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 
12 double 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