Error()
 }

 // Resetter resets a ReadCloser returned by NewReader or NewReaderDict
 // to switch to a new underlying Reader. This permits reusing a ReadCloser
 // instead of allocating a new one.
 type Resetter interface {
 	// Reset discards any buffered data and resets the Resetter as if it was
 	// newly initialized with the given reader.
 	Reset(r io.Reader, dict []byte) error
 }

 // The data structure for decoding Huffman tables is based on that of
 // zlib. There is a lookup table of a fixed bit width (huffmanChunkBits),
 // For codes smaller than the table width, there are multiple entries
 // (each combination of trailing bits has the same value). For codes
 // larger than the table width, the table contains a link to an overflow
 // table. The width of each entry in the link table is the maximum code
 // size minus the chunk width.
 //
 // Note that you can do a lookup in the table even without all bits
 // filled. Since the extra bits are zero, and the DEFLATE Huffman codes
 // have the property that shorter codes come before longer ones, the
 // bit length estimate in the result is a lower bound on the actual
 // number of bits.
 //
 // See the following:
 //	https://github.com/madler/zlib/raw/master/doc/algorithm.txt

 // chunk & 15 is number of bits
 // chunk >> 4 is value, including table link

 const (
 	huffmanChunkBits  = 9
 	huffmanNumChunks  = 1 << huffmanChunkBits
 	huffmanCountMask  = 15
 	huffmanValueShift = 4
 )

 type huffmanDecoder struct {
 	min      int                      // the minimum code length
 	chunks   [huffmanNumChunks]uint32 // chunks as described above
 	links    [][]uint32               // overflow links
 	linkMask uint32                   // mask the width of the link table
 }

 // Initialize Huffman decoding tables from array of code lengths.
 // Following this function, h is guaranteed to be initialized into a complete
 // tree (i.e., neither over-subscribed nor under-subscribed). The exception is a
 // degenerate case where the tree has only a single symbol with length 1. Empty
 // trees are permitted.
 func (h *huffmanDecoder) init(lengths []int) bool {
 	// Sanity enables additional runtime tests during Huffman
 	// table construction. It's intended to be used during
 	// development to supplement the currently ad-hoc unit tests.
 	const sanity = false

 	if h.min != 0 {
 		*h = huffmanDecoder{}
 	}

 	// Count number of codes of each length,
 	// compute min and max length.
 	var count [maxCodeLen]int
 	var min, max int
 	for _, n := range lengths {
 		if n == 0 {
 			continue
 		}
 		if min == 0 || n < min {
 			min = n
 		}
 		if n > max {
 			max = n
 		}
 		count[n]++
 	}

 	// Empty tree. The decompressor.huffSym function will fail later if the tree
 	// is used. Technically, an empty tree is only valid for the HDIST tree and
 	// not the HCLEN and HLIT tree. However, a stream with an empty HCLEN tree
 	// is guaranteed to fail since it will attempt to use the tree to decode the
 	// codes for the HLIT and HDIST trees. Similarly, an empty HLIT tree is
 	// guaranteed to fail later since the compressed data section must be
 	// composed of at least one symbol (the end-of-block marker).
 	if max == 0 {
 		return true
 	}

 	code := 0
 	var nextcode [maxCodeLen]int
 	for i := min; i <= max; i++ {
 		code <<= 1
 		nextcode[i] = code
 		code += count[i]
 	}

 	// Check that the coding is complete (i.e., that we've
 	// assigned all 2-to-the-max possible bit sequences).
 	// Exception: To be compatible with zlib, we also need to
 	// accept degenerate single-code codings. See also
 	// TestDegenerateHuffmanCoding.
 	if code != 1<<uint(max) && !(code == 1 && max == 1) {
 		return false
 	}

 	h.min = min
 	if max > huffmanChunkBits {
 		numLinks := 1 << (uint(max) - huffmanChunkBits)
 		h.linkMask = uint32(numLinks - 1)

 		// create link tables
 		link := nextcode[huffmanChunkBits+1] >> 1
 		h.links = make([][]uint32, huffmanNumChunks-link)
 		for j := uint(link); j < huffmanNumChunks; j++ {
 			reverse := int(bits.Reverse16(uint16(j)))
 			reverse >>= uint(16 - huffmanChunkBits)
 			off := j - uint(link)
 			if sanity && h.chunks[reverse] != 0 {
 				panic("impossible: overwriting existing chunk")
 			}
 			h.chunks[reverse] = uint32(off<<huffmanValueShift | (huffmanChunkBits + 1))
 			h.links[off] = make([]uint32, numLinks)
 		}
 	}

 	for i, n := range lengths {
 		if n == 0 {
 			continue
 		}
 		code := nextcode[n]
 		nextcode[n]++
 		chunk := uint32(i<<huffmanValueShift | n)
 		reverse := int(bits.Reverse16(uint16(code)))
 		reverse >>= uint(16 - n)
 		if n <= huffmanChunkBits {
 			for off := reverse; off < len(h.chunks); off += 1 << uint(n) {
 				// We should never need to overwrite
 				// an existing chunk. Also, 0 is
 				// never a valid chunk, because the
 				// lower 4 "count" bits should be
 				// between 1 and 15.
 				if sanity && h.chunks[off] != 0 {
 					panic("impossible: overwriting existing chunk")
 				}
 				h.chunks[off] = chunk
 			}
 		} else {
 			j := reverse & (huffmanNumChunks - 1)
 			if sanity && h.chunks[j]&huffmanCountMask != huffmanChunkBits+1 {
 				// Longer codes should have been
 				// associated with a link table above.
 				panic("impossible: not an indirect chunk")
 			}
 			value := h.chunks[j] >> huffmanValueShift
 			linktab := h.links[value]
 			reverse >>= huffmanChunkBits
 			for off := reverse; off < len(linktab); off += 1 << uint(n-huffmanChunkBits) {
 				if sanity && linktab[off] != 0 {
 					panic("impossible: overwriting existing chunk")
 				}
 				linktab[off] = chunk
 			}
 		}
 	}

