libwebp-0.6.0-rc1

third_party/libwebp/enc/histogram_enc.c

 // 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)
 //
 #ifdef HAVE_CONFIG_H
 #include "../webp/config.h"
 #endif
 
 #include <math.h>
 
-#include "./backward_references.h"
-#include "./histogram.h"
+#include "./backward_references_enc.h"
+#include "./histogram_enc.h"
 #include "../dsp/lossless.h"
+#include "../dsp/lossless_common.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.
@@ skipping 175 matching lines @@
 static double InitialHuffmanCost(void) {
   // 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;
   return kHuffmanCodeOfHuffmanCodeSize - kSmallBias;
 }
 
 // Finalize the Huffman cost based on streak numbers and length type (<3 or >=3)
 static double FinalHuffmanCost(const VP8LStreaks* const stats) {
+  // The constants in this function are experimental and got rounded from
+  // their original values in 1/8 when switched to 1/1024.
   double retval = InitialHuffmanCost();
+  // Second coefficient: Many zeros in the histogram are covered efficiently
+  // by a run-length encode. Originally 2/8.
   retval += stats->counts[0] * 1.5625 + 0.234375 * stats->streaks[0][1];
+  // Second coefficient: Constant values are encoded less efficiently, but still
+  // RLE'ed. Originally 6/8.
   retval += stats->counts[1] * 2.578125 + 0.703125 * stats->streaks[1][1];
+  // 0s are usually encoded more efficiently than non-0s.
+  // Originally 15/8.
   retval += 1.796875 * stats->streaks[0][0];
+  // Originally 26/8.
   retval += 3.28125 * stats->streaks[1][0];
   return retval;
 }
 
 // Get the symbol entropy for the distribution 'population'.
 // Set 'trivial_sym', if there's only one symbol present in the distribution.
 static double PopulationCost(const uint32_t* const population, int length,
                              uint32_t* const trivial_sym) {
   VP8LBitEntropy bit_entropy;
   VP8LStreaks stats;
   VP8LGetEntropyUnrefined(population, length, &bit_entropy, &stats);
   if (trivial_sym != NULL) {
     *trivial_sym = (bit_entropy.nonzeros == 1) ? bit_entropy.nonzero_code
                                                : VP8L_NON_TRIVIAL_SYM;
   }
 
   return BitsEntropyRefine(&bit_entropy) + FinalHuffmanCost(&stats);
 }
 
+// trivial_at_end is 1 if the two histograms only have one element that is
+// non-zero: both the zero-th one, or both the last one.
 static WEBP_INLINE double GetCombinedEntropy(const uint32_t* const X,
                                              const uint32_t* const Y,
-                                             int length) {
-  VP8LBitEntropy bit_entropy;
+                                             int length, int trivial_at_end) {
   VP8LStreaks stats;
-  VP8LGetCombinedEntropyUnrefined(X, Y, length, &bit_entropy, &stats);
+  if (trivial_at_end) {
+    // This configuration is due to palettization that transforms an indexed
+    // pixel into 0xff000000 | (pixel << 8) in VP8LBundleColorMap.
+    // BitsEntropyRefine is 0 for histograms with only one non-zero value.
+    // Only FinalHuffmanCost needs to be evaluated.
+    memset(&stats, 0, sizeof(stats));
+    // Deal with the non-zero value at index 0 or length-1.
+    stats.streaks[1][0] += 1;
+    // Deal with the following/previous zero streak.
+    stats.counts[0] += 1;
+    stats.streaks[0][1] += length - 1;
+    return FinalHuffmanCost(&stats);
+  } else {
+    VP8LBitEntropy bit_entropy;
+    VP8LGetCombinedEntropyUnrefined(X, Y, length, &bit_entropy, &stats);
 
-  return BitsEntropyRefine(&bit_entropy) + FinalHuffmanCost(&stats);
+    return BitsEntropyRefine(&bit_entropy) + FinalHuffmanCost(&stats);
+  }
 }
 
