zstd: Add experimental encoder dictionaries

huff0/huff0.go

@@ -119,6 +119,16 @@ type Scratch struct {
 	huffWeight     [maxSymbolValue + 1]byte
 }
 
+// TransferCTable will transfer the previously used compression table.
+func (s *Scratch) TransferCTable(src *Scratch) {
+	if cap(s.prevTable) < len(src.prevTable) {
+		s.prevTable = make(cTable, 0, maxSymbolValue+1)
+	}
+	s.prevTable = s.prevTable[:len(src.prevTable)]
+	copy(s.prevTable, src.prevTable)
+	s.prevTableLog = src.prevTableLog
+}
+
 func (s *Scratch) prepare(in []byte) (*Scratch, error) {
 	if len(in) > BlockSizeMax {
 		return nil, ErrTooBig

zstd/README.md

@@ -5,7 +5,6 @@ It offers a very wide range of compression / speed trade-off, while being backed
 A high performance compression algorithm is implemented. For now focused on speed. 
 
 This package provides [compression](#Compressor) to and [decompression](#Decompressor) of Zstandard content. 
-Note that custom dictionaries are only supported for decompression.
 
 This package is pure Go and without use of "unsafe". 
 
@@ -232,41 +231,6 @@ nyc-taxi-data-10M.csv   gzstd   1   3325605752  928656485   23876   132.83
 nyc-taxi-data-10M.csv   gzkp    1   3325605752  924718719   16388   193.53
 ```
 
-### Converters
-
-As part of the development process a *Snappy* -> *Zstandard* converter was also built.
-
-This can convert a *framed* [Snappy Stream](https://godoc.org/github.com/golang/snappy#Writer) to a zstd stream. 
-Note that a single block is not framed.
-
-Conversion is done by converting the stream directly from Snappy without intermediate full decoding.
-Therefore the compression ratio is much less than what can be done by a full decompression
-and compression, and a faulty Snappy stream may lead to a faulty Zstandard stream without
-any errors being generated.
-No CRC value is being generated and not all CRC values of the Snappy stream are checked.
-However, it provides really fast re-compression of Snappy streams.
-
-
-```
-BenchmarkSnappy_ConvertSilesia-8           1  1156001600 ns/op   183.35 MB/s
-Snappy len 103008711 -> zstd len 82687318
-
-BenchmarkSnappy_Enwik9-8           1  6472998400 ns/op   154.49 MB/s
-Snappy len 508028601 -> zstd len 390921079
-```
-
-
-```Go
-    s := zstd.SnappyConverter{}
-    n, err = s.Convert(input, output)
-    if err != nil {
-        fmt.Println("Re-compressed stream to", n, "bytes")
-    }
-```
-
-The converter `s` can be reused to avoid allocations, even after errors.
-
-
 ## Decompressor
 
 Staus: STABLE - there may still be subtle bugs, but a wide variety of content has been tested.
@@ -337,6 +301,21 @@ A re-used Decoder will still contain the dictionaries registered.
 
 When registering multiple dictionaries with the same ID, the last one will be used.
 
+It is possible to use dictionaries when compressing data.
+
+To enable a dictionary use `WithEncoderDict(dict []byte)`. Here only one dictionary will be used 
+and it will likely be used even if it doesn't improve compression. 
+
+The used dictionary must be used to decompress the content.
+
+For any real gains, the dictionary should be built with similar data. 
+If an unsuitable dictionary is used the output may be slightly larger than using no dictionary.
+Use the [zstd commandline tool](https://github.com/facebook/zstd/releases) to build a dictionary from sample data.
+For information see [zstd dictionary information](https://github.com/facebook/zstd#the-case-for-small-data-compression). 
