zstd: Add RLE detection+encoding

Add full block RLE encoding.

Fastest+Default: The first block in RLE will be detected automatically, but following will likely be backreferences.

Better+Best: Explicit RLE block detection. Will always be encoded as RLE blocks.

Full block RLE are very uncommon and the gains are small, but can happen in very sparse data, so makes sense for "stronger" settings.

Add feature #937

zstd/blockenc.go

@@ -427,6 +427,16 @@ func (b *blockEnc) encodeLits(lits []byte, raw bool) error {
 	return nil
 }
 
+// encodeRLE will encode an RLE block.
+func (b *blockEnc) encodeRLE(val byte, length uint32) {
+	var bh blockHeader
+	bh.setLast(b.last)
+	bh.setSize(length)
+	bh.setType(blockTypeRLE)
+	b.output = bh.appendTo(b.output)
+	b.output = append(b.output, val)
+}
+
 // fuzzFseEncoder can be used to fuzz the FSE encoder.
 func fuzzFseEncoder(data []byte) int {
 	if len(data) > maxSequences || len(data) < 2 {
@@ -479,6 +489,16 @@ func (b *blockEnc) encode(org []byte, raw, rawAllLits bool) error {
 	if len(b.sequences) == 0 {
 		return b.encodeLits(b.literals, rawAllLits)
 	}
+	if len(b.sequences) == 1 && len(org) > 0 && len(b.literals) <= 1 {
+		// Check common RLE cases.
+		seq := b.sequences[0]
+		if seq.litLen == uint32(len(b.literals)) && seq.offset-3 == 1 {
+			// Offset == 1 and 0 or 1 literals.
+			b.encodeRLE(org[0], b.sequences[0].matchLen+zstdMinMatch+seq.litLen)
+			return nil
+		}
+	}
+
 	// We want some difference to at least account for the headers.
 	saved := b.size - len(b.literals) - (b.size >> 6)
 	if saved < 16 {

zstd/enc_best.go

@@ -135,8 +135,20 @@ func (e *bestFastEncoder) Encode(blk *blockEnc, src []byte) {
 		break
 	}
 
+	// Add block to history
 	s := e.addBlock(src)
 	blk.size = len(src)
+
+	// Check RLE first
+	if len(src) > zstdMinMatch {
+		ml := matchLen(src[1:], src)
+		if ml == len(src)-1 {
+			blk.literals = append(blk.literals, src[0])
+			blk.sequences = append(blk.sequences, seq{litLen: 1, matchLen: uint32(len(src)-1) - zstdMinMatch, offset: 1 + 3})
+			return
+		}
+	}
+
 	if len(src) < minNonLiteralBlockSize {
 		blk.extraLits = len(src)
 		blk.literals = blk.literals[:len(src)]

zstd/enc_better.go

@@ -102,9 +102,20 @@ func (e *betterFastEncoder) Encode(blk *blockEnc, src []byte) {
 		e.cur = e.maxMatchOff
 		break
 	}
-
+	// Add block to history
 	s := e.addBlock(src)
 	blk.size = len(src)
+
+	// Check RLE first
+	if len(src) > zstdMinMatch {
+		ml := matchLen(src[1:], src)
+		if ml == len(src)-1 {
+			blk.literals = append(blk.literals, src[0])
+			blk.sequences = append(blk.sequences, seq{litLen: 1, matchLen: uint32(len(src)-1) - zstdMinMatch, offset: 1 + 3})
+			return
+		}
+	}
+
 	if len(src) < minNonLiteralBlockSize {
 		blk.extraLits = len(src)
 		blk.literals = blk.literals[:len(src)]

zstd/encoder_test.go

@@ -342,6 +342,52 @@ func TestEncoder_EncodeAllTwain(t *testing.T) {
 	}
 }
 
+func TestEncoder_EncodeRLE(t *testing.T) {
+	in := make([]byte, 1<<20)
+	testWindowSizes := testWindowSizes
+	if testing.Short() {
+		testWindowSizes = []int{1 << 20}
+	}
+
+	dec, err := NewReader(nil)
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer dec.Close()
+
+	for level := speedNotSet + 1; level < speedLast; level++ {
+		t.Run(level.String(), func(t *testing.T) {
+			if isRaceTest && level >= SpeedBestCompression {
+				t.SkipNow()
+			}
+			for _, windowSize := range testWindowSizes {
+				t.Run(fmt.Sprintf("window:%d", windowSize), func(t *testing.T) {
+					e, err := NewWriter(nil, WithEncoderLevel(level), WithWindowSize(windowSize))
+					if err != nil {
+						t.Fatal(err)
+					}
+					defer e.Close()
+					start := time.Now()
+					dst := e.EncodeAll(in, nil)
+					t.Log("Simple Encoder len", len(in), "-> zstd len", len(dst))
+					mbpersec := (float64(len(in)) / (1024 * 1024)) / (float64(time.Since(start)) / (float64(time.Second)))
+					t.Logf("Encoded %d bytes with %.2f MB/s", len(in), mbpersec)
+
+					decoded, err := dec.DecodeAll(dst, nil)
+					if err != nil {
+						t.Error(err, len(decoded))
+					}
+					if !bytes.Equal(decoded, in) {
+						os.WriteFile("testdata/"+t.Name()+"-Mark.Twain-Tom.Sawyer.txt.got", decoded, os.ModePerm)
+						t.Fatal("Decoded does not match")
+					}
+					t.Log("Encoded content matched")
+				})
+			}
+		})
+	}
+}
+
 func TestEncoder_EncodeAllPi(t *testing.T) {
 	in, err := os.ReadFile("../testdata/pi.txt")
 	if err != nil {