 	if sanity {
 		// Above we've sanity checked that we never overwrote
 		// an existing entry. Here we additionally check that
 		// we filled the tables completely.
 		for i, chunk := range h.chunks {
 			if chunk == 0 {
 				// As an exception, in the degenerate
 				// single-code case, we allow odd
 				// chunks to be missing.
 				if code == 1 && i%2 == 1 {
 					continue
 				}
 				panic("impossible: missing chunk")
 			}
 		}
 		for _, linktab := range h.links {
 			for _, chunk := range linktab {
 				if chunk == 0 {
 					panic("impossible: missing chunk")
 				}
 			}
 		}
 	}

 	return true
 }

 // The actual read interface needed by NewReader.
 // If the passed in io.Reader does not also have ReadByte,
 // the NewReader will introduce its own buffering.
 type Reader interface {
 	io.Reader
 	io.ByteReader
 }

 // Decompress state.
 type decompressor struct {
 	// Input source.
 	r       Reader
 	roffset int64

 	// Input bits, in top of b.
 	b  uint32
 	nb uint

 	// Huffman decoders for literal/length, distance.
 	h1, h2 huffmanDecoder

 	// Length arrays used to define Huffman codes.
 	bits     *[maxNumLit + maxNumDist]int
 	codebits *[numCodes]int

 	// Output history, buffer.
 	dict dictDecoder

 	// Temporary buffer (avoids repeated allocation).
 	buf [4]byte

 	// Next step in the decompression,
 	// and decompression state.
 	step      func(*decompressor)
 	stepState int
 	final     bool
 	err       error
 	toRead    []byte
 	hl, hd    *huffmanDecoder
 	copyLen   int
 	copyDist  int
 }

 func (f *decompressor) nextBlock() {
 	for f.nb < 1+2 {
 		if f.err = f.moreBits(); f.err != nil {
 			return
 		}
 	}
 	f.final = f.b&1 == 1
 	f.b >>= 1
 	typ := f.b & 3
 	f.b >>= 2
 	f.nb -= 1 + 2
 	switch typ {
 	case 0:
 		f.dataBlock()
 	case 1:
 		// compressed, fixed Huffman tables
 		f.hl = &fixedHuffmanDecoder
 		f.hd = nil
 		f.huffmanBlock()
 	case 2:
 		// compressed, dynamic Huffman tables
 		if f.err = f.readHuffman(); f.err != nil {
 			break
 		}
 		f.hl = &f.h1
 		f.hd = &f.h2
 		f.huffmanBlock()
 	default:
 		// 3 is reserved.
 		f.err = CorruptInputError(f.roffset)
 	}
 }

 func (f *decompressor) Read(b []byte) (int, error) {
 	for {
 		if len(f.toRead) > 0 {
 			n := copy(b, f.toRead)
 			f.toRead = f.toRead[n:]
 			if len(f.toRead) == 0 {
 				return n, f.err
 			}
 			return n, nil
 		}
 		if f.err != nil {
 			return 0, f.err
 		}
 		f.step(f)
 		if f.err != nil && len(f.toRead) == 0 {
 			f.toRead = f.dict.readFlush() // Flush what's left in case of error
 		}
 	}
 }

 func (f *decompressor) Close() error {
 	if f.err == io.EOF {
 		return nil
 	}
 	return f.err
 }

 // RFC 1951 section 3.2.7.
 // Compression with dynamic Huffman codes

 var codeOrder = [...]int{16, 17, 18, 0, 8, 7, 9, 6, 10, 5, 11, 4, 12, 3, 13, 2, 14, 1, 15}

 func (f *decompressor) readHuffman() error {
 	// HLIT[5], HDIST[5], HCLEN[4].
 	for f.nb < 5+5+4 {
 		if err := f.moreBits(); err != nil {
 			return err
 		}
 	}
 	nlit := int(f.b&0x1F) + 257
 	if nlit > maxNumLit {
 		return CorruptInputError(f.roffset)
 	}
 	f.b >>= 5
 	ndist := int(f.b&0x1F) + 1
 	if ndist > maxNumDist {
 		return CorruptInputError(f.roffset)
 	}
 	f.b >>= 5
 	nclen := int(f.b&0xF) + 4
 	// numCodes is 19, so nclen is always valid.
 	f.b >>= 4
 	f.nb -= 5 + 5 + 4

 	// (HCLEN+4)*3 bits: code lengths in the magic codeOrder order.
 	for i := 0; i < nclen; i++ {
 		for f.nb < 3 {
 			if err := f.moreBits(); err != nil {
 				return err
 			}
 		}
 		f.codebits[codeOrder[i]] = int(f.b & 0x7)
 		f.b >>= 3
 		f.nb -= 3
 	}
 	for i := nclen; i < len(codeOrder); i++ {
 		f.codebits[codeOrder[i]] = 0
 	}
 	if !f.h1.init(f.codebits[0:]) {
 		return CorruptInputError(f.roffset)
 	}

 	// HLIT + 257 code lengths, HDIST + 1 code lengths,
 	// using the code length Huffman code.
 	for i, n := 0, nlit+ndist; i < n; {
 		x, err := f.huffSym(&f.h1)
 		if err != nil {
 			return err
 		}
 		if x < 16 {
 			// Actual length.
 			f.bits[i] = x
 			i++
 			continue
 		}
 		// Repeat previous length or zero.
 		var rep int
 		var nb uint
 		var b int
 		switch x {
 		default:
 			return InternalError("unexpected length code")
 		case 16:
 			rep = 3
 			nb = 2
 			if i == 0 {
 				return CorruptInputError(f.roffset)
 			}
 			b = f.bits[i-1]
 		case 17:
 			rep = 3
 			nb = 3
 			b = 0
 		case 18:
 			rep = 11
 			nb = 7
 			b = 0
 		}
 		for f.nb < nb {
 			if err := f.moreBits(); err != nil {
 				return err
 			}
 		}
 		rep += int(f.b & uint32(1<<nb-1))
 		f.b >>= nb
 		f.nb -= nb
 		if i+rep > n {
 			return CorruptInputError(f.roffset)
 		}
 		for j := 0; j < rep; j++ {
 			f.bits[i] = b
 			i++
 		}
 	}

 	if !f.h1.init(f.bits[0:nlit]) || !f.h2.init(f.bits[nlit:nlit+ndist]) {
 		return CorruptInputError(f.roffset)
 	}

