summaryrefslogtreecommitdiff
path: root/vendor/github.com/klauspost/compress/zstd/blockdec.go
blob: dc587b2c949d80b51d43c30bf3a77f9f0b94b29c (plain)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
462
463
464
465
466
467
468
469
470
471
472
473
474
475
476
477
478
479
480
481
482
483
484
485
486
487
488
489
490
491
492
493
494
495
496
497
498
499
500
501
502
503
504
505
506
507
508
509
510
511
512
513
514
515
516
517
518
519
520
521
522
523
524
525
526
527
528
529
530
531
532
533
534
535
536
537
538
539
540
541
542
543
544
545
546
547
548
549
550
551
552
553
554
555
556
557
558
559
560
561
562
563
564
565
566
567
568
569
570
571
572
573
574
575
576
577
578
579
580
581
582
583
584
585
586
587
588
589
590
591
592
593
594
595
596
597
598
599
600
601
602
603
604
605
606
607
608
609
610
611
612
613
614
615
616
617
618
619
620
621
622
623
624
625
626
627
628
629
630
631
632
633
634
635
636
637
638
639
640
641
642
643
644
645
646
647
648
649
650
651
652
653
654
655
656
657
658
659
660
661
662
663
664
665
666
667
668
669
670
671
672
673
674
675
676
677
678
679
680
681
682
683
684
685
686
687
688
689
690
691
692
693
694
695
696
697
698
699
700
701
702
703
704
705
706
707
708
709
710
711
712
713
714
715
716
717
718
719
720
721
722
723
724
// Copyright 2019+ Klaus Post. All rights reserved.
// License information can be found in the LICENSE file.
// Based on work by Yann Collet, released under BSD License.

package zstd

import (
	"errors"
	"fmt"
	"io"
	"sync"

	"github.com/klauspost/compress/huff0"
	"github.com/klauspost/compress/zstd/internal/xxhash"
)

type blockType uint8

//go:generate stringer -type=blockType,literalsBlockType,seqCompMode,tableIndex

const (
	blockTypeRaw blockType = iota
	blockTypeRLE
	blockTypeCompressed
	blockTypeReserved
)

type literalsBlockType uint8

const (
	literalsBlockRaw literalsBlockType = iota
	literalsBlockRLE
	literalsBlockCompressed
	literalsBlockTreeless
)

const (
	// maxCompressedBlockSize is the biggest allowed compressed block size (128KB)
	maxCompressedBlockSize = 128 << 10

	// Maximum possible block size (all Raw+Uncompressed).
	maxBlockSize = (1 << 21) - 1

	// https://github.com/facebook/zstd/blob/dev/doc/zstd_compression_format.md#literals_section_header
	maxCompressedLiteralSize = 1 << 18
	maxRLELiteralSize        = 1 << 20
	maxMatchLen              = 131074
	maxSequences             = 0x7f00 + 0xffff

	// We support slightly less than the reference decoder to be able to
	// use ints on 32 bit archs.
	maxOffsetBits = 30
)

var (
	huffDecoderPool = sync.Pool{New: func() interface{} {
		return &huff0.Scratch{}
	}}

	fseDecoderPool = sync.Pool{New: func() interface{} {
		return &fseDecoder{}
	}}
)

type blockDec struct {
	// Raw source data of the block.
	data        []byte
	dataStorage []byte

	// Destination of the decoded data.
	dst []byte

	// Buffer for literals data.
	literalBuf []byte

	// Window size of the block.
	WindowSize uint64

	history chan *history
	input   chan struct{}
	result  chan decodeOutput
	err     error
	decWG   sync.WaitGroup

	// Frame to use for singlethreaded decoding.
	// Should not be used by the decoder itself since parent may be another frame.
	localFrame *frameDec

