-Upgraded webp to a MUCH newer version. Hoping it fixes some bugs in the process. Keeping old version just in case for now.

-Added ability to convert xml and tscn scenes to binary on export, makes loading of larger scenes faster

1 files changed, 736 insertions, 244 deletions

diff --git a/drivers/webp/enc/histogram.c b/drivers/webp/enc/histogram.c
index ca838e064d..62c320d809 100644
--- a/drivers/webp/enc/histogram.c
+++ b/drivers/webp/enc/histogram.c
@@ -1,38 +1,82 @@
 // Copyright 2012 Google Inc. All Rights Reserved.
 //
-// This code is licensed under the same terms as WebM:
-//  Software License Agreement:  http://www.webmproject.org/license/software/
-//  Additional IP Rights Grant:  http://www.webmproject.org/license/additional/
+// 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)
 //
 #ifdef HAVE_CONFIG_H
-#include "config.h"
+#include "../webp/config.h"
 #endif
 
 #include <math.h>
-#include <stdio.h>
 
 #include "./backward_references.h"
 #include "./histogram.h"
 #include "../dsp/lossless.h"
 #include "../utils/utils.h"
 
+#define MAX_COST 1.e38
+
+// Number of partitions for the three dominant (literal, red and blue) symbol
+// costs.
+#define NUM_PARTITIONS 4
+// The size of the bin-hash corresponding to the three dominant costs.
+#define BIN_SIZE (NUM_PARTITIONS * NUM_PARTITIONS * NUM_PARTITIONS)
+// Maximum number of histograms allowed in greedy combining algorithm.
+#define MAX_HISTO_GREEDY 100
+
 static void HistogramClear(VP8LHistogram* const p) {
-  memset(p->literal_, 0, sizeof(p->literal_));
-  memset(p->red_, 0, sizeof(p->red_));
-  memset(p->blue_, 0, sizeof(p->blue_));
-  memset(p->alpha_, 0, sizeof(p->alpha_));
-  memset(p->distance_, 0, sizeof(p->distance_));
-  p->bit_cost_ = 0;
+  uint32_t* const literal = p->literal_;
+  const int cache_bits = p->palette_code_bits_;
+  const int histo_size = VP8LGetHistogramSize(cache_bits);
+  memset(p, 0, histo_size);
+  p->palette_code_bits_ = cache_bits;
+  p->literal_ = literal;
+}
+
+// Swap two histogram pointers.
+static void HistogramSwap(VP8LHistogram** const A, VP8LHistogram** const B) {
+  VP8LHistogram* const tmp = *A;
+  *A = *B;
+  *B = tmp;
+}
+
+static void HistogramCopy(const VP8LHistogram* const src,
+                          VP8LHistogram* const dst) {
+  uint32_t* const dst_literal = dst->literal_;
+  const int dst_cache_bits = dst->palette_code_bits_;
+  const int histo_size = VP8LGetHistogramSize(dst_cache_bits);
+  assert(src->palette_code_bits_ == dst_cache_bits);
+  memcpy(dst, src, histo_size);
+  dst->literal_ = dst_literal;
+}
+
+int VP8LGetHistogramSize(int cache_bits) {
+  const int literal_size = VP8LHistogramNumCodes(cache_bits);
+  const size_t total_size = sizeof(VP8LHistogram) + sizeof(int) * literal_size;
+  assert(total_size <= (size_t)0x7fffffff);
+  return (int)total_size;
+}
+
+void VP8LFreeHistogram(VP8LHistogram* const histo) {
+  WebPSafeFree(histo);
+}
+
+void VP8LFreeHistogramSet(VP8LHistogramSet* const histo) {
+  WebPSafeFree(histo);
 }
 
 void VP8LHistogramStoreRefs(const VP8LBackwardRefs* const refs,
                             VP8LHistogram* const histo) {
-  int i;
-  for (i = 0; i < refs->size; ++i) {
-    VP8LHistogramAddSinglePixOrCopy(histo, &refs->refs[i]);
+  VP8LRefsCursor c = VP8LRefsCursorInit(refs);
+  while (VP8LRefsCursorOk(&c)) {
+    VP8LHistogramAddSinglePixOrCopy(histo, c.cur_pos);
+    VP8LRefsCursorNext(&c);
   }
 }
 
@@ -51,13 +95,25 @@ void VP8LHistogramInit(VP8LHistogram* const p, int palette_code_bits) {
   HistogramClear(p);
 }
 
+VP8LHistogram* VP8LAllocateHistogram(int cache_bits) {
+  VP8LHistogram* histo = NULL;
+  const int total_size = VP8LGetHistogramSize(cache_bits);
+  uint8_t* const memory = (uint8_t*)WebPSafeMalloc(total_size, sizeof(*memory));
+  if (memory == NULL) return NULL;
+  histo = (VP8LHistogram*)memory;
+  // literal_ won't necessary be aligned.
+  histo->literal_ = (uint32_t*)(memory + sizeof(VP8LHistogram));
+  VP8LHistogramInit(histo, cache_bits);
+  return histo;
+}
+
 VP8LHistogramSet* VP8LAllocateHistogramSet(int size, int cache_bits) {
   int i;
   VP8LHistogramSet* set;
-  VP8LHistogram* bulk;
-  const uint64_t total_size = (uint64_t)sizeof(*set)
-                            + size * sizeof(*set->histograms)
-                            + size * sizeof(**set->histograms);
+  const int histo_size = VP8LGetHistogramSize(cache_bits);
+  const size_t total_size =
+      sizeof(*set) + size * (sizeof(*set->histograms) +
+      histo_size + WEBP_ALIGN_CST);
   uint8_t* memory = (uint8_t*)WebPSafeMalloc(total_size, sizeof(*memory));
   if (memory == NULL) return NULL;
 