 	// As an optimization, we can initialize the min bits to read at a time
 	// for the HLIT tree to the length of the EOB marker since we know that
 	// every block must terminate with one. This preserves the property that
 	// we never read any extra bytes after the end of the DEFLATE stream.
 	if f.h1.min < f.bits[endBlockMarker] {
 		f.h1.min = f.bits[endBlockMarker]
 	}

 	return nil
 }

 // Decode a single Huffman block from f.
 // hl and hd are the Huffman states for the lit/length values
 // and the distance values, respectively. If hd == nil, using the
 // fixed distance encoding associated with fixed Huffman blocks.
 func (f *decompressor) huffmanBlock() {
 	const (
 		stateInit = iota // Zero value must be stateInit
 		stateDict
 	)

 	switch f.stepState {
 	case stateInit:
 		goto readLiteral
 	case stateDict:
 		goto copyHistory
 	}

 readLiteral:
 	// Read literal and/or (length, distance) according to RFC section 3.2.3.
 	{
 		v, err := f.huffSym(f.hl)
 		if err != nil {
 			f.err = err
 			return
 		}
 		var n uint // number of bits extra
 		var length int
 		switch {
 		case v < 256:
 			f.dict.writeByte(byte(v))
 			if f.dict.availWrite() == 0 {
 				f.toRead = f.dict.readFlush()
 				f.step = (*decompressor).huffmanBlock
 				f.stepState = stateInit
 				return
 			}
 			goto readLiteral
 		case v == 256:
 			f.finishBlock()
 			return
 		// otherwise, reference to older data
 		case v < 265:
 			length = v - (257 - 3)
 			n = 0
 		case v < 269:
 			length = v*2 - (265*2 - 11)
 			n = 1
 		case v < 273:
 			length = v*4 - (269*4 - 19)
 			n = 2
 		case v < 277:
 			length = v*8 - (273*8 - 35)
 			n = 3
 		case v < 281:
 			length = v*16 - (277*16 - 67)
 			n = 4
 		case v < 285:
 			length = v*32 - (281*32 - 131)
 			n = 5
 		case v < maxNumLit:
 			length = 258
 			n = 0
 		default:
 			f.err = CorruptInputError(f.roffset)
 			return
 		}
 		if n > 0 {
 			for f.nb < n {
 				if err = f.moreBits(); err != nil {
 					f.err = err
 					return
 				}
 			}
 			length += int(f.b & uint32(1<<n-1))
 			f.b >>= n
 			f.nb -= n
 		}

 		var dist int
 		if f.hd == nil {
 			for f.nb < 5 {
 				if err = f.moreBits(); err != nil {
 					f.err = err
 					return
 				}
 			}
 			dist = int(bits.Reverse8(uint8(f.b & 0x1F << 3)))
 			f.b >>= 5
 			f.nb -= 5
 		} else {
 			if dist, err = f.huffSym(f.hd); err != nil {
 				f.err = err
 				return
 			}
 		}

 		switch {
 		case dist < 4:
 			dist++
 		case dist < maxNumDist:
 			nb := uint(dist-2) >> 1
 			// have 1 bit in bottom of dist, need nb more.
 			extra := (dist & 1) << nb
 			for f.nb < nb {
 				if err = f.moreBits(); err != nil {
 					f.err = err
 					return
 				}
 			}
 			extra |= int(f.b & uint32(1<<nb-1))
 			f.b >>= nb
 			f.nb -= nb
 			dist = 1<<(nb+1) + 1 + extra
 		default:
 			f.err = CorruptInputError(f.roffset)
 			return
 		}

 		// No check on length; encoding can be prescient.
 		if dist > f.dict.histSize() {
 			f.err = CorruptInputError(f.roffset)
 			return
 		}

 		f.copyLen, f.copyDist = length, dist
 		goto copyHistory
 	}

 copyHistory:
 	// Perform a backwards copy according to RFC section 3.2.3.
 	{
 		cnt := f.dict.tryWriteCopy(f.copyDist, f.copyLen)
 		if cnt == 0 {
 			cnt = f.dict.writeCopy(f.copyDist, f.copyLen)
 		}
 		f.copyLen -= cnt

 		if f.dict.availWrite() == 0 || f.copyLen > 0 {
 			f.toRead = f.dict.readFlush()
 			f.step = (*decompressor).huffmanBlock // We need to continue this work
 			f.stepState = stateDict
 			return
 		}
 		goto readLiteral
 	}
 }

 // Copy a single uncompressed data block from input to output.
 func (f *decompressor) dataBlock() {
 	// Uncompressed.
 	// Discard current half-byte.
 	f.nb = 0
 	f.b = 0