+
+For now there is a fixed startup performance penalty for compressing content with dictionaries. 
+This will likely be improved over time. Just be aware to test performance when implementing.  
+
 ### Allocation-less operation
 
 The decoder has been designed to operate without allocations after a warmup. 

zstd/blockdec.go

@@ -646,7 +646,7 @@ func (b *blockDec) decodeCompressed(hist *history) error {
 		}
 	} else {
 		if hist.huffTree != nil && huff != nil {
-			if hist.dict == nil || hist.dict.litDec != hist.huffTree {
+			if hist.dict == nil || hist.dict.litEnc != hist.huffTree {
 				huffDecoderPool.Put(hist.huffTree)
 			}
 			hist.huffTree = nil

zstd/blockenc.go

@@ -14,12 +14,13 @@ import (
 )
 
 type blockEnc struct {
-	size      int
-	literals  []byte
-	sequences []seq
-	coders    seqCoders
-	litEnc    *huff0.Scratch
-	wr        bitWriter
+	size       int
+	literals   []byte
+	sequences  []seq
+	coders     seqCoders
+	litEnc     *huff0.Scratch
+	dictLitEnc *huff0.Scratch
+	wr         bitWriter
 
 	extraLits int
 	last      bool
@@ -314,19 +315,19 @@ func (b *blockEnc) encodeRawTo(dst, src []byte) []byte {
 }
 
 // encodeLits can be used if the block is only litLen.
-func (b *blockEnc) encodeLits(raw bool) error {
+func (b *blockEnc) encodeLits(lits []byte, raw bool) error {
 	var bh blockHeader
 	bh.setLast(b.last)
-	bh.setSize(uint32(len(b.literals)))
+	bh.setSize(uint32(len(lits)))
 
 	// Don't compress extremely small blocks
-	if len(b.literals) < 32 || raw {
+	if len(lits) < 8 || (len(lits) < 32 && b.dictLitEnc == nil) || raw {
 		if debug {
-			println("Adding RAW block, length", len(b.literals), "last:", b.last)
+			println("Adding RAW block, length", len(lits), "last:", b.last)
 		}
 		bh.setType(blockTypeRaw)
 		b.output = bh.appendTo(b.output)
-		b.output = append(b.output, b.literals...)
+		b.output = append(b.output, lits...)
 		return nil
 	}
 
@@ -335,33 +336,38 @@ func (b *blockEnc) encodeLits(raw bool) error {
 		reUsed, single bool
 		err            error
 	)
-	if len(b.literals) >= 1024 {
+	if b.dictLitEnc != nil {
+		b.litEnc.TransferCTable(b.dictLitEnc)
+		b.litEnc.Reuse = huff0.ReusePolicyAllow
+		b.dictLitEnc = nil
+	}
+	if len(lits) >= 1024 {
 		// Use 4 Streams.
-		out, reUsed, err = huff0.Compress4X(b.literals, b.litEnc)
-	} else if len(b.literals) > 32 {
+		out, reUsed, err = huff0.Compress4X(lits, b.litEnc)
+	} else if len(lits) > 32 {
 		// Use 1 stream
 		single = true
-		out, reUsed, err = huff0.Compress1X(b.literals, b.litEnc)
+		out, reUsed, err = huff0.Compress1X(lits, b.litEnc)
 	} else {
 		err = huff0.ErrIncompressible
 	}
 
 	switch err {
 	case huff0.ErrIncompressible:
 		if debug {
-			println("Adding RAW block, length", len(b.literals), "last:", b.last)
+			println("Adding RAW block, length", len(lits), "last:", b.last)
 		}
 		bh.setType(blockTypeRaw)
 		b.output = bh.appendTo(b.output)
-		b.output = append(b.output, b.literals...)
+		b.output = append(b.output, lits...)
 		return nil
 	case huff0.ErrUseRLE:
 		if debug {
-			println("Adding RLE block, length", len(b.literals))
+			println("Adding RLE block, length", len(lits))
 		}
 		bh.setType(blockTypeRLE)
 		b.output = bh.appendTo(b.output)
-		b.output = append(b.output, b.literals[0])
+		b.output = append(b.output, lits[0])
 		return nil
 	default:
 		return err
@@ -384,7 +390,7 @@ func (b *blockEnc) encodeLits(raw bool) error {
