1 /* Copyright 2016 Google Inc. All Rights Reserved.
2
3 Distributed under MIT license.
4 See file LICENSE for detail or copy at https://opensource.org/licenses/MIT
5 */
6
7 /**
8 * @file
9 * Common constants used in decoder and encoder API.
10 */
11
12 #ifndef BROTLI_COMMON_CONSTANTS_H_
13 #define BROTLI_COMMON_CONSTANTS_H_
14
15 #include "./platform.h"
16 #include <brotli/types.h>
17
18 /* Specification: 7.3. Encoding of the context map */
19 #define BROTLI_CONTEXT_MAP_MAX_RLE 16
20
21 /* Specification: 2. Compressed representation overview */
22 #define BROTLI_MAX_NUMBER_OF_BLOCK_TYPES 256
23
24 /* Specification: 3.3. Alphabet sizes: insert-and-copy length */
25 #define BROTLI_NUM_LITERAL_SYMBOLS 256
26 #define BROTLI_NUM_COMMAND_SYMBOLS 704
27 #define BROTLI_NUM_BLOCK_LEN_SYMBOLS 26
28 #define BROTLI_MAX_CONTEXT_MAP_SYMBOLS (BROTLI_MAX_NUMBER_OF_BLOCK_TYPES + \
29 BROTLI_CONTEXT_MAP_MAX_RLE)
30 #define BROTLI_MAX_BLOCK_TYPE_SYMBOLS (BROTLI_MAX_NUMBER_OF_BLOCK_TYPES + 2)
31
32 /* Specification: 3.5. Complex prefix codes */
33 #define BROTLI_REPEAT_PREVIOUS_CODE_LENGTH 16
34 #define BROTLI_REPEAT_ZERO_CODE_LENGTH 17
35 #define BROTLI_CODE_LENGTH_CODES (BROTLI_REPEAT_ZERO_CODE_LENGTH + 1)
36 /* "code length of 8 is repeated" */
37 #define BROTLI_INITIAL_REPEATED_CODE_LENGTH 8
38
39 /* "Large Window Brotli" */
40
41 /**
42 * The theoretical maximum number of distance bits specified for large window
43 * brotli, for 64-bit encoders and decoders. Even when in practice 32-bit
44 * encoders and decoders only support up to 30 max distance bits, the value is
45 * set to 62 because it affects the large window brotli file format.
46 * Specifically, it affects the encoding of simple huffman tree for distances,
47 * see Specification RFC 7932 chapter 3.4.
48 */
49 #define BROTLI_LARGE_MAX_DISTANCE_BITS 62U
50 #define BROTLI_LARGE_MIN_WBITS 10
51 /**
52 * The maximum supported large brotli window bits by the encoder and decoder.
53 * Large window brotli allows up to 62 bits, however the current encoder and
54 * decoder, designed for 32-bit integers, only support up to 30 bits maximum.
55 */
56 #define BROTLI_LARGE_MAX_WBITS 30
57
58 /* Specification: 4. Encoding of distances */
59 #define BROTLI_NUM_DISTANCE_SHORT_CODES 16
60 /**
61 * Maximal number of "postfix" bits.
62 *
63 * Number of "postfix" bits is stored as 2 bits in meta-block header.
64 */
65 #define BROTLI_MAX_NPOSTFIX 3
66 #define BROTLI_MAX_NDIRECT 120
67 #define BROTLI_MAX_DISTANCE_BITS 24U
68 #define BROTLI_DISTANCE_ALPHABET_SIZE(NPOSTFIX, NDIRECT, MAXNBITS) ( \
69 BROTLI_NUM_DISTANCE_SHORT_CODES + (NDIRECT) + \
70 ((MAXNBITS) << ((NPOSTFIX) + 1)))
71 /* BROTLI_NUM_DISTANCE_SYMBOLS == 1128 */
72 #define BROTLI_NUM_DISTANCE_SYMBOLS \
73 BROTLI_DISTANCE_ALPHABET_SIZE( \
74 BROTLI_MAX_NDIRECT, BROTLI_MAX_NPOSTFIX, BROTLI_LARGE_MAX_DISTANCE_BITS)
75
76 /* ((1 << 26) - 4) is the maximal distance that can be expressed in RFC 7932
77 brotli stream using NPOSTFIX = 0 and NDIRECT = 0. With other NPOSTFIX and
78 NDIRECT values distances up to ((1 << 29) + 88) could be expressed. */
79 #define BROTLI_MAX_DISTANCE 0x3FFFFFC
80
81 /* ((1 << 31) - 4) is the safe distance limit. Using this number as a limit
82 allows safe distance calculation without overflows, given the distance
83 alphabet size is limited to corresponding size
84 (see kLargeWindowDistanceCodeLimits). */
85 #define BROTLI_MAX_ALLOWED_DISTANCE 0x7FFFFFFC
86
87 /* 7.1. Context modes and context ID lookup for literals */
88 /* "context IDs for literals are in the range of 0..63" */
89 #define BROTLI_LITERAL_CONTEXT_BITS 6
90
91 /* 7.2. Context ID for distances */
92 #define BROTLI_DISTANCE_CONTEXT_BITS 2
93
94 /* 9.1. Format of the Stream Header */
95 /* Number of slack bytes for window size. Don't confuse
96 with BROTLI_NUM_DISTANCE_SHORT_CODES. */
97 #define BROTLI_WINDOW_GAP 16
98 #define BROTLI_MAX_BACKWARD_LIMIT(W) (((size_t)1 << (W)) - BROTLI_WINDOW_GAP)
99
100 typedef struct BrotliDistanceCodeLimit {
101 uint32_t max_alphabet_size;
102 uint32_t max_distance;
103 } BrotliDistanceCodeLimit;
104
105 /* This function calculates maximal size of distance alphabet, such that the
106 distances greater than the given values can not be represented.