 	// Length then ones-complement of length.
 	nr, err := io.ReadFull(f.r, f.buf[0:4])
 	f.roffset += int64(nr)
 	if err != nil {
 		f.err = noEOF(err)
 		return
 	}
 	n := int(f.buf[0]) | int(f.buf[1])<<8
 	nn := int(f.buf[2]) | int(f.buf[3])<<8
 	if uint16(nn) != uint16(^n) {
 		f.err = CorruptInputError(f.roffset)
 		return
 	}

 	if n == 0 {
 		f.toRead = f.dict.readFlush()
 		f.finishBlock()
 		return
 	}

 	f.copyLen = n
 	f.copyData()
 }

 // copyData copies f.copyLen bytes from the underlying reader into f.hist.
 // It pauses for reads when f.hist is full.
 func (f *decompressor) copyData() {
 	buf := f.dict.writeSlice()
 	if len(buf) > f.copyLen {
 		buf = buf[:f.copyLen]
 	}

 	cnt, err := io.ReadFull(f.r, buf)
 	f.roffset += int64(cnt)
 	f.copyLen -= cnt
 	f.dict.writeMark(cnt)
 	if err != nil {
 		f.err = noEOF(err)
 		return
 	}

 	if f.dict.availWrite() == 0 || f.copyLen > 0 {
 		f.toRead = f.dict.readFlush()
 		f.step = (*decompressor).copyData
 		return
 	}
 	f.finishBlock()
 }

 func (f *decompressor) finishBlock() {
 	if f.final {
 		if f.dict.availRead() > 0 {
 			f.toRead = f.dict.readFlush()
 		}
 		f.err = io.EOF
 	}
 	f.step = (*decompressor).nextBlock
 }

 // noEOF returns err, unless err == io.EOF, in which case it returns io.ErrUnexpectedEOF.
 func noEOF(e error) error {
 	if e == io.EOF {
 		return io.ErrUnexpectedEOF
 	}
 	return e
 }

 func (f *decompressor) moreBits() error {
 	c, err := f.r.ReadByte()
 	if err != nil {
 		return noEOF(err)
 	}
 	f.roffset++
 	f.b |= uint32(c) << f.nb
 	f.nb += 8
 	return nil
 }

 // Read the next Huffman-encoded symbol from f according to h.
 func (f *decompressor) huffSym(h *huffmanDecoder) (int, error) {
 	// Since a huffmanDecoder can be empty or be composed of a degenerate tree
 	// with single element, huffSym must error on these two edge cases. In both
 	// cases, the chunks slice will be 0 for the invalid sequence, leading it
 	// satisfy the n == 0 check below.
 	n := uint(h.min)
 	// Optimization. Compiler isn't smart enough to keep f.b,f.nb in registers,
 	// but is smart enough to keep local variables in registers, so use nb and b,
 	// inline call to moreBits and reassign b,nb back to f on return.
 	nb, b := f.nb, f.b
 	for {
 		for nb < n {
 			c, err := f.r.ReadByte()
 			if err != nil {
 				f.b = b
 				f.nb = nb
 				return 0, noEOF(err)
 			}
 			f.roffset++
 			b |= uint32(c) << (nb & 31)
 			nb += 8
 		}
 		chunk := h.chunks[b&(huffmanNumChunks-1)]
 		n = uint(chunk & huffmanCountMask)
 		if n > huffmanChunkBits {
 			chunk = h.links[chunk>>huffmanValueShift][(b>>huffmanChunkBits)&h.linkMask]
 			n = uint(chunk & huffmanCountMask)
 		}
 		if n <= nb {
 			if n == 0 {
 				f.b = b
 				f.nb = nb
 				f.err = CorruptInputError(f.roffset)
 				return 0, f.err
 			}
 			f.b = b >> (n & 31)
 			f.nb = nb - n
 			return int(chunk >> huffmanValueShift), nil
 		}
 	}
 }

 func makeReader(r io.Reader) Reader {
 	if rr, ok := r.(Reader); ok {
 		return rr
 	}
 	return bufio.NewReader(r)
 }

 func fixedHuffmanDecoderInit() {
 	fixedOnce.Do(func() {
 		// These come from the RFC section 3.2.6.
 		var bits [288]int
 		for i := 0; i < 144; i++ {
 			bits[i] = 8
 		}
 		for i := 144; i < 256; i++ {
 			bits[i] = 9
 		}
 		for i := 256; i < 280; i++ {
 			bits[i] = 7
 		}
 		for i := 280; i < 288; i++ {
 			bits[i] = 8
 		}
 		fixedHuffmanDecoder.init(bits[:])
 	})
 }

 func (f *decompressor) Reset(r io.Reader, dict []byte) error {
 	*f = decompressor{
 		r:        makeReader(r),
 		bits:     f.bits,
 		codebits: f.codebits,
 		dict:     f.dict,
 		step:     (*decompressor).nextBlock,
 	}
 	f.dict.init(maxMatchOffset, dict)
 	return nil
 }

 // NewReader returns a new ReadCloser that can be used
 // to read the uncompressed version of r.
 // If r does not also implement io.ByteReader,
 // the decompressor may read more data than necessary from r.
 // It is the caller's responsibility to call Close on the ReadCloser
 // when finished reading.
 //
 // The ReadCloser returned by NewReader also implements Resetter.
 func NewReader(r io.Reader) io.ReadCloser {
 	fixedHuffmanDecoderInit()

 	var f decompressor
 	f.r = makeReader(r)
 	f.bits = new([maxNumLit + maxNumDist]int)
 	f.codebits = new([numCodes]int)
 	f.step = (*decompressor).nextBlock
 	f.dict.init(maxMatchOffset, nil)
 	return &f
 }

 // NewReaderDict is like NewReader but initializes the reader
 // with a preset dictionary. The returned Reader behaves as if
 // the uncompressed data stream started with the given dictionary,
 // which has already been read. NewReaderDict is typically used
 // to read data compressed by NewWriterDict.
 //
 // The ReadCloser returned by NewReader also implements Resetter.
 func NewReaderDict(r io.Reader, dict []byte) io.ReadCloser {
 	fixedHuffmanDecoderInit()

 	var f decompressor
 	f.r = makeReader(r)
 	f.bits = new([maxNumLit + maxNumDist]int)
 	f.codebits = new([numCodes]int)
 	f.step = (*decompressor).nextBlock
 	f.dict.init(maxMatchOffset, dict)
 	return &f
 }
	// Copyright 2009 The Go Authors. All rights reserved.
	// Use of this source code is governed by a BSD-style
	// license that can be found in the LICENSE file.