 // Estimates the Entropy + Huffman + other block overhead size cost.
 double VP8LHistogramEstimateBits(const VP8LHistogram* const p) {
   return
       PopulationCost(
           p->literal_, VP8LHistogramNumCodes(p->palette_code_bits_), NULL)
       + PopulationCost(p->red_, NUM_LITERAL_CODES, NULL)
       + PopulationCost(p->blue_, NUM_LITERAL_CODES, NULL)
       + PopulationCost(p->alpha_, NUM_LITERAL_CODES, NULL)
       + PopulationCost(p->distance_, NUM_DISTANCE_CODES, NULL)
       + VP8LExtraCost(p->literal_ + NUM_LITERAL_CODES, NUM_LENGTH_CODES)
       + VP8LExtraCost(p->distance_, NUM_DISTANCE_CODES);
 }
 
 // -----------------------------------------------------------------------------
 // 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_;
+  int trivial_at_end = 0;
   assert(a->palette_code_bits_ == b->palette_code_bits_);
   *cost += GetCombinedEntropy(a->literal_, b->literal_,
-                              VP8LHistogramNumCodes(palette_code_bits));
+                              VP8LHistogramNumCodes(palette_code_bits), 0);
   *cost += VP8LExtraCostCombined(a->literal_ + NUM_LITERAL_CODES,
                                  b->literal_ + NUM_LITERAL_CODES,
                                  NUM_LENGTH_CODES);
   if (*cost > cost_threshold) return 0;
 
-  *cost += GetCombinedEntropy(a->red_, b->red_, NUM_LITERAL_CODES);
+  if (a->trivial_symbol_ != VP8L_NON_TRIVIAL_SYM &&
+      a->trivial_symbol_ == b->trivial_symbol_) {
+    // A, R and B are all 0 or 0xff.
+    const uint32_t color_a = (a->trivial_symbol_ >> 24) & 0xff;
+    const uint32_t color_r = (a->trivial_symbol_ >> 16) & 0xff;
+    const uint32_t color_b = (a->trivial_symbol_ >> 0) & 0xff;
+    if ((color_a == 0 || color_a == 0xff) &&
+        (color_r == 0 || color_r == 0xff) &&
+        (color_b == 0 || color_b == 0xff)) {
+      trivial_at_end = 1;
+    }
+  }
+
+  *cost +=
+      GetCombinedEntropy(a->red_, b->red_, NUM_LITERAL_CODES, trivial_at_end);
   if (*cost > cost_threshold) return 0;
 
-  *cost += GetCombinedEntropy(a->blue_, b->blue_, NUM_LITERAL_CODES);
+  *cost +=
+      GetCombinedEntropy(a->blue_, b->blue_, NUM_LITERAL_CODES, trivial_at_end);
   if (*cost > cost_threshold) return 0;
 
-  *cost += GetCombinedEntropy(a->alpha_, b->alpha_, NUM_LITERAL_CODES);
+  *cost += GetCombinedEntropy(a->alpha_, b->alpha_, NUM_LITERAL_CODES,
+                              trivial_at_end);
   if (*cost > cost_threshold) return 0;
 
-  *cost += GetCombinedEntropy(a->distance_, b->distance_, NUM_DISTANCE_CODES);
+  *cost +=
+      GetCombinedEntropy(a->distance_, b->distance_, NUM_DISTANCE_CODES, 0);
   *cost +=
       VP8LExtraCostCombined(a->distance_, b->distance_, NUM_DISTANCE_CODES);
   if (*cost > cost_threshold) return 0;
 
   return 1;
 }
 
+static WEBP_INLINE void HistogramAdd(const VP8LHistogram* const a,
+                                     const VP8LHistogram* const b,
+                                     VP8LHistogram* const out) {
+  VP8LHistogramAdd(a, b, out);
+  out->trivial_symbol_ = (a->trivial_symbol_ == b->trivial_symbol_)
+                       ? a->trivial_symbol_
+                       : VP8L_NON_TRIVIAL_SYM;
+}
+
 // 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);
+    HistogramAdd(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;
   }
 
   return cost - sum_cost;
 }
 
 // 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,
@@ skipping 118 matching lines @@
     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) {
+                                       uint16_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);
+    UpdateDominantCostRange(histograms[i], &cost_range);
   }
 
   // bin-hash histograms on three of the dominant (literal, red and blue)
-  // symbol costs.
+  // symbol costs and store the resulting bin_id for each histogram.
   for (i = 0; i < histo_size; ++i) {
-    const VP8LHistogram* const histo = histograms[i];
-    const int bin_id = GetHistoBinIndex(histo, &cost_range, low_effort);
-    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.
-    const int 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;
+    bin_map[i] = GetHistoBinIndex(histograms[i], &cost_range, low_effort);
   }
 }
 
