compress/flate: extract LZ77 dictionary logic into seperate struct

The LZ77 portion of DEFLATE is relatively self-contained. For the
decompression side of things, we extract this logic out for the
following reasons:
* It is easier to test just the LZ77 portion of the logic.
* It reduces the noise in the inflate.go

Also, we adjust the way that callbacks are handled in the inflate.
Instead of using functions to abstract the logical componets of
huffmanBlock(), use goto statements to jump between the necessary
sections. This is faster since it avoids a function call and is
arguably more readable.

benchmark                              old MB/s     new MB/s     speedup
BenchmarkDecodeDigitsSpeed1e4-4        53.62        60.11        1.12x
BenchmarkDecodeDigitsSpeed1e5-4        61.90        69.07        1.12x
BenchmarkDecodeDigitsSpeed1e6-4        63.24        70.58        1.12x
BenchmarkDecodeDigitsDefault1e4-4      54.10        59.00        1.09x
BenchmarkDecodeDigitsDefault1e5-4      69.50        74.07        1.07x
BenchmarkDecodeDigitsDefault1e6-4      71.54        75.85        1.06x
BenchmarkDecodeDigitsCompress1e4-4     54.39        58.94        1.08x
BenchmarkDecodeDigitsCompress1e5-4     69.21        73.96        1.07x
BenchmarkDecodeDigitsCompress1e6-4     71.14        75.75        1.06x
BenchmarkDecodeTwainSpeed1e4-4         53.15        58.13        1.09x
BenchmarkDecodeTwainSpeed1e5-4         66.56        72.29        1.09x
BenchmarkDecodeTwainSpeed1e6-4         69.13        75.11        1.09x
BenchmarkDecodeTwainDefault1e4-4       56.00        60.23        1.08x
BenchmarkDecodeTwainDefault1e5-4       77.84        82.27        1.06x
BenchmarkDecodeTwainDefault1e6-4       82.07        86.85        1.06x
BenchmarkDecodeTwainCompress1e4-4      56.13        60.38        1.08x
BenchmarkDecodeTwainCompress1e5-4      78.23        82.62        1.06x
BenchmarkDecodeTwainCompress1e6-4      82.38        86.73        1.05x

Change-Id: I8c6ae0e6bed652dd0570fc113c999977f5e71636
Reviewed-on: https://go-review.googlesource.com/16528Reviewed-by: Matthew Dempsky <mdempsky@google.com>
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/compress/flate/copy.go

-// Copyright 2012 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
-
-// forwardCopy is like the built-in copy function except that it always goes
-// forward from the start, even if the dst and src overlap.
-// It is equivalent to:
-//   for i := 0; i < n; i++ {
-//     mem[dst+i] = mem[src+i]
-//   }
-func forwardCopy(mem []byte, dst, src, n int) {
-	if dst <= src {
-		copy(mem[dst:dst+n], mem[src:src+n])
-		return
-	}
-	for {
-		if dst >= src+n {
-			copy(mem[dst:dst+n], mem[src:src+n])
-			return
-		}
-		// There is some forward overlap.  The destination
-		// will be filled with a repeated pattern of mem[src:src+k].
-		// We copy one instance of the pattern here, then repeat.
-		// Each time around this loop k will double.
-		k := dst - src
-		copy(mem[dst:dst+k], mem[src:src+k])
-		n -= k
-		dst += k
-	}
-}

