summaryrefslogtreecommitdiffstats
path: root/media/libwebp/src/enc/backward_references_enc.c
blob: dc98bf171943a051d6354768e8f4f9683a6aecec (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
725
726
727
728
729
730
731
732
733
734
735
736
737
738
739
740
741
742
743
744
745
746
747
748
749
750
751
752
753
754
755
756
757
758
759
760
761
762
763
764
765
766
767
768
769
770
771
772
773
774
775
776
777
778
779
780
781
782
783
784
785
786
787
788
789
790
791
792
793
794
795
796
797
798
799
800
801
802
803
804
805
806
807
808
809
810
811
812
813
814
815
816
817
818
819
820
821
822
823
824
825
826
827
828
829
830
831
832
833
834
835
836
837
838
839
840
841
842
843
844
845
846
847
848
849
850
851
852
853
854
855
856
857
858
859
860
861
862
863
864
865
866
867
868
869
870
871
872
873
874
875
876
877
878
879
880
881
882
883
884
885
886
887
888
889
890
891
892
893
894
895
896
897
898
899
900
901
902
903
904
905
906
907
908
909
910
911
912
913
914
915
916
917
918
919
920
921
922
923
924
925
926
927
928
929
930
931
932
933
934
935
936
937
938
939
940
941
942
943
944
945
946
947
948
949
950
951
952
953
954
955
956
957
958
959
960
961
962
963
964
965
966
967
968
969
970
971
972
973
974
975
976
977
978
979
980
981
982
983
984
985
986
987
988
989
990
991
992
993
994
995
996
997
998
999
1000
1001
1002
1003
1004
1005
1006
1007
1008
1009
1010
1011
1012
1013
1014
1015
1016
1017
1018
1019
1020
1021
1022
1023
1024
1025
1026
1027
1028
1029
1030
1031
1032
1033
1034
1035
1036
1037
1038
1039
1040
1041
1042
1043
1044
1045
1046
1047
1048
1049
1050
1051
1052
1053
1054
1055
1056
1057
1058
1059
1060
1061
1062
1063
1064
1065
// Copyright 2012 Google Inc. All Rights Reserved.
//
// Use of this source code is governed by a BSD-style license
// that can be found in the COPYING file in the root of the source
// tree. An additional intellectual property rights grant can be found
// in the file PATENTS. All contributing project authors may
// be found in the AUTHORS file in the root of the source tree.
// -----------------------------------------------------------------------------
//
// Author: Jyrki Alakuijala (jyrki@google.com)
//

#include "src/enc/backward_references_enc.h"

#include <assert.h>
#include <float.h>
#include <math.h>

#include "src/dsp/dsp.h"
#include "src/dsp/lossless.h"
#include "src/dsp/lossless_common.h"
#include "src/enc/histogram_enc.h"
#include "src/enc/vp8i_enc.h"
#include "src/utils/color_cache_utils.h"
#include "src/utils/utils.h"
#include "src/webp/encode.h"

#define MIN_BLOCK_SIZE 256  // minimum block size for backward references

#define MAX_ENTROPY    (1e30f)

// 1M window (4M bytes) minus 120 special codes for short distances.
#define WINDOW_SIZE ((1 << WINDOW_SIZE_BITS) - 120)

// Minimum number of pixels for which it is cheaper to encode a
// distance + length instead of each pixel as a literal.
#define MIN_LENGTH 4

// -----------------------------------------------------------------------------

static const uint8_t plane_to_code_lut[128] = {
 96,   73,  55,  39,  23,  13,   5,  1,  255, 255, 255, 255, 255, 255, 255, 255,
 101,  78,  58,  42,  26,  16,   8,  2,    0,   3,  9,   17,  27,  43,  59,  79,
 102,  86,  62,  46,  32,  20,  10,  6,    4,   7,  11,  21,  33,  47,  63,  87,
 105,  90,  70,  52,  37,  28,  18,  14,  12,  15,  19,  29,  38,  53,  71,  91,
 110,  99,  82,  66,  48,  35,  30,  24,  22,  25,  31,  36,  49,  67,  83, 100,
 115, 108,  94,  76,  64,  50,  44,  40,  34,  41,  45,  51,  65,  77,  95, 109,
 118, 113, 103,  92,  80,  68,  60,  56,  54,  57,  61,  69,  81,  93, 104, 114,
 119, 116, 111, 106,  97,  88,  84,  74,  72,  75,  85,  89,  98, 107, 112, 117
};

extern int VP8LDistanceToPlaneCode(int xsize, int dist);
int VP8LDistanceToPlaneCode(int xsize, int dist) {
  const int yoffset = dist / xsize;
  const int xoffset = dist - yoffset * xsize;
  if (xoffset <= 8 && yoffset < 8) {
    return plane_to_code_lut[yoffset * 16 + 8 - xoffset] + 1;
  } else if (xoffset > xsize - 8 && yoffset < 7) {
    return plane_to_code_lut[(yoffset + 1) * 16 + 8 + (xsize - xoffset)] + 1;
  }
  return dist + 120;
}

