zlib, flatex: Remove code related to dicts

Git never uses them

9 files changed, 171 insertions, 414 deletions

diff --git a/internal/flatex/decompress_bytes.go b/internal/flatex/decompress_bytes.go
index ce6d0558..0d95084c 100644
--- a/internal/flatex/decompress_bytes.go
+++ b/internal/flatex/decompress_bytes.go
@@ -7,8 +7,6 @@ import (
 	"git.sr.ht/~runxiyu/furgit/internal/bufpool"
 )
 
-// bufferDecompressor wraps the custom slice inflater so byte-slice
-// decompressions avoid repeated allocations.
 type bufferDecompressor struct {
 	inflater sliceInflater
 }
@@ -23,27 +21,15 @@ var bufferDecompressorPool = sync.Pool{
 	},
 }
 
-// Decompress inflates the provided DEFLATE stream and returns the full output
-// in a pooled bufpool.Buffer along with the number of consumed bytes from src.
 func Decompress(src []byte) (*bufpool.Buffer, int, error) {
-	return DecompressDictSized(src, nil, 0)
+	return DecompressSized(src, 0)
 }
 
-// DecompressDict inflates the provided DEFLATE stream using dict as the preset
-// dictionary and returns the full output in a pooled bufpool.Buffer. The second
-// returned value reports how many bytes of src were consumed.
-func DecompressDict(src []byte, dict []byte) (*bufpool.Buffer, int, error) {
-	return DecompressDictSized(src, dict, 0)
-}
-
-// DecompressDictSized is like DecompressDict but allows providing an expected
-// output size to pre-size the destination buffer and avoid repeated growth.
-// A non-positive sizeHint falls back to the default buffer capacity.
-func DecompressDictSized(src []byte, dict []byte, sizeHint int) (*bufpool.Buffer, int, error) {
+func DecompressSized(src []byte, sizeHint int) (*bufpool.Buffer, int, error) {
 	d := bufferDecompressorPool.Get().(*bufferDecompressor)
 	defer bufferDecompressorPool.Put(d)
 
-	if err := d.inflater.reset(src, dict); err != nil {
+	if err := d.inflater.reset(src); err != nil {
 		return nil, 0, err
 	}
 
@@ -65,7 +51,7 @@ func DecompressDictSized(src []byte, dict []byte, sizeHint int) (*bufpool.Buffer
 		}
 		d.inflater.step(&d.inflater)
 		if d.inflater.err != nil && len(d.inflater.toRead) == 0 {
-			d.inflater.toRead = d.inflater.dict.readFlush()
+			d.inflater.toRead = d.inflater.window.readFlush()
 		}
 	}
 }
diff --git a/internal/flatex/decompress_test.go b/internal/flatex/decompress_test.go
index 7c290555..c991ea74 100644
--- a/internal/flatex/decompress_test.go
+++ b/internal/flatex/decompress_test.go
@@ -6,18 +6,10 @@ import (
 	"testing"
 )
 
-func compressDeflate(t *testing.T, payload, dict []byte) []byte {
+func compressDeflate(t *testing.T, payload []byte) []byte {
 	t.Helper()
 	var buf bytes.Buffer
-	var (
-		w   *stdflate.Writer
-		err error
-	)
-	if dict != nil {
-		w, err = stdflate.NewWriterDict(&buf, stdflate.DefaultCompression, dict)
-	} else {
-		w, err = stdflate.NewWriter(&buf, stdflate.DefaultCompression)
-	}
+	w, err := stdflate.NewWriter(&buf, stdflate.DefaultCompression)
 	if err != nil {
 		t.Fatalf("NewWriter: %v", err)
 	}
@@ -32,7 +24,7 @@ func compressDeflate(t *testing.T, payload, dict []byte) []byte {
 
 func TestDecompress(t *testing.T) {
 	payload := bytes.Repeat([]byte("golang"), 32)
-	compressed := compressDeflate(t, payload, nil)
+	compressed := compressDeflate(t, payload)
 
 	out, _, err := Decompress(compressed)
 	if err != nil {
@@ -45,44 +37,14 @@ func TestDecompress(t *testing.T) {
 	}
 }
 