 		lh.setType(literalsBlockCompressed)
 	}
 	// Set sizes
-	lh.setSizes(len(out), len(b.literals), single)
+	lh.setSizes(len(out), len(lits), single)
 	bh.setSize(uint32(len(out) + lh.size() + 1))
 
 	// Write block headers.
@@ -444,13 +450,19 @@ func fuzzFseEncoder(data []byte) int {
 }
 
 // encode will encode the block and append the output in b.output.
-func (b *blockEnc) encode(raw, rawAllLits bool) error {
+// Previous offset codes must be pushed if more blocks are expected.
+func (b *blockEnc) encode(org []byte, raw, rawAllLits bool) error {
 	if len(b.sequences) == 0 {
-		return b.encodeLits(rawAllLits)
+		return b.encodeLits(b.literals, rawAllLits)
 	}
-	// We want some difference
-	if len(b.literals) > (b.size - (b.size >> 5)) {
-		return errIncompressible
+	// We want some difference to at least account for the headers.
+	saved := b.size - len(b.literals) - (b.size >> 5)
+	if saved < 16 {
+		if org == nil {
+			return errIncompressible
+		}
+		b.popOffsets()
+		return b.encodeLits(org, rawAllLits)
 	}
 
 	var bh blockHeader
@@ -466,6 +478,11 @@ func (b *blockEnc) encode(raw, rawAllLits bool) error {
 		reUsed, single bool
 		err            error
 	)
+	if b.dictLitEnc != nil {
+		b.litEnc.TransferCTable(b.dictLitEnc)
+		b.litEnc.Reuse = huff0.ReusePolicyAllow
+		b.dictLitEnc = nil
+	}
 	if len(b.literals) >= 1024 && !raw {
 		// Use 4 Streams.
 		out, reUsed, err = huff0.Compress4X(b.literals, b.litEnc)

zstd/decoder.go

@@ -187,7 +187,7 @@ func (d *Decoder) Reset(r io.Reader) error {
 		d.current.err = err
 		d.current.flushed = true
 		if debug {
-			println("sync decode to ", len(dst), "bytes, err:", err)
+			println("sync decode to", len(dst), "bytes, err:", err)
 		}
 		return nil
 	}
@@ -303,6 +303,9 @@ func (d *Decoder) DecodeAll(input, dst []byte) ([]byte, error) {
 		frame.history.reset()
 		err := frame.reset(&frame.bBuf)
 		if err == io.EOF {
+			if debug {
+				println("frame reset return EOF")
+			}
 			return dst, nil
 		}
 		if frame.DictionaryID != nil {
@@ -341,6 +344,9 @@ func (d *Decoder) DecodeAll(input, dst []byte) ([]byte, error) {
 			return dst, err
 		}
 		if len(frame.bBuf) == 0 {
+			if debug {
+				println("frame dbuf empty")
+			}
 			break
 		}
 	}

zstd/dict.go

@@ -13,14 +13,31 @@ import (
 type dict struct {
 	id uint32
 
-	litDec              *huff0.Scratch
+	litEnc              *huff0.Scratch
 	llDec, ofDec, mlDec sequenceDec
-	offsets             [3]int
-	content             []byte
+	//llEnc, ofEnc, mlEnc []*fseEncoder
+	offsets [3]int
+	content []byte
 }
 
 var dictMagic = [4]byte{0x37, 0xa4, 0x30, 0xec}
 
+// ID returns the dictionary id or 0 if d is nil.
+func (d *dict) ID() uint32 {
+	if d == nil {
+		return 0
+	}
+	return d.id
+}
+
+// DictContentSize returns the dictionary content size or 0 if d is nil.
+func (d *dict) DictContentSize() int {
+	if d == nil {
+		return 0
+	}
+	return len(d.content)
+}
+
 // Load a dictionary as described in
 // https://github.com/facebook/zstd/blob/master/doc/zstd_compression_format.md#dictionary-format
 func loadDict(b []byte) (*dict, error) {
@@ -43,10 +60,11 @@ func loadDict(b []byte) (*dict, error) {
 
 	// Read literal table
 	var err error
-	d.litDec, b, err = huff0.ReadTable(b[8:], nil)
+	d.litEnc, b, err = huff0.ReadTable(b[8:], nil)
 	if err != nil {
 		return nil, err
 	}
+	d.litEnc.Reuse = huff0.ReusePolicyMust
 
 	br := byteReader{
 		b:   b,