// Returns the exact index where array1 and array2 are different. For an index
// inferior or equal to best_len_match, the return value just has to be strictly
// inferior to best_len_match. The current behavior is to return 0 if this index
// is best_len_match, and the index itself otherwise.
// If no two elements are the same, it returns max_limit.
static WEBP_INLINE int FindMatchLength(const uint32_t* const array1,
                                       const uint32_t* const array2,
                                       int best_len_match, int max_limit) {
  // Before 'expensive' linear match, check if the two arrays match at the
  // current best length index.
  if (array1[best_len_match] != array2[best_len_match]) return 0;

  return VP8LVectorMismatch(array1, array2, max_limit);
}

// -----------------------------------------------------------------------------
//  VP8LBackwardRefs

struct PixOrCopyBlock {
  PixOrCopyBlock* next_;   // next block (or NULL)
  PixOrCopy* start_;       // data start
  int size_;               // currently used size
};

extern void VP8LClearBackwardRefs(VP8LBackwardRefs* const refs);
void VP8LClearBackwardRefs(VP8LBackwardRefs* const refs) {
  assert(refs != NULL);
  if (refs->tail_ != NULL) {
    *refs->tail_ = refs->free_blocks_;  // recycle all blocks at once
  }
  refs->free_blocks_ = refs->refs_;
  refs->tail_ = &refs->refs_;
  refs->last_block_ = NULL;
  refs->refs_ = NULL;
}

void VP8LBackwardRefsClear(VP8LBackwardRefs* const refs) {
  assert(refs != NULL);
  VP8LClearBackwardRefs(refs);
  while (refs->free_blocks_ != NULL) {
    PixOrCopyBlock* const next = refs->free_blocks_->next_;
    WebPSafeFree(refs->free_blocks_);
    refs->free_blocks_ = next;
  }
}

// Swaps the content of two VP8LBackwardRefs.
static void BackwardRefsSwap(VP8LBackwardRefs* const refs1,
                             VP8LBackwardRefs* const refs2) {
  const int point_to_refs1 =
      (refs1->tail_ != NULL && refs1->tail_ == &refs1->refs_);
  const int point_to_refs2 =
      (refs2->tail_ != NULL && refs2->tail_ == &refs2->refs_);
  const VP8LBackwardRefs tmp = *refs1;
  *refs1 = *refs2;
  *refs2 = tmp;
  if (point_to_refs2) refs1->tail_ = &refs1->refs_;
  if (point_to_refs1) refs2->tail_ = &refs2->refs_;
}

void VP8LBackwardRefsInit(VP8LBackwardRefs* const refs, int block_size) {
  assert(refs != NULL);
  memset(refs, 0, sizeof(*refs));
  refs->tail_ = &refs->refs_;
  refs->block_size_ =
      (block_size < MIN_BLOCK_SIZE) ? MIN_BLOCK_SIZE : block_size;
}

VP8LRefsCursor VP8LRefsCursorInit(const VP8LBackwardRefs* const refs) {
  VP8LRefsCursor c;
  c.cur_block_ = refs->refs_;
  if (refs->refs_ != NULL) {
    c.cur_pos = c.cur_block_->start_;
    c.last_pos_ = c.cur_pos + c.cur_block_->size_;
  } else {
    c.cur_pos = NULL;
    c.last_pos_ = NULL;
  }
  return c;
}

void VP8LRefsCursorNextBlock(VP8LRefsCursor* const c) {
  PixOrCopyBlock* const b = c->cur_block_->next_;
  c->cur_pos = (b == NULL) ? NULL : b->start_;
  c->last_pos_ = (b == NULL) ? NULL : b->start_ + b->size_;
  c->cur_block_ = b;
}

// Create a new block, either from the free list or allocated
static PixOrCopyBlock* BackwardRefsNewBlock(VP8LBackwardRefs* const refs) {
  PixOrCopyBlock* b = refs->free_blocks_;
  if (b == NULL) {   // allocate new memory chunk
    const size_t total_size =
        sizeof(*b) + refs->block_size_ * sizeof(*b->start_);
    b = (PixOrCopyBlock*)WebPSafeMalloc(1ULL, total_size);
    if (b == NULL) {
      refs->error_ |= 1;
      return NULL;
    }
    b->start_ = (PixOrCopy*)((uint8_t*)b + sizeof(*b));  // not always aligned
  } else {  // recycle from free-list
    refs->free_blocks_ = b->next_;
  }
  *refs->tail_ = b;
  refs->tail_ = &b->next_;
  refs->last_block_ = b;
  b->next_ = NULL;
  b->size_ = 0;
  return b;
}

// Return 1 on success, 0 on error.
static int BackwardRefsClone(const VP8LBackwardRefs* const from,
                             VP8LBackwardRefs* const to) {
  const PixOrCopyBlock* block_from = from->refs_;
  VP8LClearBackwardRefs(to);
  while (block_from != NULL) {
    PixOrCopyBlock* const block_to = BackwardRefsNewBlock(to);
    if (block_to == NULL) return 0;
    memcpy(block_to->start_, block_from->start_,
           block_from->size_ * sizeof(PixOrCopy));
    block_to->size_ = block_from->size_;
    block_from = block_from->next_;
  }
  return 1;
}

extern void VP8LBackwardRefsCursorAdd(VP8LBackwardRefs* const refs,
                                      const PixOrCopy v);
void VP8LBackwardRefsCursorAdd(VP8LBackwardRefs* const refs,
                               const PixOrCopy v) {
  PixOrCopyBlock* b = refs->last_block_;
  if (b == NULL || b->size_ == refs->block_size_) {
    b = BackwardRefsNewBlock(refs);
    if (b == NULL) return;   // refs->error_ is set
  }
  b->start_[b->size_++] = v;
}