-func TestDecompressDict(t *testing.T) {
-	dict := []byte("furgit dictionary payload")
-	payload := append([]byte(nil), dict...)
-	payload = append(payload, []byte(" -- and some more data repeated -- and some more data repeated")...)
-
-	compressed := compressDeflate(t, payload, dict)
-
-	out, _, err := DecompressDict(compressed, dict)
-	if err != nil {
-		t.Fatalf("DecompressDict: %v", err)
-	}
-	defer out.Release()
-
-	if !bytes.Equal(out.Bytes(), payload) {
-		t.Fatalf("unexpected payload: got %q", out.Bytes())
-	}
-}
-
-func TestDecompressDictMissing(t *testing.T) {
-	dict := []byte("shared prefix to enforce dictionary usage")
-	payload := append([]byte(nil), dict...)
-	payload = append(payload, []byte(" trailing data to force reference")...)
-
-	compressed := compressDeflate(t, payload, dict)
-
-	if _, _, err := Decompress(compressed); err == nil {
-		t.Fatalf("expected error when dictionary missing")
-	}
-}
-
-func TestDecompressDictSizedUsesHint(t *testing.T) {
+func TestDecompressSizedUsesHint(t *testing.T) {
 	payload := []byte("short")
-	compressed := compressDeflate(t, payload, nil)
+	compressed := compressDeflate(t, payload)
 
 	const hint = 1 << 19
-	out, _, err := DecompressDictSized(compressed, nil, hint)
+	out, _, err := DecompressSized(compressed, hint)
 	if err != nil {
-		t.Fatalf("DecompressDictSized: %v", err)
+		t.Fatalf("DecompressSized: %v", err)
 	}
 	defer out.Release()
 
diff --git a/internal/flatex/dict_decoder.go b/internal/flatex/dict_decoder.go
deleted file mode 100644
index 7a81e640..00000000
--- a/internal/flatex/dict_decoder.go
+++ /dev/null
@@ -1,182 +0,0 @@
-// 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 flatex
-
-// 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.
-	for dstPos < endPos {
-		dstPos += copy(dd.hist[dstPos:endPos], dd.hist[srcPos:dstPos])
-	}
-
-	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
-}
diff --git a/internal/flatex/inflate.go b/internal/flatex/inflate.go
index ca85b355..1d32e8cd 100644
--- a/internal/flatex/inflate.go
+++ b/internal/flatex/inflate.go
@@ -70,13 +70,13 @@ 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]
+// Resetter resets a ReadCloser returned by [NewReader]
 // 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
+	Reset(r io.Reader) error
 }
 
 // The data structure for decoding Huffman tables is based on that of
@@ -287,7 +287,7 @@ type decompressor struct {
 	codebits *[numCodes]int
 
 	// Output history, buffer.
-	dict dictDecoder
+	window windowDecoder
 
 	// Temporary buffer (avoids repeated allocation).
 	buf [4]byte
@@ -352,7 +352,7 @@ func (f *decompressor) Read(b []byte) (int, error) {
 		}
 		f.step(f)
 		if f.err != nil && len(f.toRead) == 0 {
-			f.toRead = f.dict.readFlush() // Flush what's left in case of error
+			f.toRead = f.window.readFlush()
 		}
 	}
 }
@@ -506,9 +506,9 @@ readLiteral:
 		var length int
 		switch {
 		case v < 256:
-			f.dict.writeByte(byte(v))
-			if f.dict.availWrite() == 0 {
-				f.toRead = f.dict.readFlush()
+			f.window.writeByte(byte(v))
+			if f.window.availWrite() == 0 {
+				f.toRead = f.window.readFlush()
 				f.step = (*decompressor).huffmanBlock
 				f.stepState = stateInit
 				return
@@ -596,7 +596,7 @@ readLiteral:
 		}
 