	// Block is RLE, this is the size.
	RLESize uint32
	tmp     [4]byte

	Type blockType

	// Is this the last block of a frame?
	Last bool

	// Use less memory
	lowMem bool
}

func (b *blockDec) String() string {
	if b == nil {
		return "<nil>"
	}
	return fmt.Sprintf("Steam Size: %d, Type: %v, Last: %t, Window: %d", len(b.data), b.Type, b.Last, b.WindowSize)
}

func newBlockDec(lowMem bool) *blockDec {
	b := blockDec{
		lowMem:  lowMem,
		result:  make(chan decodeOutput, 1),
		input:   make(chan struct{}, 1),
		history: make(chan *history, 1),
	}
	b.decWG.Add(1)
	go b.startDecoder()
	return &b
}

// reset will reset the block.
// Input must be a start of a block and will be at the end of the block when returned.
func (b *blockDec) reset(br byteBuffer, windowSize uint64) error {
	b.WindowSize = windowSize
	tmp, err := br.readSmall(3)
	if err != nil {
		println("Reading block header:", err)
		return err
	}
	bh := uint32(tmp[0]) | (uint32(tmp[1]) << 8) | (uint32(tmp[2]) << 16)
	b.Last = bh&1 != 0
	b.Type = blockType((bh >> 1) & 3)
	// find size.
	cSize := int(bh >> 3)
	maxSize := maxBlockSize
	switch b.Type {
	case blockTypeReserved:
		return ErrReservedBlockType
	case blockTypeRLE:
		b.RLESize = uint32(cSize)
		if b.lowMem {
			maxSize = cSize
		}
		cSize = 1
	case blockTypeCompressed:
		if debugDecoder {
			println("Data size on stream:", cSize)
		}
		b.RLESize = 0
		maxSize = maxCompressedBlockSize
		if windowSize < maxCompressedBlockSize && b.lowMem {
			maxSize = int(windowSize)
		}
		if cSize > maxCompressedBlockSize || uint64(cSize) > b.WindowSize {
			if debugDecoder {
				printf("compressed block too big: csize:%d block: %+v\n", uint64(cSize), b)
			}
			return ErrCompressedSizeTooBig
		}
	case blockTypeRaw:
		b.RLESize = 0
		// We do not need a destination for raw blocks.
		maxSize = -1
	default:
		panic("Invalid block type")
	}

	// Read block data.
	if cap(b.dataStorage) < cSize {
		if b.lowMem || cSize > maxCompressedBlockSize {
			b.dataStorage = make([]byte, 0, cSize)
		} else {
			b.dataStorage = make([]byte, 0, maxCompressedBlockSize)
		}
	}
	if cap(b.dst) <= maxSize {
		b.dst = make([]byte, 0, maxSize+1)
	}
	b.data, err = br.readBig(cSize, b.dataStorage)
	if err != nil {
		if debugDecoder {
			println("Reading block:", err, "(", cSize, ")", len(b.data))
			printf("%T", br)
		}
		return err
	}
	return nil
}

// sendEOF will make the decoder send EOF on this frame.
func (b *blockDec) sendErr(err error) {
	b.Last = true
	b.Type = blockTypeReserved
	b.err = err
	b.input <- struct{}{}
}

// Close will release resources.
// Closed blockDec cannot be reset.
func (b *blockDec) Close() {
	close(b.input)
	close(b.history)
	close(b.result)
	b.decWG.Wait()
}

// decodeAsync will prepare decoding the block when it receives input.
// This will separate output and history.
func (b *blockDec) startDecoder() {
	defer b.decWG.Done()
	for range b.input {
		//println("blockDec: Got block input")
		switch b.Type {
		case blockTypeRLE:
			if cap(b.dst) < int(b.RLESize) {
				if b.lowMem {
					b.dst = make([]byte, b.RLESize)
				} else {
					b.dst = make([]byte, maxBlockSize)
				}
			}
			o := decodeOutput{
				d:   b,
				b:   b.dst[:b.RLESize],
				err: nil,
			}
			v := b.data[0]
			for i := range o.b {
				o.b[i] = v
			}
			hist := <-b.history
			hist.append(o.b)
			b.result <- o
		case blockTypeRaw:
			o := decodeOutput{
				d:   b,
				b:   b.data,
				err: nil,
			}
			hist := <-b.history
			hist.append(o.b)
			b.result <- o
		case blockTypeCompressed:
			b.dst = b.dst[:0]
			err := b.decodeCompressed(nil)
			o := decodeOutput{
				d:   b,
				b:   b.dst,
				err: err,
			}
			if debugDecoder {
				println("Decompressed to", len(b.dst), "bytes, error:", err)
			}
			b.result <- o
		case blockTypeReserved:
			// Used for returning errors.
			<-b.history
			b.result <- decodeOutput{
				d:   b,
				b:   nil,
				err: b.err,
			}
		default:
			panic("Invalid block type")
		}
		if debugDecoder {
			println("blockDec: Finished block")
		}
	}
}

