bit_cost_inc.h revision 26fa459c 
1/* NOLINT(build/header_guard) */
2/* Copyright 2013 Google Inc. All Rights Reserved.
3
4   Distributed under MIT license.
5   See file LICENSE for detail or copy at https://opensource.org/licenses/MIT
6*/
7
8/* template parameters: FN */
9
10#define HistogramType FN(Histogram)
11
12double FN(BrotliPopulationCost)(const HistogramType* histogram) {
13  static const double kOneSymbolHistogramCost = 12;
14  static const double kTwoSymbolHistogramCost = 20;
15  static const double kThreeSymbolHistogramCost = 28;
16  static const double kFourSymbolHistogramCost = 37;
17  const size_t data_size = FN(HistogramDataSize)();
18  int count = 0;
19  size_t s[5];
20  double bits = 0.0;
21  size_t i;
22  if (histogram->total_count_ == 0) {
23    return kOneSymbolHistogramCost;
24  }
25  for (i = 0; i < data_size; ++i) {
26    if (histogram->data_[i] > 0) {
27      s[count] = i;
28      ++count;
29      if (count > 4) break;
30    }
31  }
32  if (count == 1) {
33    return kOneSymbolHistogramCost;
34  }
35  if (count == 2) {
36    return (kTwoSymbolHistogramCost + (double)histogram->total_count_);
37  }
38  if (count == 3) {
39    const uint32_t histo0 = histogram->data_[s[0]];
40    const uint32_t histo1 = histogram->data_[s[1]];
41    const uint32_t histo2 = histogram->data_[s[2]];
42    const uint32_t histomax =
43        BROTLI_MAX(uint32_t, histo0, BROTLI_MAX(uint32_t, histo1, histo2));
44    return (kThreeSymbolHistogramCost +
45            2 * (histo0 + histo1 + histo2) - histomax);
46  }
47  if (count == 4) {
48    uint32_t histo[4];
49    uint32_t h23;
50    uint32_t histomax;
51    for (i = 0; i < 4; ++i) {
52      histo[i] = histogram->data_[s[i]];
53    }
54    /* Sort */
55    for (i = 0; i < 4; ++i) {
56      size_t j;
57      for (j = i + 1; j < 4; ++j) {
58        if (histo[j] > histo[i]) {
59          BROTLI_SWAP(uint32_t, histo, j, i);
60        }
61      }
62    }
63    h23 = histo[2] + histo[3];
64    histomax = BROTLI_MAX(uint32_t, h23, histo[0]);
65    return (kFourSymbolHistogramCost +
66            3 * h23 + 2 * (histo[0] + histo[1]) - histomax);
67  }
68
69  {
70    /* In this loop we compute the entropy of the histogram and simultaneously
71       build a simplified histogram of the code length codes where we use the
72       zero repeat code 17, but we don't use the non-zero repeat code 16. */
73    size_t max_depth = 1;
74    uint32_t depth_histo[BROTLI_CODE_LENGTH_CODES] = { 0 };
75    const double log2total = FastLog2(histogram->total_count_);
76    for (i = 0; i < data_size;) {
77      if (histogram->data_[i] > 0) {
78        /* Compute -log2(P(symbol)) = -log2(count(symbol)/total_count) =
79                                    = log2(total_count) - log2(count(symbol)) */
80        double log2p = log2total - FastLog2(histogram->data_[i]);
81        /* Approximate the bit depth by round(-log2(P(symbol))) */
82        size_t depth = (size_t)(log2p + 0.5);
83        bits += histogram->data_[i] * log2p;
84        if (depth > 15) {
85          depth = 15;
86        }
87        if (depth > max_depth) {
88          max_depth = depth;
89        }
90        ++depth_histo[depth];
91        ++i;
92      } else {
93        /* Compute the run length of zeros and add the appropriate number of 0
94           and 17 code length codes to the code length code histogram. */
95        uint32_t reps = 1;
96        size_t k;
97        for (k = i + 1; k < data_size && histogram->data_[k] == 0; ++k) {
98          ++reps;
99        }
100        i += reps;
101        if (i == data_size) {
102          /* Don't add any cost for the last zero run, since these are encoded
103             only implicitly. */
104          break;
105        }
106        if (reps < 3) {
107          depth_histo[0] += reps;
108        } else {
109          reps -= 2;
110          while (reps > 0) {
111            ++depth_histo[BROTLI_REPEAT_ZERO_CODE_LENGTH];
112            /* Add the 3 extra bits for the 17 code length code. */
113            bits += 3;
114            reps >>= 3;
115          }
116        }
117      }
118    }
119    /* Add the estimated encoding cost of the code length code histogram. */
120    bits += (double)(18 + 2 * max_depth);
121    /* Add the entropy of the code length code histogram. */
122    bits += BitsEntropy(depth_histo, BROTLI_CODE_LENGTH_CODES);
123  }
124  return bits;
125}
126
127#undef HistogramType
128