107
108 This limits are designed to support fast and safe 32-bit decoders.
109 "32-bit" means that signed integer values up to ((1 << 31) - 1) could be
110 safely expressed.
111
112 Brotli distance alphabet symbols do not represent consecutive distance
113 ranges. Each distance alphabet symbol (excluding direct distances and short
114 codes), represent interleaved (for NPOSTFIX > 0) range of distances.
115 A "group" of consecutive (1 << NPOSTFIX) symbols represent non-interleaved
116 range. Two consecutive groups require the same amount of "extra bits".
117
118 It is important that distance alphabet represents complete "groups".
119 To avoid complex logic on encoder side about interleaved ranges
120 it was decided to restrict both sides to complete distance code "groups".
121 */
BrotliCalculateDistanceCodeLimit(uint32_t max_distance,uint32_t npostfix,uint32_t ndirect)122 BROTLI_UNUSED_FUNCTION BrotliDistanceCodeLimit BrotliCalculateDistanceCodeLimit(
123 uint32_t max_distance, uint32_t npostfix, uint32_t ndirect) {
124 BrotliDistanceCodeLimit result;
125 /* Marking this function as unused, because not all files
126 including "constants.h" use it -> compiler warns about that. */
127 BROTLI_UNUSED(&BrotliCalculateDistanceCodeLimit);
128 if (max_distance <= ndirect) {
129 /* This case never happens / exists only for the sake of completeness. */
130 result.max_alphabet_size = max_distance + BROTLI_NUM_DISTANCE_SHORT_CODES;
131 result.max_distance = max_distance;
132 return result;
133 } else {
134 /* The first prohibited value. */
135 uint32_t forbidden_distance = max_distance + 1;
136 /* Subtract "directly" encoded region. */
137 uint32_t offset = forbidden_distance - ndirect - 1;
138 uint32_t ndistbits = 0;
139 uint32_t tmp;
140 uint32_t half;
141 uint32_t group;
142 /* Postfix for the last dcode in the group. */
143 uint32_t postfix = (1u << npostfix) - 1;
144 uint32_t extra;
145 uint32_t start;
146 /* Remove postfix and "head-start". */
147 offset = (offset >> npostfix) + 4;
148 /* Calculate the number of distance bits. */
149 tmp = offset / 2;
150 /* Poor-man's log2floor, to avoid extra dependencies. */
151 while (tmp != 0) {ndistbits++; tmp = tmp >> 1;}
152 /* One bit is covered with subrange addressing ("half"). */
153 ndistbits--;
154 /* Find subrange. */
155 half = (offset >> ndistbits) & 1;
156 /* Calculate the "group" part of dcode. */
157 group = ((ndistbits - 1) << 1) | half;
158 /* Calculated "group" covers the prohibited distance value. */
159 if (group == 0) {
160 /* This case is added for correctness; does not occur for limit > 128. */
161 result.max_alphabet_size = ndirect + BROTLI_NUM_DISTANCE_SHORT_CODES;
162 result.max_distance = ndirect;
163 return result;
164 }
165 /* Decrement "group", so it is the last permitted "group". */
166 group--;
167 /* After group was decremented, ndistbits and half must be recalculated. */
168 ndistbits = (group >> 1) + 1;
169 /* The last available distance in the subrange has all extra bits set. */
170 extra = (1u << ndistbits) - 1;
171 /* Calculate region start. NB: ndistbits >= 1. */
172 start = (1u << (ndistbits + 1)) - 4;
173 /* Move to subregion. */
174 start += (group & 1) << ndistbits;
175 /* Calculate the alphabet size. */
176 result.max_alphabet_size = ((group << npostfix) | postfix) + ndirect +
177 BROTLI_NUM_DISTANCE_SHORT_CODES + 1;
178 /* Calculate the maximal distance representable by alphabet. */
179 result.max_distance = ((start + extra) << npostfix) + postfix + ndirect + 1;
180 return result;
181 }
182 }
183
184 #endif /* BROTLI_COMMON_CONSTANTS_H_ */
185