// decodeAsync will prepare decoding the block when it receives the history.
// If history is provided, it will not fetch it from the channel.
func (b *blockDec) decodeBuf(hist *history) error {
	switch b.Type {
	case blockTypeRLE:
		if cap(b.dst) < int(b.RLESize) {
			if b.lowMem {
				b.dst = make([]byte, b.RLESize)
			} else {
				b.dst = make([]byte, maxBlockSize)
			}
		}
		b.dst = b.dst[:b.RLESize]
		v := b.data[0]
		for i := range b.dst {
			b.dst[i] = v
		}
		hist.appendKeep(b.dst)
		return nil
	case blockTypeRaw:
		hist.appendKeep(b.data)
		return nil
	case blockTypeCompressed:
		saved := b.dst
		b.dst = hist.b
		hist.b = nil
		err := b.decodeCompressed(hist)
		if debugDecoder {
			println("Decompressed to total", len(b.dst), "bytes, hash:", xxhash.Sum64(b.dst), "error:", err)
		}
		hist.b = b.dst
		b.dst = saved
		return err
	case blockTypeReserved:
		// Used for returning errors.
		return b.err
	default:
		panic("Invalid block type")
	}
}

// decodeCompressed will start decompressing a block.
// If no history is supplied the decoder will decodeAsync as much as possible
// before fetching from blockDec.history
func (b *blockDec) decodeCompressed(hist *history) error {
	in := b.data
	delayedHistory := hist == nil