@@ -65,12 +121,15 @@ VP8LHistogramSet* VP8LAllocateHistogramSet(int size, int cache_bits) {
   memory += sizeof(*set);
   set->histograms = (VP8LHistogram**)memory;
   memory += size * sizeof(*set->histograms);
-  bulk = (VP8LHistogram*)memory;
   set->max_size = size;
   set->size = size;
   for (i = 0; i < size; ++i) {
-    set->histograms[i] = bulk + i;
+    memory = (uint8_t*)WEBP_ALIGN(memory);
+    set->histograms[i] = (VP8LHistogram*)memory;
+    // literal_ won't necessary be aligned.
+    set->histograms[i]->literal_ = (uint32_t*)(memory + sizeof(VP8LHistogram));
     VP8LHistogramInit(set->histograms[i], cache_bits);
+    memory += histo_size;
   }
   return set;
 }
@@ -85,151 +144,183 @@ void VP8LHistogramAddSinglePixOrCopy(VP8LHistogram* const histo,
     ++histo->literal_[PixOrCopyLiteral(v, 1)];
     ++histo->blue_[PixOrCopyLiteral(v, 0)];
   } else if (PixOrCopyIsCacheIdx(v)) {
-    int literal_ix = 256 + NUM_LENGTH_CODES + PixOrCopyCacheIdx(v);
+    const int literal_ix =
+        NUM_LITERAL_CODES + NUM_LENGTH_CODES + PixOrCopyCacheIdx(v);
     ++histo->literal_[literal_ix];
   } else {
-    int code, extra_bits_count, extra_bits_value;
-    PrefixEncode(PixOrCopyLength(v),
-                 &code, &extra_bits_count, &extra_bits_value);
-    ++histo->literal_[256 + code];
-    PrefixEncode(PixOrCopyDistance(v),
-                 &code, &extra_bits_count, &extra_bits_value);
+    int code, extra_bits;
+    VP8LPrefixEncodeBits(PixOrCopyLength(v), &code, &extra_bits);
+    ++histo->literal_[NUM_LITERAL_CODES + code];
+    VP8LPrefixEncodeBits(PixOrCopyDistance(v), &code, &extra_bits);
     ++histo->distance_[code];
   }
 }
 
+// -----------------------------------------------------------------------------
+// Various histogram combine/cost-eval functions
+
+static int GetCombinedHistogramEntropy(const VP8LHistogram* const a,
+                                       const VP8LHistogram* const b,
+                                       double cost_threshold,
+                                       double* cost) {
+  const int palette_code_bits = a->palette_code_bits_;
+  assert(a->palette_code_bits_ == b->palette_code_bits_);
+  *cost += VP8LGetCombinedEntropy(a->literal_, b->literal_,
+                                  VP8LHistogramNumCodes(palette_code_bits));
+  *cost += VP8LExtraCostCombined(a->literal_ + NUM_LITERAL_CODES,
+                                 b->literal_ + NUM_LITERAL_CODES,
+                                 NUM_LENGTH_CODES);
+  if (*cost > cost_threshold) return 0;
+
+  *cost += VP8LGetCombinedEntropy(a->red_, b->red_, NUM_LITERAL_CODES);
+  if (*cost > cost_threshold) return 0;
+
+  *cost += VP8LGetCombinedEntropy(a->blue_, b->blue_, NUM_LITERAL_CODES);
+  if (*cost > cost_threshold) return 0;
+
+  *cost += VP8LGetCombinedEntropy(a->alpha_, b->alpha_, NUM_LITERAL_CODES);
+  if (*cost > cost_threshold) return 0;
+
+  *cost += VP8LGetCombinedEntropy(a->distance_, b->distance_,
+                                  NUM_DISTANCE_CODES);
+  *cost += VP8LExtraCostCombined(a->distance_, b->distance_,
+                                 NUM_DISTANCE_CODES);
+  if (*cost > cost_threshold) return 0;
+
+  return 1;
+}
 
-
-static double BitsEntropy(const int* const array, int n) {
-  double retval = 0.;
-  int sum = 0;
-  int nonzeros = 0;
-  int max_val = 0;
-  int i;
-  double mix;
-  for (i = 0; i < n; ++i) {
-    if (array[i] != 0) {
-      sum += array[i];
-      ++nonzeros;
-      retval -= VP8LFastSLog2(array[i]);
-      if (max_val < array[i]) {
-        max_val = array[i];
-      }
-    }
+// Performs out = a + b, computing the cost C(a+b) - C(a) - C(b) while comparing
+// to the threshold value 'cost_threshold'. The score returned is
+//  Score = C(a+b) - C(a) - C(b), where C(a) + C(b) is known and fixed.
+// Since the previous score passed is 'cost_threshold', we only need to compare
+// the partial cost against 'cost_threshold + C(a) + C(b)' to possibly bail-out
+// early.
+static double HistogramAddEval(const VP8LHistogram* const a,
+                               const VP8LHistogram* const b,
+                               VP8LHistogram* const out,
+                               double cost_threshold) {
+  double cost = 0;
+  const double sum_cost = a->bit_cost_ + b->bit_cost_;
+  cost_threshold += sum_cost;
+
+  if (GetCombinedHistogramEntropy(a, b, cost_threshold, &cost)) {
+    VP8LHistogramAdd(a, b, out);
+    out->bit_cost_ = cost;
+    out->palette_code_bits_ = a->palette_code_bits_;
+    out->trivial_symbol_ = (a->trivial_symbol_ == b->trivial_symbol_) ?
+        a->trivial_symbol_ : VP8L_NON_TRIVIAL_SYM;
   }
-  retval += VP8LFastSLog2(sum);
 
-  if (nonzeros < 5) {
-    if (nonzeros <= 1) {
-      return 0;
-    }
-    // Two symbols, they will be 0 and 1 in a Huffman code.
-    // Let's mix in a bit of entropy to favor good clustering when
-    // distributions of these are combined.
-    if (nonzeros == 2) {
-      return 0.99 * sum + 0.01 * retval;
-    }
-    // No matter what the entropy says, we cannot be better than min_limit
-    // with Huffman coding. I am mixing a bit of entropy into the
-    // min_limit since it produces much better (~0.5 %) compression results
-    // perhaps because of better entropy clustering.
-    if (nonzeros == 3) {
-      mix = 0.95;
-    } else {
-      mix = 0.7;  // nonzeros == 4.
-    }
-  } else {
-    mix = 0.627;
-  }
+  return cost - sum_cost;
+}
 