 		// No check on length; encoding can be prescient.
-		if dist > f.dict.histSize() {
+		if dist > f.window.histSize() {
 			f.err = CorruptInputError(f.roffset)
 			return
 		}
@@ -608,14 +608,14 @@ readLiteral:
 copyHistory:
 	// Perform a backwards copy according to RFC section 3.2.3.
 	{
-		cnt := f.dict.tryWriteCopy(f.copyDist, f.copyLen)
+		cnt := f.window.tryWriteCopy(f.copyDist, f.copyLen)
 		if cnt == 0 {
-			cnt = f.dict.writeCopy(f.copyDist, f.copyLen)
+			cnt = f.window.writeCopy(f.copyDist, f.copyLen)
 		}
 		f.copyLen -= cnt
 
-		if f.dict.availWrite() == 0 || f.copyLen > 0 {
-			f.toRead = f.dict.readFlush()
+		if f.window.availWrite() == 0 || f.copyLen > 0 {
+			f.toRead = f.window.readFlush()
 			f.step = (*decompressor).huffmanBlock // We need to continue this work
 			f.stepState = stateDict
 			return
@@ -646,7 +646,7 @@ func (f *decompressor) dataBlock() {
 	}
 
 	if n == 0 {
-		f.toRead = f.dict.readFlush()
+		f.toRead = f.window.readFlush()
 		f.finishBlock()
 		return
 	}
@@ -658,7 +658,7 @@ 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() {
-	buf := f.dict.writeSlice()
+	buf := f.window.writeSlice()
 	if len(buf) > f.copyLen {
 		buf = buf[:f.copyLen]
 	}
@@ -666,14 +666,14 @@ func (f *decompressor) copyData() {
 	cnt, err := io.ReadFull(f.r, buf)
 	f.roffset += int64(cnt)
 	f.copyLen -= cnt
-	f.dict.writeMark(cnt)
+	f.window.writeMark(cnt)
 	if err != nil {
 		f.err = noEOF(err)
 		return
 	}
 
-	if f.dict.availWrite() == 0 || f.copyLen > 0 {
-		f.toRead = f.dict.readFlush()
+	if f.window.availWrite() == 0 || f.copyLen > 0 {
+		f.toRead = f.window.readFlush()
 		f.step = (*decompressor).copyData
 		return
 	}
@@ -682,8 +682,8 @@ func (f *decompressor) copyData() {
 
 func (f *decompressor) finishBlock() {
 	if f.final {
-		if f.dict.availRead() > 0 {
-			f.toRead = f.dict.readFlush()
+		if f.window.availRead() > 0 {
+			f.toRead = f.window.readFlush()
 		}
 		f.err = io.EOF
 	}
@@ -788,16 +788,16 @@ func fixedHuffmanDecoderInit() {
 	})
 }
 
-func (f *decompressor) Reset(r io.Reader, dict []byte) error {
+func (f *decompressor) Reset(r io.Reader) error {
 	*f = decompressor{
 		rBuf:     f.rBuf,
 		bits:     f.bits,
 		codebits: f.codebits,
-		dict:     f.dict,
+		window:   f.window,
 		step:     (*decompressor).nextBlock,
 	}
 	f.makeReader(r)
-	f.dict.init(maxMatchOffset, dict)
+	f.window.init(maxMatchOffset)
 	return nil
 }
 
@@ -817,25 +817,6 @@ func NewReader(r io.Reader) io.ReadCloser {
 	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 NewReaderDict also implements [Resetter].
-func NewReaderDict(r io.Reader, dict []byte) io.ReadCloser {
-	fixedHuffmanDecoderInit()
-
-	var f decompressor
-	f.makeReader(r)
-	f.bits = new([maxNumLit + maxNumDist]int)
-	f.codebits = new([numCodes]int)
-	f.step = (*decompressor).nextBlock
-	f.dict.init(maxMatchOffset, dict)
+	f.window.init(maxMatchOffset)
 	return &f
 }
diff --git a/internal/flatex/slice_inflate.go b/internal/flatex/slice_inflate.go
index d16a6441..3e07e744 100644
--- a/internal/flatex/slice_inflate.go
+++ b/internal/flatex/slice_inflate.go
@@ -21,7 +21,7 @@ type sliceInflater struct {
 	bits     *[maxNumLit + maxNumDist]int
 	codebits *[numCodes]int
 