	if delayedHistory {
		// We must always grab history.
		defer func() {
			if hist == nil {
				<-b.history
			}
		}()
	}
	// There must be at least one byte for Literals_Block_Type and one for Sequences_Section_Header
	if len(in) < 2 {
		return ErrBlockTooSmall
	}
	litType := literalsBlockType(in[0] & 3)
	var litRegenSize int
	var litCompSize int
	sizeFormat := (in[0] >> 2) & 3
	var fourStreams bool
	switch litType {
	case literalsBlockRaw, literalsBlockRLE:
		switch sizeFormat {
		case 0, 2:
			// Regenerated_Size uses 5 bits (0-31). Literals_Section_Header uses 1 byte.
			litRegenSize = int(in[0] >> 3)
			in = in[1:]
		case 1:
			// Regenerated_Size uses 12 bits (0-4095). Literals_Section_Header uses 2 bytes.
			litRegenSize = int(in[0]>>4) + (int(in[1]) << 4)
			in = in[2:]
		case 3:
			//  Regenerated_Size uses 20 bits (0-1048575). Literals_Section_Header uses 3 bytes.
			if len(in) < 3 {
				println("too small: litType:", litType, " sizeFormat", sizeFormat, len(in))
				return ErrBlockTooSmall
			}
			litRegenSize = int(in[0]>>4) + (int(in[1]) << 4) + (int(in[2]) << 12)
			in = in[3:]
		}
	case literalsBlockCompressed, literalsBlockTreeless:
		switch sizeFormat {
		case 0, 1:
			// Both Regenerated_Size and Compressed_Size use 10 bits (0-1023).
			if len(in) < 3 {
				println("too small: litType:", litType, " sizeFormat", sizeFormat, len(in))
				return ErrBlockTooSmall
			}
			n := uint64(in[0]>>4) + (uint64(in[1]) << 4) + (uint64(in[2]) << 12)
			litRegenSize = int(n & 1023)
			litCompSize = int(n >> 10)
			fourStreams = sizeFormat == 1
			in = in[3:]
		case 2:
			fourStreams = true
			if len(in) < 4 {
				println("too small: litType:", litType, " sizeFormat", sizeFormat, len(in))
				return ErrBlockTooSmall
			}
			n := uint64(in[0]>>4) + (uint64(in[1]) << 4) + (uint64(in[2]) << 12) + (uint64(in[3]) << 20)
			litRegenSize = int(n & 16383)
			litCompSize = int(n >> 14)
			in = in[4:]
		case 3:
			fourStreams = true
			if len(in) < 5 {
				println("too small: litType:", litType, " sizeFormat", sizeFormat, len(in))
				return ErrBlockTooSmall
			}
			n := uint64(in[0]>>4) + (uint64(in[1]) << 4) + (uint64(in[2]) << 12) + (uint64(in[3]) << 20) + (uint64(in[4]) << 28)
			litRegenSize = int(n & 262143)
			litCompSize = int(n >> 18)
			in = in[5:]
		}
	}
	if debugDecoder {
		println("literals type:", litType, "litRegenSize:", litRegenSize, "litCompSize:", litCompSize, "sizeFormat:", sizeFormat, "4X:", fourStreams)
	}
	var literals []byte
	var huff *huff0.Scratch
	switch litType {
	case literalsBlockRaw:
		if len(in) < litRegenSize {
			println("too small: litType:", litType, " sizeFormat", sizeFormat, "remain:", len(in), "want:", litRegenSize)
			return ErrBlockTooSmall
		}
		literals = in[:litRegenSize]
		in = in[litRegenSize:]
		//printf("Found %d uncompressed literals\n", litRegenSize)
	case literalsBlockRLE:
		if len(in) < 1 {
			println("too small: litType:", litType, " sizeFormat", sizeFormat, "remain:", len(in), "want:", 1)
			return ErrBlockTooSmall
		}
		if cap(b.literalBuf) < litRegenSize {
			if b.lowMem {
				b.literalBuf = make([]byte, litRegenSize)
			} else {
				if litRegenSize > maxCompressedLiteralSize {
					// Exceptional
					b.literalBuf = make([]byte, litRegenSize)
				} else {
					b.literalBuf = make([]byte, litRegenSize, maxCompressedLiteralSize)

				}
			}
		}
		literals = b.literalBuf[:litRegenSize]
		v := in[0]
		for i := range literals {
			literals[i] = v
		}
		in = in[1:]
		if debugDecoder {
			printf("Found %d RLE compressed literals\n", litRegenSize)
		}
	case literalsBlockTreeless:
		if len(in) < litCompSize {
			println("too small: litType:", litType, " sizeFormat", sizeFormat, "remain:", len(in), "want:", litCompSize)
			return ErrBlockTooSmall
		}
		// Store compressed literals, so we defer decoding until we get history.
		literals = in[:litCompSize]
		in = in[litCompSize:]
		if debugDecoder {
			printf("Found %d compressed literals\n", litCompSize)
		}
	case literalsBlockCompressed:
		if len(in) < litCompSize {
			println("too small: litType:", litType, " sizeFormat", sizeFormat, "remain:", len(in), "want:", litCompSize)
			return ErrBlockTooSmall
		}
		literals = in[:litCompSize]
		in = in[litCompSize:]
		huff = huffDecoderPool.Get().(*huff0.Scratch)
		var err error
		// Ensure we have space to store it.
		if cap(b.literalBuf) < litRegenSize {
			if b.lowMem {
				b.literalBuf = make([]byte, 0, litRegenSize)
			} else {
				b.literalBuf = make([]byte, 0, maxCompressedLiteralSize)
			}
		}
		if huff == nil {
			huff = &huff0.Scratch{}
		}
		huff, literals, err = huff0.ReadTable(literals, huff)
		if err != nil {
			println("reading huffman table:", err)
			return err
		}
		// Use our out buffer.
		if fourStreams {
			literals, err = huff.Decoder().Decompress4X(b.literalBuf[:0:litRegenSize], literals)
		} else {
			literals, err = huff.Decoder().Decompress1X(b.literalBuf[:0:litRegenSize], literals)
		}
		if err != nil {
			println("decoding compressed literals:", err)
			return err
		}
		// Make sure we don't leak our literals buffer
		if len(literals) != litRegenSize {
			return fmt.Errorf("literal output size mismatch want %d, got %d", litRegenSize, len(literals))
		}
		if debugDecoder {
			printf("Decompressed %d literals into %d bytes\n", litCompSize, litRegenSize)
		}
	}