src/compress/flate/copy_test.go

-// Copyright 2012 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
-
-import (
-	"testing"
-)
-
-func TestForwardCopy(t *testing.T) {
-	testCases := []struct {
-		dst0, dst1 int
-		src0, src1 int
-		want       string
-	}{
-		{0, 9, 0, 9, "012345678"},
-		{0, 5, 4, 9, "45678"},
-		{4, 9, 0, 5, "01230"},
-		{1, 6, 3, 8, "34567"},
-		{3, 8, 1, 6, "12121"},
-		{0, 9, 3, 6, "345"},
-		{3, 6, 0, 9, "012"},
-		{1, 6, 0, 9, "00000"},
-		{0, 4, 7, 8, "7"},
-		{0, 1, 6, 8, "6"},
-		{4, 4, 6, 9, ""},
-		{2, 8, 6, 6, ""},
-		{0, 0, 0, 0, ""},
-	}
-	for _, tc := range testCases {
-		b := []byte("0123456789")
-		n := tc.dst1 - tc.dst0
-		if tc.src1-tc.src0 < n {
-			n = tc.src1 - tc.src0
-		}
-		forwardCopy(b, tc.dst0, tc.src0, n)
-		got := string(b[tc.dst0 : tc.dst0+n])
-		if got != tc.want {
-			t.Errorf("dst=b[%d:%d], src=b[%d:%d]: got %q, want %q",
-				tc.dst0, tc.dst1, tc.src0, tc.src1, got, tc.want)
-		}
-		// Check that the bytes outside of dst[:n] were not modified.
-		for i, x := range b {
-			if i >= tc.dst0 && i < tc.dst0+n {
-				continue
-			}
-			if int(x) != '0'+i {
-				t.Errorf("dst=b[%d:%d], src=b[%d:%d]: copy overrun at b[%d]: got '%c', want '%c'",
-					tc.dst0, tc.dst1, tc.src0, tc.src1, i, x, '0'+i)
-			}
-		}
-	}
-}

