libwebp-0.6.0-rc1

third_party/libwebp/enc/histogram.c

Index: third_party/libwebp/enc/histogram.c
diff --git a/third_party/libwebp/enc/histogram.c b/third_party/libwebp/enc/histogram.c
deleted file mode 100644
index 36b7f2262599c94728d30966a8370eb538722f51..0000000000000000000000000000000000000000
--- a/third_party/libwebp/enc/histogram.c
+++ /dev/null
@@ -1,938 +0,0 @@
-// 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 "../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) {
-  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) {
-  VP8LRefsCursor c = VP8LRefsCursorInit(refs);
-  while (VP8LRefsCursorOk(&c)) {
-    VP8LHistogramAddSinglePixOrCopy(histo, c.cur_pos);
-    VP8LRefsCursorNext(&c);
-  }
-}
-
-void VP8LHistogramCreate(VP8LHistogram* const p,
-                         const VP8LBackwardRefs* const refs,
-                         int palette_code_bits) {
-  if (palette_code_bits >= 0) {
-    p->palette_code_bits_ = palette_code_bits;
-  }
-  HistogramClear(p);
-  VP8LHistogramStoreRefs(refs, p);
-}
-
-void VP8LHistogramInit(VP8LHistogram* const p, int palette_code_bits) {
-  p->palette_code_bits_ = 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;
-  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;
-
-  set = (VP8LHistogramSet*)memory;
-  memory += sizeof(*set);
-  set->histograms = (VP8LHistogram**)memory;
-  memory += size * sizeof(*set->histograms);
-  set->max_size = size;
-  set->size = size;
-  for (i = 0; i < size; ++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;
-}
-
-// -----------------------------------------------------------------------------
-
-void VP8LHistogramAddSinglePixOrCopy(VP8LHistogram* const histo,
-                                     const PixOrCopy* const v) {
-  if (PixOrCopyIsLiteral(v)) {
-    ++histo->alpha_[PixOrCopyLiteral(v, 3)];
-    ++histo->red_[PixOrCopyLiteral(v, 2)];
-    ++histo->literal_[PixOrCopyLiteral(v, 1)];
-    ++histo->blue_[PixOrCopyLiteral(v, 0)];
-  } else if (PixOrCopyIsCacheIdx(v)) {
-    const int literal_ix =
-        NUM_LITERAL_CODES + NUM_LENGTH_CODES + PixOrCopyCacheIdx(v);
-    ++histo->literal_[literal_ix];
-  } else {
-    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];
-  }
-}
-
-// -----------------------------------------------------------------------------
-// Entropy-related functions.
-
-static WEBP_INLINE double BitsEntropyRefine(const VP8LBitEntropy* entropy) {
-  double mix;
-  if (entropy->nonzeros < 5) {
-    if (entropy->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 (entropy->nonzeros == 2) {
-      return 0.99 * entropy->sum + 0.01 * entropy->entropy;
-    }
-    // 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 (entropy->nonzeros == 3) {
-      mix = 0.95;
-    } else {
-      mix = 0.7;  // nonzeros == 4.
-    }
-  } else {
-    mix = 0.627;
-  }
-
-  {
-    double min_limit = 2 * entropy->sum - entropy->max_val;
-    min_limit = mix * min_limit + (1.0 - mix) * entropy->entropy;
-    return (entropy->entropy < min_limit) ? min_limit : entropy->entropy;
-  }
-}
-
-double VP8LBitsEntropy(const uint32_t* const array, int n,
-                       uint32_t* const trivial_symbol) {
-  VP8LBitEntropy entropy;
-  VP8LBitsEntropyUnrefined(array, n, &entropy);
-  if (trivial_symbol != NULL) {
-    *trivial_symbol =
-        (entropy.nonzeros == 1) ? entropy.nonzero_code : VP8L_NON_TRIVIAL_SYM;
-  }
-
-  return BitsEntropyRefine(&entropy);
-}
-
-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) {
-  double retval = InitialHuffmanCost();
-  retval += stats->counts[0] * 1.5625 + 0.234375 * stats->streaks[0][1];
-  retval += stats->counts[1] * 2.578125 + 0.703125 * stats->streaks[1][1];
-  retval += 1.796875 * stats->streaks[0][0];
-  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);
-}
-
-static WEBP_INLINE double GetCombinedEntropy(const uint32_t* const X,
-                                             const uint32_t* const Y,
-                                             int length) {
-  VP8LBitEntropy bit_entropy;
-  VP8LStreaks stats;
-  VP8LGetCombinedEntropyUnrefined(X, Y, length, &bit_entropy, &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_;
-  assert(a->palette_code_bits_ == b->palette_code_bits_);
-  *cost += GetCombinedEntropy(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 += GetCombinedEntropy(a->red_, b->red_, NUM_LITERAL_CODES);
-  if (*cost > cost_threshold) return 0;
-
