zstd: Fix reuse of huff0 when data hard to compress

zstd would reject huff0 compressed literals if the improvement was too small.

However, this would update the huff0 state to contain a new table which could be reused. In that case a wrong table could be used for the next block.

We move the rejection code to huff0, so the state can be properly maintained.

Fixes #170

huff0/compress.go

@@ -54,6 +54,12 @@ func compress(in []byte, s *Scratch, compressor func(src []byte) ([]byte, error)
 		canReuse = s.canUseTable(s.prevTable)
 	}
 
+	// We want the output size to be less than this:
+	wantSize := len(in)
+	if s.WantLogLess > 0 {
+		wantSize -= wantSize >> s.WantLogLess
+	}
+
 	// Reset for next run.
 	s.clearCount = true
 	s.maxCount = 0
@@ -77,7 +83,7 @@ func compress(in []byte, s *Scratch, compressor func(src []byte) ([]byte, error)
 		s.cTable = s.prevTable
 		s.Out, err = compressor(in)
 		s.cTable = keepTable
-		if err == nil && len(s.Out) < len(in) {
+		if err == nil && len(s.Out) < wantSize {
 			s.OutData = s.Out
 			return s.Out, true, nil
 		}
@@ -100,16 +106,16 @@ func compress(in []byte, s *Scratch, compressor func(src []byte) ([]byte, error)
 		hSize := len(s.Out)
 		oldSize := s.prevTable.estimateSize(s.count[:s.symbolLen])
 		newSize := s.cTable.estimateSize(s.count[:s.symbolLen])
-		if oldSize <= hSize+newSize || hSize+12 >= len(in) {
+		if oldSize <= hSize+newSize || hSize+12 >= wantSize {
 			// Retain cTable even if we re-use.
 			keepTable := s.cTable
 			s.cTable = s.prevTable
 			s.Out, err = compressor(in)
+			s.cTable = keepTable
 			if err != nil {
 				return nil, false, err
 			}
-			s.cTable = keepTable
-			if len(s.Out) >= len(in) {
+			if len(s.Out) >= wantSize {
 				return nil, false, ErrIncompressible
 			}
 			s.OutData = s.Out
@@ -131,7 +137,7 @@ func compress(in []byte, s *Scratch, compressor func(src []byte) ([]byte, error)
 		s.OutTable = nil
 		return nil, false, err
 	}
-	if len(s.Out) >= len(in) {
+	if len(s.Out) >= wantSize {
 		s.OutTable = nil
 		return nil, false, ErrIncompressible
 	}

huff0/huff0.go

@@ -89,6 +89,12 @@ type Scratch struct {
 	// Reuse will specify the reuse policy
 	Reuse ReusePolicy
 
+	// WantLogLess allows to specify a log 2 reduction that should at least be achieved,
+	// otherwise the block will be returned as incompressible.
+	// The reduction should then at least be (input size >> WantLogLess)
+	// If WantLogLess == 0 any improvement will do.
+	WantLogLess uint8
+
 	// MaxDecodedSize will set the maximum allowed output size.
 	// This value will automatically be set to BlockSizeMax if not set.
 	// Decoders will return ErrMaxDecodedSizeExceeded is this limit is exceeded.

zstd/blockenc.go

@@ -51,7 +51,7 @@ func (b *blockEnc) init() {
 		b.coders.llEnc = &fseEncoder{}
 		b.coders.llPrev = &fseEncoder{}
 	}
-	b.litEnc = &huff0.Scratch{}
+	b.litEnc = &huff0.Scratch{WantLogLess: 4}
 	b.reset(nil)
 }
 
@@ -415,16 +415,10 @@ func (b *blockEnc) encode() error {
 	if len(b.literals) >= 1024 {
 		// Use 4 Streams.
 		out, reUsed, err = huff0.Compress4X(b.literals, b.litEnc)
-		if len(out) > len(b.literals)-len(b.literals)>>4 {
-			err = huff0.ErrIncompressible
-		}
 	} else if len(b.literals) > 32 {
 		// Use 1 stream
 		single = true
 		out, reUsed, err = huff0.Compress1X(b.literals, b.litEnc)
-		if len(out) > len(b.literals)-len(b.literals)>>4 {
-			err = huff0.ErrIncompressible
-		}
 	} else {
 		err = huff0.ErrIncompressible
 	}
@@ -711,7 +705,7 @@ func (b *blockEnc) encode() error {
 	return nil
 }
 
-var errIncompressible = errors.New("uncompressible")
+var errIncompressible = errors.New("incompressible")
 
 func (b *blockEnc) genCodes() {
 	if len(b.sequences) == 0 {

zstd/enc_dfast.go

@@ -235,7 +235,7 @@ encodeLoop:
 				if debug && s-t > e.maxMatchOff {
 					panic("s - t >e.maxMatchOff")
 				}
-				if debug {
+				if debugMatches {
 					println("long match")
 				}
 				break
@@ -259,7 +259,7 @@ encodeLoop:
 					// but the likelihood of both the first 4 bytes and the hash matching should be enough.
 					t = candidateL.offset - e.cur
 					s += checkAt
-					if debug {
+					if debugMatches {
 						println("long match (after short)")
 					}
 					break
@@ -275,7 +275,7 @@ encodeLoop:
 				if debug && t < 0 {
 					panic("t<0")
 				}
-				if debug {
+				if debugMatches {
 					println("short match")
 				}
 				break

zstd/zstd.go

@@ -11,6 +11,7 @@ import (
 
 const debug = false
 const debugSequences = false
+const debugMatches = false
 
 // force encoder to use predefined tables.
 const forcePreDef = false