src/compress/flate/dict_decoder.go

+// Copyright 2016 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
+
+// dictDecoder implements the LZ77 sliding dictionary as used in decompression.
+// LZ77 decompresses data through sequences of two forms of commands:
+//
+//	* Literal insertions: Runs of one or more symbols are inserted into the data
+//	stream as is. This is accomplished through the writeByte method for a
+//	single symbol, or combinations of writeSlice/writeMark for multiple symbols.
+//	Any valid stream must start with a literal insertion if no preset dictionary
+//	is used.
+//
+//	* Backward copies: Runs of one or more symbols are copied from previously
+//	emitted data. Backward copies come as the tuple (dist, length) where dist
+//	determines how far back in the stream to copy from and length determines how
+//	many bytes to copy. Note that it is valid for the length to be greater than
+//	the distance. Since LZ77 uses forward copies, that situation is used to
+//	perform a form of run-length encoding on repeated runs of symbols.
+//	The writeCopy and tryWriteCopy are used to implement this command.
+//
+// For performance reasons, this implementation performs little to no sanity
+// checks about the arguments. As such, the invariants documented for each
+// method call must be respected.
+type dictDecoder struct {
+	hist []byte // Sliding window history
+
+	// Invariant: 0 <= rdPos <= wrPos <= len(hist)
+	wrPos int  // Current output position in buffer
+	rdPos int  // Have emitted hist[:rdPos] already
+	full  bool // Has a full window length been written yet?
+}
+
+// init initializes dictDecoder to have a sliding window dictionary of the given
+// size. If a preset dict is provided, it will initialize the dictionary with
+// the contents of dict.
+func (dd *dictDecoder) init(size int, dict []byte) {
+	*dd = dictDecoder{hist: dd.hist}
+
+	if cap(dd.hist) < size {
+		dd.hist = make([]byte, size)
+	}
+	dd.hist = dd.hist[:size]
+
+	if len(dict) > len(dd.hist) {
+		dict = dict[len(dict)-len(dd.hist):]
+	}
+	dd.wrPos = copy(dd.hist, dict)
+	if dd.wrPos == len(dd.hist) {
+		dd.wrPos = 0
+		dd.full = true
+	}
+	dd.rdPos = dd.wrPos
+}
+
+// histSize reports the total amount of historical data in the dictionary.
+func (dd *dictDecoder) histSize() int {
+	if dd.full {
+		return len(dd.hist)
+	}
+	return dd.wrPos
+}
+
+// availRead reports the number of bytes that can be flushed by readFlush.
+func (dd *dictDecoder) availRead() int {
+	return dd.wrPos - dd.rdPos
+}
+
+// availWrite reports the available amount of output buffer space.
+func (dd *dictDecoder) availWrite() int {
+	return len(dd.hist) - dd.wrPos
+}
+
+// writeSlice returns a slice of the available buffer to write data to.
+//
+// This invariant will be kept: len(s) <= availWrite()
+func (dd *dictDecoder) writeSlice() []byte {
+	return dd.hist[dd.wrPos:]
+}
+
+// writeMark advances the writer pointer by cnt.
+//
+// This invariant must be kept: 0 <= cnt <= availWrite()
+func (dd *dictDecoder) writeMark(cnt int) {
+	dd.wrPos += cnt
+}
+
+// writeByte writes a single byte to the dictionary.
+//
+// This invariant must be kept: 0 < availWrite()