-// 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;
-}
-
+// Compact image_histo[] by merging some histograms with same bin_id together if
+// it's advantageous.
 static VP8LHistogram* HistogramCombineEntropyBin(
     VP8LHistogramSet* const image_histo,
     VP8LHistogram* cur_combo,
-    int16_t* const bin_map, int bin_depth, int num_bins,
+    const uint16_t* const bin_map, int bin_map_size, int num_bins,
     double combine_cost_factor, int low_effort) {
-  int bin_id;
   VP8LHistogram** const histograms = image_histo->histograms;
+  int idx;
+  // Work in-place: processed histograms are put at the beginning of
+  // image_histo[]. At the end, we just have to truncate the array.
+  int size = 0;
+  struct {
+    int16_t first;    // position of the histogram that accumulates all
+                      // histograms with the same bin_id
+    uint16_t num_combine_failures;   // number of combine failures per bin_id
+  } bin_info[BIN_SIZE];
 
-  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.;
+  assert(num_bins <= BIN_SIZE);
+  for (idx = 0; idx < num_bins; ++idx) {
+    bin_info[idx].first = -1;
+    bin_info[idx].num_combine_failures = 0;
+  }
+
+  for (idx = 0; idx < bin_map_size; ++idx) {
+    const int bin_id = bin_map[idx];
+    const int first = bin_info[bin_id].first;
+    assert(size <= idx);
+    if (first == -1) {
+      // just move histogram #idx to its final position
+      histograms[size] = histograms[idx];
+      bin_info[bin_id].first = size++;
+    } else if (low_effort) {
+      HistogramAdd(histograms[idx], histograms[first], histograms[first]);
+    } else {
+      // try to merge #idx into #first (both share the same bin_id)
+      const double bit_cost = histograms[idx]->bit_cost_;
+      const double bit_cost_thresh = -bit_cost * combine_cost_factor;
+      const double curr_cost_diff =
+          HistogramAddEval(histograms[first], histograms[idx],
+                           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[idx]->trivial_symbol_ == VP8L_NON_TRIVIAL_SYM) &&
+             (histograms[first]->trivial_symbol_ == VP8L_NON_TRIVIAL_SYM));
+        const int max_combine_failures = 32;
+        if (try_combine ||
+            bin_info[bin_id].num_combine_failures >= max_combine_failures) {
+          // move the (better) merged histogram to its final slot
+          HistogramSwap(&cur_combo, &histograms[first]);
+        } else {
+          histograms[size++] = histograms[idx];
+          ++bin_info[bin_id].num_combine_failures;
+        }
       } 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;
-            }
-          }
-        }
+        histograms[size++] = histograms[idx];
       }
     }
-    if (low_effort) {
-      // Update the bit_cost for the merged histograms (per bin index).
-      UpdateHistogramCost(histograms[idx1]);
+  }
+  image_histo->size = size;
+  if (low_effort) {
+    // for low_effort case, update the final cost when everything is merged
+    for (idx = 0; idx < size; ++idx) {
+      UpdateHistogramCost(histograms[idx]);
     }
   }
-  HistogramCompactBins(image_histo);
   return cur_combo;
 }
 
-static uint32_t MyRand(uint32_t *seed) {
-  *seed *= 16807U;
+static uint32_t MyRand(uint32_t* const seed) {
+  *seed = (*seed * 16807ull) & 0xffffffffu;
   if (*seed == 0) {
     *seed = 1;
   }
   return *seed;
 }
 
 // -----------------------------------------------------------------------------
 // Histogram pairs priority queue
 
 // Pair of histograms. Negative idx1 value means that pair is out-of-date.
@@ skipping 105 matching lines @@
       // Initialize positions array.
       PreparePair(histograms, i, j, &histo_queue.queue[histo_queue.size]);
       UpdateQueueFront(&histo_queue);
     }
   }
 
   while (image_histo_size > 1 && histo_queue.size > 0) {
     HistogramPair* copy_to;
     const int idx1 = histo_queue.queue[0].idx1;
     const int idx2 = histo_queue.queue[0].idx2;
-    VP8LHistogramAdd(histograms[idx2], histograms[idx1], histograms[idx1]);
+    HistogramAdd(histograms[idx2], histograms[idx1], histograms[idx1]);
     histograms[idx1]->bit_cost_ = histo_queue.queue[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;
 
