Update libwebp to 0.6.1

* lossless performance and compression improvements + a new 'cruncher' mode (-m 6 -q 100)
* ARM performance improvements with clang (15-20% w/ndk r15c)
* webp-js: emscripten/webassembly based javascript decoder
* miscellaneous bug & build fixes

1 files changed, 943 insertions, 0 deletions

diff --git a/thirdparty/libwebp/src/enc/backward_references_enc.c b/thirdparty/libwebp/src/enc/backward_references_enc.c
new file mode 100644
index 0000000000..39230188b9
--- /dev/null
+++ b/thirdparty/libwebp/src/enc/backward_references_enc.c
@@ -0,0 +1,943 @@
+// 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 <assert.h>
+#include <math.h>
+
+#include "src/enc/backward_references_enc.h"
+#include "src/enc/histogram_enc.h"
+#include "src/dsp/lossless.h"
+#include "src/dsp/lossless_common.h"
+#include "src/dsp/dsp.h"
+#include "src/utils/color_cache_utils.h"
+#include "src/utils/utils.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;
+  }
+}
+
+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;
+}
+
+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;
+}
+
+// -----------------------------------------------------------------------------
+
+#define HASH_MULTIPLIER_HI (0xc6a4a793ULL)
+#define HASH_MULTIPLIER_LO (0x5bd1e996ULL)
+
+static WEBP_INLINE uint32_t GetPixPairHash64(const uint32_t* const argb) {
+  uint32_t key;
+  key  = (argb[1] * HASH_MULTIPLIER_HI) & 0xffffffffu;
+  key += (argb[0] * HASH_MULTIPLIER_LO) & 0xffffffffu;
+  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 int size = xsize * ysize;
+  const int iter_max = GetMaxItersForQuality(quality);
+  const uint32_t window_size = GetWindowSizeForHashChain(quality, xsize);
+  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 0;
+
+  // 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;
+    }
+  }
+  // Process the penultimate pixel.
+  chain[pos] = hash_to_first_index[GetPixPairHash64(argb + pos)];
+
+  WebPSafeFree(hash_to_first_index);
+
+  // 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;
+      }
+    }
+  }
+  return 1;
+}
+
+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;
+  double 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;
+    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 {
+      // 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 safely ignore them.
+      int len = PixOrCopyLength(v);
+      uint32_t argb_prev = *argb ^ 0xffffffffu;
+      // 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 double 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 VP8LBackwardRefs* GetBackwardReferences(
+    int width, int height, const uint32_t* const argb, int quality,
+    int lz77_types_to_try, int* const cache_bits,
+    const VP8LHashChain* const hash_chain, VP8LBackwardRefs* best,
+    VP8LBackwardRefs* worst) {
+  const int cache_bits_initial = *cache_bits;
+  double bit_cost_best = -1;
+  VP8LHistogram* histo = NULL;
+  int lz77_type, lz77_type_best = 0;
+  VP8LHashChain hash_chain_box;
+  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;
+    double bit_cost;
+    int cache_bits_tmp = cache_bits_initial;
+    if ((lz77_types_to_try & lz77_type) == 0) continue;
+    switch (lz77_type) {
+      case kLZ77RLE:
+        res = BackwardReferencesRle(width, height, argb, 0, worst);
+        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, worst);
+        break;
+      case kLZ77Box:
+        if (!VP8LHashChainInit(&hash_chain_box, width * height)) goto Error;
+        res = BackwardReferencesLz77Box(width, height, argb, 0, hash_chain,
+                                        &hash_chain_box, worst);
+        break;
+      default:
+        assert(0);
+    }
+    if (!res) goto Error;
+
+    // Next, try with a color cache and update the references.
+    if (!CalculateBestCacheSize(argb, quality, worst, &cache_bits_tmp)) {
+      goto Error;
+    }
+    if (cache_bits_tmp > 0) {
+      if (!BackwardRefsWithLocalCache(argb, cache_bits_tmp, worst)) {
+        goto Error;
+      }
+    }
+
+    // Keep the best backward references.
+    VP8LHistogramCreate(histo, worst, cache_bits_tmp);
+    bit_cost = VP8LHistogramEstimateBits(histo);
+    if (lz77_type_best == 0 || bit_cost < bit_cost_best) {
+      VP8LBackwardRefs* const tmp = worst;
+      worst = best;
+      best = tmp;
+      bit_cost_best = bit_cost;
+      *cache_bits = cache_bits_tmp;
+      lz77_type_best = lz77_type;
+    }
+  }
+  assert(lz77_type_best > 0);
+
+  // Improve on simple LZ77 but only for high quality (TraceBackwards is
+  // costly).
+  if ((lz77_type_best == kLZ77Standard || lz77_type_best == kLZ77Box) &&
+      quality >= 25) {
+    const VP8LHashChain* const hash_chain_tmp =
+        (lz77_type_best == kLZ77Standard) ? hash_chain : &hash_chain_box;
+    if (VP8LBackwardReferencesTraceBackwards(width, height, argb, *cache_bits,
+                                             hash_chain_tmp, best, worst)) {
+      double bit_cost_trace;
+      VP8LHistogramCreate(histo, worst, *cache_bits);
+      bit_cost_trace = VP8LHistogramEstimateBits(histo);
+      if (bit_cost_trace < bit_cost_best) best = worst;
+    }
+  }
+
+  BackwardReferences2DLocality(width, best);
+
+Error:
+  VP8LHashChainClear(&hash_chain_box);
+  VP8LFreeHistogram(histo);
+  return best;
+}
+
+VP8LBackwardRefs* VP8LGetBackwardReferences(
+    int width, int height, const uint32_t* const argb, int quality,
+    int low_effort, int lz77_types_to_try, int* const cache_bits,
+    const VP8LHashChain* const hash_chain, VP8LBackwardRefs* const refs_tmp1,
+    VP8LBackwardRefs* const refs_tmp2) {
+  if (low_effort) {
+    return GetBackwardReferencesLowEffort(width, height, argb, cache_bits,
+                                          hash_chain, refs_tmp1);
+  } else {
+    return GetBackwardReferences(width, height, argb, quality,
+                                 lz77_types_to_try, cache_bits, hash_chain,
+                                 refs_tmp1, refs_tmp2);
+  }
+}