-	dict dictDecoder
+	window windowDecoder
 
 	toRead    []byte
 	step      func(*sliceInflater)
@@ -33,18 +33,18 @@ type sliceInflater struct {
 	copyDist  int
 }
 
-func (f *sliceInflater) reset(src []byte, dict []byte) error {
+func (f *sliceInflater) reset(src []byte) error {
 	bits := f.bits
 	codebits := f.codebits
-	dictState := f.dict
+	windowState := f.window
 	*f = sliceInflater{
 		input:    src,
 		bits:     bits,
 		codebits: codebits,
-		dict:     dictState,
+		window:   windowState,
 		step:     (*sliceInflater).nextBlock,
 	}
-	f.dict.init(maxMatchOffset, dict)
+	f.window.init(maxMatchOffset)
 	return nil
 }
 
@@ -103,9 +103,9 @@ readLiteral:
 		var length int
 		switch {
 		case v < 256:
-			f.dict.writeByte(byte(v))
-			if f.dict.availWrite() == 0 {
-				f.toRead = f.dict.readFlush()
+			f.window.writeByte(byte(v))
+			if f.window.availWrite() == 0 {
+				f.toRead = f.window.readFlush()
 				f.step = (*sliceInflater).huffmanBlock
 				f.stepState = stateInit
 				return
@@ -190,7 +190,7 @@ readLiteral:
 			return
 		}
 
-		if dist > f.dict.histSize() {
+		if dist > f.window.histSize() {
 			f.err = CorruptInputError(f.roffset)
 			return
 		}
@@ -201,14 +201,14 @@ readLiteral:
 
 copyHistory:
 	{
-		cnt := f.dict.tryWriteCopy(f.copyDist, f.copyLen)
+		cnt := f.window.tryWriteCopy(f.copyDist, f.copyLen)
 		if cnt == 0 {
-			cnt = f.dict.writeCopy(f.copyDist, f.copyLen)
+			cnt = f.window.writeCopy(f.copyDist, f.copyLen)
 		}
 		f.copyLen -= cnt
 
-		if f.dict.availWrite() == 0 || f.copyLen > 0 {
-			f.toRead = f.dict.readFlush()
+		if f.window.availWrite() == 0 || f.copyLen > 0 {
+			f.toRead = f.window.readFlush()
 			f.step = (*sliceInflater).huffmanBlock
 			f.stepState = stateDict
 			return
@@ -237,7 +237,7 @@ func (f *sliceInflater) dataBlock() {
 	}
 
 	if n == 0 {
-		f.toRead = f.dict.readFlush()
+		f.toRead = f.window.readFlush()
 		f.finishBlock()
 		return
 	}
@@ -252,9 +252,9 @@ func (f *sliceInflater) copyData() {
 			f.finishBlock()
 			return
 		}
-		buf := f.dict.writeSlice()
+		buf := f.window.writeSlice()
 		if len(buf) == 0 {
-			f.toRead = f.dict.readFlush()
+			f.toRead = f.window.readFlush()
 			f.step = (*sliceInflater).copyData
 			return
 		}
@@ -270,9 +270,9 @@ func (f *sliceInflater) copyData() {
 		f.pos += n
 		f.roffset += int64(n)
 		f.copyLen -= n
-		f.dict.writeMark(n)
-		if f.dict.availWrite() == 0 {
-			f.toRead = f.dict.readFlush()
+		f.window.writeMark(n)
+		if f.window.availWrite() == 0 {
+			f.toRead = f.window.readFlush()
 			f.step = (*sliceInflater).copyData
 			return
 		}
@@ -281,8 +281,8 @@ func (f *sliceInflater) copyData() {
 
