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 | |