-  {
-    double min_limit = 2 * sum - max_val;
-    min_limit = mix * min_limit + (1.0 - mix) * retval;
-    return (retval < min_limit) ? min_limit : retval;
-  }
+// Same as HistogramAddEval(), except that the resulting histogram
+// is not stored. Only the cost C(a+b) - C(a) is evaluated. We omit
+// the term C(b) which is constant over all the evaluations.
+static double HistogramAddThresh(const VP8LHistogram* const a,
+                                 const VP8LHistogram* const b,
+                                 double cost_threshold) {
+  double cost = -a->bit_cost_;
+  GetCombinedHistogramEntropy(a, b, cost_threshold, &cost);
+  return cost;
 }
 
-double VP8LHistogramEstimateBitsBulk(const VP8LHistogram* const p) {
-  double retval = BitsEntropy(&p->literal_[0], VP8LHistogramNumCodes(p))
-                + BitsEntropy(&p->red_[0], 256)
-                + BitsEntropy(&p->blue_[0], 256)
-                + BitsEntropy(&p->alpha_[0], 256)
-                + BitsEntropy(&p->distance_[0], NUM_DISTANCE_CODES);
-  // Compute the extra bits cost.
-  int i;
-  for (i = 2; i < NUM_LENGTH_CODES - 2; ++i) {
-    retval +=
-        (i >> 1) * p->literal_[256 + i + 2];
-  }
-  for (i = 2; i < NUM_DISTANCE_CODES - 2; ++i) {
-    retval += (i >> 1) * p->distance_[i + 2];
-  }
-  return retval;
-}
-
-
-// Returns the cost encode the rle-encoded entropy code.
-// The constants in this function are experimental.
-static double HuffmanCost(const int* const population, int length) {
-  // Small bias because Huffman code length is typically not stored in
-  // full length.
-  static const int kHuffmanCodeOfHuffmanCodeSize = CODE_LENGTH_CODES * 3;
-  static const double kSmallBias = 9.1;
-  double retval = kHuffmanCodeOfHuffmanCodeSize - kSmallBias;
-  int streak = 0;
-  int i = 0;
-  for (; i < length - 1; ++i) {
-    ++streak;
-    if (population[i] == population[i + 1]) {
-      continue;
-    }
- last_streak_hack:
-    // population[i] points now to the symbol in the streak of same values.
-    if (streak > 3) {
-      if (population[i] == 0) {
-        retval += 1.5625 + 0.234375 * streak;
-      } else {
-        retval += 2.578125 + 0.703125 * streak;
-      }
-    } else {
-      if (population[i] == 0) {
-        retval += 1.796875 * streak;
-      } else {
-        retval += 3.28125 * streak;
-      }
-    }
-    streak = 0;
-  }
-  if (i == length - 1) {
-    ++streak;
-    goto last_streak_hack;
+// -----------------------------------------------------------------------------
+
+// The structure to keep track of cost range for the three dominant entropy
+// symbols.
+// TODO(skal): Evaluate if float can be used here instead of double for
+// representing the entropy costs.
+typedef struct {
+  double literal_max_;
+  double literal_min_;
+  double red_max_;
+  double red_min_;
+  double blue_max_;
+  double blue_min_;
+} DominantCostRange;
+
+static void DominantCostRangeInit(DominantCostRange* const c) {
+  c->literal_max_ = 0.;
+  c->literal_min_ = MAX_COST;
+  c->red_max_ = 0.;
+  c->red_min_ = MAX_COST;
+  c->blue_max_ = 0.;
+  c->blue_min_ = MAX_COST;
+}
+
+static void UpdateDominantCostRange(
+    const VP8LHistogram* const h, DominantCostRange* const c) {
+  if (c->literal_max_ < h->literal_cost_) c->literal_max_ = h->literal_cost_;
+  if (c->literal_min_ > h->literal_cost_) c->literal_min_ = h->literal_cost_;
+  if (c->red_max_ < h->red_cost_) c->red_max_ = h->red_cost_;
+  if (c->red_min_ > h->red_cost_) c->red_min_ = h->red_cost_;
+  if (c->blue_max_ < h->blue_cost_) c->blue_max_ = h->blue_cost_;
+  if (c->blue_min_ > h->blue_cost_) c->blue_min_ = h->blue_cost_;
+}
+
+static void UpdateHistogramCost(VP8LHistogram* const h) {
+  uint32_t alpha_sym, red_sym, blue_sym;
+  const double alpha_cost = VP8LPopulationCost(h->alpha_, NUM_LITERAL_CODES,
+                                               &alpha_sym);
+  const double distance_cost =
+      VP8LPopulationCost(h->distance_, NUM_DISTANCE_CODES, NULL) +
+      VP8LExtraCost(h->distance_, NUM_DISTANCE_CODES);
+  const int num_codes = VP8LHistogramNumCodes(h->palette_code_bits_);
+  h->literal_cost_ = VP8LPopulationCost(h->literal_, num_codes, NULL) +
+                     VP8LExtraCost(h->literal_ + NUM_LITERAL_CODES,
+                                   NUM_LENGTH_CODES);
+  h->red_cost_ = VP8LPopulationCost(h->red_, NUM_LITERAL_CODES, &red_sym);
+  h->blue_cost_ = VP8LPopulationCost(h->blue_, NUM_LITERAL_CODES, &blue_sym);
+  h->bit_cost_ = h->literal_cost_ + h->red_cost_ + h->blue_cost_ +
+                 alpha_cost + distance_cost;
+  if ((alpha_sym | red_sym | blue_sym) == VP8L_NON_TRIVIAL_SYM) {
+    h->trivial_symbol_ = VP8L_NON_TRIVIAL_SYM;
+  } else {
+    h->trivial_symbol_ =
+        ((uint32_t)alpha_sym << 24) | (red_sym << 16) | (blue_sym << 0);
   }
-  return retval;
 }
 
-// Estimates the Huffman dictionary + other block overhead size.
-static double HistogramEstimateBitsHeader(const VP8LHistogram* const p) {
-  return HuffmanCost(&p->alpha_[0], 256) +
-         HuffmanCost(&p->red_[0], 256) +
-         HuffmanCost(&p->literal_[0], VP8LHistogramNumCodes(p)) +
-         HuffmanCost(&p->blue_[0], 256) +
-         HuffmanCost(&p->distance_[0], NUM_DISTANCE_CODES);
+static int GetBinIdForEntropy(double min, double max, double val) {
+  const double range = max - min + 1e-6;
+  const double delta = val - min;
+  return (int)(NUM_PARTITIONS * delta / range);
 }
 