	// Package flate implements the DEFLATE compressed data format, described in
	// RFC 1951. The gzip and zlib packages implement access to DEFLATE-based file
	// formats.
	package flate

	import (
	"bufio"
	"io"
	"math/bits"
	"strconv"
	"sync"
	)

	const (
	maxCodeLen = 16 // max length of Huffman code
	// The next three numbers come from the RFC section 3.2.7, with the
	// additional proviso in section 3.2.5 which implies that distance codes
	// 30 and 31 should never occur in compressed data.
	maxNumLit = 286
	maxNumDist = 30
	numCodes = 19 // number of codes in Huffman meta-code
	)

	// Initialize the fixedHuffmanDecoder only once upon first use.
	var fixedOnce sync.Once
	var fixedHuffmanDecoder huffmanDecoder

	// A CorruptInputError reports the presence of corrupt input at a given offset.
	type CorruptInputError int64

	func (e CorruptInputError) Error() string {
	return "flate: corrupt input before offset " + strconv.FormatInt(int64(e), 10)
	}

	// An InternalError reports an error in the flate code itself.
	type InternalError string

	func (e InternalError) Error() string { return "flate: internal error: " + string(e) }

	// A ReadError reports an error encountered while reading input.
	//
	// Deprecated: No longer returned.
	type ReadError struct {
	Offset int64 // byte offset where error occurred
	Err error // error returned by underlying Read
	}

	func (e *ReadError) Error() string {
	return "flate: read error at offset " + strconv.FormatInt(e.Offset, 10) + ": " + e.Err.Error()
	}

	// A WriteError reports an error encountered while writing output.
	//
	// Deprecated: No longer returned.
	type WriteError struct {
	Offset int64 // byte offset where error occurred
	Err error // error returned by underlying Write
	}

	func (e *WriteError) Error() string {
	return "flate: write error at offset " + strconv.FormatInt(e.Offset, 10) + ": " + e.Err.Error()
	}

	// Resetter resets a ReadCloser returned by NewReader or NewReaderDict
	// to switch to a new underlying Reader. This permits reusing a ReadCloser
	// instead of allocating a new one.
	type Resetter interface {
	// Reset discards any buffered data and resets the Resetter as if it was
	// newly initialized with the given reader.
	Reset(r io.Reader, dict []byte) error
	}

	// The data structure for decoding Huffman tables is based on that of
	// zlib. There is a lookup table of a fixed bit width (huffmanChunkBits),
	// For codes smaller than the table width, there are multiple entries
	// (each combination of trailing bits has the same value). For codes
	// larger than the table width, the table contains a link to an overflow
	// table. The width of each entry in the link table is the maximum code
	// size minus the chunk width.
	//
	// Note that you can do a lookup in the table even without all bits
	// filled. Since the extra bits are zero, and the DEFLATE Huffman codes
	// have the property that shorter codes come before longer ones, the
	// bit length estimate in the result is a lower bound on the actual
	// number of bits.
	//
	// See the following:
	// https://github.com/madler/zlib/raw/master/doc/algorithm.txt

	// chunk & 15 is number of bits
	// chunk >> 4 is value, including table link

	const (
	huffmanChunkBits = 9
	huffmanNumChunks = 1 << huffmanChunkBits
	huffmanCountMask = 15
	huffmanValueShift = 4
	)

	type huffmanDecoder struct {
	min int // the minimum code length
	chunks [huffmanNumChunks]uint32 // chunks as described above
	links [][]uint32 // overflow links
	linkMask uint32 // mask the width of the link table
	}

	// Initialize Huffman decoding tables from array of code lengths.
	// Following this function, h is guaranteed to be initialized into a complete
	// tree (i.e., neither over-subscribed nor under-subscribed). The exception is a
	// degenerate case where the tree has only a single symbol with length 1. Empty
	// trees are permitted.
	func (h *huffmanDecoder) init(lengths []int) bool {
	// Sanity enables additional runtime tests during Huffman
	// table construction. It's intended to be used during
	// development to supplement the currently ad-hoc unit tests.
	const sanity = false

	if h.min != 0 {
	*h = huffmanDecoder{}
	}

	// Count number of codes of each length,
	// compute min and max length.
	var count [maxCodeLen]int
	var min, max int
	for _, n := range lengths {
	if n == 0 {
	continue
	}
	if min == 0 \|\| n < min {
	min = n
	}
	if n > max {
	max = n
	}
	count[n]++
	}

	// Empty tree. The decompressor.huffSym function will fail later if the tree
	// is used. Technically, an empty tree is only valid for the HDIST tree and
	// not the HCLEN and HLIT tree. However, a stream with an empty HCLEN tree
	// is guaranteed to fail since it will attempt to use the tree to decode the
	// codes for the HLIT and HDIST trees. Similarly, an empty HLIT tree is
	// guaranteed to fail later since the compressed data section must be
	// composed of at least one symbol (the end-of-block marker).
	if max == 0 {
	return true
	}

	code := 0
	var nextcode [maxCodeLen]int
	for i := min; i <= max; i++ {
	code <<= 1
	nextcode[i] = code
	code += count[i]
	}

	// Check that the coding is complete (i.e., that we've
	// assigned all 2-to-the-max possible bit sequences).
	// Exception: To be compatible with zlib, we also need to
	// accept degenerate single-code codings. See also
	// TestDegenerateHuffmanCoding.
	if code != 1<<uint(max) && !(code == 1 && max == 1) {
	return false
	}

	h.min = min
	if max > huffmanChunkBits {
	numLinks := 1 << (uint(max) - huffmanChunkBits)
	h.linkMask = uint32(numLinks - 1)