+func (dd *dictDecoder) writeByte(c byte) {
+	dd.hist[dd.wrPos] = c
+	dd.wrPos++
+}
+
+// writeCopy copies a string at a given (dist, length) to the output.
+// This returns the number of bytes copied and may be less than the requested
+// length if the available space in the output buffer is too small.
+//
+// This invariant must be kept: 0 < dist <= histSize()
+func (dd *dictDecoder) writeCopy(dist, length int) int {
+	dstBase := dd.wrPos
+	dstPos := dstBase
+	srcPos := dstPos - dist
+	endPos := dstPos + length
+	if endPos > len(dd.hist) {
+		endPos = len(dd.hist)
+	}
+
+	// Copy non-overlapping section after destination position.
+	//
+	// This section is non-overlapping in that the copy length for this section
+	// is always less than or equal to the backwards distance. This can occur
+	// if a distance refers to data that wraps-around in the buffer.
+	// Thus, a backwards copy is performed here; that is, the exact bytes in
+	// the source prior to the copy is placed in the destination.
+	if srcPos < 0 {
+		srcPos += len(dd.hist)
+		dstPos += copy(dd.hist[dstPos:endPos], dd.hist[srcPos:])
+		srcPos = 0
+	}
+
+	// Copy possibly overlapping section before destination position.
+	//
+	// This section can overlap if the copy length for this section is larger
+	// than the backwards distance. This is allowed by LZ77 so that repeated
+	// strings can be succinctly represented using (dist, length) pairs.
+	// Thus, a forwards copy is performed here; that is, the bytes copied is
+	// possibly dependent on the resulting bytes in the destination as the copy
+	// progresses along. This is functionally equivalent to the following:
+	//
+	//	for i := 0; i < endPos-dstPos; i++ {
+	//		dd.hist[dstPos+i] = dd.hist[srcPos+i]
+	//	}
+	//	dstPos = endPos
+	//
+	for dstPos < endPos {
+		dstPos += copy(dd.hist[dstPos:endPos], dd.hist[srcPos:dstPos])
+	}
+
+	dd.wrPos = dstPos
+	return dstPos - dstBase
+}
+
+// tryWriteCopy tries to copy a string at a given (distance, length) to the
+// output. This specialized version is optimized for short distances.
+//
+// This method is designed to be inlined for performance reasons.
+//
+// This invariant must be kept: 0 < dist <= histSize()
+func (dd *dictDecoder) tryWriteCopy(dist, length int) int {
+	dstPos := dd.wrPos
+	endPos := dstPos + length
+	if dstPos < dist || endPos > len(dd.hist) {
+		return 0
+	}
+	dstBase := dstPos
+	srcPos := dstPos - dist
+
+	// Copy possibly overlapping section before destination position.
+loop:
+	dstPos += copy(dd.hist[dstPos:endPos], dd.hist[srcPos:dstPos])
+	if dstPos < endPos {
+		goto loop // Avoid for-loop so that this function can be inlined
+	}
+
+	dd.wrPos = dstPos
+	return dstPos - dstBase
+}
+
+// readFlush returns a slice of the historical buffer that is ready to be
+// emitted to the user. The data returned by readFlush must be fully consumed
+// before calling any other dictDecoder methods.
+func (dd *dictDecoder) readFlush() []byte {
+	toRead := dd.hist[dd.rdPos:dd.wrPos]
+	dd.rdPos = dd.wrPos
+	if dd.wrPos == len(dd.hist) {
+		dd.wrPos, dd.rdPos = 0, 0
+		dd.full = true
+	}
+	return toRead
+}

