Initial attempt to make a compressor with less overhead

io.Reader actually has massive overhead...

5 files changed, 330 insertions, 0 deletions

diff --git a/internal/bench/decompress.go b/internal/bench/decompress.go
index c4a4d274..2eab0ad2 100644
--- a/internal/bench/decompress.go
+++ b/internal/bench/decompress.go
@@ -1,3 +1,5 @@
+//go:build ignore
+
 package main
 
 import (
diff --git a/internal/flate/decompress_bytes.go b/internal/flate/decompress_bytes.go
new file mode 100644
index 00000000..2cd9fd89
--- /dev/null
+++ b/internal/flate/decompress_bytes.go
@@ -0,0 +1,99 @@
+package flate
+
+import (
+	"io"
+	"sync"
+
+	"git.sr.ht/~runxiyu/furgit/internal/bufpool"
+)
+
+// byteSliceReader implements Reader over an in-memory byte slice.
+type byteSliceReader struct {
+	data []byte
+	off  int
+}
+
+func (r *byteSliceReader) Reset(data []byte) {
+	r.data = data
+	r.off = 0
+}
+
+func (r *byteSliceReader) Read(p []byte) (int, error) {
+	if r.off >= len(r.data) {
+		return 0, io.EOF
+	}
+	n := copy(p, r.data[r.off:])
+	r.off += n
+	return n, nil
+}
+
+func (r *byteSliceReader) ReadByte() (byte, error) {
+	if r.off >= len(r.data) {
+		return 0, io.EOF
+	}
+	b := r.data[r.off]
+	r.off++
+	return b, nil
+}
+
+// bufferDecompressor wraps the core decompressor with pooling state so that
+// byte-slice decompressions avoid repeated allocations.
+type bufferDecompressor struct {
+	dec    decompressor
+	reader byteSliceReader
+}
+
+var bufferDecompressorPool = sync.Pool{
+	New: func() any {
+		fixedHuffmanDecoderInit()
+		d := &bufferDecompressor{}
+		d.dec.bits = new([maxNumLit + maxNumDist]int)
+		d.dec.codebits = new([numCodes]int)
+		d.dec.step = (*decompressor).nextBlock
+		return d
+	},
+}
+
+// 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 DecompressDict(src, nil)
+}
+
+// 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) {
+	d := bufferDecompressorPool.Get().(*bufferDecompressor)
+	defer func() {
+		d.reader.Reset(nil)
+		bufferDecompressorPool.Put(d)
+	}()
+
+	d.reader.Reset(src)
+	if err := d.dec.Reset(&d.reader, dict); err != nil {
+		return bufpool.Buffer{}, 0, err
+	}
+
+	out := bufpool.Borrow(bufpool.DefaultBufferCap)
+	out.Resize(0)
+
+	for {
+		if len(d.dec.toRead) > 0 {
+			out.Append(d.dec.toRead)
+			d.dec.toRead = nil
+			continue
+		}
+		if d.dec.err != nil {
+			if d.dec.err == io.EOF {
+				return out, d.reader.off, nil
+			}
+			out.Release()
+			return bufpool.Buffer{}, 0, d.dec.err
+		}
+		d.dec.step(&d.dec)
+		if d.dec.err != nil && len(d.dec.toRead) == 0 {
+			d.dec.toRead = d.dec.dict.readFlush()
+		}
+	}
+}
diff --git a/internal/flate/decompress_test.go b/internal/flate/decompress_test.go
new file mode 100644
index 00000000..089159d6
--- /dev/null
+++ b/internal/flate/decompress_test.go
@@ -0,0 +1,76 @@
+package flate
+
+import (
+	"bytes"
+	stdflate "compress/flate"
+	"testing"
+)
+
+func compressDeflate(t *testing.T, payload, dict []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)
+	}
+	if err != nil {
+		t.Fatalf("NewWriter: %v", err)
+	}
+	if _, err := w.Write(payload); err != nil {
+		t.Fatalf("Write: %v", err)
+	}
+	if err := w.Close(); err != nil {
+		t.Fatalf("Close: %v", err)
+	}
+	return buf.Bytes()
+}
+
+func TestDecompress(t *testing.T) {