	// create link tables
	link := nextcode[huffmanChunkBits+1] >> 1
	h.links = make([][]uint32, huffmanNumChunks-link)
	for j := uint(link); j < huffmanNumChunks; j++ {
	reverse := int(bits.Reverse16(uint16(j)))
	reverse >>= uint(16 - huffmanChunkBits)
	off := j - uint(link)
	if sanity && h.chunks[reverse] != 0 {
	panic("impossible: overwriting existing chunk")
	}
	h.chunks[reverse] = uint32(off<<huffmanValueShift \| (huffmanChunkBits + 1))
	h.links[off] = make([]uint32, numLinks)
	}
	}

	for i, n := range lengths {
	if n == 0 {
	continue
	}
	code := nextcode[n]
	nextcode[n]++
	chunk := uint32(i<<huffmanValueShift \| n)
	reverse := int(bits.Reverse16(uint16(code)))
	reverse >>= uint(16 - n)
	if n <= huffmanChunkBits {
	for off := reverse; off < len(h.chunks); off += 1 << uint(n) {
	// We should never need to overwrite
	// an existing chunk. Also, 0 is
	// never a valid chunk, because the
	// lower 4 "count" bits should be
	// between 1 and 15.
	if sanity && h.chunks[off] != 0 {
	panic("impossible: overwriting existing chunk")
	}
	h.chunks[off] = chunk
	}
	} else {
	j := reverse & (huffmanNumChunks - 1)
	if sanity && h.chunks[j]&huffmanCountMask != huffmanChunkBits+1 {
	// Longer codes should have been
	// associated with a link table above.
	panic("impossible: not an indirect chunk")
	}
	value := h.chunks[j] >> huffmanValueShift
	linktab := h.links[value]
	reverse >>= huffmanChunkBits
	for off := reverse; off < len(linktab); off += 1 << uint(n-huffmanChunkBits) {
	if sanity && linktab[off] != 0 {
	panic("impossible: overwriting existing chunk")
	}
	linktab[off] = chunk
	}
	}
	}

	if sanity {
	// Above we've sanity checked that we never overwrote
	// an existing entry. Here we additionally check that
	// we filled the tables completely.
	for i, chunk := range h.chunks {
	if chunk == 0 {
	// As an exception, in the degenerate
	// single-code case, we allow odd
	// chunks to be missing.
	if code == 1 && i%2 == 1 {
	continue
	}
	panic("impossible: missing chunk")
	}
	}
	for _, linktab := range h.links {
	for _, chunk := range linktab {
	if chunk == 0 {
	panic("impossible: missing chunk")
	}
	}
	}
	}

	return true
	}

	// The actual read interface needed by NewReader.
	// If the passed in io.Reader does not also have ReadByte,
	// the NewReader will introduce its own buffering.
	type Reader interface {
	io.Reader
	io.ByteReader
	}

	// Decompress state.
	type decompressor struct {
	// Input source.
	r Reader
	roffset int64

	// Input bits, in top of b.
	b uint32
	nb uint

	// Huffman decoders for literal/length, distance.
	h1, h2 huffmanDecoder

	// Length arrays used to define Huffman codes.
	bits *[maxNumLit + maxNumDist]int
	codebits *[numCodes]int

	// Output history, buffer.
	dict dictDecoder

	// Temporary buffer (avoids repeated allocation).
	buf [4]byte

	// Next step in the decompression,
	// and decompression state.
	step func(*decompressor)
	stepState int
	final bool
	err error
	toRead []byte
	hl, hd *huffmanDecoder
	copyLen int
	copyDist int
	}

	func (f *decompressor) nextBlock() {
	for f.nb < 1+2 {
	if f.err = f.moreBits(); f.err != nil {
	return
	}
	}
	f.final = f.b&1 == 1
	f.b >>= 1
	typ := f.b & 3
	f.b >>= 2
	f.nb -= 1 + 2
	switch typ {
	case 0:
	f.dataBlock()
	case 1:
	// compressed, fixed Huffman tables
	f.hl = &fixedHuffmanDecoder
	f.hd = nil
	f.huffmanBlock()
	case 2:
	// compressed, dynamic Huffman tables
	if f.err = f.readHuffman(); f.err != nil {
	break
	}
	f.hl = &f.h1
	f.hd = &f.h2
	f.huffmanBlock()
	default:
	// 3 is reserved.
	f.err = CorruptInputError(f.roffset)
	}
	}

	func (f *decompressor) Read(b []byte) (int, error) {
	for {
	if len(f.toRead) > 0 {
	n := copy(b, f.toRead)
	f.toRead = f.toRead[n:]
	if len(f.toRead) == 0 {
	return n, f.err
	}
	return n, nil
	}
	if f.err != nil {
	return 0, f.err
	}
	f.step(f)
	if f.err != nil && len(f.toRead) == 0 {
	f.toRead = f.dict.readFlush() // Flush what's left in case of error
	}
	}
	}

	func (f *decompressor) Close() error {
	if f.err == io.EOF {
	return nil
	}
	return f.err
	}

	// RFC 1951 section 3.2.7.
	// Compression with dynamic Huffman codes

	var codeOrder = [...]int{16, 17, 18, 0, 8, 7, 9, 6, 10, 5, 11, 4, 12, 3, 13, 2, 14, 1, 15}

	func (f *decompressor) readHuffman() error {
	// HLIT[5], HDIST[5], HCLEN[4].
	for f.nb < 5+5+4 {
	if err := f.moreBits(); err != nil {
	return err
	}
	}
	nlit := int(f.b&0x1F) + 257
	if nlit > maxNumLit {
	return CorruptInputError(f.roffset)
	}
	f.b >>= 5
	ndist := int(f.b&0x1F) + 1
	if ndist > maxNumDist {
	return CorruptInputError(f.roffset)
	}
	f.b >>= 5
	nclen := int(f.b&0xF) + 4
	// numCodes is 19, so nclen is always valid.
	f.b >>= 4
	f.nb -= 5 + 5 + 4

	// (HCLEN+4)*3 bits: code lengths in the magic codeOrder order.
	for i := 0; i < nclen; i++ {
	for f.nb < 3 {
	if err := f.moreBits(); err != nil {
	return err
	}
	}
	f.codebits[codeOrder[i]] = int(f.b & 0x7)
	f.b >>= 3
	f.nb -= 3
	}
	for i := nclen; i < len(codeOrder); i++ {
	f.codebits[codeOrder[i]] = 0
	}
	if !f.h1.init(f.codebits[0:]) {
	return CorruptInputError(f.roffset)
	}