// -----------------------------------------------------------------------------
// Hash chains

int VP8LHashChainInit(VP8LHashChain* const p, int size) {
  assert(p->size_ == 0);
  assert(p->offset_length_ == NULL);
  assert(size > 0);
  p->offset_length_ =
      (uint32_t*)WebPSafeMalloc(size, sizeof(*p->offset_length_));
  if (p->offset_length_ == NULL) return 0;
  p->size_ = size;

  return 1;
}

void VP8LHashChainClear(VP8LHashChain* const p) {
  assert(p != NULL);
  WebPSafeFree(p->offset_length_);

  p->size_ = 0;
  p->offset_length_ = NULL;
}

// -----------------------------------------------------------------------------

static const uint32_t kHashMultiplierHi = 0xc6a4a793u;
static const uint32_t kHashMultiplierLo = 0x5bd1e996u;

static WEBP_UBSAN_IGNORE_UNSIGNED_OVERFLOW WEBP_INLINE
uint32_t GetPixPairHash64(const uint32_t* const argb) {
  uint32_t key;
  key  = argb[1] * kHashMultiplierHi;
  key += argb[0] * kHashMultiplierLo;
  key = key >> (32 - HASH_BITS);
  return key;
}

// Returns the maximum number of hash chain lookups to do for a
// given compression quality. Return value in range [8, 86].
static int GetMaxItersForQuality(int quality) {
  return 8 + (quality * quality) / 128;
}

static int GetWindowSizeForHashChain(int quality, int xsize) {
  const int max_window_size = (quality > 75) ? WINDOW_SIZE
                            : (quality > 50) ? (xsize << 8)
                            : (quality > 25) ? (xsize << 6)
                            : (xsize << 4);
  assert(xsize > 0);
  return (max_window_size > WINDOW_SIZE) ? WINDOW_SIZE : max_window_size;
}

static WEBP_INLINE int MaxFindCopyLength(int len) {
  return (len < MAX_LENGTH) ? len : MAX_LENGTH;
}

int VP8LHashChainFill(VP8LHashChain* const p, int quality,
                      const uint32_t* const argb, int xsize, int ysize,
                      int low_effort, const WebPPicture* const pic,
                      int percent_range, int* const percent) {
  const int size = xsize * ysize;
  const int iter_max = GetMaxItersForQuality(quality);
  const uint32_t window_size = GetWindowSizeForHashChain(quality, xsize);
  int remaining_percent = percent_range;
  int percent_start = *percent;
  int pos;
  int argb_comp;
  uint32_t base_position;
  int32_t* hash_to_first_index;
  // Temporarily use the p->offset_length_ as a hash chain.
  int32_t* chain = (int32_t*)p->offset_length_;
  assert(size > 0);
  assert(p->size_ != 0);
  assert(p->offset_length_ != NULL);

  if (size <= 2) {
    p->offset_length_[0] = p->offset_length_[size - 1] = 0;
    return 1;
  }

  hash_to_first_index =
      (int32_t*)WebPSafeMalloc(HASH_SIZE, sizeof(*hash_to_first_index));
  if (hash_to_first_index == NULL) {
    return WebPEncodingSetError(pic, VP8_ENC_ERROR_OUT_OF_MEMORY);
  }

  percent_range = remaining_percent / 2;
  remaining_percent -= percent_range;

  // Set the int32_t array to -1.
  memset(hash_to_first_index, 0xff, HASH_SIZE * sizeof(*hash_to_first_index));
  // Fill the chain linking pixels with the same hash.
  argb_comp = (argb[0] == argb[1]);
  for (pos = 0; pos < size - 2;) {
    uint32_t hash_code;
    const int argb_comp_next = (argb[pos + 1] == argb[pos + 2]);
    if (argb_comp && argb_comp_next) {
      // Consecutive pixels with the same color will share the same hash.
      // We therefore use a different hash: the color and its repetition
      // length.
      uint32_t tmp[2];
      uint32_t len = 1;
      tmp[0] = argb[pos];
      // Figure out how far the pixels are the same.
      // The last pixel has a different 64 bit hash, as its next pixel does
      // not have the same color, so we just need to get to the last pixel equal
      // to its follower.
      while (pos + (int)len + 2 < size && argb[pos + len + 2] == argb[pos]) {
        ++len;
      }
      if (len > MAX_LENGTH) {
        // Skip the pixels that match for distance=1 and length>MAX_LENGTH
        // because they are linked to their predecessor and we automatically
        // check that in the main for loop below. Skipping means setting no
        // predecessor in the chain, hence -1.
        memset(chain + pos, 0xff, (len - MAX_LENGTH) * sizeof(*chain));
        pos += len - MAX_LENGTH;
        len = MAX_LENGTH;
      }
      // Process the rest of the hash chain.
      while (len) {
        tmp[1] = len--;
        hash_code = GetPixPairHash64(tmp);
        chain[pos] = hash_to_first_index[hash_code];
        hash_to_first_index[hash_code] = pos++;
      }
      argb_comp = 0;
    } else {
      // Just move one pixel forward.
      hash_code = GetPixPairHash64(argb + pos);
      chain[pos] = hash_to_first_index[hash_code];
      hash_to_first_index[hash_code] = pos++;
      argb_comp = argb_comp_next;
    }

    if (!WebPReportProgress(
            pic, percent_start + percent_range * pos / (size - 2), percent)) {
      WebPSafeFree(hash_to_first_index);
      return 0;
    }
  }
  // Process the penultimate pixel.
  chain[pos] = hash_to_first_index[GetPixPairHash64(argb + pos)];

  WebPSafeFree(hash_to_first_index);

  percent_start += percent_range;
  if (!WebPReportProgress(pic, percent_start, percent)) return 0;
  percent_range = remaining_percent;