src/compress/flate/dict_decoder_test.go

+// Copyright 2016 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
+
+import (
+	"bytes"
+	"strings"
+	"testing"
+)
+
+func TestDictDecoder(t *testing.T) {
+	const (
+		abc  = "ABC\n"
+		fox  = "The quick brown fox jumped over the lazy dog!\n"
+		poem = "The Road Not Taken\nRobert Frost\n" +
+			"\n" +
+			"Two roads diverged in a yellow wood,\n" +
+			"And sorry I could not travel both\n" +
+			"And be one traveler, long I stood\n" +
+			"And looked down one as far as I could\n" +
+			"To where it bent in the undergrowth;\n" +
+			"\n" +
+			"Then took the other, as just as fair,\n" +
+			"And having perhaps the better claim,\n" +
+			"Because it was grassy and wanted wear;\n" +
+			"Though as for that the passing there\n" +
+			"Had worn them really about the same,\n" +
+			"\n" +
+			"And both that morning equally lay\n" +
+			"In leaves no step had trodden black.\n" +
+			"Oh, I kept the first for another day!\n" +
+			"Yet knowing how way leads on to way,\n" +
+			"I doubted if I should ever come back.\n" +
+			"\n" +
+			"I shall be telling this with a sigh\n" +
+			"Somewhere ages and ages hence:\n" +
+			"Two roads diverged in a wood, and I-\n" +
+			"I took the one less traveled by,\n" +
+			"And that has made all the difference.\n"
+	)
+
+	var poemRefs = []struct {
+		dist   int // Backward distance (0 if this is an insertion)
+		length int // Length of copy or insertion
+	}{
+		{0, 38}, {33, 3}, {0, 48}, {79, 3}, {0, 11}, {34, 5}, {0, 6}, {23, 7},
+		{0, 8}, {50, 3}, {0, 2}, {69, 3}, {34, 5}, {0, 4}, {97, 3}, {0, 4},
+		{43, 5}, {0, 6}, {7, 4}, {88, 7}, {0, 12}, {80, 3}, {0, 2}, {141, 4},
+		{0, 1}, {196, 3}, {0, 3}, {157, 3}, {0, 6}, {181, 3}, {0, 2}, {23, 3},
+		{77, 3}, {28, 5}, {128, 3}, {110, 4}, {70, 3}, {0, 4}, {85, 6}, {0, 2},
+		{182, 6}, {0, 4}, {133, 3}, {0, 7}, {47, 5}, {0, 20}, {112, 5}, {0, 1},
+		{58, 3}, {0, 8}, {59, 3}, {0, 4}, {173, 3}, {0, 5}, {114, 3}, {0, 4},
+		{92, 5}, {0, 2}, {71, 3}, {0, 2}, {76, 5}, {0, 1}, {46, 3}, {96, 4},
+		{130, 4}, {0, 3}, {360, 3}, {0, 3}, {178, 5}, {0, 7}, {75, 3}, {0, 3},
+		{45, 6}, {0, 6}, {299, 6}, {180, 3}, {70, 6}, {0, 1}, {48, 3}, {66, 4},
+		{0, 3}, {47, 5}, {0, 9}, {325, 3}, {0, 1}, {359, 3}, {318, 3}, {0, 2},
+		{199, 3}, {0, 1}, {344, 3}, {0, 3}, {248, 3}, {0, 10}, {310, 3}, {0, 3},
+		{93, 6}, {0, 3}, {252, 3}, {157, 4}, {0, 2}, {273, 5}, {0, 14}, {99, 4},
+		{0, 1}, {464, 4}, {0, 2}, {92, 4}, {495, 3}, {0, 1}, {322, 4}, {16, 4},
+		{0, 3}, {402, 3}, {0, 2}, {237, 4}, {0, 2}, {432, 4}, {0, 1}, {483, 5},
+		{0, 2}, {294, 4}, {0, 2}, {306, 3}, {113, 5}, {0, 1}, {26, 4}, {164, 3},
+		{488, 4}, {0, 1}, {542, 3}, {248, 6}, {0, 5}, {205, 3}, {0, 8}, {48, 3},
+		{449, 6}, {0, 2}, {192, 3}, {328, 4}, {9, 5}, {433, 3}, {0, 3}, {622, 25},
+		{615, 5}, {46, 5}, {0, 2}, {104, 3}, {475, 10}, {549, 3}, {0, 4}, {597, 8},
+		{314, 3}, {0, 1}, {473, 6}, {317, 5}, {0, 1}, {400, 3}, {0, 3}, {109, 3},
+		{151, 3}, {48, 4}, {0, 4}, {125, 3}, {108, 3}, {0, 2},
+	}
+
+	var got, want bytes.Buffer
+	var dd dictDecoder
+	dd.init(1<<11, nil)
+
+	var writeCopy = func(dist, length int) {
+		for length > 0 {
+			cnt := dd.tryWriteCopy(dist, length)
+			if cnt == 0 {
+				cnt = dd.writeCopy(dist, length)
+			}
+
+			length -= cnt
+			if dd.availWrite() == 0 {
+				got.Write(dd.readFlush())
+			}
+		}
+	}
+	var writeString = func(str string) {
+		for len(str) > 0 {
+			cnt := copy(dd.writeSlice(), str)
+			str = str[cnt:]
+			dd.writeMark(cnt)
+			if dd.availWrite() == 0 {
+				got.Write(dd.readFlush())
+			}
+		}
+	}
+
+	writeString(".")
+	want.WriteByte('.')
+
+	str := poem
+	for _, ref := range poemRefs {
+		if ref.dist == 0 {
+			writeString(str[:ref.length])
+		} else {
+			writeCopy(ref.dist, ref.length)
+		}
+		str = str[ref.length:]
+	}
+	want.WriteString(poem)
+
+	writeCopy(dd.histSize(), 33)
+	want.Write(want.Bytes()[:33])
+
+	writeString(abc)
+	writeCopy(len(abc), 59*len(abc))
+	want.WriteString(strings.Repeat(abc, 60))
+
+	writeString(fox)
+	writeCopy(len(fox), 9*len(fox))
+	want.WriteString(strings.Repeat(fox, 10))
+
+	writeString(".")
+	writeCopy(1, 9)
+	want.WriteString(strings.Repeat(".", 10))
+
+	writeString(strings.ToUpper(poem))
+	writeCopy(len(poem), 7*len(poem))
+	want.WriteString(strings.Repeat(strings.ToUpper(poem), 8))
+
+	writeCopy(dd.histSize(), 10)
+	want.Write(want.Bytes()[want.Len()-dd.histSize():][:10])
+
+	got.Write(dd.readFlush())
+	if got.String() != want.String() {
+		t.Errorf("final string mismatch:\ngot  %q\nwant %q", got.String(), want.String())
+	}
+}

