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/port.h>
17 #include <brotli/types.h>
18
19 /* Specification: 7.3. Encoding of the context map */
20 #define BROTLI_CONTEXT_MAP_MAX_RLE 16
21
22 /* Specification: 2. Compressed representation overview */
23 #define BROTLI_MAX_NUMBER_OF_BLOCK_TYPES 256
24
25 /* Specification: 3.3. Alphabet sizes: insert-and-copy length */
26 #define BROTLI_NUM_LITERAL_SYMBOLS 256
27 #define BROTLI_NUM_COMMAND_SYMBOLS 704
28 #define BROTLI_NUM_BLOCK_LEN_SYMBOLS 26
29 #define BROTLI_MAX_CONTEXT_MAP_SYMBOLS (BROTLI_MAX_NUMBER_OF_BLOCK_TYPES + \
30 BROTLI_CONTEXT_MAP_MAX_RLE)
31 #define BROTLI_MAX_BLOCK_TYPE_SYMBOLS (BROTLI_MAX_NUMBER_OF_BLOCK_TYPES + 2)
32
33 /* Specification: 3.5. Complex prefix codes */
34 #define BROTLI_REPEAT_PREVIOUS_CODE_LENGTH 16
35 #define BROTLI_REPEAT_ZERO_CODE_LENGTH 17
36 #define BROTLI_CODE_LENGTH_CODES (BROTLI_REPEAT_ZERO_CODE_LENGTH + 1)
37 /* "code length of 8 is repeated" */
38 #define BROTLI_INITIAL_REPEATED_CODE_LENGTH 8
39
40 /* "Large Window Brotli" */
41
42 /**
43 * The theoretical maximum number of distance bits specified for large window
44 * brotli, for 64-bit encoders and decoders. Even when in practice 32-bit
45 * encoders and decoders only support up to 30 max distance bits, the value is
46 * set to 62 because it affects the large window brotli file format.
47 * Specifically, it affects the encoding of simple huffman tree for distances,
48 * see Specification RFC 7932 chapter 3.4.
49 */
50 #define BROTLI_LARGE_MAX_DISTANCE_BITS 62U
51 #define BROTLI_LARGE_MIN_WBITS 10
52 /**
53 * The maximum supported large brotli window bits by the encoder and decoder.
54 * Large window brotli allows up to 62 bits, however the current encoder and
55 * decoder, designed for 32-bit integers, only support up to 30 bits maximum.
56 */
57 #define BROTLI_LARGE_MAX_WBITS 30
58
59 /* Specification: 4. Encoding of distances */
60 #define BROTLI_NUM_DISTANCE_SHORT_CODES 16
61 /**
62 * Maximal number of "postfix" bits.
63 *
64 * Number of "postfix" bits is stored as 2 bits in meta-block header.
65 */
66 #define BROTLI_MAX_NPOSTFIX 3
67 #define BROTLI_MAX_NDIRECT 120
68 #define BROTLI_MAX_DISTANCE_BITS 24U
69 #define BROTLI_DISTANCE_ALPHABET_SIZE(NPOSTFIX, NDIRECT, MAXNBITS) ( \
70 BROTLI_NUM_DISTANCE_SHORT_CODES + (NDIRECT) + \
71 ((MAXNBITS) << ((NPOSTFIX) + 1)))
72 /* BROTLI_NUM_DISTANCE_SYMBOLS == 1128 */
73 #define BROTLI_NUM_DISTANCE_SYMBOLS \
74 BROTLI_DISTANCE_ALPHABET_SIZE( \
75 BROTLI_MAX_NDIRECT, BROTLI_MAX_NPOSTFIX, BROTLI_LARGE_MAX_DISTANCE_BITS)
76
77 /* ((1 << 26) - 4) is the maximal distance that can be expressed in RFC 7932
78 brotli stream using NPOSTFIX = 0 and NDIRECT = 0. With other NPOSTFIX and
79 NDIRECT values distances up to ((1 << 29) + 88) could be expressed. */
80 #define BROTLI_MAX_DISTANCE 0x3FFFFFC
81
82 /* ((1 << 31) - 4) is the safe distance limit. Using this number as a limit
83 allows safe distance calculation without overflows, given the distance
84 alphabet size is limited to corresponding size
85 (see kLargeWindowDistanceCodeLimits). */
86 #define BROTLI_MAX_ALLOWED_DISTANCE 0x7FFFFFFC
87
88
89 /* Specification: 4. Encoding of Literal Insertion Lengths and Copy Lengths */
90 #define BROTLI_NUM_INS_COPY_CODES 24
91
92 /* 7.1. Context modes and context ID lookup for literals */
93 /* "context IDs for literals are in the range of 0..63" */
94 #define BROTLI_LITERAL_CONTEXT_BITS 6
95
96 /* 7.2. Context ID for distances */
97 #define BROTLI_DISTANCE_CONTEXT_BITS 2
98
99 /* 9.1. Format of the Stream Header */
100 /* Number of slack bytes for window size. Don't confuse
101 with BROTLI_NUM_DISTANCE_SHORT_CODES. */
102 #define BROTLI_WINDOW_GAP 16
103 #define BROTLI_MAX_BACKWARD_LIMIT(W) (((size_t)1 << (W)) - BROTLI_WINDOW_GAP)
104
105 typedef struct BrotliDistanceCodeLimit {
106 uint32_t max_alphabet_size;
107 uint32_t max_distance;
108 } BrotliDistanceCodeLimit;
109
110 /* This function calculates maximal size of distance alphabet, such that the
111 distances greater than the given values can not be represented.