  // Find the best match interval at each pixel, defined by an offset to the
  // pixel and a length. The right-most pixel cannot match anything to the right
  // (hence a best length of 0) and the left-most pixel nothing to the left
  // (hence an offset of 0).
  assert(size > 2);
  p->offset_length_[0] = p->offset_length_[size - 1] = 0;
  for (base_position = size - 2; base_position > 0;) {
    const int max_len = MaxFindCopyLength(size - 1 - base_position);
    const uint32_t* const argb_start = argb + base_position;
    int iter = iter_max;
    int best_length = 0;
    uint32_t best_distance = 0;
    uint32_t best_argb;
    const int min_pos =
        (base_position > window_size) ? base_position - window_size : 0;
    const int length_max = (max_len < 256) ? max_len : 256;
    uint32_t max_base_position;

    pos = chain[base_position];
    if (!low_effort) {
      int curr_length;
      // Heuristic: use the comparison with the above line as an initialization.
      if (base_position >= (uint32_t)xsize) {
        curr_length = FindMatchLength(argb_start - xsize, argb_start,
                                      best_length, max_len);
        if (curr_length > best_length) {
          best_length = curr_length;
          best_distance = xsize;
        }
        --iter;
      }
      // Heuristic: compare to the previous pixel.
      curr_length =
          FindMatchLength(argb_start - 1, argb_start, best_length, max_len);
      if (curr_length > best_length) {
        best_length = curr_length;
        best_distance = 1;
      }
      --iter;
      // Skip the for loop if we already have the maximum.
      if (best_length == MAX_LENGTH) pos = min_pos - 1;
    }
    best_argb = argb_start[best_length];

    for (; pos >= min_pos && --iter; pos = chain[pos]) {
      int curr_length;
      assert(base_position > (uint32_t)pos);

      if (argb[pos + best_length] != best_argb) continue;

      curr_length = VP8LVectorMismatch(argb + pos, argb_start, max_len);
      if (best_length < curr_length) {
        best_length = curr_length;
        best_distance = base_position - pos;
        best_argb = argb_start[best_length];
        // Stop if we have reached a good enough length.
        if (best_length >= length_max) break;
      }
    }
    // We have the best match but in case the two intervals continue matching
    // to the left, we have the best matches for the left-extended pixels.
    max_base_position = base_position;
    while (1) {
      assert(best_length <= MAX_LENGTH);
      assert(best_distance <= WINDOW_SIZE);
      p->offset_length_[base_position] =
          (best_distance << MAX_LENGTH_BITS) | (uint32_t)best_length;
      --base_position;
      // Stop if we don't have a match or if we are out of bounds.
      if (best_distance == 0 || base_position == 0) break;
      // Stop if we cannot extend the matching intervals to the left.
      if (base_position < best_distance ||
          argb[base_position - best_distance] != argb[base_position]) {
        break;
      }
      // Stop if we are matching at its limit because there could be a closer
      // matching interval with the same maximum length. Then again, if the
      // matching interval is as close as possible (best_distance == 1), we will
      // never find anything better so let's continue.
      if (best_length == MAX_LENGTH && best_distance != 1 &&
          base_position + MAX_LENGTH < max_base_position) {
        break;
      }
      if (best_length < MAX_LENGTH) {
        ++best_length;
        max_base_position = base_position;
      }
    }

    if (!WebPReportProgress(pic,
                            percent_start + percent_range *
                                                (size - 2 - base_position) /
                                                (size - 2),
                            percent)) {
      return 0;
    }
  }

  return WebPReportProgress(pic, percent_start + percent_range, percent);
}

static WEBP_INLINE void AddSingleLiteral(uint32_t pixel, int use_color_cache,
                                         VP8LColorCache* const hashers,
                                         VP8LBackwardRefs* const refs) {
  PixOrCopy v;
  if (use_color_cache) {
    const uint32_t key = VP8LColorCacheGetIndex(hashers, pixel);
    if (VP8LColorCacheLookup(hashers, key) == pixel) {
      v = PixOrCopyCreateCacheIdx(key);
    } else {
      v = PixOrCopyCreateLiteral(pixel);
      VP8LColorCacheSet(hashers, key, pixel);
    }
  } else {
    v = PixOrCopyCreateLiteral(pixel);
  }
  VP8LBackwardRefsCursorAdd(refs, v);
}

static int BackwardReferencesRle(int xsize, int ysize,
                                 const uint32_t* const argb,
                                 int cache_bits, VP8LBackwardRefs* const refs) {
  const int pix_count = xsize * ysize;
  int i, k;
  const int use_color_cache = (cache_bits > 0);
  VP8LColorCache hashers;

  if (use_color_cache && !VP8LColorCacheInit(&hashers, cache_bits)) {
    return 0;
  }
  VP8LClearBackwardRefs(refs);
  // Add first pixel as literal.
  AddSingleLiteral(argb[0], use_color_cache, &hashers, refs);
  i = 1;
  while (i < pix_count) {
    const int max_len = MaxFindCopyLength(pix_count - i);
    const int rle_len = FindMatchLength(argb + i, argb + i - 1, 0, max_len);
    const int prev_row_len = (i < xsize) ? 0 :
        FindMatchLength(argb + i, argb + i - xsize, 0, max_len);
    if (rle_len >= prev_row_len && rle_len >= MIN_LENGTH) {
      VP8LBackwardRefsCursorAdd(refs, PixOrCopyCreateCopy(1, rle_len));
      // We don't need to update the color cache here since it is always the
      // same pixel being copied, and that does not change the color cache
      // state.
      i += rle_len;
    } else if (prev_row_len >= MIN_LENGTH) {
      VP8LBackwardRefsCursorAdd(refs, PixOrCopyCreateCopy(xsize, prev_row_len));
      if (use_color_cache) {
        for (k = 0; k < prev_row_len; ++k) {
          VP8LColorCacheInsert(&hashers, argb[i + k]);
        }
      }
      i += prev_row_len;
    } else {
      AddSingleLiteral(argb[i], use_color_cache, &hashers, refs);
      i++;
    }
  }
  if (use_color_cache) VP8LColorCacheClear(&hashers);
  return !refs->error_;
}

