Add stateless gzip/deflate

flate/deflate.go

@@ -322,8 +322,6 @@ func (d *compressor) writeStoredBlock(buf []byte) error {
 	return d.w.err
 }
 
-const hashmul = 0x1e35a7bd
-
 // hash4 returns a hash representation of the first 4 bytes
 // of the supplied slice.
 // The caller must ensure that len(b) >= 4.

flate/stateless.go

@@ -0,0 +1,252 @@
+package flate
+
+import (
+	"io"
+	"math"
+)
+
+const (
+	maxStatelessBlock = math.MaxInt16
+
+	slTableBits  = 13
+	slTableSize  = 1 << slTableBits
+	slTableShift = 32 - slTableBits
+)
+
+type statelessWriter struct {
+	dst    io.Writer
+	closed bool
+}
+
+func (s *statelessWriter) Close() error {
+	if s.closed {
+		return nil
+	}
+	s.closed = true
+	// Emit EOF block
+	return StatelessDeflate(s.dst, nil, true)
+}
+
+func (s *statelessWriter) Write(p []byte) (n int, err error) {
+	err = StatelessDeflate(s.dst, p, false)
+	if err != nil {
+		return 0, err
+	}
+	return len(p), nil
+}
+
+func (s *statelessWriter) Reset(w io.Writer) {
+	s.dst = w
+	s.closed = false
+}
+
+// NewStatelessWriter will do compression but without maintaining any state
+// between Write calls.
+// There will be no memory kept between Write calls,
+// but compression and speed will be suboptimal.
+// Because of this, the size of actual Write calls will affect output size.
+func NewStatelessWriter(dst io.Writer) io.WriteCloser {
+	return &statelessWriter{dst: dst}
+}
+
+// StatelessDeflate allows to compress directly to a Writer without retaining state.
+// When returning everything will be flushed.
+func StatelessDeflate(out io.Writer, in []byte, eof bool) error {
+	var dst tokens
+	bw := newHuffmanBitWriter(out)
+	if eof && len(in) == 0 {
+		// Just write an EOF block.
+		// Could be faster...
+		bw.writeStoredHeader(0, true)
+		bw.flush()
+		return bw.err
+	}
+
+	for len(in) > 0 {
+		todo := in
+		if len(todo) > maxStatelessBlock {
+			todo = todo[:maxStatelessBlock]
+		}
+		in = in[len(todo):]
+		// Compress
+		statelessEnc(&dst, todo)
+		isEof := eof && len(in) == 0
+
+		if dst.n == 0 {
+			bw.writeStoredHeader(len(todo), isEof)
+			if bw.err != nil {
+				return bw.err
+			}
+			bw.writeBytes(todo)
+		} else if int(dst.n) > len(todo)-len(todo)>>4 {
+			// If we removed less than 1/16th, huffman compress the block.
+			bw.writeBlockHuff(isEof, todo, false)
+		} else {
+			bw.writeBlockDynamic(&dst, isEof, todo, false)
+		}
+		if bw.err != nil {
+			return bw.err
+		}
+		dst.Reset()
+	}
+	if !eof {
+		// Align.
+		bw.writeStoredHeader(0, false)
+	}
+	bw.flush()
+	return bw.err
+}
+
+func hashSL(u uint32) uint32 {
+	return (u * 0x1e35a7bd) >> slTableShift
+}
+
+func load3216(b []byte, i int16) uint32 {
+	// Help the compiler eliminate bounds checks on the read so it can be done in a single read.
+	b = b[i:]
+	b = b[:4]
+	return uint32(b[0]) | uint32(b[1])<<8 | uint32(b[2])<<16 | uint32(b[3])<<24
+}
+
+func load6416(b []byte, i int16) uint64 {
+	// Help the compiler eliminate bounds checks on the read so it can be done in a single read.
+	b = b[i:]
+	b = b[:8]
+	return uint64(b[0]) | uint64(b[1])<<8 | uint64(b[2])<<16 | uint64(b[3])<<24 |
+		uint64(b[4])<<32 | uint64(b[5])<<40 | uint64(b[6])<<48 | uint64(b[7])<<56
+}
+
+func statelessEnc(dst *tokens, src []byte) {
+	const (
+		inputMargin            = 12 - 1
+		minNonLiteralBlockSize = 1 + 1 + inputMargin