 func (f *sliceInflater) finishBlock() {
 	if f.final {
-		if f.dict.availRead() > 0 {
-			f.toRead = f.dict.readFlush()
+		if f.window.availRead() > 0 {
+			f.toRead = f.window.readFlush()
 		}
 		f.err = io.EOF
 	}
diff --git a/internal/flatex/window_decoder.go b/internal/flatex/window_decoder.go
new file mode 100644
index 00000000..492c6a96
--- /dev/null
+++ b/internal/flatex/window_decoder.go
@@ -0,0 +1,101 @@
+package flatex
+
+// windowDecoder implements the sliding window used in decompression.
+type windowDecoder struct {
+	hist []byte
+
+	wrPos int
+	rdPos int
+	full  bool
+}
+
+func (wd *windowDecoder) init(size int) {
+	*wd = windowDecoder{hist: wd.hist}
+
+	if cap(wd.hist) < size {
+		wd.hist = make([]byte, size)
+	}
+	wd.hist = wd.hist[:size]
+
+	wd.wrPos = 0
+	wd.rdPos = 0
+	wd.full = false
+}
+
+func (wd *windowDecoder) histSize() int {
+	if wd.full {
+		return len(wd.hist)
+	}
+	return wd.wrPos
+}
+
+func (wd *windowDecoder) availRead() int {
+	return wd.wrPos - wd.rdPos
+}
+
+func (wd *windowDecoder) availWrite() int {
+	return len(wd.hist) - wd.wrPos
+}
+
+func (wd *windowDecoder) writeSlice() []byte {
+	return wd.hist[wd.wrPos:]
+}
+
+func (wd *windowDecoder) writeMark(cnt int) {
+	wd.wrPos += cnt
+}
+
+func (wd *windowDecoder) writeByte(c byte) {
+	wd.hist[wd.wrPos] = c
+	wd.wrPos++
+}
+
+func (wd *windowDecoder) writeCopy(dist, length int) int {
+	dstBase := wd.wrPos
+	dstPos := dstBase
+	srcPos := dstPos - dist
+	endPos := dstPos + length
+	if endPos > len(wd.hist) {
+		endPos = len(wd.hist)
+	}
+
+	if srcPos < 0 {
+		srcPos += len(wd.hist)
+		dstPos += copy(wd.hist[dstPos:endPos], wd.hist[srcPos:])
+		srcPos = 0
+	}
+
+	for dstPos < endPos {
+		dstPos += copy(wd.hist[dstPos:endPos], wd.hist[srcPos:dstPos])
+	}
+
+	wd.wrPos = dstPos
+	return dstPos - dstBase
+}
+
+func (wd *windowDecoder) tryWriteCopy(dist, length int) int {
+	dstPos := wd.wrPos
+	endPos := dstPos + length
+	if dstPos < dist || endPos > len(wd.hist) {
+		return 0
+	}
+	dstBase := dstPos
+	srcPos := dstPos - dist
+
+	for dstPos < endPos {
+		dstPos += copy(wd.hist[dstPos:endPos], wd.hist[srcPos:dstPos])
+	}
+
+	wd.wrPos = dstPos
+	return dstPos - dstBase
+}
+
+func (wd *windowDecoder) readFlush() []byte {
+	toRead := wd.hist[wd.rdPos:wd.wrPos]
+	wd.rdPos = wd.wrPos
+	if wd.wrPos == len(wd.hist) {
+		wd.wrPos, wd.rdPos = 0, 0
+		wd.full = true
+	}
+	return toRead
+}
diff --git a/internal/zlibx/decompress.go b/internal/zlibx/decompress.go
index 68a92587..df4a34be 100644
--- a/internal/zlibx/decompress.go
+++ b/internal/zlibx/decompress.go
@@ -9,28 +9,11 @@ import (
 	"git.sr.ht/~runxiyu/furgit/internal/flatex"
 )
 
-// Decompress inflates the provided zlib wrapped stream and returns the
-// uncompressed data inside a pooled bufpool.Buffer.
 func Decompress(src []byte) (*bufpool.Buffer, error) {
 	return DecompressSized(src, 0)
 }
 