static int BackwardReferencesLz77(int xsize, int ysize,
                                  const uint32_t* const argb, int cache_bits,
                                  const VP8LHashChain* const hash_chain,
                                  VP8LBackwardRefs* const refs) {
  int i;
  int i_last_check = -1;
  int ok = 0;
  int cc_init = 0;
  const int use_color_cache = (cache_bits > 0);
  const int pix_count = xsize * ysize;
  VP8LColorCache hashers;

  if (use_color_cache) {
    cc_init = VP8LColorCacheInit(&hashers, cache_bits);
    if (!cc_init) goto Error;
  }
  VP8LClearBackwardRefs(refs);
  for (i = 0; i < pix_count;) {
    // Alternative#1: Code the pixels starting at 'i' using backward reference.
    int offset = 0;
    int len = 0;
    int j;
    VP8LHashChainFindCopy(hash_chain, i, &offset, &len);
    if (len >= MIN_LENGTH) {
      const int len_ini = len;
      int max_reach = 0;
      const int j_max =
          (i + len_ini >= pix_count) ? pix_count - 1 : i + len_ini;
      // Only start from what we have not checked already.
      i_last_check = (i > i_last_check) ? i : i_last_check;
      // We know the best match for the current pixel but we try to find the
      // best matches for the current pixel AND the next one combined.
      // The naive method would use the intervals:
      // [i,i+len) + [i+len, length of best match at i+len)
      // while we check if we can use:
      // [i,j) (where j<=i+len) + [j, length of best match at j)
      for (j = i_last_check + 1; j <= j_max; ++j) {
        const int len_j = VP8LHashChainFindLength(hash_chain, j);
        const int reach =
            j + (len_j >= MIN_LENGTH ? len_j : 1);  // 1 for single literal.
        if (reach > max_reach) {
          len = j - i;
          max_reach = reach;
          if (max_reach >= pix_count) break;
        }
      }
    } else {
      len = 1;
    }
    // Go with literal or backward reference.
    assert(len > 0);
    if (len == 1) {
      AddSingleLiteral(argb[i], use_color_cache, &hashers, refs);
    } else {
      VP8LBackwardRefsCursorAdd(refs, PixOrCopyCreateCopy(offset, len));
      if (use_color_cache) {
        for (j = i; j < i + len; ++j) VP8LColorCacheInsert(&hashers, argb[j]);
      }
    }
    i += len;
  }

  ok = !refs->error_;
 Error:
  if (cc_init) VP8LColorCacheClear(&hashers);
  return ok;
}

// Compute an LZ77 by forcing matches to happen within a given distance cost.
// We therefore limit the algorithm to the lowest 32 values in the PlaneCode
// definition.
#define WINDOW_OFFSETS_SIZE_MAX 32
static int BackwardReferencesLz77Box(int xsize, int ysize,
                                     const uint32_t* const argb, int cache_bits,
                                     const VP8LHashChain* const hash_chain_best,
                                     VP8LHashChain* hash_chain,
                                     VP8LBackwardRefs* const refs) {
  int i;
  const int pix_count = xsize * ysize;
  uint16_t* counts;
  int window_offsets[WINDOW_OFFSETS_SIZE_MAX] = {0};
  int window_offsets_new[WINDOW_OFFSETS_SIZE_MAX] = {0};
  int window_offsets_size = 0;
  int window_offsets_new_size = 0;
  uint16_t* const counts_ini =
      (uint16_t*)WebPSafeMalloc(xsize * ysize, sizeof(*counts_ini));
  int best_offset_prev = -1, best_length_prev = -1;
  if (counts_ini == NULL) return 0;

  // counts[i] counts how many times a pixel is repeated starting at position i.
  i = pix_count - 2;
  counts = counts_ini + i;
  counts[1] = 1;
  for (; i >= 0; --i, --counts) {
    if (argb[i] == argb[i + 1]) {
      // Max out the counts to MAX_LENGTH.
      counts[0] = counts[1] + (counts[1] != MAX_LENGTH);
    } else {
      counts[0] = 1;
    }
  }

  // Figure out the window offsets around a pixel. They are stored in a
  // spiraling order around the pixel as defined by VP8LDistanceToPlaneCode.
  {
    int x, y;
    for (y = 0; y <= 6; ++y) {
      for (x = -6; x <= 6; ++x) {
        const int offset = y * xsize + x;
        int plane_code;
        // Ignore offsets that bring us after the pixel.
        if (offset <= 0) continue;
        plane_code = VP8LDistanceToPlaneCode(xsize, offset) - 1;
        if (plane_code >= WINDOW_OFFSETS_SIZE_MAX) continue;
        window_offsets[plane_code] = offset;
      }
    }
    // For narrow images, not all plane codes are reached, so remove those.
    for (i = 0; i < WINDOW_OFFSETS_SIZE_MAX; ++i) {
      if (window_offsets[i] == 0) continue;
      window_offsets[window_offsets_size++] = window_offsets[i];
    }
    // Given a pixel P, find the offsets that reach pixels unreachable from P-1
    // with any of the offsets in window_offsets[].
    for (i = 0; i < window_offsets_size; ++i) {
      int j;
      int is_reachable = 0;
      for (j = 0; j < window_offsets_size && !is_reachable; ++j) {
        is_reachable |= (window_offsets[i] == window_offsets[j] + 1);
      }
      if (!is_reachable) {
        window_offsets_new[window_offsets_new_size] = window_offsets[i];
        ++window_offsets_new_size;
      }
    }
  }

