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 <brotli/port.h>
16 #include <brotli/types.h>
17
18 #include "platform.h"
19
20 /* Specification: 7.3. Encoding of the context map */
21 #define BROTLI_CONTEXT_MAP_MAX_RLE 16
22
23 /* Specification: 2. Compressed representation overview */
24 #define BROTLI_MAX_NUMBER_OF_BLOCK_TYPES 256
25
26 /* Specification: 3.3. Alphabet sizes: insert-and-copy length */
27 #define BROTLI_NUM_LITERAL_SYMBOLS 256
28 #define BROTLI_NUM_COMMAND_SYMBOLS 704
29 #define BROTLI_NUM_BLOCK_LEN_SYMBOLS 26
30 #define BROTLI_MAX_CONTEXT_MAP_SYMBOLS (BROTLI_MAX_NUMBER_OF_BLOCK_TYPES + \
31 BROTLI_CONTEXT_MAP_MAX_RLE)
32 #define BROTLI_MAX_BLOCK_TYPE_SYMBOLS (BROTLI_MAX_NUMBER_OF_BLOCK_TYPES + 2)
33
34 /* Specification: 3.5. Complex prefix codes */
35 #define BROTLI_REPEAT_PREVIOUS_CODE_LENGTH 16
36 #define BROTLI_REPEAT_ZERO_CODE_LENGTH 17
37 #define BROTLI_CODE_LENGTH_CODES (BROTLI_REPEAT_ZERO_CODE_LENGTH + 1)
38 /* "code length of 8 is repeated" */
39 #define BROTLI_INITIAL_REPEATED_CODE_LENGTH 8
40
41 /* "Large Window Brotli" */
42
43 /**
44 * The theoretical maximum number of distance bits specified for large window
45 * brotli, for 64-bit encoders and decoders. Even when in practice 32-bit
46 * encoders and decoders only support up to 30 max distance bits, the value is
47 * set to 62 because it affects the large window brotli file format.
48 * Specifically, it affects the encoding of simple huffman tree for distances,
49 * see Specification RFC 7932 chapter 3.4.
50 */
51 #define BROTLI_LARGE_MAX_DISTANCE_BITS 62U
52 #define BROTLI_LARGE_MIN_WBITS 10
53 /**
54 * The maximum supported large brotli window bits by the encoder and decoder.
55 * Large window brotli allows up to 62 bits, however the current encoder and
56 * decoder, designed for 32-bit integers, only support up to 30 bits maximum.
57 */
58 #define BROTLI_LARGE_MAX_WBITS 30
59
60 /* Specification: 4. Encoding of distances */
61 #define BROTLI_NUM_DISTANCE_SHORT_CODES 16
62 /**
63 * Maximal number of "postfix" bits.
64 *
65 * Number of "postfix" bits is stored as 2 bits in meta-block header.
66 */
67 #define BROTLI_MAX_NPOSTFIX 3
68 #define BROTLI_MAX_NDIRECT 120
69 #define BROTLI_MAX_DISTANCE_BITS 24U
70 #define BROTLI_DISTANCE_ALPHABET_SIZE(NPOSTFIX, NDIRECT, MAXNBITS) ( \
71 BROTLI_NUM_DISTANCE_SHORT_CODES + (NDIRECT) + \
72 ((MAXNBITS) << ((NPOSTFIX) + 1)))
73 /* BROTLI_NUM_DISTANCE_SYMBOLS == 1128 */
74 #define BROTLI_NUM_DISTANCE_SYMBOLS \
75 BROTLI_DISTANCE_ALPHABET_SIZE( \
76 BROTLI_MAX_NDIRECT, BROTLI_MAX_NPOSTFIX, BROTLI_LARGE_MAX_DISTANCE_BITS)
77
78 /* ((1 << 26) - 4) is the maximal distance that can be expressed in RFC 7932
79 brotli stream using NPOSTFIX = 0 and NDIRECT = 0. With other NPOSTFIX and
80 NDIRECT values distances up to ((1 << 29) + 88) could be expressed. */
81 #define BROTLI_MAX_DISTANCE 0x3FFFFFC
82
83 /* ((1 << 31) - 4) is the safe distance limit. Using this number as a limit
84 allows safe distance calculation without overflows, given the distance
85 alphabet size is limited to corresponding size
86 (see kLargeWindowDistanceCodeLimits). */
87 #define BROTLI_MAX_ALLOWED_DISTANCE 0x7FFFFFFC
88
89
90 /* Specification: 4. Encoding of Literal Insertion Lengths and Copy Lengths */
91 #define BROTLI_NUM_INS_COPY_CODES 24
92
93 /* 7.1. Context modes and context ID lookup for literals */
94 /* "context IDs for literals are in the range of 0..63" */
95 #define BROTLI_LITERAL_CONTEXT_BITS 6
96
97 /* 7.2. Context ID for distances */
98 #define BROTLI_DISTANCE_CONTEXT_BITS 2
99
100 /* 9.1. Format of the Stream Header */
101 /* Number of slack bytes for window size. Don't confuse
102 with BROTLI_NUM_DISTANCE_SHORT_CODES. */
103 #define BROTLI_WINDOW_GAP 16
104 #define BROTLI_MAX_BACKWARD_LIMIT(W) (((size_t)1 << (W)) - BROTLI_WINDOW_GAP)
105
106 typedef struct BrotliDistanceCodeLimit {
107 uint32_t max_alphabet_size;
108 uint32_t max_distance;
109 } BrotliDistanceCodeLimit;
110
111 /* This function calculates maximal size of distance alphabet, such that the
112 distances greater than the given values can not be represented.