+	payload := bytes.Repeat([]byte("golang"), 32)
+	compressed := compressDeflate(t, payload, nil)
+
+	out, _, err := Decompress(compressed)
+	if err != nil {
+		t.Fatalf("Decompress: %v", err)
+	}
+	defer out.Release()
+
+	if !bytes.Equal(out.Bytes(), payload) {
+		t.Fatalf("unexpected payload: got %q", out.Bytes())
+	}
+}
+
+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")
+	}
+}
diff --git a/internal/zlib/decompress.go b/internal/zlib/decompress.go
new file mode 100644
index 00000000..55f6d3e2
--- /dev/null
+++ b/internal/zlib/decompress.go
@@ -0,0 +1,69 @@
+package zlib
+
+import (
+	"encoding/binary"
+	"io"
+
+	"git.sr.ht/~runxiyu/furgit/internal/adler32"
+	"git.sr.ht/~runxiyu/furgit/internal/bufpool"
+	"git.sr.ht/~runxiyu/furgit/internal/flate"
+)
+
+// 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 DecompressDict(src, nil)
+}
+
+// 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) {
+	if len(src) < 6 {
+		return bufpool.Buffer{}, io.ErrUnexpectedEOF
+	}
+
+	cmf := src[0]
+	flg := src[1]
+	if (cmf&0x0f != zlibDeflate) || (cmf>>4 > zlibMaxWindow) || (binary.BigEndian.Uint16(src[:2])%31 != 0) {
+		return bufpool.Buffer{}, ErrHeader
+	}
+
+	offset := 2
+	haveDict := flg&0x20 != 0
+	if haveDict {
+		if len(src) < offset+4 {
+			return bufpool.Buffer{}, io.ErrUnexpectedEOF
+		}
+		if dict == nil {
+			return bufpool.Buffer{}, ErrDictionary
+		}
+		checksum := binary.BigEndian.Uint32(src[offset : offset+4])
+		if checksum != adler32.Checksum(dict) {
+			return bufpool.Buffer{}, ErrDictionary
+		}
+		offset += 4
+	}
+
+	if len(src[offset:]) < 4 {
+		return bufpool.Buffer{}, io.ErrUnexpectedEOF
+	}
+
+	deflateData := src[offset:]
+	out, consumed, err := flate.DecompressDict(deflateData, dict)
+	if err != nil {
+		return bufpool.Buffer{}, err
+	}
+
+	checksumPos := offset + consumed
+	if checksumPos+4 > len(src) {
+		out.Release()
+		return bufpool.Buffer{}, io.ErrUnexpectedEOF
+	}
+	expected := binary.BigEndian.Uint32(src[checksumPos : checksumPos+4])
+	if expected != adler32.Checksum(out.Bytes()) {
+		out.Release()
+		return bufpool.Buffer{}, ErrChecksum
+	}
+	return out, nil
+}
diff --git a/internal/zlib/decompress_test.go b/internal/zlib/decompress_test.go
new file mode 100644
index 00000000..bb517ae6
--- /dev/null
+++ b/internal/zlib/decompress_test.go
@@ -0,0 +1,84 @@
+package zlib
+
+import (
+	"bytes"
+	stdzlib "compress/zlib"
+	"testing"
+)
+
+func compressZlib(t *testing.T, payload, dict []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)
+	}
+	if _, err := w.Write(payload); err != nil {
+		t.Fatalf("Write: %v", err)
+	}
+	if err := w.Close(); err != nil {
+		t.Fatalf("Close: %v", err)
+	}
+	return buf.Bytes()
+}
+
+func TestDecompress(t *testing.T) {
+	payload := []byte("hello, zlib world!")
+	compressed := compressZlib(t, payload, nil)
+
+	out, err := Decompress(compressed)
+	if err != nil {
+		t.Fatalf("Decompress: %v", err)
+	}
+	defer out.Release()
+
+	if !bytes.Equal(out.Bytes(), payload) {
+		t.Fatalf("unexpected payload %q", out.Bytes())
+	}
+}
+
+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[len(compressed)-1] ^= 0xff
+
+	if _, err := Decompress(compressed); err != ErrChecksum {
+		t.Fatalf("expected ErrChecksum, got %v", err)
+	}
+}