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