	// HLIT + 257 code lengths, HDIST + 1 code lengths,
	// using the code length Huffman code.
	for i, n := 0, nlit+ndist; i < n; {
	x, err := f.huffSym(&f.h1)
	if err != nil {
	return err
	}
	if x < 16 {
	// Actual length.
	f.bits[i] = x
	i++
	continue
	}
	// Repeat previous length or zero.
	var rep int
	var nb uint
	var b int
	switch x {
	default:
	return InternalError("unexpected length code")
	case 16:
	rep = 3
	nb = 2
	if i == 0 {
	return CorruptInputError(f.roffset)
	}
	b = f.bits[i-1]
	case 17:
	rep = 3
	nb = 3
	b = 0
	case 18:
	rep = 11
	nb = 7
	b = 0
	}
	for f.nb < nb {
	if err := f.moreBits(); err != nil {
	return err
	}
	}
	rep += int(f.b & uint32(1<<nb-1))
	f.b >>= nb
	f.nb -= nb
	if i+rep > n {
	return CorruptInputError(f.roffset)
	}
	for j := 0; j < rep; j++ {
	f.bits[i] = b
	i++
	}
	}

	if !f.h1.init(f.bits[0:nlit]) \|\| !f.h2.init(f.bits[nlit:nlit+ndist]) {
	return CorruptInputError(f.roffset)
	}

	// As an optimization, we can initialize the min bits to read at a time
	// for the HLIT tree to the length of the EOB marker since we know that
	// every block must terminate with one. This preserves the property that
	// we never read any extra bytes after the end of the DEFLATE stream.
	if f.h1.min < f.bits[endBlockMarker] {
	f.h1.min = f.bits[endBlockMarker]
	}

	return nil
	}

	// Decode a single Huffman block from f.
	// hl and hd are the Huffman states for the lit/length values
	// and the distance values, respectively. If hd == nil, using the
	// fixed distance encoding associated with fixed Huffman blocks.
	func (f *decompressor) huffmanBlock() {
	const (
	stateInit = iota // Zero value must be stateInit
	stateDict
	)

	switch f.stepState {
	case stateInit:
	goto readLiteral
	case stateDict:
	goto copyHistory
	}

	readLiteral:
	// Read literal and/or (length, distance) according to RFC section 3.2.3.
	{
	v, err := f.huffSym(f.hl)
	if err != nil {
	f.err = err
	return
	}
	var n uint // number of bits extra
	var length int
	switch {
	case v < 256:
	f.dict.writeByte(byte(v))
	if f.dict.availWrite() == 0 {
	f.toRead = f.dict.readFlush()
	f.step = (*decompressor).huffmanBlock
	f.stepState = stateInit
	return
	}
	goto readLiteral
	case v == 256:
	f.finishBlock()
	return
	// otherwise, reference to older data
	case v < 265:
	length = v - (257 - 3)
	n = 0
	case v < 269:
	length = v2 - (2652 - 11)
	n = 1
	case v < 273:
	length = v4 - (2694 - 19)
	n = 2
	case v < 277:
	length = v8 - (2738 - 35)
	n = 3
	case v < 281:
	length = v16 - (27716 - 67)
	n = 4
	case v < 285:
	length = v32 - (28132 - 131)
	n = 5
	case v < maxNumLit:
	length = 258
	n = 0
	default:
	f.err = CorruptInputError(f.roffset)
	return
	}
	if n > 0 {
	for f.nb < n {
	if err = f.moreBits(); err != nil {
	f.err = err
	return
	}
	}
	length += int(f.b & uint32(1<<n-1))
	f.b >>= n
	f.nb -= n
	}

	var dist int
	if f.hd == nil {
	for f.nb < 5 {
	if err = f.moreBits(); err != nil {
	f.err = err
	return
	}
	}
	dist = int(bits.Reverse8(uint8(f.b & 0x1F << 3)))
	f.b >>= 5
	f.nb -= 5
	} else {
	if dist, err = f.huffSym(f.hd); err != nil {
	f.err = err
	return
	}
	}

	switch {
	case dist < 4:
	dist++
	case dist < maxNumDist:
	nb := uint(dist-2) >> 1
	// have 1 bit in bottom of dist, need nb more.
	extra := (dist & 1) << nb
	for f.nb < nb {
	if err = f.moreBits(); err != nil {
	f.err = err
	return
	}
	}
	extra \|= int(f.b & uint32(1<<nb-1))
	f.b >>= nb
	f.nb -= nb
	dist = 1<<(nb+1) + 1 + extra
	default:
	f.err = CorruptInputError(f.roffset)
	return
	}

	// No check on length; encoding can be prescient.
	if dist > f.dict.histSize() {
	f.err = CorruptInputError(f.roffset)
	return
	}

	f.copyLen, f.copyDist = length, dist
	goto copyHistory
	}

	copyHistory:
	// Perform a backwards copy according to RFC section 3.2.3.
	{
	cnt := f.dict.tryWriteCopy(f.copyDist, f.copyLen)
	if cnt == 0 {
	cnt = f.dict.writeCopy(f.copyDist, f.copyLen)
	}
	f.copyLen -= cnt

	if f.dict.availWrite() == 0 \|\| f.copyLen > 0 {
	f.toRead = f.dict.readFlush()
	f.step = (*decompressor).huffmanBlock // We need to continue this work
	f.stepState = stateDict
	return
	}
	goto readLiteral
	}
	}

	// Copy a single uncompressed data block from input to output.
	func (f *decompressor) dataBlock() {
	// Uncompressed.
	// Discard current half-byte.
	f.nb = 0
	f.b = 0

