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metablock_inc.h
Go to the documentation of this file.
1/* NOLINT(build/header_guard) */
2/* Copyright 2015 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/* Greedy block splitter for one block category (literal, command or distance).
13*/
14typedef struct FN(BlockSplitter) {
15 /* Alphabet size of particular block category. */
16 size_t alphabet_size_;
17 /* We collect at least this many symbols for each block. */
18 size_t min_block_size_;
19 /* We merge histograms A and B if
20 entropy(A+B) < entropy(A) + entropy(B) + split_threshold_,
21 where A is the current histogram and B is the histogram of the last or the
22 second last block type. */
23 double split_threshold_;
24
25 size_t num_blocks_;
26 BlockSplit* split_; /* not owned */
27 HistogramType* histograms_; /* not owned */
28 size_t* histograms_size_; /* not owned */
29
30 /* The number of symbols that we want to collect before deciding on whether
31 or not to merge the block with a previous one or emit a new block. */
32 size_t target_block_size_;
33 /* The number of symbols in the current histogram. */
34 size_t block_size_;
35 /* Offset of the current histogram. */
36 size_t curr_histogram_ix_;
37 /* Offset of the histograms of the previous two block types. */
38 size_t last_histogram_ix_[2];
39 /* Entropy of the previous two block types. */
40 double last_entropy_[2];
41 /* The number of times we merged the current block with the last one. */
42 size_t merge_last_count_;
43} FN(BlockSplitter);
44
45static void FN(InitBlockSplitter)(
46 MemoryManager* m, FN(BlockSplitter)* self, size_t alphabet_size,
47 size_t min_block_size, double split_threshold, size_t num_symbols,
48 BlockSplit* split, HistogramType** histograms, size_t* histograms_size) {
49 size_t max_num_blocks = num_symbols / min_block_size + 1;
50 /* We have to allocate one more histogram than the maximum number of block
51 types for the current histogram when the meta-block is too big. */
52 size_t max_num_types =
53 BROTLI_MIN(size_t, max_num_blocks, BROTLI_MAX_NUMBER_OF_BLOCK_TYPES + 1);
54 self->alphabet_size_ = alphabet_size;
55 self->min_block_size_ = min_block_size;
56 self->split_threshold_ = split_threshold;
57 self->num_blocks_ = 0;
58 self->split_ = split;
59 self->histograms_size_ = histograms_size;
60 self->target_block_size_ = min_block_size;
61 self->block_size_ = 0;
62 self->curr_histogram_ix_ = 0;
63 self->merge_last_count_ = 0;
64 BROTLI_ENSURE_CAPACITY(m, uint8_t,
65 split->types, split->types_alloc_size, max_num_blocks);
66 BROTLI_ENSURE_CAPACITY(m, uint32_t,
67 split->lengths, split->lengths_alloc_size, max_num_blocks);
68 if (BROTLI_IS_OOM(m)) return;
69 self->split_->num_blocks = max_num_blocks;
70 BROTLI_DCHECK(*histograms == 0);
71 *histograms_size = max_num_types;
72 *histograms = BROTLI_ALLOC(m, HistogramType, *histograms_size);
73 self->histograms_ = *histograms;
74 if (BROTLI_IS_OOM(m) || BROTLI_IS_NULL(*histograms)) return;
75 /* Clear only current histogram. */
76 FN(HistogramClear)(&self->histograms_[0]);
77 self->last_histogram_ix_[0] = self->last_histogram_ix_[1] = 0;
78}
79
80/* Does either of three things:
81 (1) emits the current block with a new block type;
82 (2) emits the current block with the type of the second last block;
83 (3) merges the current block with the last block. */
84static void FN(BlockSplitterFinishBlock)(
85 FN(BlockSplitter)* self, BROTLI_BOOL is_final) {
86 BlockSplit* split = self->split_;
87 double* last_entropy = self->last_entropy_;
88 HistogramType* histograms = self->histograms_;
89 self->block_size_ =
90 BROTLI_MAX(size_t, self->block_size_, self->min_block_size_);
91 if (self->num_blocks_ == 0) {
92 /* Create first block. */
93 split->lengths[0] = (uint32_t)self->block_size_;
94 split->types[0] = 0;
95 last_entropy[0] =
96 BitsEntropy(histograms[0].data_, self->alphabet_size_);
97 last_entropy[1] = last_entropy[0];
98 ++self->num_blocks_;
99 ++split->num_types;
100 ++self->curr_histogram_ix_;
101 if (self->curr_histogram_ix_ < *self->histograms_size_)
102 FN(HistogramClear)(&histograms[self->curr_histogram_ix_]);
103 self->block_size_ = 0;
104 } else if (self->block_size_ > 0) {
105 double entropy = BitsEntropy(histograms[self->curr_histogram_ix_].data_,