	// Decode Sequences
	// https://github.com/facebook/zstd/blob/dev/doc/zstd_compression_format.md#sequences-section
	if len(in) < 1 {
		return ErrBlockTooSmall
	}
	seqHeader := in[0]
	nSeqs := 0
	switch {
	case seqHeader == 0:
		in = in[1:]
	case seqHeader < 128:
		nSeqs = int(seqHeader)
		in = in[1:]
	case seqHeader < 255:
		if len(in) < 2 {
			return ErrBlockTooSmall
		}
		nSeqs = int(seqHeader-128)<<8 | int(in[1])
		in = in[2:]
	case seqHeader == 255:
		if len(in) < 3 {
			return ErrBlockTooSmall
		}
		nSeqs = 0x7f00 + int(in[1]) + (int(in[2]) << 8)
		in = in[3:]
	}

	var seqs = &sequenceDecs{}
	if nSeqs > 0 {
		if len(in) < 1 {
			return ErrBlockTooSmall
		}
		br := byteReader{b: in, off: 0}
		compMode := br.Uint8()
		br.advance(1)
		if debugDecoder {
			printf("Compression modes: 0b%b", compMode)
		}
		for i := uint(0); i < 3; i++ {
			mode := seqCompMode((compMode >> (6 - i*2)) & 3)
			if debugDecoder {
				println("Table", tableIndex(i), "is", mode)
			}
			var seq *sequenceDec
			switch tableIndex(i) {
			case tableLiteralLengths:
				seq = &seqs.litLengths
			case tableOffsets:
				seq = &seqs.offsets
			case tableMatchLengths:
				seq = &seqs.matchLengths
			default:
				panic("unknown table")
			}
			switch mode {
			case compModePredefined:
				seq.fse = &fsePredef[i]
			case compModeRLE:
				if br.remain() < 1 {
					return ErrBlockTooSmall
				}
				v := br.Uint8()
				br.advance(1)
				dec := fseDecoderPool.Get().(*fseDecoder)
				symb, err := decSymbolValue(v, symbolTableX[i])
				if err != nil {
					printf("RLE Transform table (%v) error: %v", tableIndex(i), err)
					return err
				}
				dec.setRLE(symb)
				seq.fse = dec
				if debugDecoder {
					printf("RLE set to %+v, code: %v", symb, v)
				}
			case compModeFSE:
				println("Reading table for", tableIndex(i))
				dec := fseDecoderPool.Get().(*fseDecoder)
				err := dec.readNCount(&br, uint16(maxTableSymbol[i]))
				if err != nil {
					println("Read table error:", err)
					return err
				}
				err = dec.transform(symbolTableX[i])
				if err != nil {
					println("Transform table error:", err)
					return err
				}
				if debugDecoder {
					println("Read table ok", "symbolLen:", dec.symbolLen)
				}
				seq.fse = dec
			case compModeRepeat:
				seq.repeat = true
			}
			if br.overread() {
				return io.ErrUnexpectedEOF
			}
		}
		in = br.unread()
	}

	// Wait for history.
	// All time spent after this is critical since it is strictly sequential.
	if hist == nil {
		hist = <-b.history
		if hist.error {
			return ErrDecoderClosed
		}
	}