-  *cost += GetCombinedEntropy(a->blue_, b->blue_, NUM_LITERAL_CODES);
-  if (*cost > cost_threshold) return 0;
-
-  *cost += GetCombinedEntropy(a->alpha_, b->alpha_, NUM_LITERAL_CODES);
-  if (*cost > cost_threshold) return 0;
-
-  *cost += GetCombinedEntropy(a->distance_, b->distance_, NUM_DISTANCE_CODES);
-  *cost +=
-      VP8LExtraCostCombined(a->distance_, b->distance_, NUM_DISTANCE_CODES);
-  if (*cost > cost_threshold) return 0;
-
-  return 1;
-}
-
-// 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;
-  }
-
-  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,
-                                 double cost_threshold) {
-  double cost = -a->bit_cost_;
-  GetCombinedHistogramEntropy(a, b, cost_threshold, &cost);
-  return cost;
-}
-
-// -----------------------------------------------------------------------------
-
-// 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 =
-      PopulationCost(h->alpha_, NUM_LITERAL_CODES, &alpha_sym);
-  const double distance_cost =
-      PopulationCost(h->distance_, NUM_DISTANCE_CODES, NULL) +
-      VP8LExtraCost(h->distance_, NUM_DISTANCE_CODES);
-  const int num_codes = VP8LHistogramNumCodes(h->palette_code_bits_);
-  h->literal_cost_ = PopulationCost(h->literal_, num_codes, NULL) +
-                     VP8LExtraCost(h->literal_ + NUM_LITERAL_CODES,
-                                   NUM_LENGTH_CODES);
-  h->red_cost_ = PopulationCost(h->red_, NUM_LITERAL_CODES, &red_sym);
-  h->blue_cost_ = PopulationCost(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);
-  }
-}
-
-static int GetBinIdForEntropy(double min, double max, double val) {
-  const double range = max - min;
-  if (range > 0.) {
-    const double delta = val - min;
-    return (int)((NUM_PARTITIONS - 1e-6) * delta / range);
-  } else {
-    return 0;
-  }
-}
-
-static int GetHistoBinIndex(const VP8LHistogram* const h,
-                            const DominantCostRange* const c, int low_effort) {
-  int bin_id = GetBinIdForEntropy(c->literal_min_, c->literal_max_,
-                                  h->literal_cost_);
-  assert(bin_id < NUM_PARTITIONS);
-  if (!low_effort) {
-    bin_id = bin_id * NUM_PARTITIONS
-           + GetBinIdForEntropy(c->red_min_, c->red_max_, h->red_cost_);
-    bin_id = bin_id * NUM_PARTITIONS
-           + GetBinIdForEntropy(c->blue_min_, c->blue_max_, h->blue_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_histo->histograms;
-  VP8LRefsCursor c = VP8LRefsCursorInit(backward_refs);
-  assert(histo_bits > 0);
-  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);
-    while (x >= xsize) {
-      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) {
-    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;
-  }
-}
-
-// 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) {
-  *seed *= 16807U;
-  if (*seed == 0) {
-    *seed = 1;
-  }
-  return *seed;
-}
-
-// -----------------------------------------------------------------------------
-// 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* queue;
-  int size;
-  int max_size;
-} HistoQueue;
-
-static int HistoQueueInit(HistoQueue* const histo_queue, const int max_index) {
-  histo_queue->size = 0;
-  // max_index^2 for the queue size is safe. If you look at
-  // HistogramCombineGreedy, and imagine that UpdateQueueFront always pushes
-  // data to the queue, you insert at most:
-  // - max_index*(max_index-1)/2 (the first two for loops)
-  // - max_index - 1 in the last for loop at the first iteration of the while
-  //   loop, max_index - 2 at the second iteration ... therefore
-  //   max_index*(max_index-1)/2 overall too
-  histo_queue->max_size = max_index * max_index;
-  // We allocate max_size + 1 because the last element at index "size" is
-  // used as temporary data (and it could be up to max_size).
-  histo_queue->queue = (HistogramPair*)WebPSafeMalloc(
-      histo_queue->max_size + 1, sizeof(*histo_queue->queue));
-  return histo_queue->queue != NULL;
-}
-
-static void HistoQueueClear(HistoQueue* const histo_queue) {
-  assert(histo_queue != NULL);
-  WebPSafeFree(histo_queue->queue);
-}
-
-static void SwapHistogramPairs(HistogramPair *p1,
-                               HistogramPair *p2) {
-  const HistogramPair tmp = *p1;
-  *p1 = *p2;
-  *p2 = tmp;
-}
-
-// Given a valid priority queue in range [0, queue_size) this function checks
-// whether histo_queue[queue_size] should be accepted and swaps it with the
-// front if it is smaller. Otherwise, it leaves it as is.