	// Length then ones-complement of length.
	nr, err := io.ReadFull(f.r, f.buf[0:4])
	f.roffset += int64(nr)
	if err != nil {
	f.err = noEOF(err)
	return
	}
	n := int(f.buf[0]) \| int(f.buf[1])<<8
	nn := int(f.buf[2]) \| int(f.buf[3])<<8
	if uint16(nn) != uint16(^n) {
	f.err = CorruptInputError(f.roffset)
	return
	}

	if n == 0 {
	f.toRead = f.dict.readFlush()
	f.finishBlock()
	return
	}

	f.copyLen = n
	f.copyData()
	}

	// copyData copies f.copyLen bytes from the underlying reader into f.hist.
	// It pauses for reads when f.hist is full.
	func (f *decompressor) copyData() {
	buf := f.dict.writeSlice()
	if len(buf) > f.copyLen {
	buf = buf[:f.copyLen]
	}

	cnt, err := io.ReadFull(f.r, buf)
	f.roffset += int64(cnt)
	f.copyLen -= cnt
	f.dict.writeMark(cnt)
	if err != nil {
	f.err = noEOF(err)
	return
	}

	if f.dict.availWrite() == 0 \|\| f.copyLen > 0 {
	f.toRead = f.dict.readFlush()
	f.step = (*decompressor).copyData
	return
	}
	f.finishBlock()
	}

	func (f *decompressor) finishBlock() {
	if f.final {
	if f.dict.availRead() > 0 {
	f.toRead = f.dict.readFlush()
	}
	f.err = io.EOF
	}
	f.step = (*decompressor).nextBlock
	}

	// noEOF returns err, unless err == io.EOF, in which case it returns io.ErrUnexpectedEOF.
	func noEOF(e error) error {
	if e == io.EOF {
	return io.ErrUnexpectedEOF
	}
	return e
	}

	func (f *decompressor) moreBits() error {
	c, err := f.r.ReadByte()
	if err != nil {
	return noEOF(err)
	}
	f.roffset++
	f.b \|= uint32(c) << f.nb
	f.nb += 8
	return nil
	}

	// Read the next Huffman-encoded symbol from f according to h.
	func (f decompressor) huffSym(h huffmanDecoder) (int, error) {
	// Since a huffmanDecoder can be empty or be composed of a degenerate tree
	// with single element, huffSym must error on these two edge cases. In both
	// cases, the chunks slice will be 0 for the invalid sequence, leading it
	// satisfy the n == 0 check below.
	n := uint(h.min)
	// Optimization. Compiler isn't smart enough to keep f.b,f.nb in registers,
	// but is smart enough to keep local variables in registers, so use nb and b,
	// inline call to moreBits and reassign b,nb back to f on return.
	nb, b := f.nb, f.b
	for {
	for nb < n {
	c, err := f.r.ReadByte()
	if err != nil {
	f.b = b
	f.nb = nb
	return 0, noEOF(err)
	}
	f.roffset++
	b \|= uint32(c) << (nb & 31)
	nb += 8
	}
	chunk := h.chunks[b&(huffmanNumChunks-1)]
	n = uint(chunk & huffmanCountMask)
	if n > huffmanChunkBits {
	chunk = h.links[chunk>>huffmanValueShift][(b>>huffmanChunkBits)&h.linkMask]
	n = uint(chunk & huffmanCountMask)
	}
	if n <= nb {
	if n == 0 {
	f.b = b
	f.nb = nb
	f.err = CorruptInputError(f.roffset)
	return 0, f.err
	}
	f.b = b >> (n & 31)
	f.nb = nb - n
	return int(chunk >> huffmanValueShift), nil
	}
	}
	}

	func makeReader(r io.Reader) Reader {
	if rr, ok := r.(Reader); ok {
	return rr
	}
	return bufio.NewReader(r)
	}

	func fixedHuffmanDecoderInit() {
	fixedOnce.Do(func() {
	// These come from the RFC section 3.2.6.
	var bits [288]int
	for i := 0; i < 144; i++ {
	bits[i] = 8
	}
	for i := 144; i < 256; i++ {
	bits[i] = 9
	}
	for i := 256; i < 280; i++ {
	bits[i] = 7
	}
	for i := 280; i < 288; i++ {
	bits[i] = 8
	}
	fixedHuffmanDecoder.init(bits[:])
	})
	}

	func (f *decompressor) Reset(r io.Reader, dict []byte) error {
	*f = decompressor{
	r: makeReader(r),
	bits: f.bits,
	codebits: f.codebits,
	dict: f.dict,
	step: (*decompressor).nextBlock,
	}
	f.dict.init(maxMatchOffset, dict)
	return nil
	}

	// NewReader returns a new ReadCloser that can be used
	// to read the uncompressed version of r.
	// If r does not also implement io.ByteReader,
	// the decompressor may read more data than necessary from r.
	// It is the caller's responsibility to call Close on the ReadCloser
	// when finished reading.
	//
	// The ReadCloser returned by NewReader also implements Resetter.
	func NewReader(r io.Reader) io.ReadCloser {
	fixedHuffmanDecoderInit()

	var f decompressor
	f.r = makeReader(r)
	f.bits = new([maxNumLit + maxNumDist]int)
	f.codebits = new([numCodes]int)
	f.step = (*decompressor).nextBlock
	f.dict.init(maxMatchOffset, nil)
	return &f
	}

	// NewReaderDict is like NewReader but initializes the reader
	// with a preset dictionary. The returned Reader behaves as if
	// the uncompressed data stream started with the given dictionary,
	// which has already been read. NewReaderDict is typically used
	// to read data compressed by NewWriterDict.
	//
	// The ReadCloser returned by NewReader also implements Resetter.
	func NewReaderDict(r io.Reader, dict []byte) io.ReadCloser {
	fixedHuffmanDecoderInit()

	var f decompressor
	f.r = makeReader(r)
	f.bits = new([maxNumLit + maxNumDist]int)
	f.codebits = new([numCodes]int)
	f.step = (*decompressor).nextBlock
	f.dict.init(maxMatchOffset, dict)
	return &f
	}