-// DecompressSized inflates the provided zlib stream, using sizeHint to
-// preallocate the output buffer when known (e.g. packfile entries).
 func DecompressSized(src []byte, sizeHint int) (*bufpool.Buffer, error) {
-	return DecompressDictSized(src, nil, sizeHint)
-}
-
-// DecompressDict is like Decompress but accepts a preset dictionary. The
-// dictionary must match the checksum embedded in the stream if the dictionary
-// flag is present.
-func DecompressDict(src []byte, dict []byte) (*bufpool.Buffer, error) {
-	return DecompressDictSized(src, dict, 0)
-}
-
-// DecompressDictSized is like DecompressDict but allows providing an expected
-// uncompressed size to avoid buffer growth copies.
-func DecompressDictSized(src []byte, dict []byte, sizeHint int) (*bufpool.Buffer, error) {
 	if len(src) < 6 {
 		return nil, io.ErrUnexpectedEOF
 	}
@@ -42,19 +25,8 @@ func DecompressDictSized(src []byte, dict []byte, sizeHint int) (*bufpool.Buffer
 	}
 
 	offset := 2
-	haveDict := flg&0x20 != 0
-	if haveDict {
-		if len(src) < offset+4 {
-			return nil, io.ErrUnexpectedEOF
-		}
-		if dict == nil {
-			return nil, ErrDictionary
-		}
-		checksum := binary.BigEndian.Uint32(src[offset : offset+4])
-		if checksum != adler32.Checksum(dict) {
-			return nil, ErrDictionary
-		}
-		offset += 4
+	if flg&0x20 != 0 {
+		return nil, ErrHeader
 	}
 
 	if len(src[offset:]) < 4 {
@@ -62,7 +34,7 @@ func DecompressDictSized(src []byte, dict []byte, sizeHint int) (*bufpool.Buffer
 	}
 
 	deflateData := src[offset:]
-	out, consumed, err := flatex.DecompressDictSized(deflateData, dict, sizeHint)
+	out, consumed, err := flatex.DecompressSized(deflateData, sizeHint)
 	if err != nil {
 		return nil, err
 	}
diff --git a/internal/zlibx/decompress_test.go b/internal/zlibx/decompress_test.go
index 3dfc07a5..8dcc6c02 100644
--- a/internal/zlibx/decompress_test.go
+++ b/internal/zlibx/decompress_test.go
@@ -6,21 +6,10 @@ import (
 	"testing"
 )
 
-func compressZlib(t *testing.T, payload, dict []byte) []byte {
+func compressZlib(t *testing.T, payload []byte) []byte {
 	t.Helper()
 	var buf bytes.Buffer
-	var (
-		w   *stdzlib.Writer
-		err error
-	)
-	if dict != nil {
-		w, err = stdzlib.NewWriterLevelDict(&buf, stdzlib.DefaultCompression, dict)
-	} else {
-		w = stdzlib.NewWriter(&buf)
-	}
-	if err != nil {
-		t.Fatalf("NewWriter: %v", err)
-	}
+	w := stdzlib.NewWriter(&buf)
 	if _, err := w.Write(payload); err != nil {
 		t.Fatalf("Write: %v", err)
 	}
@@ -32,7 +21,7 @@ func compressZlib(t *testing.T, payload, dict []byte) []byte {
 
 func TestDecompress(t *testing.T) {
 	payload := []byte("hello, zlib world!")
-	compressed := compressZlib(t, payload, nil)
+	compressed := compressZlib(t, payload)
 
 	out, err := Decompress(compressed)
 	if err != nil {
@@ -45,37 +34,9 @@ func TestDecompress(t *testing.T) {
 	}
 }
 
-func TestDecompressDict(t *testing.T) {
-	dict := []byte("git dictionary for zlib")
-	payload := append([]byte(nil), dict...)
-	payload = append(payload, []byte(" -- extended body -- extended body")...)
-	compressed := compressZlib(t, payload, dict)
-
-	out, err := DecompressDict(compressed, dict)
-	if err != nil {
-		t.Fatalf("DecompressDict: %v", err)
-	}
-	defer out.Release()
-
-	if !bytes.Equal(out.Bytes(), payload) {
-		t.Fatalf("unexpected payload %q", out.Bytes())
-	}
-}
-
-func TestDecompressDictMissing(t *testing.T) {
-	dict := []byte("preset dictionary")
-	payload := append([]byte(nil), dict...)
-	payload = append(payload, []byte(" .. more data ..")...)
-	compressed := compressZlib(t, payload, dict)
-
-	if _, err := Decompress(compressed); err != ErrDictionary {
-		t.Fatalf("expected ErrDictionary, got %v", err)
-	}
-}
-
 func TestDecompressChecksumError(t *testing.T) {
 	payload := []byte("checksum check")
-	compressed := compressZlib(t, payload, nil)
+	compressed := compressZlib(t, payload)
 	compressed[len(compressed)-1] ^= 0xff
 
