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: EXPORT_FN, FN */
9
EXPORT_FN(CreateBackwardReferences)10 static BROTLI_NOINLINE void EXPORT_FN(CreateBackwardReferences)(
11 size_t num_bytes, size_t position,
12 const uint8_t* ringbuffer, size_t ringbuffer_mask,
13 ContextLut literal_context_lut, const BrotliEncoderParams* params,
14 Hasher* hasher, int* dist_cache, size_t* last_insert_len,
15 Command* commands, size_t* num_commands, size_t* num_literals) {
16 HASHER()* privat = &hasher->privat.FN(_);
17 /* Set maximum distance, see section 9.1. of the spec. */
18 const size_t max_backward_limit = BROTLI_MAX_BACKWARD_LIMIT(params->lgwin);
19 const size_t position_offset = params->stream_offset;
20
21 const Command* const orig_commands = commands;
22 size_t insert_length = *last_insert_len;
23 const size_t pos_end = position + num_bytes;
24 const size_t store_end = num_bytes >= FN(StoreLookahead)() ?
25 position + num_bytes - FN(StoreLookahead)() + 1 : position;
26
27 /* For speed up heuristics for random data. */
28 const size_t random_heuristics_window_size =
29 LiteralSpreeLengthForSparseSearch(params);
30 size_t apply_random_heuristics = position + random_heuristics_window_size;
31 const size_t gap = 0;
32
33 /* Minimum score to accept a backward reference. */
34 const score_t kMinScore = BROTLI_SCORE_BASE + 100;
35
36 BROTLI_UNUSED(literal_context_lut);
37
38 FN(PrepareDistanceCache)(privat, dist_cache);
39
40 while (position + FN(HashTypeLength)() < pos_end) {
41 size_t max_length = pos_end - position;
42 size_t max_distance = BROTLI_MIN(size_t, position, max_backward_limit);
43 size_t dictionary_start = BROTLI_MIN(size_t,
44 position + position_offset, max_backward_limit);
45 HasherSearchResult sr;
46 sr.len = 0;
47 sr.len_code_delta = 0;
48 sr.distance = 0;
49 sr.score = kMinScore;
50 FN(FindLongestMatch)(privat, ¶ms->dictionary,
51 ringbuffer, ringbuffer_mask, dist_cache, position, max_length,
52 max_distance, dictionary_start + gap, params->dist.max_distance, &sr);
53 if (sr.score > kMinScore) {
54 /* Found a match. Let's look for something even better ahead. */
55 int delayed_backward_references_in_row = 0;
56 --max_length;
57 for (;; --max_length) {
58 const score_t cost_diff_lazy = 175;
59 HasherSearchResult sr2;
60 sr2.len = params->quality < MIN_QUALITY_FOR_EXTENSIVE_REFERENCE_SEARCH ?
61 BROTLI_MIN(size_t, sr.len - 1, max_length) : 0;
62 sr2.len_code_delta = 0;
63 sr2.distance = 0;
64 sr2.score = kMinScore;
65 max_distance = BROTLI_MIN(size_t, position + 1, max_backward_limit);
66 dictionary_start = BROTLI_MIN(size_t,
67 position + 1 + position_offset, max_backward_limit);
68 FN(FindLongestMatch)(privat,
69 ¶ms->dictionary,
70 ringbuffer, ringbuffer_mask, dist_cache, position + 1, max_length,
71 max_distance, dictionary_start + gap, params->dist.max_distance,
72 &sr2);
73 if (sr2.score >= sr.score + cost_diff_lazy) {
74 /* Ok, let's just write one byte for now and start a match from the
75 next byte. */
76 ++position;
77 ++insert_length;
78 sr = sr2;
79 if (++delayed_backward_references_in_row < 4 &&
80 position + FN(HashTypeLength)() < pos_end) {
81 continue;
82 }
83 }
84 break;
85 }
86 apply_random_heuristics =
87 position + 2 * sr.len + random_heuristics_window_size;
88 dictionary_start = BROTLI_MIN(size_t,
89 position + position_offset, max_backward_limit);
90 {
91 /* The first 16 codes are special short-codes,
92 and the minimum offset is 1. */
93 size_t distance_code = ComputeDistanceCode(
94 sr.distance, dictionary_start + gap, dist_cache);
95 if ((sr.distance <= (dictionary_start + gap)) && distance_code > 0) {
96 dist_cache[3] = dist_cache[2];
97 dist_cache[2] = dist_cache[1];
98 dist_cache[1] = dist_cache[0];
99 dist_cache[0] = (int)sr.distance;
100 FN(PrepareDistanceCache)(privat, dist_cache);
101 }
102 InitCommand(commands++, ¶ms->dist, insert_length,
103 sr.len, sr.len_code_delta, distance_code);
104 }
105 *num_literals += insert_length;
106 insert_length = 0;
107 /* Put the hash keys into the table, if there are enough bytes left.
108 Depending on the hasher implementation, it can push all positions
109 in the given range or only a subset of them.
110 Avoid hash poisoning with RLE data. */
111 {
112 size_t range_start = position + 2;
113 size_t range_end = BROTLI_MIN(size_t, position + sr.len, store_end);
114 if (sr.distance < (sr.len >> 2)) {
115 range_start = BROTLI_MIN(size_t, range_end, BROTLI_MAX(size_t,
116 range_start, position + sr.len - (sr.distance << 2)));
117 }
118 FN(StoreRange)(privat, ringbuffer, ringbuffer_mask, range_start,
119 range_end);
120 }
121 position += sr.len;
122 } else {
123 ++insert_length;
124 ++position;
125 /* If we have not seen matches for a long time, we can skip some
126 match lookups. Unsuccessful match lookups are very very expensive
127 and this kind of a heuristic speeds up compression quite
128 a lot. */
129 if (position > apply_random_heuristics) {
130 /* Going through uncompressible data, jump. */
131 if (position >
132 apply_random_heuristics + 4 * random_heuristics_window_size) {
133 /* It is quite a long time since we saw a copy, so we assume
134 that this data is not compressible, and store hashes less
135 often. Hashes of non compressible data are less likely to
136 turn out to be useful in the future, too, so we store less of
137 them to not to flood out the hash table of good compressible
138 data. */
139 const size_t kMargin =
140 BROTLI_MAX(size_t, FN(StoreLookahead)() - 1, 4);
141 size_t pos_jump =
142 BROTLI_MIN(size_t, position + 16, pos_end - kMargin);
143 for (; position < pos_jump; position += 4) {
144 FN(Store)(privat, ringbuffer, ringbuffer_mask, position);
145 insert_length += 4;
146 }
147 } else {
148 const size_t kMargin =
149 BROTLI_MAX(size_t, FN(StoreLookahead)() - 1, 2);
150 size_t pos_jump =
151 BROTLI_MIN(size_t, position + 8, pos_end - kMargin);
152 for (; position < pos_jump; position += 2) {
153 FN(Store)(privat, ringbuffer, ringbuffer_mask, position);
154 insert_length += 2;
155 }
156 }
157 }
158 }
159 }
160 insert_length += pos_end - position;
161 *last_insert_len = insert_length;
162 *num_commands += (size_t)(commands - orig_commands);
163 }
164