-static void UpdateQueueFront(HistoQueue* const histo_queue) {
-  if (histo_queue->queue[histo_queue->size].cost_diff >= 0) return;
-
-  if (histo_queue->queue[histo_queue->size].cost_diff <
-      histo_queue->queue[0].cost_diff) {
-    SwapHistogramPairs(histo_queue->queue,
-                       histo_queue->queue + histo_queue->size);
-  }
-  ++histo_queue->size;
-
-  // We cannot add more elements than the capacity.
-  // The allocation adds an extra element to the official capacity so that
-  // histo_queue->queue[histo_queue->max_size] is read/written within bound.
-  assert(histo_queue->size <= histo_queue->max_size);
-}
-
-// -----------------------------------------------------------------------------
-
-static void PreparePair(VP8LHistogram** histograms, int idx1, int idx2,
-                        HistogramPair* const pair) {
-  VP8LHistogram* h1;
-  VP8LHistogram* h2;
-  double sum_cost;
-
-  if (idx1 > idx2) {
-    const int tmp = idx2;
-    idx2 = idx1;
-    idx1 = tmp;
-  }
-  pair->idx1 = idx1;
-  pair->idx2 = idx2;
-  h1 = histograms[idx1];
-  h2 = histograms[idx2];
-  sum_cost = h1->bit_cost_ + h2->bit_cost_;
-  pair->cost_combo = 0.;
-  GetCombinedHistogramEntropy(h1, h2, sum_cost, &pair->cost_combo);
-  pair->cost_diff = pair->cost_combo - sum_cost;
-}
-
-// Combines histograms by continuously choosing the one with the highest cost
-// reduction.
-static int HistogramCombineGreedy(VP8LHistogramSet* const image_histo) {
-  int ok = 0;
-  int image_histo_size = image_histo->size;
-  int i, j;
-  VP8LHistogram** const histograms = image_histo->histograms;
-  // Indexes of remaining histograms.
-  int* const clusters =
-      (int*)WebPSafeMalloc(image_histo_size, sizeof(*clusters));
-  // Priority queue of histogram pairs.
-  HistoQueue histo_queue;
-
-  if (!HistoQueueInit(&histo_queue, 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.
-      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]);
-    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
-    // therefore pop the head of the queue.
-    copy_to = histo_queue.queue;
-    for (i = 0; i < histo_queue.size; ++i) {
-      HistogramPair* const p = histo_queue.queue + i;
-      if (p->idx1 == idx1 || p->idx2 == idx1 ||
-          p->idx1 == idx2 || p->idx2 == idx2) {
-        // Do not copy the invalid pair.
-        continue;
-      }
-      if (p->cost_diff < histo_queue.queue[0].cost_diff) {
-        // Replace the top of the queue if we found better.
-        SwapHistogramPairs(histo_queue.queue, p);
-      }
-      SwapHistogramPairs(copy_to, p);
-      ++copy_to;
-    }
-    histo_queue.size = (int)(copy_to - histo_queue.queue);
-
-    // Push new pairs formed with combined histogram to the queue.
-    for (i = 0; i < image_histo_size; ++i) {
-      if (clusters[i] != idx1) {
-        PreparePair(histograms, idx1, clusters[i],
-                    &histo_queue.queue[histo_queue.size]);
-        UpdateQueueFront(&histo_queue);
-      }
-    }
-  }
-  // 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);
-  HistoQueueClear(&histo_queue);
-  return ok;
-}
-
-static void 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;
-  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 = -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_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;
-      }
-
-      // 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?
-        HistogramSwap(&best_combo, &tmp_histo);
-        best_cost_diff = curr_cost_diff;
-        best_idx1 = idx1;
-        best_idx2 = idx2;
-      }
-    }
-
-    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 (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) {
-      break;
-    }
-  }
-  image_histo->size = image_histo_size;
-}
-
-// -----------------------------------------------------------------------------
-// Histogram refinement
-
-// 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,
-                           uint16_t* const symbols) {
-  int i;
-  VP8LHistogram** const in_histo = in->histograms;
-  VP8LHistogram** const out_histo = out->histograms;
-  const int in_size = in->size;
-  const int out_size = out->size;
-  if (out_size > 1) {
-    for (i = 0; i < in_size; ++i) {
-      int best_out = 0;
-      double best_bits = MAX_COST;
-      int k;
-      for (k = 0; k < out_size; ++k) {
-        const double cur_bits =
-            HistogramAddThresh(out_histo[k], in_histo[i], best_bits);
-        if (k == 0 || cur_bits < best_bits) {
-          best_bits = cur_bits;
-          best_out = k;
-        }
-      }
-      symbols[i] = best_out;
-    }
-  } else {
-    assert(out_size == 1);
-    for (i = 0; i < in_size; ++i) {
-      symbols[i] = 0;
-    }
-  }
-
-  // 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]);
-  }
-}
-
-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;
-  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));
-    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;
-}