src/compress/flate/inflate.go

@@ -282,29 +282,28 @@ type decompressor struct {
 	codebits *[numCodes]int
 
 	// Output history, buffer.
-	hist  *[maxHist]byte
-	hp    int  // current output position in buffer
-	hw    int  // have written hist[0:hw] already
-	hfull bool // buffer has filled at least once
+	dict dictDecoder
 
 	// Temporary buffer (avoids repeated allocation).
 	buf [4]byte
 
 	// Next step in the decompression,
 	// and decompression state.
-	step     func(*decompressor)
-	final    bool
-	err      error
-	toRead   []byte
-	hl, hd   *huffmanDecoder
-	copyLen  int
-	copyDist int
+	step      func(*decompressor)
+	stepState int
+	final     bool
+	err       error
+	toRead    []byte
+	hl, hd    *huffmanDecoder
+	copyLen   int
+	copyDist  int
 }
 
 func (f *decompressor) nextBlock() {
 	if f.final {
-		if f.hw != f.hp {
-			f.flush((*decompressor).nextBlock)
+		if f.dict.availRead() > 0 {
+			f.toRead = f.dict.readFlush()
+			f.step = (*decompressor).nextBlock
 			return
 		}
 		f.err = io.EOF
@@ -353,6 +352,7 @@ func (f *decompressor) Read(b []byte) (int, error) {
 			return 0, f.err
 		}
 		f.step(f)
+		f.woffset += int64(len(f.toRead))
 	}
 }
 
@@ -481,7 +481,21 @@ func (f *decompressor) readHuffman() error {
 // and the distance values, respectively.  If hd == nil, using the
 // fixed distance encoding associated with fixed Huffman blocks.
 func (f *decompressor) huffmanBlock() {
-	for {
+	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
@@ -491,14 +505,14 @@ func (f *decompressor) huffmanBlock() {
 		var length int
 		switch {
 		case v < 256:
-			f.hist[f.hp] = byte(v)
-			f.hp++
-			if f.hp == len(f.hist) {
-				// After the flush, continue this loop.
-				f.flush((*decompressor).huffmanBlock)
+			f.dict.writeByte(byte(v))
+			if f.dict.availWrite() == 0 {
+				f.toRead = f.dict.readFlush()
+				f.step = (*decompressor).huffmanBlock
+				f.stepState = stateInit
 				return
 			}
-			continue
+			goto readLiteral
 		case v == 256:
 			// Done with huffman block; read next block.
 			f.step = (*decompressor).nextBlock
@@ -581,61 +595,33 @@ func (f *decompressor) huffmanBlock() {
 			return
 		}
 
-		// Copy history[-dist:-dist+length] into output.
-		if dist > len(f.hist) {
-			f.err = InternalError("bad history distance")
-			return
-		}
-
 		// No check on length; encoding can be prescient.
-		if !f.hfull && dist > f.hp {
+		if dist > f.dict.histSize() {
 			f.err = CorruptInputError(f.roffset)
 			return
 		}
 
 		f.copyLen, f.copyDist = length, dist
-		if f.copyHist() {
-			return
-		}
+		goto copyHistory
 	}
-}
 
-// copyHist copies f.copyLen bytes from f.hist (f.copyDist bytes ago) to itself.
-// It reports whether the f.hist buffer is full.
-func (f *decompressor) copyHist() bool {
-	p := f.hp - f.copyDist
-	if p < 0 {
-		p += len(f.hist)
-	}
-	for f.copyLen > 0 {
-		n := f.copyLen
-		if x := len(f.hist) - f.hp; n > x {
-			n = x
-		}
-		if x := len(f.hist) - p; n > x {
-			n = x
-		}
-		forwardCopy(f.hist[:], f.hp, p, n)
-		p += n
-		f.hp += n
-		f.copyLen -= n
-		if f.hp == len(f.hist) {
-			// After flush continue copying out of history.
-			f.flush((*decompressor).copyHuff)
-			return true
-		}
-		if p == len(f.hist) {
-			p = 0
+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)
 		}
-	}
-	return false
-}
+		f.copyLen -= cnt
 
-func (f *decompressor) copyHuff() {
-	if f.copyHist() {
-		return
+		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
 	}
-	f.huffmanBlock()
 }
 
 // Copy a single uncompressed data block from input to output.
@@ -663,8 +649,8 @@ func (f *decompressor) dataBlock() {
 	}
 
 	if n == 0 {
-		// 0-length block means sync
-		f.flush((*decompressor).nextBlock)
+		f.toRead = f.dict.readFlush()
+		f.step = (*decompressor).nextBlock
 		return
 	}
 