  hash_chain->offset_length_[0] = 0;
  for (i = 1; i < pix_count; ++i) {
    int ind;
    int best_length = VP8LHashChainFindLength(hash_chain_best, i);
    int best_offset;
    int do_compute = 1;

    if (best_length >= MAX_LENGTH) {
      // Do not recompute the best match if we already have a maximal one in the
      // window.
      best_offset = VP8LHashChainFindOffset(hash_chain_best, i);
      for (ind = 0; ind < window_offsets_size; ++ind) {
        if (best_offset == window_offsets[ind]) {
          do_compute = 0;
          break;
        }
      }
    }
    if (do_compute) {
      // Figure out if we should use the offset/length from the previous pixel
      // as an initial guess and therefore only inspect the offsets in
      // window_offsets_new[].
      const int use_prev =
          (best_length_prev > 1) && (best_length_prev < MAX_LENGTH);
      const int num_ind =
          use_prev ? window_offsets_new_size : window_offsets_size;
      best_length = use_prev ? best_length_prev - 1 : 0;
      best_offset = use_prev ? best_offset_prev : 0;
      // Find the longest match in a window around the pixel.
      for (ind = 0; ind < num_ind; ++ind) {
        int curr_length = 0;
        int j = i;
        int j_offset =
            use_prev ? i - window_offsets_new[ind] : i - window_offsets[ind];
        if (j_offset < 0 || argb[j_offset] != argb[i]) continue;
        // The longest match is the sum of how many times each pixel is
        // repeated.
        do {
          const int counts_j_offset = counts_ini[j_offset];
          const int counts_j = counts_ini[j];
          if (counts_j_offset != counts_j) {
            curr_length +=
                (counts_j_offset < counts_j) ? counts_j_offset : counts_j;
            break;
          }
          // The same color is repeated counts_pos times at j_offset and j.
          curr_length += counts_j_offset;
          j_offset += counts_j_offset;
          j += counts_j_offset;
        } while (curr_length <= MAX_LENGTH && j < pix_count &&
                 argb[j_offset] == argb[j]);
        if (best_length < curr_length) {
          best_offset =
              use_prev ? window_offsets_new[ind] : window_offsets[ind];
          if (curr_length >= MAX_LENGTH) {
            best_length = MAX_LENGTH;
            break;
          } else {
            best_length = curr_length;
          }
        }
      }
    }

    assert(i + best_length <= pix_count);
    assert(best_length <= MAX_LENGTH);
    if (best_length <= MIN_LENGTH) {
      hash_chain->offset_length_[i] = 0;
      best_offset_prev = 0;
      best_length_prev = 0;
    } else {
      hash_chain->offset_length_[i] =
          (best_offset << MAX_LENGTH_BITS) | (uint32_t)best_length;
      best_offset_prev = best_offset;
      best_length_prev = best_length;
    }
  }
  hash_chain->offset_length_[0] = 0;
  WebPSafeFree(counts_ini);

  return BackwardReferencesLz77(xsize, ysize, argb, cache_bits, hash_chain,
                                refs);
}

// -----------------------------------------------------------------------------

static void BackwardReferences2DLocality(int xsize,
                                         const VP8LBackwardRefs* const refs) {
  VP8LRefsCursor c = VP8LRefsCursorInit(refs);
  while (VP8LRefsCursorOk(&c)) {
    if (PixOrCopyIsCopy(c.cur_pos)) {
      const int dist = c.cur_pos->argb_or_distance;
      const int transformed_dist = VP8LDistanceToPlaneCode(xsize, dist);
      c.cur_pos->argb_or_distance = transformed_dist;
    }
    VP8LRefsCursorNext(&c);
  }
}

// Evaluate optimal cache bits for the local color cache.
// The input *best_cache_bits sets the maximum cache bits to use (passing 0
// implies disabling the local color cache). The local color cache is also
// disabled for the lower (<= 25) quality.
// Returns 0 in case of memory error.
static int CalculateBestCacheSize(const uint32_t* argb, int quality,
                                  const VP8LBackwardRefs* const refs,
                                  int* const best_cache_bits) {
  int i;
  const int cache_bits_max = (quality <= 25) ? 0 : *best_cache_bits;
  float entropy_min = MAX_ENTROPY;
  int cc_init[MAX_COLOR_CACHE_BITS + 1] = { 0 };
  VP8LColorCache hashers[MAX_COLOR_CACHE_BITS + 1];
  VP8LRefsCursor c = VP8LRefsCursorInit(refs);
  VP8LHistogram* histos[MAX_COLOR_CACHE_BITS + 1] = { NULL };
  int ok = 0;

  assert(cache_bits_max >= 0 && cache_bits_max <= MAX_COLOR_CACHE_BITS);

  if (cache_bits_max == 0) {
    *best_cache_bits = 0;
    // Local color cache is disabled.
    return 1;
  }

  // Allocate data.
  for (i = 0; i <= cache_bits_max; ++i) {
    histos[i] = VP8LAllocateHistogram(i);
    if (histos[i] == NULL) goto Error;
    VP8LHistogramInit(histos[i], i, /*init_arrays=*/ 1);
    if (i == 0) continue;
    cc_init[i] = VP8LColorCacheInit(&hashers[i], i);
    if (!cc_init[i]) goto Error;
  }