	// Decode treeless literal block.
	if litType == literalsBlockTreeless {
		// TODO: We could send the history early WITHOUT the stream history.
		//   This would allow decoding treeless literals before the byte history is available.
		//   Silencia stats: Treeless 4393, with: 32775, total: 37168, 11% treeless.
		//   So not much obvious gain here.

		if hist.huffTree == nil {
			return errors.New("literal block was treeless, but no history was defined")
		}
		// Ensure we have space to store it.
		if cap(b.literalBuf) < litRegenSize {
			if b.lowMem {
				b.literalBuf = make([]byte, 0, litRegenSize)
			} else {
				b.literalBuf = make([]byte, 0, maxCompressedLiteralSize)
			}
		}
		var err error
		// Use our out buffer.
		huff = hist.huffTree
		if fourStreams {
			literals, err = huff.Decoder().Decompress4X(b.literalBuf[:0:litRegenSize], literals)
		} else {
			literals, err = huff.Decoder().Decompress1X(b.literalBuf[:0:litRegenSize], literals)
		}
		// Make sure we don't leak our literals buffer
		if err != nil {
			println("decompressing literals:", err)
			return err
		}
		if len(literals) != litRegenSize {
			return fmt.Errorf("literal output size mismatch want %d, got %d", litRegenSize, len(literals))
		}
	} else {
		if hist.huffTree != nil && huff != nil {
			if hist.dict == nil || hist.dict.litEnc != hist.huffTree {
				huffDecoderPool.Put(hist.huffTree)
			}
			hist.huffTree = nil
		}
	}
	if huff != nil {
		hist.huffTree = huff
	}
	if debugDecoder {
		println("Final literals:", len(literals), "hash:", xxhash.Sum64(literals), "and", nSeqs, "sequences.")
	}

	if nSeqs == 0 {
		// Decompressed content is defined entirely as Literals Section content.
		b.dst = append(b.dst, literals...)
		if delayedHistory {
			hist.append(literals)
		}
		return nil
	}

	seqs, err := seqs.mergeHistory(&hist.decoders)
	if err != nil {
		return err
	}
	if debugDecoder {
		println("History merged ok")
	}
	br := &bitReader{}
	if err := br.init(in); err != nil {
		return err
	}

	// TODO: Investigate if sending history without decoders are faster.
	//   This would allow the sequences to be decoded async and only have to construct stream history.
	//   If only recent offsets were not transferred, this would be an obvious win.
	// 	 Also, if first 3 sequences don't reference recent offsets, all sequences can be decoded.

	hbytes := hist.b
	if len(hbytes) > hist.windowSize {
		hbytes = hbytes[len(hbytes)-hist.windowSize:]
		// We do not need history any more.
		if hist.dict != nil {
			hist.dict.content = nil
		}
	}

	if err := seqs.initialize(br, hist, literals, b.dst); err != nil {
		println("initializing sequences:", err)
		return err
	}

	err = seqs.decode(nSeqs, br, hbytes)
	if err != nil {
		return err
	}
	if !br.finished() {
		return fmt.Errorf("%d extra bits on block, should be 0", br.remain())
	}

	err = br.close()
	if err != nil {
		printf("Closing sequences: %v, %+v\n", err, *br)
	}
	if len(b.data) > maxCompressedBlockSize {
		return fmt.Errorf("compressed block size too large (%d)", len(b.data))
	}
	// Set output and release references.
	b.dst = seqs.out
	seqs.out, seqs.literals, seqs.hist = nil, nil, nil

	if !delayedHistory {
		// If we don't have delayed history, no need to update.
		hist.recentOffsets = seqs.prevOffset
		return nil
	}
	if b.Last {
		// if last block we don't care about history.
		println("Last block, no history returned")
		hist.b = hist.b[:0]
		return nil
	}
	hist.append(b.dst)
	hist.recentOffsets = seqs.prevOffset
	if debugDecoder {
		println("Finished block with literals:", len(literals), "and", nSeqs, "sequences.")
	}

	return nil
}