-double VP8LHistogramEstimateBits(const VP8LHistogram* const p) {
-  return HistogramEstimateBitsHeader(p) + VP8LHistogramEstimateBitsBulk(p);
+static int GetHistoBinIndexLowEffort(
+    const VP8LHistogram* const h, const DominantCostRange* const c) {
+  const int bin_id = GetBinIdForEntropy(c->literal_min_, c->literal_max_,
+                                        h->literal_cost_);
+  assert(bin_id < NUM_PARTITIONS);
+  return bin_id;
 }
 
-static void HistogramBuildImage(int xsize, int histo_bits,
-                                const VP8LBackwardRefs* const backward_refs,
-                                VP8LHistogramSet* const image) {
-  int i;
+static int GetHistoBinIndex(
+    const VP8LHistogram* const h, const DominantCostRange* const c) {
+  const int bin_id =
+      GetBinIdForEntropy(c->blue_min_, c->blue_max_, h->blue_cost_) +
+      NUM_PARTITIONS * GetBinIdForEntropy(c->red_min_, c->red_max_,
+                                          h->red_cost_) +
+      NUM_PARTITIONS * NUM_PARTITIONS * GetBinIdForEntropy(c->literal_min_,
+                                                           c->literal_max_,
+                                                           h->literal_cost_);
+  assert(bin_id < BIN_SIZE);
+  return bin_id;
+}
+
+// Construct the histograms from backward references.
+static void HistogramBuild(
+    int xsize, int histo_bits, const VP8LBackwardRefs* const backward_refs,
+    VP8LHistogramSet* const image_histo) {
   int x = 0, y = 0;
   const int histo_xsize = VP8LSubSampleSize(xsize, histo_bits);
-  VP8LHistogram** const histograms = image->histograms;
+  VP8LHistogram** const histograms = image_histo->histograms;
+  VP8LRefsCursor c = VP8LRefsCursorInit(backward_refs);
   assert(histo_bits > 0);
-  for (i = 0; i < backward_refs->size; ++i) {
-    const PixOrCopy* const v = &backward_refs->refs[i];
+  while (VP8LRefsCursorOk(&c)) {
+    const PixOrCopy* const v = c.cur_pos;
     const int ix = (y >> histo_bits) * histo_xsize + (x >> histo_bits);
     VP8LHistogramAddSinglePixOrCopy(histograms[ix], v);
     x += PixOrCopyLength(v);
@@ -237,7 +328,134 @@ static void HistogramBuildImage(int xsize, int histo_bits,
       x -= xsize;
       ++y;
     }
+    VP8LRefsCursorNext(&c);
+  }
+}
+
+// Copies the histograms and computes its bit_cost.
+static void HistogramCopyAndAnalyze(
+    VP8LHistogramSet* const orig_histo, VP8LHistogramSet* const image_histo) {
+  int i;
+  const int histo_size = orig_histo->size;
+  VP8LHistogram** const orig_histograms = orig_histo->histograms;
+  VP8LHistogram** const histograms = image_histo->histograms;
+  for (i = 0; i < histo_size; ++i) {
+    VP8LHistogram* const histo = orig_histograms[i];
+    UpdateHistogramCost(histo);
+    // Copy histograms from orig_histo[] to image_histo[].
+    HistogramCopy(histo, histograms[i]);
+  }
+}
+
+// Partition histograms to different entropy bins for three dominant (literal,
+// red and blue) symbol costs and compute the histogram aggregate bit_cost.
+static void HistogramAnalyzeEntropyBin(VP8LHistogramSet* const image_histo,
+                                       int16_t* const bin_map, int low_effort) {
+  int i;
+  VP8LHistogram** const histograms = image_histo->histograms;
+  const int histo_size = image_histo->size;
+  const int bin_depth = histo_size + 1;
+  DominantCostRange cost_range;
+  DominantCostRangeInit(&cost_range);
+
+  // Analyze the dominant (literal, red and blue) entropy costs.
+  for (i = 0; i < histo_size; ++i) {
+    VP8LHistogram* const histo = histograms[i];
+    UpdateDominantCostRange(histo, &cost_range);
+  }
+
+  // bin-hash histograms on three of the dominant (literal, red and blue)
+  // symbol costs.
+  for (i = 0; i < histo_size; ++i) {
+    int num_histos;
+    VP8LHistogram* const histo = histograms[i];
+    const int16_t bin_id = low_effort ?
+        (int16_t)GetHistoBinIndexLowEffort(histo, &cost_range) :
+        (int16_t)GetHistoBinIndex(histo, &cost_range);
+    const int bin_offset = bin_id * bin_depth;
+    // bin_map[n][0] for every bin 'n' maintains the counter for the number of
+    // histograms in that bin.
+    // Get and increment the num_histos in that bin.
+    num_histos = ++bin_map[bin_offset];
+    assert(bin_offset + num_histos < bin_depth * BIN_SIZE);
+    // Add histogram i'th index at num_histos (last) position in the bin_map.
+    bin_map[bin_offset + num_histos] = i;
+  }
+}
+
+// Compact the histogram set by removing unused entries.
+static void HistogramCompactBins(VP8LHistogramSet* const image_histo) {
+  VP8LHistogram** const histograms = image_histo->histograms;
+  int i, j;
+
+  for (i = 0, j = 0; i < image_histo->size; ++i) {
+    if (histograms[i] != NULL && histograms[i]->bit_cost_ != 0.) {
+      if (j < i) {
+        histograms[j] = histograms[i];
+        histograms[i] = NULL;
+      }
+      ++j;
+    }
   }
+  image_histo->size = j;
+}
+
+static VP8LHistogram* HistogramCombineEntropyBin(
+    VP8LHistogramSet* const image_histo,
+    VP8LHistogram* cur_combo,
+    int16_t* const bin_map, int bin_depth, int num_bins,
+    double combine_cost_factor, int low_effort) {
+  int bin_id;
+  VP8LHistogram** const histograms = image_histo->histograms;
+
+  for (bin_id = 0; bin_id < num_bins; ++bin_id) {
+    const int bin_offset = bin_id * bin_depth;
+    const int num_histos = bin_map[bin_offset];
+    const int idx1 = bin_map[bin_offset + 1];
+    int num_combine_failures = 0;
+    int n;
+    for (n = 2; n <= num_histos; ++n) {
+      const int idx2 = bin_map[bin_offset + n];
+      if (low_effort) {
+        // Merge all histograms with the same bin index, irrespective of cost of
+        // the merged histograms.
+        VP8LHistogramAdd(histograms[idx1], histograms[idx2], histograms[idx1]);
+        histograms[idx2]->bit_cost_ = 0.;
+      } else {
+        const double bit_cost_idx2 = histograms[idx2]->bit_cost_;
+        if (bit_cost_idx2 > 0.) {
+          const double bit_cost_thresh = -bit_cost_idx2 * combine_cost_factor;
+          const double curr_cost_diff =
+              HistogramAddEval(histograms[idx1], histograms[idx2],
+                               cur_combo, bit_cost_thresh);
+          if (curr_cost_diff < bit_cost_thresh) {
+            // Try to merge two histograms only if the combo is a trivial one or
+            // the two candidate histograms are already non-trivial.
+            // For some images, 'try_combine' turns out to be false for a lot of
+            // histogram pairs. In that case, we fallback to combining
+            // histograms as usual to avoid increasing the header size.
+            const int try_combine =
+                (cur_combo->trivial_symbol_ != VP8L_NON_TRIVIAL_SYM) ||
+                ((histograms[idx1]->trivial_symbol_ == VP8L_NON_TRIVIAL_SYM) &&
+                 (histograms[idx2]->trivial_symbol_ == VP8L_NON_TRIVIAL_SYM));
+            const int max_combine_failures = 32;
+            if (try_combine || (num_combine_failures >= max_combine_failures)) {
+              HistogramSwap(&cur_combo, &histograms[idx1]);
+              histograms[idx2]->bit_cost_ = 0.;
+            } else {
+              ++num_combine_failures;
+            }
+          }
+        }
+      }
+    }
+    if (low_effort) {
+      // Update the bit_cost for the merged histograms (per bin index).
+      UpdateHistogramCost(histograms[idx1]);
+    }
+  }
+  HistogramCompactBins(image_histo);
+  return cur_combo;
 }
 