113
114 This limits are designed to support fast and safe 32-bit decoders.
115 "32-bit" means that signed integer values up to ((1 << 31) - 1) could be
116 safely expressed.
117
118 Brotli distance alphabet symbols do not represent consecutive distance
119 ranges. Each distance alphabet symbol (excluding direct distances and short
120 codes), represent interleaved (for NPOSTFIX > 0) range of distances.
121 A "group" of consecutive (1 << NPOSTFIX) symbols represent non-interleaved
122 range. Two consecutive groups require the same amount of "extra bits".
123
124 It is important that distance alphabet represents complete "groups".
125 To avoid complex logic on encoder side about interleaved ranges
126 it was decided to restrict both sides to complete distance code "groups".
127 */
BrotliCalculateDistanceCodeLimit(uint32_t max_distance,uint32_t npostfix,uint32_t ndirect)128 BROTLI_UNUSED_FUNCTION BrotliDistanceCodeLimit BrotliCalculateDistanceCodeLimit(
129 uint32_t max_distance, uint32_t npostfix, uint32_t ndirect) {
130 BrotliDistanceCodeLimit result;
131 /* Marking this function as unused, because not all files
132 including "constants.h" use it -> compiler warns about that. */
133 BROTLI_UNUSED(&BrotliCalculateDistanceCodeLimit);
134 if (max_distance <= ndirect) {
135 /* This case never happens / exists only for the sake of completeness. */
136 result.max_alphabet_size = max_distance + BROTLI_NUM_DISTANCE_SHORT_CODES;
137 result.max_distance = max_distance;
138 return result;
139 } else {
140 /* The first prohibited value. */
141 uint32_t forbidden_distance = max_distance + 1;
142 /* Subtract "directly" encoded region. */
143 uint32_t offset = forbidden_distance - ndirect - 1;
144 uint32_t ndistbits = 0;
145 uint32_t tmp;
146 uint32_t half;
147 uint32_t group;
148 /* Postfix for the last dcode in the group. */
149 uint32_t postfix = (1u << npostfix) - 1;
150 uint32_t extra;
151 uint32_t start;
152 /* Remove postfix and "head-start". */
153 offset = (offset >> npostfix) + 4;
154 /* Calculate the number of distance bits. */
155 tmp = offset / 2;
156 /* Poor-man's log2floor, to avoid extra dependencies. */
157 while (tmp != 0) {ndistbits++; tmp = tmp >> 1;}
158 /* One bit is covered with subrange addressing ("half"). */
159 ndistbits--;
160 /* Find subrange. */
161 half = (offset >> ndistbits) & 1;
162 /* Calculate the "group" part of dcode. */
163 group = ((ndistbits - 1) << 1) | half;
164 /* Calculated "group" covers the prohibited distance value. */
165 if (group == 0) {
166 /* This case is added for correctness; does not occur for limit > 128. */
167 result.max_alphabet_size = ndirect + BROTLI_NUM_DISTANCE_SHORT_CODES;
168 result.max_distance = ndirect;
169 return result;
170 }
171 /* Decrement "group", so it is the last permitted "group". */
172 group--;
173 /* After group was decremented, ndistbits and half must be recalculated. */
174 ndistbits = (group >> 1) + 1;
175 /* The last available distance in the subrange has all extra bits set. */
176 extra = (1u << ndistbits) - 1;
177 /* Calculate region start. NB: ndistbits >= 1. */
178 start = (1u << (ndistbits + 1)) - 4;
179 /* Move to subregion. */
180 start += (group & 1) << ndistbits;
181 /* Calculate the alphabet size. */
182 result.max_alphabet_size = ((group << npostfix) | postfix) + ndirect +
183 BROTLI_NUM_DISTANCE_SHORT_CODES + 1;
184 /* Calculate the maximal distance representable by alphabet. */
185 result.max_distance = ((start + extra) << npostfix) + postfix + ndirect + 1;
186 return result;
187 }
188 }
189
190 /* Represents the range of values belonging to a prefix code:
191 [offset, offset + 2^nbits) */
192 typedef struct {
193 uint16_t offset;
194 uint8_t nbits;
195 } BrotliPrefixCodeRange;
196
197 /* "Soft-private", it is exported, but not "advertised" as API. */
198 BROTLI_COMMON_API extern const BrotliPrefixCodeRange
199 _kBrotliPrefixCodeRanges[BROTLI_NUM_BLOCK_LEN_SYMBOLS];
200
201 #endif /* BROTLI_COMMON_CONSTANTS_H_ */
202