  // Find the cache_bits giving the lowest entropy. The search is done in a
  // brute-force way as the function (entropy w.r.t cache_bits) can be
  // anything in practice.
  while (VP8LRefsCursorOk(&c)) {
    const PixOrCopy* const v = c.cur_pos;
    if (PixOrCopyIsLiteral(v)) {
      const uint32_t pix = *argb++;
      const uint32_t a = (pix >> 24) & 0xff;
      const uint32_t r = (pix >> 16) & 0xff;
      const uint32_t g = (pix >>  8) & 0xff;
      const uint32_t b = (pix >>  0) & 0xff;
      // The keys of the caches can be derived from the longest one.
      int key = VP8LHashPix(pix, 32 - cache_bits_max);
      // Do not use the color cache for cache_bits = 0.
      ++histos[0]->blue_[b];
      ++histos[0]->literal_[g];
      ++histos[0]->red_[r];
      ++histos[0]->alpha_[a];
      // Deal with cache_bits > 0.
      for (i = cache_bits_max; i >= 1; --i, key >>= 1) {
        if (VP8LColorCacheLookup(&hashers[i], key) == pix) {
          ++histos[i]->literal_[NUM_LITERAL_CODES + NUM_LENGTH_CODES + key];
        } else {
          VP8LColorCacheSet(&hashers[i], key, pix);
          ++histos[i]->blue_[b];
          ++histos[i]->literal_[g];
          ++histos[i]->red_[r];
          ++histos[i]->alpha_[a];
        }
      }
    } else {
      int code, extra_bits, extra_bits_value;
      // We should compute the contribution of the (distance,length)
      // histograms but those are the same independently from the cache size.
      // As those constant contributions are in the end added to the other
      // histogram contributions, we can ignore them, except for the length
      // prefix that is part of the literal_ histogram.
      int len = PixOrCopyLength(v);
      uint32_t argb_prev = *argb ^ 0xffffffffu;
      VP8LPrefixEncode(len, &code, &extra_bits, &extra_bits_value);
      for (i = 0; i <= cache_bits_max; ++i) {
        ++histos[i]->literal_[NUM_LITERAL_CODES + code];
      }
      // Update the color caches.
      do {
        if (*argb != argb_prev) {
          // Efficiency: insert only if the color changes.
          int key = VP8LHashPix(*argb, 32 - cache_bits_max);
          for (i = cache_bits_max; i >= 1; --i, key >>= 1) {
            hashers[i].colors_[key] = *argb;
          }
          argb_prev = *argb;
        }
        argb++;
      } while (--len != 0);
    }
    VP8LRefsCursorNext(&c);
  }

  for (i = 0; i <= cache_bits_max; ++i) {
    const float entropy = VP8LHistogramEstimateBits(histos[i]);
    if (i == 0 || entropy < entropy_min) {
      entropy_min = entropy;
      *best_cache_bits = i;
    }
  }
  ok = 1;
 Error:
  for (i = 0; i <= cache_bits_max; ++i) {
    if (cc_init[i]) VP8LColorCacheClear(&hashers[i]);
    VP8LFreeHistogram(histos[i]);
  }
  return ok;
}

// Update (in-place) backward references for specified cache_bits.
static int BackwardRefsWithLocalCache(const uint32_t* const argb,
                                      int cache_bits,
                                      VP8LBackwardRefs* const refs) {
  int pixel_index = 0;
  VP8LColorCache hashers;
  VP8LRefsCursor c = VP8LRefsCursorInit(refs);
  if (!VP8LColorCacheInit(&hashers, cache_bits)) return 0;

  while (VP8LRefsCursorOk(&c)) {
    PixOrCopy* const v = c.cur_pos;
    if (PixOrCopyIsLiteral(v)) {
      const uint32_t argb_literal = v->argb_or_distance;
      const int ix = VP8LColorCacheContains(&hashers, argb_literal);
      if (ix >= 0) {
        // hashers contains argb_literal
        *v = PixOrCopyCreateCacheIdx(ix);
      } else {
        VP8LColorCacheInsert(&hashers, argb_literal);
      }
      ++pixel_index;
    } else {
      // refs was created without local cache, so it can not have cache indexes.
      int k;
      assert(PixOrCopyIsCopy(v));
      for (k = 0; k < v->len; ++k) {
        VP8LColorCacheInsert(&hashers, argb[pixel_index++]);
      }
    }
    VP8LRefsCursorNext(&c);
  }
  VP8LColorCacheClear(&hashers);
  return 1;
}

static VP8LBackwardRefs* GetBackwardReferencesLowEffort(
    int width, int height, const uint32_t* const argb,
    int* const cache_bits, const VP8LHashChain* const hash_chain,
    VP8LBackwardRefs* const refs_lz77) {
  *cache_bits = 0;
  if (!BackwardReferencesLz77(width, height, argb, 0, hash_chain, refs_lz77)) {
    return NULL;
  }
  BackwardReferences2DLocality(width, refs_lz77);
  return refs_lz77;
}

extern int VP8LBackwardReferencesTraceBackwards(
    int xsize, int ysize, const uint32_t* const argb, int cache_bits,
    const VP8LHashChain* const hash_chain,
    const VP8LBackwardRefs* const refs_src, VP8LBackwardRefs* const refs_dst);
static int GetBackwardReferences(int width, int height,
                                 const uint32_t* const argb, int quality,
                                 int lz77_types_to_try, int cache_bits_max,
                                 int do_no_cache,
                                 const VP8LHashChain* const hash_chain,
                                 VP8LBackwardRefs* const refs,
                                 int* const cache_bits_best) {
  VP8LHistogram* histo = NULL;
  int i, lz77_type;
  // Index 0 is for a color cache, index 1 for no cache (if needed).
  int lz77_types_best[2] = {0, 0};
  float bit_costs_best[2] = {FLT_MAX, FLT_MAX};
  VP8LHashChain hash_chain_box;
  VP8LBackwardRefs* const refs_tmp = &refs[do_no_cache ? 2 : 1];
  int status = 0;
  memset(&hash_chain_box, 0, sizeof(hash_chain_box));