 static uint32_t MyRand(uint32_t *seed) {
@@ -248,159 +466,433 @@ static uint32_t MyRand(uint32_t *seed) {
   return *seed;
 }
 
-static int HistogramCombine(const VP8LHistogramSet* const in,
-                            VP8LHistogramSet* const out, int num_pairs) {
+// -----------------------------------------------------------------------------
+// Histogram pairs priority queue
+
+// Pair of histograms. Negative idx1 value means that pair is out-of-date.
+typedef struct {
+  int idx1;
+  int idx2;
+  double cost_diff;
+  double cost_combo;
+} HistogramPair;
+
+typedef struct {
+  HistogramPair* heap;
+  int* positions;
+  int size;
+  int max_index;
+} HistoHeap;
+
+static int HistoHeapInit(HistoHeap* const histo_heap, const int max_index) {
+  histo_heap->size = 0;
+  histo_heap->max_index = max_index;
+  histo_heap->heap = WebPSafeMalloc(max_index * max_index,
+                                    sizeof(*histo_heap->heap));
+  histo_heap->positions = WebPSafeMalloc(max_index * max_index,
+                                         sizeof(*histo_heap->positions));
+  return histo_heap->heap != NULL && histo_heap->positions != NULL;
+}
+
+static void HistoHeapClear(HistoHeap* const histo_heap) {
+  assert(histo_heap != NULL);
+  WebPSafeFree(histo_heap->heap);
+  WebPSafeFree(histo_heap->positions);
+}
+
+static void SwapHistogramPairs(HistogramPair *p1,
+                               HistogramPair *p2) {
+  const HistogramPair tmp = *p1;
+  *p1 = *p2;
+  *p2 = tmp;
+}
+
+// Given a valid min-heap in range [0, heap_size-1) this function places value
+// heap[heap_size-1] into right location within heap and sets its position in
+// positions array.
+static void HeapPush(HistoHeap* const histo_heap) {
+  HistogramPair* const heap = histo_heap->heap - 1;
+  int* const positions = histo_heap->positions;
+  const int max_index = histo_heap->max_index;
+  int v;
+  ++histo_heap->size;
+  v = histo_heap->size;
+  while (v > 1 && heap[v].cost_diff < heap[v >> 1].cost_diff) {
+    SwapHistogramPairs(&heap[v], &heap[v >> 1]);
+    // Change position of moved pair in heap.
+    if (heap[v].idx1 >= 0) {
+      const int pos = heap[v].idx1 * max_index + heap[v].idx2;
+      assert(pos >= 0 && pos < max_index * max_index);
+      positions[pos] = v;
+    }
+    v >>= 1;
+  }
+  positions[heap[v].idx1 * max_index + heap[v].idx2] = v;
+}
+
+// Given a valid min-heap in range [0, heap_size) this function shortens heap
+// range by one and places element with the lowest value to (heap_size-1).
+static void HeapPop(HistoHeap* const histo_heap) {
+  HistogramPair* const heap = histo_heap->heap - 1;
+  int* const positions = histo_heap->positions;
+  const int heap_size = histo_heap->size;
+  const int max_index = histo_heap->max_index;
+  int v = 1;
+  if (heap[v].idx1 >= 0) {
+    positions[heap[v].idx1 * max_index + heap[v].idx2] = -1;
+  }
+  SwapHistogramPairs(&heap[v], &heap[heap_size]);
+  while ((v << 1) < heap_size) {
+    int son = (heap[v << 1].cost_diff < heap[v].cost_diff) ? (v << 1) : v;
+    if (((v << 1) + 1) < heap_size &&
+        heap[(v << 1) + 1].cost_diff < heap[son].cost_diff) {
+      son = (v << 1) + 1;
+    }
+    if (son == v) break;
+    SwapHistogramPairs(&heap[v], &heap[son]);
+    // Change position of moved pair in heap.
+    if (heap[v].idx1 >= 0) {
+      positions[heap[v].idx1 * max_index + heap[v].idx2] = v;
+    }
+    v = son;
+  }
+  if (heap[v].idx1 >= 0) {
+    positions[heap[v].idx1 * max_index + heap[v].idx2] = v;
+  }
+  --histo_heap->size;
+}
+
+// -----------------------------------------------------------------------------
+
+static void PreparePair(VP8LHistogram** histograms, int idx1, int idx2,
+                        HistogramPair* const pair,
+                        VP8LHistogram* const histos) {
+  if (idx1 > idx2) {
+    const int tmp = idx2;
+    idx2 = idx1;
+    idx1 = tmp;
+  }
+  pair->idx1 = idx1;
+  pair->idx2 = idx2;
+  pair->cost_diff =
+      HistogramAddEval(histograms[idx1], histograms[idx2], histos, 0);
+  pair->cost_combo = histos->bit_cost_;
+}
+
+#define POSITION_INVALID (-1)
+
+// Invalidates pairs intersecting (idx1, idx2) in heap.
+static void InvalidatePairs(int idx1, int idx2,
+                            const HistoHeap* const histo_heap) {
+  HistogramPair* const heap = histo_heap->heap - 1;
+  int* const positions = histo_heap->positions;
+  const int max_index = histo_heap->max_index;
+  int i;
+  for (i = 0; i < idx1; ++i) {
+    const int pos = positions[i * max_index + idx1];
+    if (pos >= 0) {
+      heap[pos].idx1 = POSITION_INVALID;
+    }
+  }
+  for (i = idx1 + 1; i < max_index; ++i) {
+    const int pos = positions[idx1 * max_index + i];
+    if (pos >= 0) {
+      heap[pos].idx1 = POSITION_INVALID;
+    }
+  }
+  for (i = 0; i < idx2; ++i) {
+    const int pos = positions[i * max_index + idx2];
+    if (pos >= 0) {
+      heap[pos].idx1 = POSITION_INVALID;
+    }
+  }
+  for (i = idx2 + 1; i < max_index; ++i) {
+    const int pos = positions[idx2 * max_index + i];
+    if (pos >= 0) {
+      heap[pos].idx1 = POSITION_INVALID;
+    }
+  }
+}
+
+// Combines histograms by continuously choosing the one with the highest cost
+// reduction.
+static int HistogramCombineGreedy(VP8LHistogramSet* const image_histo,
+                                  VP8LHistogram* const histos) {
   int ok = 0;
-  int i, iter;
+  int image_histo_size = image_histo->size;
+  int i, j;
+  VP8LHistogram** const histograms = image_histo->histograms;
+  // Indexes of remaining histograms.
+  int* const clusters = WebPSafeMalloc(image_histo_size, sizeof(*clusters));
+  // Heap of histogram pairs.
+  HistoHeap histo_heap;
+