112
113 This limits are designed to support fast and safe 32-bit decoders.
114 "32-bit" means that signed integer values up to ((1 << 31) - 1) could be
115 safely expressed.
116
117 Brotli distance alphabet symbols do not represent consecutive distance
118 ranges. Each distance alphabet symbol (excluding direct distances and short
119 codes), represent interleaved (for NPOSTFIX > 0) range of distances.
120 A "group" of consecutive (1 << NPOSTFIX) symbols represent non-interleaved
121 range. Two consecutive groups require the same amount of "extra bits".
122
123 It is important that distance alphabet represents complete "groups".
124 To avoid complex logic on encoder side about interleaved ranges
125 it was decided to restrict both sides to complete distance code "groups".
126 */
BrotliCalculateDistanceCodeLimit(uint32_t max_distance,uint32_t npostfix,uint32_t ndirect)127 BROTLI_UNUSED_FUNCTION BrotliDistanceCodeLimit BrotliCalculateDistanceCodeLimit(
128 uint32_t max_distance, uint32_t npostfix, uint32_t ndirect) {
129 BrotliDistanceCodeLimit result;
130 /* Marking this function as unused, because not all files
131 including "constants.h" use it -> compiler warns about that. */
132 BROTLI_UNUSED(&BrotliCalculateDistanceCodeLimit);
133 if (max_distance <= ndirect) {
134 /* This case never happens / exists only for the sake of completeness. */
135 result.max_alphabet_size = max_distance + BROTLI_NUM_DISTANCE_SHORT_CODES;
136 result.max_distance = max_distance;
137 return result;
138 } else {
139 /* The first prohibited value. */
140 uint32_t forbidden_distance = max_distance + 1;
141 /* Subtract "directly" encoded region. */
142 uint32_t offset = forbidden_distance - ndirect - 1;
143 uint32_t ndistbits = 0;
144 uint32_t tmp;
145 uint32_t half;
146 uint32_t group;
147 /* Postfix for the last dcode in the group. */
148 uint32_t postfix = (1u << npostfix) - 1;
149 uint32_t extra;
150 uint32_t start;
151 /* Remove postfix and "head-start". */
152 offset = (offset >> npostfix) + 4;
153 /* Calculate the number of distance bits. */
154 tmp = offset / 2;
155 /* Poor-man's log2floor, to avoid extra dependencies. */
156 while (tmp != 0) {ndistbits++; tmp = tmp >> 1;}
157 /* One bit is covered with subrange addressing ("half"). */
158 ndistbits--;
159 /* Find subrange. */
160 half = (offset >> ndistbits) & 1;
161 /* Calculate the "group" part of dcode. */
162 group = ((ndistbits - 1) << 1) | half;
163 /* Calculated "group" covers the prohibited distance value. */
164 if (group == 0) {
165 /* This case is added for correctness; does not occur for limit > 128. */
166 result.max_alphabet_size = ndirect + BROTLI_NUM_DISTANCE_SHORT_CODES;
167 result.max_distance = ndirect;
168 return result;
169 }
170 /* Decrement "group", so it is the last permitted "group". */
171 group--;
172 /* After group was decremented, ndistbits and half must be recalculated. */
173 ndistbits = (group >> 1) + 1;
174 /* The last available distance in the subrange has all extra bits set. */
175 extra = (1u << ndistbits) - 1;
176 /* Calculate region start. NB: ndistbits >= 1. */
177 start = (1u << (ndistbits + 1)) - 4;
178 /* Move to subregion. */
179 start += (group & 1) << ndistbits;
180 /* Calculate the alphabet size. */
181 result.max_alphabet_size = ((group << npostfix) | postfix) + ndirect +
182 BROTLI_NUM_DISTANCE_SHORT_CODES + 1;
183 /* Calculate the maximal distance representable by alphabet. */
184 result.max_distance = ((start + extra) << npostfix) + postfix + ndirect + 1;
185 return result;
186 }
187 }
188
189 /* Represents the range of values belonging to a prefix code:
190 [offset, offset + 2^nbits) */
191 typedef struct {
192 uint16_t offset;
193 uint8_t nbits;
194 } BrotliPrefixCodeRange;
195
196 /* "Soft-private", it is exported, but not "advertised" as API. */
197 BROTLI_COMMON_API extern const BrotliPrefixCodeRange
198 _kBrotliPrefixCodeRanges[BROTLI_NUM_BLOCK_LEN_SYMBOLS];
199
200 #endif /* BROTLI_COMMON_CONSTANTS_H_ */
201