+	)
+
+	type tableEntry struct {
+		offset int16
+	}
+
+	var table [slTableSize]tableEntry
+
+	// This check isn't in the Snappy implementation, but there, the caller
+	// instead of the callee handles this case.
+	if len(src) < minNonLiteralBlockSize {
+		// We do not fill the token table.
+		// This will be picked up by caller.
+		dst.n = uint16(len(src))
+		return
+	}
+
+	s := int16(1)
+	nextEmit := int16(0)
+	// sLimit is when to stop looking for offset/length copies. The inputMargin
+	// lets us use a fast path for emitLiteral in the main loop, while we are
+	// looking for copies.
+	sLimit := int16(len(src) - inputMargin)
+
+	// nextEmit is where in src the next emitLiteral should start from.
+	cv := load3216(src, s)
+
+	for {
+		const skipLog = 5
+		const doEvery = 2
+
+		nextS := s
+		var candidate tableEntry
+		for {
+			nextHash := hashSL(cv)
+			candidate = table[nextHash]
+			nextS = s + doEvery + (s-nextEmit)>>skipLog
+			if nextS > sLimit || nextS <= 0 {
+				goto emitRemainder
+			}
+
+			now := load6416(src, nextS)
+			table[nextHash] = tableEntry{offset: s}
+			nextHash = hashSL(uint32(now))
+
+			if cv == load3216(src, candidate.offset) {
+				table[nextHash] = tableEntry{offset: nextS}
+				break
+			}
+
+			// Do one right away...
+			cv = uint32(now)
+			s = nextS
+			nextS++
+			candidate = table[nextHash]
+			now >>= 8
+			table[nextHash] = tableEntry{offset: s}
+
+			if cv == load3216(src, candidate.offset) {
+				table[nextHash] = tableEntry{offset: nextS}
+				break
+			}
+			cv = uint32(now)
+			s = nextS
+		}
+
+		// A 4-byte match has been found. We'll later see if more than 4 bytes
+		// match. But, prior to the match, src[nextEmit:s] are unmatched. Emit
+		// them as literal bytes.
+		for {
+			// Invariant: we have a 4-byte match at s, and no need to emit any
+			// literal bytes prior to s.
+
+			// Extend the 4-byte match as long as possible.
+			t := candidate.offset
+			l := int16(matchLen(src[s+4:], src[t+4:]) + 4)
+
+			// Extend backwards
+			for t > 0 && s > nextEmit && src[t-1] == src[s-1] {
+				s--
+				t--
+				l++
+			}
+			if nextEmit < s {
+				emitLiteral(dst, src[nextEmit:s])
+			}
+
+			// Save the match found
+			dst.AddMatchLong(int32(l), uint32(s-t-baseMatchOffset))
+			s += l
+			nextEmit = s
+			if nextS >= s {
+				s = nextS + 1
+			}
+			if s >= sLimit {
+				goto emitRemainder
+			}
+
+			// We could immediately start working at s now, but to improve
+			// compression we first update the hash table at s-2 and at s. If
+			// another emitCopy is not our next move, also calculate nextHash
+			// at s+1. At least on GOARCH=amd64, these three hash calculations
+			// are faster as one load64 call (with some shifts) instead of
+			// three load32 calls.
+			x := load6416(src, s-2)
+			o := s - 2
+			prevHash := hashSL(uint32(x))
+			table[prevHash] = tableEntry{offset: o}
+			x >>= 16
+			currHash := hashSL(uint32(x))
+			candidate = table[currHash]
+			table[currHash] = tableEntry{offset: o + 2}
+
+			if uint32(x) != load3216(src, candidate.offset) {
+				cv = uint32(x >> 8)
+				s++
+				break
+			}
+		}
+	}
+
+emitRemainder:
+	if int(nextEmit) < len(src) {
+		// If nothing was added, don't encode literals.
+		if dst.n == 0 {
+			return
+		}
+		emitLiteral(dst, src[nextEmit:])
+	}
+}

