zstd: Respect WithAllLitEntropyCompression (#792)

* zstd: Respect WithAllLitEntropyCompression

With no sequences (matches) literal compression was turned off.

Re-enable this and simplify code. Set as default for "Better" and "Best".

* Fall back to raw block when unable to improve.

zstd/blockenc.go

@@ -473,7 +473,7 @@ func (b *blockEnc) encode(org []byte, raw, rawAllLits bool) error {
 		return b.encodeLits(b.literals, rawAllLits)
 	}
 	// We want some difference to at least account for the headers.
-	saved := b.size - len(b.literals) - (b.size >> 5)
+	saved := b.size - len(b.literals) - (b.size >> 6)
 	if saved < 16 {
 		if org == nil {
 			return errIncompressible
@@ -779,10 +779,12 @@ func (b *blockEnc) encode(org []byte, raw, rawAllLits bool) error {
 	}
 	b.output = wr.out
 
+	// Maybe even add a bigger margin.
 	if len(b.output)-3-bhOffset >= b.size {
-		// Maybe even add a bigger margin.
+		// Discard and encode as raw block.
+		b.encodeRaw(org)
 		b.litEnc.Reuse = huff0.ReusePolicyNone
-		return errIncompressible
+		return nil
 	}
 
 	// Size is output minus block header.

zstd/encoder.go

@@ -277,23 +277,9 @@ func (e *Encoder) nextBlock(final bool) error {
 			s.eofWritten = true
 		}
 
-		err := errIncompressible
-		// If we got the exact same number of literals as input,
-		// assume the literals cannot be compressed.
-		if len(src) != len(blk.literals) || len(src) != e.o.blockSize {
-			err = blk.encode(src, e.o.noEntropy, !e.o.allLitEntropy)
-		}
-		switch err {
-		case errIncompressible:
-			if debugEncoder {
-				println("Storing incompressible block as raw")
-			}
-			blk.encodeRaw(src)
-			// In fast mode, we do not transfer offsets, so we don't have to deal with changing the.
-		case nil:
-		default:
-			s.err = err
-			return err
+		s.err = blk.encode(src, e.o.noEntropy, !e.o.allLitEntropy)
+		if s.err != nil {
+			return s.err
 		}
 		_, s.err = s.w.Write(blk.output)
 		s.nWritten += int64(len(blk.output))
@@ -343,22 +329,8 @@ func (e *Encoder) nextBlock(final bool) error {
 				}
 				s.wWg.Done()
 			}()
-			err := errIncompressible
-			// If we got the exact same number of literals as input,
-			// assume the literals cannot be compressed.
-			if len(src) != len(blk.literals) || len(src) != e.o.blockSize {
-				err = blk.encode(src, e.o.noEntropy, !e.o.allLitEntropy)
-			}
-			switch err {
-			case errIncompressible:
-				if debugEncoder {
-					println("Storing incompressible block as raw")
-				}
-				blk.encodeRaw(src)
-				// In fast mode, we do not transfer offsets, so we don't have to deal with changing the.
-			case nil:
-			default:
-				s.writeErr = err
+			s.writeErr = blk.encode(src, e.o.noEntropy, !e.o.allLitEntropy)
+			if s.writeErr != nil {
 				return
 			}
 			_, s.writeErr = s.w.Write(blk.output)
@@ -568,25 +540,15 @@ func (e *Encoder) EncodeAll(src, dst []byte) []byte {
 
 		// If we got the exact same number of literals as input,
 		// assume the literals cannot be compressed.
-		err := errIncompressible
 		oldout := blk.output
-		if len(blk.literals) != len(src) || len(src) != e.o.blockSize {
-			// Output directly to dst
-			blk.output = dst
-			err = blk.encode(src, e.o.noEntropy, !e.o.allLitEntropy)
-		}
+		// Output directly to dst
+		blk.output = dst
 
-		switch err {
-		case errIncompressible:
-			if debugEncoder {
-				println("Storing incompressible block as raw")
-			}
-			dst = blk.encodeRawTo(dst, src)
-		case nil:
-			dst = blk.output
-		default:
+		err := blk.encode(src, e.o.noEntropy, !e.o.allLitEntropy)
+		if err != nil {
 			panic(err)
 		}
+		dst = blk.output
 		blk.output = oldout
 	} else {
 		enc.Reset(e.o.dict, false)
@@ -605,25 +567,11 @@ func (e *Encoder) EncodeAll(src, dst []byte) []byte {
 			if len(src) == 0 {
 				blk.last = true
 			}
-			err := errIncompressible
-			// If we got the exact same number of literals as input,
-			// assume the literals cannot be compressed.
-			if len(blk.literals) != len(todo) || len(todo) != e.o.blockSize {
-				err = blk.encode(todo, e.o.noEntropy, !e.o.allLitEntropy)
-			}
-
-			switch err {
-			case errIncompressible:
-				if debugEncoder {
-					println("Storing incompressible block as raw")
-				}
-				dst = blk.encodeRawTo(dst, todo)
-				blk.popOffsets()
-			case nil:
-				dst = append(dst, blk.output...)
-			default:
+			err := blk.encode(todo, e.o.noEntropy, !e.o.allLitEntropy)
+			if err != nil {
 				panic(err)
 			}
+			dst = append(dst, blk.output...)
 			blk.reset(nil)
 		}
 	}

zstd/encoder_options.go

@@ -39,7 +39,7 @@ func (o *encoderOptions) setDefault() {
 		blockSize:     maxCompressedBlockSize,
 		windowSize:    8 << 20,
 		level:         SpeedDefault,
-		allLitEntropy: true,
+		allLitEntropy: false,
 		lowMem:        false,
 	}
 }
@@ -238,7 +238,7 @@ func WithEncoderLevel(l EncoderLevel) EOption {
 			}
 		}
 		if !o.customALEntropy {
-			o.allLitEntropy = l > SpeedFastest
+			o.allLitEntropy = l > SpeedDefault
 		}
 
 		return nil