@@ -675,44 +661,29 @@ func (f *decompressor) dataBlock() {
 // 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() {
-	n := f.copyLen
-	for n > 0 {
-		m := len(f.hist) - f.hp
-		if m > n {
-			m = n
-		}
-		m, err := io.ReadFull(f.r, f.hist[f.hp:f.hp+m])
-		f.roffset += int64(m)
-		if err != nil {
-			if err == io.EOF {
-				err = io.ErrUnexpectedEOF
-			}
-			f.err = err
-			return
-		}
-		n -= m
-		f.hp += m
-		if f.hp == len(f.hist) {
-			f.copyLen = n
-			f.flush((*decompressor).copyData)
-			return
-		}
+	buf := f.dict.writeSlice()
+	if len(buf) > f.copyLen {
+		buf = buf[:f.copyLen]
 	}
-	f.step = (*decompressor).nextBlock
-}
 
-func (f *decompressor) setDict(dict []byte) {
-	if len(dict) > len(f.hist) {
-		// Will only remember the tail.
-		dict = dict[len(dict)-len(f.hist):]
+	cnt, err := io.ReadFull(f.r, buf)
+	f.roffset += int64(cnt)
+	f.copyLen -= cnt
+	f.dict.writeMark(cnt)
+	if err != nil {
+		if err == io.EOF {
+			err = io.ErrUnexpectedEOF
+		}
+		f.err = err
+		return
 	}
 
-	f.hp = copy(f.hist[:], dict)
-	if f.hp == len(f.hist) {
-		f.hp = 0
-		f.hfull = true
+	if f.dict.availWrite() == 0 || f.copyLen > 0 {
+		f.toRead = f.dict.readFlush()
+		f.step = (*decompressor).copyData
+		return
 	}
-	f.hw = f.hp
+	f.step = (*decompressor).nextBlock
 }
 
 func (f *decompressor) moreBits() error {
@@ -760,19 +731,6 @@ func (f *decompressor) huffSym(h *huffmanDecoder) (int, error) {
 	}
 }
 
-// Flush any buffered output to the underlying writer.
-func (f *decompressor) flush(step func(*decompressor)) {
-	f.toRead = f.hist[f.hw:f.hp]
-	f.woffset += int64(f.hp - f.hw)
-	f.hw = f.hp
-	if f.hp == len(f.hist) {
-		f.hp = 0
-		f.hw = 0
-		f.hfull = true
-	}
-	f.step = step
-}
-
 func makeReader(r io.Reader) Reader {
 	if rr, ok := r.(Reader); ok {
 		return rr
@@ -805,12 +763,10 @@ func (f *decompressor) Reset(r io.Reader, dict []byte) error {
 		r:        makeReader(r),
 		bits:     f.bits,
 		codebits: f.codebits,
-		hist:     f.hist,
+		dict:     f.dict,
 		step:     (*decompressor).nextBlock,
 	}
-	if dict != nil {
-		f.setDict(dict)
-	}
+	f.dict.init(maxHist, nil)
 	return nil
 }
 
@@ -827,10 +783,10 @@ func NewReader(r io.Reader) io.ReadCloser {
 
 	var f decompressor
 	f.r = makeReader(r)
-	f.hist = new([maxHist]byte)
 	f.bits = new([maxNumLit + maxNumDist]int)
 	f.codebits = new([numCodes]int)
 	f.step = (*decompressor).nextBlock
+	f.dict.init(maxHist, nil)
 	return &f
 }
 
@@ -846,10 +802,9 @@ func NewReaderDict(r io.Reader, dict []byte) io.ReadCloser {
 
 	var f decompressor
 	f.r = makeReader(r)
-	f.hist = new([maxHist]byte)
 	f.bits = new([maxNumLit + maxNumDist]int)
 	f.codebits = new([numCodes]int)
 	f.step = (*decompressor).nextBlock
-	f.setDict(dict)
+	f.dict.init(maxHist, dict)
 	return &f
 }