+  if (!HistoHeapInit(&histo_heap, image_histo_size) || clusters == NULL) {
+    goto End;
+  }
+
+  for (i = 0; i < image_histo_size; ++i) {
+    // Initialize clusters indexes.
+    clusters[i] = i;
+    for (j = i + 1; j < image_histo_size; ++j) {
+      // Initialize positions array.
+      histo_heap.positions[i * histo_heap.max_index + j] = POSITION_INVALID;
+      PreparePair(histograms, i, j, &histo_heap.heap[histo_heap.size], histos);
+      if (histo_heap.heap[histo_heap.size].cost_diff < 0) {
+        HeapPush(&histo_heap);
+      }
+    }
+  }
+
+  while (image_histo_size > 1 && histo_heap.size > 0) {
+    const int idx1 = histo_heap.heap[0].idx1;
+    const int idx2 = histo_heap.heap[0].idx2;
+    VP8LHistogramAdd(histograms[idx2], histograms[idx1], histograms[idx1]);
+    histograms[idx1]->bit_cost_ = histo_heap.heap[0].cost_combo;
+    // Remove merged histogram.
+    for (i = 0; i + 1 < image_histo_size; ++i) {
+      if (clusters[i] >= idx2) {
+        clusters[i] = clusters[i + 1];
+      }
+    }
+    --image_histo_size;
+
+    // Invalidate pairs intersecting the just combined best pair.
+    InvalidatePairs(idx1, idx2, &histo_heap);
+
+    // Pop invalid pairs from the top of the heap.
+    while (histo_heap.size > 0 && histo_heap.heap[0].idx1 < 0) {
+      HeapPop(&histo_heap);
+    }
+
+    // Push new pairs formed with combined histogram to the heap.
+    for (i = 0; i < image_histo_size; ++i) {
+      if (clusters[i] != idx1) {
+        PreparePair(histograms, idx1, clusters[i],
+                    &histo_heap.heap[histo_heap.size], histos);
+        if (histo_heap.heap[histo_heap.size].cost_diff < 0) {
+          HeapPush(&histo_heap);
+        }
+      }
+    }
+  }
+  // Move remaining histograms to the beginning of the array.
+  for (i = 0; i < image_histo_size; ++i) {
+    if (i != clusters[i]) {  // swap the two histograms
+      HistogramSwap(&histograms[i], &histograms[clusters[i]]);
+    }
+  }
+
+  image_histo->size = image_histo_size;
+  ok = 1;
+
+ End:
+  WebPSafeFree(clusters);
+  HistoHeapClear(&histo_heap);
+  return ok;
+}
+
+static VP8LHistogram* HistogramCombineStochastic(
+    VP8LHistogramSet* const image_histo,
+    VP8LHistogram* tmp_histo,
+    VP8LHistogram* best_combo,
+    int quality, int min_cluster_size) {
+  int iter;
   uint32_t seed = 0;
   int tries_with_no_success = 0;
-  const int min_cluster_size = 2;
-  int out_size = in->size;
-  const int outer_iters = in->size * 3;
-  VP8LHistogram* const histos = (VP8LHistogram*)malloc(2 * sizeof(*histos));
-  VP8LHistogram* cur_combo = histos + 0;    // trial merged histogram
-  VP8LHistogram* best_combo = histos + 1;   // best merged histogram so far
-  if (histos == NULL) goto End;
-
-  // Copy histograms from in[] to out[].
-  assert(in->size <= out->size);
-  for (i = 0; i < in->size; ++i) {
-    in->histograms[i]->bit_cost_ = VP8LHistogramEstimateBits(in->histograms[i]);
-    *out->histograms[i] = *in->histograms[i];
-  }
-
-  // Collapse similar histograms in 'out'.
-  for (iter = 0; iter < outer_iters && out_size >= min_cluster_size; ++iter) {
-    // We pick the best pair to be combined out of 'inner_iters' pairs.
+  int image_histo_size = image_histo->size;
+  const int iter_mult = (quality < 25) ? 2 : 2 + (quality - 25) / 8;
+  const int outer_iters = image_histo_size * iter_mult;
+  const int num_pairs = image_histo_size / 2;
+  const int num_tries_no_success = outer_iters / 2;
+  VP8LHistogram** const histograms = image_histo->histograms;
+
+  // Collapse similar histograms in 'image_histo'.
+  ++min_cluster_size;
+  for (iter = 0;
+       iter < outer_iters && image_histo_size >= min_cluster_size;
+       ++iter) {
     double best_cost_diff = 0.;
-    int best_idx1 = 0, best_idx2 = 1;
+    int best_idx1 = -1, best_idx2 = 1;
     int j;
+    const int num_tries =
+        (num_pairs < image_histo_size) ? num_pairs : image_histo_size;
     seed += iter;
-    for (j = 0; j < num_pairs; ++j) {
+    for (j = 0; j < num_tries; ++j) {
       double curr_cost_diff;
       // Choose two histograms at random and try to combine them.
-      const uint32_t idx1 = MyRand(&seed) % out_size;
-      const uint32_t tmp = ((j & 7) + 1) % (out_size - 1);
-      const uint32_t diff = (tmp < 3) ? tmp : MyRand(&seed) % (out_size - 1);
-      const uint32_t idx2 = (idx1 + diff + 1) % out_size;
+      const uint32_t idx1 = MyRand(&seed) % image_histo_size;
+      const uint32_t tmp = (j & 7) + 1;
+      const uint32_t diff =
+          (tmp < 3) ? tmp : MyRand(&seed) % (image_histo_size - 1);
+      const uint32_t idx2 = (idx1 + diff + 1) % image_histo_size;
       if (idx1 == idx2) {
         continue;
       }
-      *cur_combo = *out->histograms[idx1];
-      VP8LHistogramAdd(cur_combo, out->histograms[idx2]);
-      cur_combo->bit_cost_ = VP8LHistogramEstimateBits(cur_combo);
+
       // Calculate cost reduction on combining.
-      curr_cost_diff = cur_combo->bit_cost_
-                     - out->histograms[idx1]->bit_cost_
-                     - out->histograms[idx2]->bit_cost_;
-      if (best_cost_diff > curr_cost_diff) {    // found a better pair?
-        {     // swap cur/best combo histograms
-          VP8LHistogram* const tmp_histo = cur_combo;
-          cur_combo = best_combo;
-          best_combo = tmp_histo;
-        }
+      curr_cost_diff = HistogramAddEval(histograms[idx1], histograms[idx2],
+                                        tmp_histo, best_cost_diff);
+      if (curr_cost_diff < best_cost_diff) {    // found a better pair?
+        HistogramSwap(&best_combo, &tmp_histo);
         best_cost_diff = curr_cost_diff;
         best_idx1 = idx1;
         best_idx2 = idx2;
       }
     }
 
