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 const BrotliDictionary* dictionary,
12 const uint16_t* dictionary_hash, size_t num_bytes, size_t position,
13 const uint8_t* ringbuffer, size_t ringbuffer_mask,
14 const BrotliEncoderParams* params, HasherHandle hasher, int* dist_cache,
15 size_t* last_insert_len, Command* commands, size_t* num_commands,
16 size_t* num_literals) {
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
20 const Command* const orig_commands = commands;
21 size_t insert_length = *last_insert_len;
22 const size_t pos_end = position + num_bytes;
23 const size_t store_end = num_bytes >= FN(StoreLookahead)() ?
24 position + num_bytes - FN(StoreLookahead)() + 1 : position;
25
26 /* For speed up heuristics for random data. */
27 const size_t random_heuristics_window_size =
28 LiteralSpreeLengthForSparseSearch(params);
29 size_t apply_random_heuristics = position + random_heuristics_window_size;
30 const size_t gap = 0;
31
32 /* Minimum score to accept a backward reference. */
33 const score_t kMinScore = BROTLI_SCORE_BASE + 100;
34
35 FN(PrepareDistanceCache)(hasher, dist_cache);
36
37 while (position + FN(HashTypeLength)() < pos_end) {
38 size_t max_length = pos_end - position;
39 size_t max_distance = BROTLI_MIN(size_t, position, max_backward_limit);
40 HasherSearchResult sr;
41 sr.len = 0;
42 sr.len_code_delta = 0;
43 sr.distance = 0;
44 sr.score = kMinScore;
45 FN(FindLongestMatch)(hasher, dictionary, dictionary_hash, ringbuffer,
46 ringbuffer_mask, dist_cache, position,
47 max_length, max_distance, gap, &sr);
48 if (sr.score > kMinScore) {
49 /* Found a match. Let's look for something even better ahead. */
50 int delayed_backward_references_in_row = 0;
51 --max_length;
52 for (;; --max_length) {
53 const score_t cost_diff_lazy = 175;
54 HasherSearchResult sr2;
55 sr2.len = params->quality < MIN_QUALITY_FOR_EXTENSIVE_REFERENCE_SEARCH ?
56 BROTLI_MIN(size_t, sr.len - 1, max_length) : 0;
57 sr2.len_code_delta = 0;
58 sr2.distance = 0;
59 sr2.score = kMinScore;
60 max_distance = BROTLI_MIN(size_t, position + 1, max_backward_limit);
61 FN(FindLongestMatch)(hasher, dictionary, dictionary_hash, ringbuffer,
62 ringbuffer_mask, dist_cache, position + 1,
63 max_length, max_distance, gap, &sr2);
64 if (sr2.score >= sr.score + cost_diff_lazy) {
65 /* Ok, let's just write one byte for now and start a match from the
66 next byte. */
67 ++position;
68 ++insert_length;
69 sr = sr2;
70 if (++delayed_backward_references_in_row < 4 &&
71 position + FN(HashTypeLength)() < pos_end) {
72 continue;
73 }
74 }
75 break;
76 }
77 apply_random_heuristics =
78 position + 2 * sr.len + random_heuristics_window_size;
79 max_distance = BROTLI_MIN(size_t, position, max_backward_limit);
80 {
81 /* The first 16 codes are special short-codes,
82 and the minimum offset is 1. */
83 size_t distance_code =
84 ComputeDistanceCode(sr.distance, max_distance + gap, dist_cache);
85 if ((sr.distance <= (max_distance + gap)) && distance_code > 0) {
86 dist_cache[3] = dist_cache[2];
87 dist_cache[2] = dist_cache[1];
88 dist_cache[1] = dist_cache[0];
89 dist_cache[0] = (int)sr.distance;
90 FN(PrepareDistanceCache)(hasher, dist_cache);
91 }
92 InitCommand(commands++, insert_length, sr.len, sr.len_code_delta,
93 distance_code);
94 }
95 *num_literals += insert_length;
96 insert_length = 0;
97 /* Put the hash keys into the table, if there are enough bytes left.
98 Depending on the hasher implementation, it can push all positions
99 in the given range or only a subset of them.
100 Avoid hash poisoning with RLE data. */
101 {
102 size_t range_start = position + 2;
103 size_t range_end = BROTLI_MIN(size_t, position + sr.len, store_end);
104 if (sr.distance < (sr.len >> 2)) {
105 range_start = BROTLI_MIN(size_t, range_end, BROTLI_MAX(size_t,
106 range_start, position + sr.len - (sr.distance << 2)));
107 }
108 FN(StoreRange)(hasher, ringbuffer, ringbuffer_mask, range_start,
109 range_end);
110 }
111 position += sr.len;
112 } else {
113 ++insert_length;
114 ++position;
115 /* If we have not seen matches for a long time, we can skip some
116 match lookups. Unsuccessful match lookups are very very expensive
117 and this kind of a heuristic speeds up compression quite
118 a lot. */
119 if (position > apply_random_heuristics) {
120 /* Going through uncompressible data, jump. */
121 if (position >
122 apply_random_heuristics + 4 * random_heuristics_window_size) {
123 /* It is quite a long time since we saw a copy, so we assume
124 that this data is not compressible, and store hashes less
125 often. Hashes of non compressible data are less likely to
126 turn out to be useful in the future, too, so we store less of
127 them to not to flood out the hash table of good compressible
128 data. */
129 const size_t kMargin =
130 BROTLI_MAX(size_t, FN(StoreLookahead)() - 1, 4);
131 size_t pos_jump =
132 BROTLI_MIN(size_t, position + 16, pos_end - kMargin);
133 for (; position < pos_jump; position += 4) {
134 FN(Store)(hasher, ringbuffer, ringbuffer_mask, position);
135 insert_length += 4;
136 }
137 } else {
138 const size_t kMargin =
139 BROTLI_MAX(size_t, FN(StoreLookahead)() - 1, 2);
140 size_t pos_jump =
141 BROTLI_MIN(size_t, position + 8, pos_end - kMargin);
142 for (; position < pos_jump; position += 2) {
143 FN(Store)(hasher, ringbuffer, ringbuffer_mask, position);
144 insert_length += 2;
145 }
146 }
147 }
148 }
149 }
150 insert_length += pos_end - position;
151 *last_insert_len = insert_length;
152 *num_commands += (size_t)(commands - orig_commands);
153 }
154