flate/writer_test.go

@@ -144,6 +144,9 @@ func BenchmarkEncodeDigitsDefault1e6(b *testing.B)  { benchmarkEncoder(b, digits
 func BenchmarkEncodeDigitsCompress1e4(b *testing.B) { benchmarkEncoder(b, digits, compress, 1e4) }
 func BenchmarkEncodeDigitsCompress1e5(b *testing.B) { benchmarkEncoder(b, digits, compress, 1e5) }
 func BenchmarkEncodeDigitsCompress1e6(b *testing.B) { benchmarkEncoder(b, digits, compress, 1e6) }
+func BenchmarkEncodeDigitsSL1e4(b *testing.B)       { benchmarkStatelessEncoder(b, digits, 1e4) }
+func BenchmarkEncodeDigitsSL1e5(b *testing.B)       { benchmarkStatelessEncoder(b, digits, 1e5) }
+func BenchmarkEncodeDigitsSL1e6(b *testing.B)       { benchmarkStatelessEncoder(b, digits, 1e6) }
 func BenchmarkEncodeTwainConstant1e4(b *testing.B)  { benchmarkEncoder(b, twain, constant, 1e4) }
 func BenchmarkEncodeTwainConstant1e5(b *testing.B)  { benchmarkEncoder(b, twain, constant, 1e5) }
 func BenchmarkEncodeTwainConstant1e6(b *testing.B)  { benchmarkEncoder(b, twain, constant, 1e6) }
@@ -156,6 +159,42 @@ func BenchmarkEncodeTwainDefault1e6(b *testing.B)   { benchmarkEncoder(b, twain,
 func BenchmarkEncodeTwainCompress1e4(b *testing.B)  { benchmarkEncoder(b, twain, compress, 1e4) }
 func BenchmarkEncodeTwainCompress1e5(b *testing.B)  { benchmarkEncoder(b, twain, compress, 1e5) }
 func BenchmarkEncodeTwainCompress1e6(b *testing.B)  { benchmarkEncoder(b, twain, compress, 1e6) }
+func BenchmarkEncodeTwainSL1e4(b *testing.B)        { benchmarkStatelessEncoder(b, twain, 1e4) }
+func BenchmarkEncodeTwainSL1e5(b *testing.B)        { benchmarkStatelessEncoder(b, twain, 1e5) }
+func BenchmarkEncodeTwainSL1e6(b *testing.B)        { benchmarkStatelessEncoder(b, twain, 1e6) }
+
+func benchmarkStatelessEncoder(b *testing.B, testfile, n int) {
+	b.SetBytes(int64(n))
+	buf0, err := ioutil.ReadFile(testfiles[testfile])
+	if err != nil {
+		b.Fatal(err)
+	}
+	if len(buf0) == 0 {
+		b.Fatalf("test file %q has no data", testfiles[testfile])
+	}
+	buf1 := make([]byte, n)
+	for i := 0; i < n; i += len(buf0) {
+		if len(buf0) > n-i {
+			buf0 = buf0[:n-i]
+		}
+		copy(buf1[i:], buf0)
+	}
+	buf0 = nil
+	runtime.GC()
+	b.ResetTimer()
+	b.ReportAllocs()
+	for i := 0; i < b.N; i++ {
+		w := NewStatelessWriter(ioutil.Discard)
+		_, err = w.Write(buf1)
+		if err != nil {
+			b.Fatal(err)
+		}
+		err = w.Close()
+		if err != nil {
+			b.Fatal(err)
+		}
+	}
+}
 
 // A writer that fails after N writes.
 type errorWriter struct {

gzip/gzip.go

@@ -22,6 +22,13 @@ const (
 	DefaultCompression  = flate.DefaultCompression
 	ConstantCompression = flate.ConstantCompression
 	HuffmanOnly         = flate.HuffmanOnly
+
+	// StatelessCompression will do compression but without maintaining any state
+	// between Write calls.
+	// There will be no memory kept between Write calls,
+	// but compression and speed will be suboptimal.
+	// Because of this, the size of actual Write calls will affect output size.
+	StatelessCompression = -3
 )
 
 // A Writer is an io.WriteCloser.
@@ -59,7 +66,7 @@ func NewWriter(w io.Writer) *Writer {
 // integer value between BestSpeed and BestCompression inclusive. The error
 // returned will be nil if the level is valid.
 func NewWriterLevel(w io.Writer, level int) (*Writer, error) {
-	if level < HuffmanOnly || level > BestCompression {
+	if level < StatelessCompression || level > BestCompression {
 		return nil, fmt.Errorf("gzip: invalid compression level: %d", level)
 	}
 	z := new(Writer)
@@ -69,9 +76,12 @@ func NewWriterLevel(w io.Writer, level int) (*Writer, error) {
 
 func (z *Writer) init(w io.Writer, level int) {
 	compressor := z.compressor
-	if compressor != nil {
-		compressor.Reset(w)
+	if level != StatelessCompression {
+		if compressor != nil {
+			compressor.Reset(w)
+		}
 	}
+
 	*z = Writer{
 		Header: Header{
 			OS: 255, // unknown
@@ -189,12 +199,16 @@ func (z *Writer) Write(p []byte) (int, error) {
 				return n, z.err
 			}
 		}
-		if z.compressor == nil {
+
+		if z.compressor == nil && z.level != StatelessCompression {
 			z.compressor, _ = flate.NewWriter(z.w, z.level)
 		}
 	}
 	z.size += uint32(len(p))
 	z.digest = crc32.Update(z.digest, crc32.IEEETable, p)
+	if z.level == StatelessCompression {
+		return len(p), flate.StatelessDeflate(z.w, p, false)
+	}
 	n, z.err = z.compressor.Write(p)
 	return n, z.err
 }
@@ -211,7 +225,7 @@ func (z *Writer) Flush() error {
 	if z.err != nil {
 		return z.err
 	}
-	if z.closed {
+	if z.closed || z.level == StatelessCompression {
 		return nil
 	}
 	if !z.wroteHeader {
@@ -240,7 +254,11 @@ func (z *Writer) Close() error {
 			return z.err
 		}
 	}
-	z.err = z.compressor.Close()
+	if z.level == StatelessCompression {
+		z.err = flate.StatelessDeflate(z.w, nil, true)
+	} else {
+		z.err = z.compressor.Close()
+	}
 	if z.err != nil {
 		return z.err
 	}