106 self->alphabet_size_);
107 HistogramType combined_histo[2];
108 double combined_entropy[2];
109 double diff[2];
110 size_t j;
111 for (j = 0; j < 2; ++j) {
112 size_t last_histogram_ix = self->last_histogram_ix_[j];
113 combined_histo[j] = histograms[self->curr_histogram_ix_];
114 FN(HistogramAddHistogram)(&combined_histo[j],
115 &histograms[last_histogram_ix]);
116 combined_entropy[j] = BitsEntropy(
117 &combined_histo[j].data_[0], self->alphabet_size_);
118 diff[j] = combined_entropy[j] - entropy - last_entropy[j];
119 }
120
121 if (split->num_types < BROTLI_MAX_NUMBER_OF_BLOCK_TYPES &&
122 diff[0] > self->split_threshold_ &&
123 diff[1] > self->split_threshold_) {
124 /* Create new block. */
125 split->lengths[self->num_blocks_] = (uint32_t)self->block_size_;
126 split->types[self->num_blocks_] = (uint8_t)split->num_types;
127 self->last_histogram_ix_[1] = self->last_histogram_ix_[0];
128 self->last_histogram_ix_[0] = (uint8_t)split->num_types;
129 last_entropy[1] = last_entropy[0];
130 last_entropy[0] = entropy;
131 ++self->num_blocks_;
132 ++split->num_types;
133 ++self->curr_histogram_ix_;
134 if (self->curr_histogram_ix_ < *self->histograms_size_)
135 FN(HistogramClear)(&histograms[self->curr_histogram_ix_]);
136 self->block_size_ = 0;
137 self->merge_last_count_ = 0;
138 self->target_block_size_ = self->min_block_size_;
139 } else if (diff[1] < diff[0] - 20.0) {
140 /* Combine this block with second last block. */
141 split->lengths[self->num_blocks_] = (uint32_t)self->block_size_;
142 split->types[self->num_blocks_] = split->types[self->num_blocks_ - 2];
143 BROTLI_SWAP(size_t, self->last_histogram_ix_, 0, 1);
144 histograms[self->last_histogram_ix_[0]] = combined_histo[1];
145 last_entropy[1] = last_entropy[0];
146 last_entropy[0] = combined_entropy[1];
147 ++self->num_blocks_;
148 self->block_size_ = 0;
149 FN(HistogramClear)(&histograms[self->curr_histogram_ix_]);
150 self->merge_last_count_ = 0;
151 self->target_block_size_ = self->min_block_size_;
152 } else {
153 /* Combine this block with last block. */
154 split->lengths[self->num_blocks_ - 1] += (uint32_t)self->block_size_;
155 histograms[self->last_histogram_ix_[0]] = combined_histo[0];
156 last_entropy[0] = combined_entropy[0];
157 if (split->num_types == 1) {
158 last_entropy[1] = last_entropy[0];
159 }
160 self->block_size_ = 0;
161 FN(HistogramClear)(&histograms[self->curr_histogram_ix_]);
162 if (++self->merge_last_count_ > 1) {
163 self->target_block_size_ += self->min_block_size_;
164 }
165 }
166 }
167 if (is_final) {
168 *self->histograms_size_ = split->num_types;
169 split->num_blocks = self->num_blocks_;
170 }
171}
172
173/* Adds the next symbol to the current histogram. When the current histogram
174 reaches the target size, decides on merging the block. */
175static void FN(BlockSplitterAddSymbol)(FN(BlockSplitter)* self, size_t symbol) {
176 FN(HistogramAdd)(&self->histograms_[self->curr_histogram_ix_], symbol);
177 ++self->block_size_;
178 if (self->block_size_ == self->target_block_size_) {
179 FN(BlockSplitterFinishBlock)(self, /* is_final = */ BROTLI_FALSE);
180 }
181}
182
183#undef HistogramType
BROTLI_MAX_NUMBER_OF_BLOCK_TYPES
#define BROTLI_MAX_NUMBER_OF_BLOCK_TYPES
Definition constants.h:23
FN
#define FN(X)
Definition backward_references.c:51
HistogramType
#define HistogramType
Definition bit_cost_inc.h:10
BROTLI_ALLOC
#define BROTLI_ALLOC(M, T, N)
Definition memory.h:44
BROTLI_ENSURE_CAPACITY
#define BROTLI_ENSURE_CAPACITY(M, T, A, C, R)
Definition memory.h:81
BROTLI_IS_OOM
#define BROTLI_IS_OOM(M)
Definition memory.h:54
BROTLI_IS_NULL
#define BROTLI_IS_NULL(A)
Definition memory.h:68
pzstd::split
Buffer split(Buffer &buffer, ZSTD_outBuffer &outBuffer)
Definition Pzstd.cpp:248
run.diff
str diff(bytes a, bytes b)
Definition run.py:91
BlockSplit
Definition block_splitter.h:22
MemoryManager
Definition memory.h:26
BROTLI_SWAP
#define BROTLI_SWAP(T, A, I, J)
BROTLI_MAX
#define BROTLI_MAX(T, A, B)
BROTLI_MIN
#define BROTLI_MIN(T, A, B)
BROTLI_DCHECK
#define BROTLI_DCHECK(x)
Definition platform.h:484
BROTLI_FALSE
#define BROTLI_FALSE
Definition types.h:53
BROTLI_BOOL
#define BROTLI_BOOL
Definition types.h:49
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