     // Remove pairs intersecting the just combined best pair. This will
@@ skipping 45 matching lines @@
                                        VP8LHistogram* best_combo,
                                        int quality, int min_cluster_size) {
   int iter;
   uint32_t seed = 0;
   int tries_with_no_success = 0;
   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;
+  int idx2_max = image_histo_size - 1;
+  int do_brute_dorce = 0;
   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 = -1, best_idx2 = 1;
     int j;
-    const int num_tries =
+    int num_tries =
         (num_pairs < image_histo_size) ? num_pairs : image_histo_size;
+    // Use a brute force approach if:
+    // - stochastic has not worked for a while and
+    // - if the number of iterations for brute force is less than the number of
+    // iterations if we never find a match ever again stochastically (hence
+    // num_tries times the number of remaining outer iterations).
+    do_brute_dorce =
+        (tries_with_no_success > 10) &&
+        (idx2_max * (idx2_max + 1) < 2 * num_tries * (outer_iters - iter));
+    if (do_brute_dorce) num_tries = idx2_max;
+
     seed += iter;
     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) % 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;
+      uint32_t idx1, idx2;
+      if (do_brute_dorce) {
+        // Use a brute force approach.
+        idx1 = (uint32_t)j;
+        idx2 = (uint32_t)idx2_max;
+      } else {
+        const uint32_t tmp = (j & 7) + 1;
+        const uint32_t diff =
+            (tmp < 3) ? tmp : MyRand(&seed) % (image_histo_size - 1);
+        idx1 = MyRand(&seed) % image_histo_size;
+        idx2 = (idx1 + diff + 1) % image_histo_size;
+        if (idx1 == idx2) {
+          continue;
+        }
       }
 
       // Calculate cost reduction on combining.
       curr_cost_diff = HistogramAddEval(histograms[idx1], histograms[idx2],
                                         tmp_histo, best_cost_diff);
-      if (curr_cost_diff < best_cost_diff) {    // found a better pair?
+      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 (do_brute_dorce) --idx2_max;
 
     if (best_idx1 >= 0) {
       HistogramSwap(&best_combo, &histograms[best_idx1]);
       // swap best_idx2 slot with last one (which is now unused)
       --image_histo_size;
+      if (idx2_max >= image_histo_size) idx2_max = image_histo_size - 1;
       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 >= num_tries_no_success) {
+    if (++tries_with_no_success >= num_tries_no_success || idx2_max == 0) {
       break;
     }
   }
   image_histo->size = image_histo_size;
 }
 
 // -----------------------------------------------------------------------------
 // Histogram refinement
 
 // Find the best 'out' histogram for each of the 'in' histograms.
@@ skipping 28 matching lines @@
     }
   }
 
   // Recompute each out based on raw and symbols.
   for (i = 0; i < out_size; ++i) {
     HistogramClear(out_histo[i]);
   }
 
   for (i = 0; i < in_size; ++i) {
     const int idx = symbols[i];
-    VP8LHistogramAdd(in_histo[i], out_histo[idx], out_histo[idx]);
+    HistogramAdd(in_histo[i], out_histo[idx], out_histo[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 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 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;
+  // 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).
+  const int entropy_combine_num_bins = low_effort ? NUM_PARTITIONS : BIN_SIZE;
   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 int bin_map_size = orig_histo->size;
+    // Reuse histogram_symbols storage. By definition, it's guaranteed to be ok.
+    uint16_t* const bin_map = histogram_symbols;
     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,
+    cur_combo = HistogramCombineEntropyBin(image_histo, cur_combo,
+                                           bin_map, bin_map_size,
+                                           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));
     HistogramCombineStochastic(image_histo, tmp_histos->histograms[0],
                                cur_combo, quality, threshold_size);
     if ((image_histo->size <= threshold_size) &&
         !HistogramCombineGreedy(image_histo)) {
       goto Error;
     }
   }
 
   // TODO(vikasa): Optimize HistogramRemap for low-effort compression mode also.
   // Find the optimal map from original histograms to the final ones.
   HistogramRemap(orig_histo, image_histo, histogram_symbols);
 
   ok = 1;
 
  Error:
-  WebPSafeFree(bin_map);
   VP8LFreeHistogramSet(orig_histo);
   return ok;
 }