  histo = VP8LAllocateHistogram(MAX_COLOR_CACHE_BITS);
  if (histo == NULL) goto Error;

  for (lz77_type = 1; lz77_types_to_try;
       lz77_types_to_try &= ~lz77_type, lz77_type <<= 1) {
    int res = 0;
    float bit_cost = 0.f;
    if ((lz77_types_to_try & lz77_type) == 0) continue;
    switch (lz77_type) {
      case kLZ77RLE:
        res = BackwardReferencesRle(width, height, argb, 0, refs_tmp);
        break;
      case kLZ77Standard:
        // Compute LZ77 with no cache (0 bits), as the ideal LZ77 with a color
        // cache is not that different in practice.
        res = BackwardReferencesLz77(width, height, argb, 0, hash_chain,
                                     refs_tmp);
        break;
      case kLZ77Box:
        if (!VP8LHashChainInit(&hash_chain_box, width * height)) goto Error;
        res = BackwardReferencesLz77Box(width, height, argb, 0, hash_chain,
                                        &hash_chain_box, refs_tmp);
        break;
      default:
        assert(0);
    }
    if (!res) goto Error;

    // Start with the no color cache case.
    for (i = 1; i >= 0; --i) {
      int cache_bits = (i == 1) ? 0 : cache_bits_max;

      if (i == 1 && !do_no_cache) continue;

      if (i == 0) {
        // Try with a color cache.
        if (!CalculateBestCacheSize(argb, quality, refs_tmp, &cache_bits)) {
          goto Error;
        }
        if (cache_bits > 0) {
          if (!BackwardRefsWithLocalCache(argb, cache_bits, refs_tmp)) {
            goto Error;
          }
        }
      }

      if (i == 0 && do_no_cache && cache_bits == 0) {
        // No need to re-compute bit_cost as it was computed at i == 1.
      } else {
        VP8LHistogramCreate(histo, refs_tmp, cache_bits);
        bit_cost = VP8LHistogramEstimateBits(histo);
      }

      if (bit_cost < bit_costs_best[i]) {
        if (i == 1) {
          // Do not swap as the full cache analysis would have the wrong
          // VP8LBackwardRefs to start with.
          if (!BackwardRefsClone(refs_tmp, &refs[1])) goto Error;
        } else {
          BackwardRefsSwap(refs_tmp, &refs[0]);
        }
        bit_costs_best[i] = bit_cost;
        lz77_types_best[i] = lz77_type;
        if (i == 0) *cache_bits_best = cache_bits;
      }
    }
  }
  assert(lz77_types_best[0] > 0);
  assert(!do_no_cache || lz77_types_best[1] > 0);

  // Improve on simple LZ77 but only for high quality (TraceBackwards is
  // costly).
  for (i = 1; i >= 0; --i) {
    if (i == 1 && !do_no_cache) continue;
    if ((lz77_types_best[i] == kLZ77Standard ||
         lz77_types_best[i] == kLZ77Box) &&
        quality >= 25) {
      const VP8LHashChain* const hash_chain_tmp =
          (lz77_types_best[i] == kLZ77Standard) ? hash_chain : &hash_chain_box;
      const int cache_bits = (i == 1) ? 0 : *cache_bits_best;
      float bit_cost_trace;
      if (!VP8LBackwardReferencesTraceBackwards(width, height, argb, cache_bits,
                                                hash_chain_tmp, &refs[i],
                                                refs_tmp)) {
        goto Error;
      }
      VP8LHistogramCreate(histo, refs_tmp, cache_bits);
      bit_cost_trace = VP8LHistogramEstimateBits(histo);
      if (bit_cost_trace < bit_costs_best[i]) {
        BackwardRefsSwap(refs_tmp, &refs[i]);
      }
    }

    BackwardReferences2DLocality(width, &refs[i]);

    if (i == 1 && lz77_types_best[0] == lz77_types_best[1] &&
        *cache_bits_best == 0) {
      // If the best cache size is 0 and we have the same best LZ77, just copy
      // the data over and stop here.
      if (!BackwardRefsClone(&refs[1], &refs[0])) goto Error;
      break;
    }
  }
  status = 1;

 Error:
  VP8LHashChainClear(&hash_chain_box);
  VP8LFreeHistogram(histo);
  return status;
}

int VP8LGetBackwardReferences(
    int width, int height, const uint32_t* const argb, int quality,
    int low_effort, int lz77_types_to_try, int cache_bits_max, int do_no_cache,
    const VP8LHashChain* const hash_chain, VP8LBackwardRefs* const refs,
    int* const cache_bits_best, const WebPPicture* const pic, int percent_range,
    int* const percent) {
  if (low_effort) {
    VP8LBackwardRefs* refs_best;
    *cache_bits_best = cache_bits_max;
    refs_best = GetBackwardReferencesLowEffort(
        width, height, argb, cache_bits_best, hash_chain, refs);
    if (refs_best == NULL) {
      return WebPEncodingSetError(pic, VP8_ENC_ERROR_OUT_OF_MEMORY);
    }
    // Set it in first position.
    BackwardRefsSwap(refs_best, &refs[0]);
  } else {
    if (!GetBackwardReferences(width, height, argb, quality, lz77_types_to_try,
                               cache_bits_max, do_no_cache, hash_chain, refs,
                               cache_bits_best)) {
      return WebPEncodingSetError(pic, VP8_ENC_ERROR_OUT_OF_MEMORY);
    }
  }

  return WebPReportProgress(pic, *percent + percent_range, percent);
}