-    if (best_cost_diff < 0.0) {
-      *out->histograms[best_idx1] = *best_combo;
+    if (best_idx1 >= 0) {
+      HistogramSwap(&best_combo, &histograms[best_idx1]);
       // swap best_idx2 slot with last one (which is now unused)
-      --out_size;
-      if (best_idx2 != out_size) {
-        out->histograms[best_idx2] = out->histograms[out_size];
-        out->histograms[out_size] = NULL;   // just for sanity check.
+      --image_histo_size;
+      if (best_idx2 != image_histo_size) {
+        HistogramSwap(&histograms[image_histo_size], &histograms[best_idx2]);
+        histograms[image_histo_size] = NULL;
       }
       tries_with_no_success = 0;
     }
-    if (++tries_with_no_success >= 50) {
+    if (++tries_with_no_success >= num_tries_no_success) {
       break;
     }
   }
-  out->size = out_size;
-  ok = 1;
-
- End:
-  free(histos);
-  return ok;
+  image_histo->size = image_histo_size;
+  return best_combo;
 }
 
 // -----------------------------------------------------------------------------
 // Histogram refinement
 
-// What is the bit cost of moving square_histogram from
-// cur_symbol to candidate_symbol.
-// TODO(skal): we don't really need to copy the histogram and Add(). Instead
-// we just need VP8LDualHistogramEstimateBits(A, B) estimation function.
-static double HistogramDistance(const VP8LHistogram* const square_histogram,
-                                const VP8LHistogram* const candidate) {
-  const double previous_bit_cost = candidate->bit_cost_;
-  double new_bit_cost;
-  VP8LHistogram modified_histo;
-  modified_histo = *candidate;
-  VP8LHistogramAdd(&modified_histo, square_histogram);
-  new_bit_cost = VP8LHistogramEstimateBits(&modified_histo);
-
-  return new_bit_cost - previous_bit_cost;
-}
-
 // Find the best 'out' histogram for each of the 'in' histograms.
 // Note: we assume that out[]->bit_cost_ is already up-to-date.
-static void HistogramRemap(const VP8LHistogramSet* const in,
-                           const VP8LHistogramSet* const out,
+static void HistogramRemap(const VP8LHistogramSet* const orig_histo,
+                           const VP8LHistogramSet* const image_histo,
                            uint16_t* const symbols) {
   int i;
-  for (i = 0; i < in->size; ++i) {
-    int best_out = 0;
-    double best_bits = HistogramDistance(in->histograms[i], out->histograms[0]);
-    int k;
-    for (k = 1; k < out->size; ++k) {
-      const double cur_bits =
-          HistogramDistance(in->histograms[i], out->histograms[k]);
-      if (cur_bits < best_bits) {
-        best_bits = cur_bits;
-        best_out = k;
+  VP8LHistogram** const orig_histograms = orig_histo->histograms;
+  VP8LHistogram** const histograms = image_histo->histograms;
+  const int orig_histo_size = orig_histo->size;
+  const int image_histo_size = image_histo->size;
+  if (image_histo_size > 1) {
+    for (i = 0; i < orig_histo_size; ++i) {
+      int best_out = 0;
+      double best_bits =
+          HistogramAddThresh(histograms[0], orig_histograms[i], MAX_COST);
+      int k;
+      for (k = 1; k < image_histo_size; ++k) {
+        const double cur_bits =
+            HistogramAddThresh(histograms[k], orig_histograms[i], best_bits);
+        if (cur_bits < best_bits) {
+          best_bits = cur_bits;
+          best_out = k;
+        }
       }
+      symbols[i] = best_out;
+    }
+  } else {
+    assert(image_histo_size == 1);
+    for (i = 0; i < orig_histo_size; ++i) {
+      symbols[i] = 0;
     }
-    symbols[i] = best_out;
   }
 