 	if _, err := Decompress(compressed); err != ErrChecksum {
@@ -85,7 +46,7 @@ func TestDecompressChecksumError(t *testing.T) {
 
 func TestDecompressSizedUsesHint(t *testing.T) {
 	payload := []byte("tiny payload")
-	compressed := compressZlib(t, payload, nil)
+	compressed := compressZlib(t, payload)
 
 	const hint = 1 << 20
 	out, err := DecompressSized(compressed, hint)
diff --git a/internal/zlibx/reader.go b/internal/zlibx/reader.go
index 9a4b4315..6e3a53ea 100644
--- a/internal/zlibx/reader.go
+++ b/internal/zlibx/reader.go
@@ -55,8 +55,6 @@ const (
 var (
 	// ErrChecksum is returned when reading ZLIB data that has an invalid checksum.
 	ErrChecksum = errors.New("zlib: invalid checksum")
-	// ErrDictionary is returned when reading ZLIB data that has an invalid dictionary.
-	ErrDictionary = errors.New("zlib: invalid dictionary")
 	// ErrHeader is returned when reading ZLIB data that has an invalid header.
 	ErrHeader = errors.New("zlib: invalid header")
 )
@@ -82,19 +80,12 @@ type reader struct {
 // data than necessary from r.
 // It is the caller's responsibility to call Close on the ReadCloser when done.
 func NewReader(r io.Reader) (io.ReadCloser, error) {
-	return NewReaderDict(r, nil)
-}
-
-// NewReaderDict is like [NewReader] but uses a preset dictionary.
-// NewReaderDict ignores the dictionary if the compressed data does not refer to it.
-// If the compressed data refers to a different dictionary, NewReaderDict returns [ErrDictionary].
-func NewReaderDict(r io.Reader, dict []byte) (io.ReadCloser, error) {
 	v := pool.Get()
 	z, ok := v.(*reader)
 	if !ok {
 		panic("zlib: pool returned unexpected type")
 	}
-	err := z.Reset(r, dict)
+	err := z.Reset(r)
 	if err != nil {
 		return nil, err
 	}
@@ -147,7 +138,7 @@ func (z *reader) Close() error {
 	return nil
 }
 
-func (z *reader) Reset(r io.Reader, dict []byte) error {
+func (z *reader) Reset(r io.Reader) error {
 	*z = reader{decompressor: z.decompressor}
 	if fr, ok := r.(flatex.Reader); ok {
 		z.r = fr
@@ -168,30 +159,15 @@ func (z *reader) Reset(r io.Reader, dict []byte) error {
 		z.err = ErrHeader
 		return z.err
 	}
-	haveDict := z.scratch[1]&0x20 != 0
-	if haveDict {
-		_, z.err = io.ReadFull(z.r, z.scratch[0:4])
-		if z.err != nil {
-			if z.err == io.EOF {
-				z.err = io.ErrUnexpectedEOF
-			}
-			return z.err
-		}
-		checksum := binary.BigEndian.Uint32(z.scratch[:4])
-		if checksum != adler32.Checksum(dict) {
-			z.err = ErrDictionary
-			return z.err
-		}
+	if z.scratch[1]&0x20 != 0 {
+		z.err = ErrHeader
+		return z.err
 	}
 
 	if z.decompressor == nil {
-		if haveDict {
-			z.decompressor = flatex.NewReaderDict(z.r, dict)
-		} else {
-			z.decompressor = flatex.NewReader(z.r)
-		}
+		z.decompressor = flatex.NewReader(z.r)
 	} else {
-		z.err = z.decompressor.(flatex.Resetter).Reset(z.r, dict)
+		z.err = z.decompressor.(flatex.Resetter).Reset(z.r)
 		if z.err != nil {
 			return z.err
 		}