   // Recompute each out based on raw and symbols.
-  for (i = 0; i < out->size; ++i) {
-    HistogramClear(out->histograms[i]);
+  for (i = 0; i < image_histo_size; ++i) {
+    HistogramClear(histograms[i]);
   }
-  for (i = 0; i < in->size; ++i) {
-    VP8LHistogramAdd(out->histograms[symbols[i]], in->histograms[i]);
+
+  for (i = 0; i < orig_histo_size; ++i) {
+    const int idx = symbols[i];
+    VP8LHistogramAdd(orig_histograms[i], histograms[idx], histograms[idx]);
   }
 }
 
+static double GetCombineCostFactor(int histo_size, int quality) {
+  double combine_cost_factor = 0.16;
+  if (quality < 90) {
+    if (histo_size > 256) combine_cost_factor /= 2.;
+    if (histo_size > 512) combine_cost_factor /= 2.;
+    if (histo_size > 1024) combine_cost_factor /= 2.;
+    if (quality <= 50) combine_cost_factor /= 2.;
+  }
+  return combine_cost_factor;
+}
+
 int VP8LGetHistoImageSymbols(int xsize, int ysize,
                              const VP8LBackwardRefs* const refs,
-                             int quality, int histo_bits, int cache_bits,
-                             VP8LHistogramSet* const image_in,
+                             int quality, int low_effort,
+                             int histo_bits, int cache_bits,
+                             VP8LHistogramSet* const image_histo,
+                             VP8LHistogramSet* const tmp_histos,
                              uint16_t* const histogram_symbols) {
   int ok = 0;
   const int histo_xsize = histo_bits ? VP8LSubSampleSize(xsize, histo_bits) : 1;
   const int histo_ysize = histo_bits ? VP8LSubSampleSize(ysize, histo_bits) : 1;
-  const int num_histo_pairs = 10 + quality / 2;  // For HistogramCombine().
-  const int histo_image_raw_size = histo_xsize * histo_ysize;
-  VP8LHistogramSet* const image_out =
-      VP8LAllocateHistogramSet(histo_image_raw_size, cache_bits);
-  if (image_out == NULL) return 0;
-
-  // Build histogram image.
-  HistogramBuildImage(xsize, histo_bits, refs, image_out);
-  // Collapse similar histograms.
-  if (!HistogramCombine(image_out, image_in, num_histo_pairs)) {
-    goto Error;
+  const int image_histo_raw_size = histo_xsize * histo_ysize;
+  const int entropy_combine_num_bins = low_effort ? NUM_PARTITIONS : BIN_SIZE;
+
+  // The bin_map for every bin follows following semantics:
+  // bin_map[n][0] = num_histo; // The number of histograms in that bin.
+  // bin_map[n][1] = index of first histogram in that bin;
+  // bin_map[n][num_histo] = index of last histogram in that bin;
+  // bin_map[n][num_histo + 1] ... bin_map[n][bin_depth - 1] = unused indices.
+  const int bin_depth = image_histo_raw_size + 1;
+  int16_t* bin_map = NULL;
+  VP8LHistogramSet* const orig_histo =
+      VP8LAllocateHistogramSet(image_histo_raw_size, cache_bits);
+  VP8LHistogram* cur_combo;
+  const int entropy_combine =
+      (orig_histo->size > entropy_combine_num_bins * 2) && (quality < 100);
+
+  if (orig_histo == NULL) goto Error;
+
+  // Don't attempt linear bin-partition heuristic for:
+  // histograms of small sizes, as bin_map will be very sparse and;
+  // Maximum quality (q==100), to preserve the compression gains at that level.
+  if (entropy_combine) {
+    const int bin_map_size = bin_depth * entropy_combine_num_bins;
+    bin_map = (int16_t*)WebPSafeCalloc(bin_map_size, sizeof(*bin_map));
+    if (bin_map == NULL) goto Error;
   }
+
+  // Construct the histograms from backward references.
+  HistogramBuild(xsize, histo_bits, refs, orig_histo);
+  // Copies the histograms and computes its bit_cost.
+  HistogramCopyAndAnalyze(orig_histo, image_histo);
+
+  cur_combo = tmp_histos->histograms[1];  // pick up working slot
+  if (entropy_combine) {
+    const double combine_cost_factor =
+        GetCombineCostFactor(image_histo_raw_size, quality);
+    HistogramAnalyzeEntropyBin(orig_histo, bin_map, low_effort);
+    // Collapse histograms with similar entropy.
+    cur_combo = HistogramCombineEntropyBin(image_histo, cur_combo, bin_map,
+                                           bin_depth, entropy_combine_num_bins,
+                                           combine_cost_factor, low_effort);
+  }
+
+  // Don't combine the histograms using stochastic and greedy heuristics for
+  // low-effort compression mode.
+  if (!low_effort || !entropy_combine) {
+    const float x = quality / 100.f;
+    // cubic ramp between 1 and MAX_HISTO_GREEDY:
+    const int threshold_size = (int)(1 + (x * x * x) * (MAX_HISTO_GREEDY - 1));
+    cur_combo = HistogramCombineStochastic(image_histo,
+                                           tmp_histos->histograms[0],
+                                           cur_combo, quality, threshold_size);
+    if ((image_histo->size <= threshold_size) &&
+        !HistogramCombineGreedy(image_histo, cur_combo)) {
+      goto Error;
+    }
+  }
+
+  // TODO(vikasa): Optimize HistogramRemap for low-effort compression mode also.
   // Find the optimal map from original histograms to the final ones.
-  HistogramRemap(image_out, image_in, histogram_symbols);
+  HistogramRemap(orig_histo, image_histo, histogram_symbols);
+
   ok = 1;
 
-Error:
-  free(image_out);
+ Error:
+  WebPSafeFree(bin_map);
+  VP8LFreeHistogramSet(orig_histo);
   return ok;
 }