| /* Copyright 2016 Google Inc. All Rights Reserved. |
| |
| Distributed under MIT license. |
| See file LICENSE for detail or copy at https://opensource.org/licenses/MIT |
| */ |
| |
| /** |
| * @file |
| * Common constants used in decoder and encoder API. |
| */ |
| |
| #ifndef BROTLI_COMMON_CONSTANTS_H_ |
| #define BROTLI_COMMON_CONSTANTS_H_ |
| |
| #include "./platform.h" |
| #include <brotli/port.h> |
| #include <brotli/types.h> |
| |
| /* Specification: 7.3. Encoding of the context map */ |
| #define BROTLI_CONTEXT_MAP_MAX_RLE 16 |
| |
| /* Specification: 2. Compressed representation overview */ |
| #define BROTLI_MAX_NUMBER_OF_BLOCK_TYPES 256 |
| |
| /* Specification: 3.3. Alphabet sizes: insert-and-copy length */ |
| #define BROTLI_NUM_LITERAL_SYMBOLS 256 |
| #define BROTLI_NUM_COMMAND_SYMBOLS 704 |
| #define BROTLI_NUM_BLOCK_LEN_SYMBOLS 26 |
| #define BROTLI_MAX_CONTEXT_MAP_SYMBOLS (BROTLI_MAX_NUMBER_OF_BLOCK_TYPES + \ |
| BROTLI_CONTEXT_MAP_MAX_RLE) |
| #define BROTLI_MAX_BLOCK_TYPE_SYMBOLS (BROTLI_MAX_NUMBER_OF_BLOCK_TYPES + 2) |
| |
| /* Specification: 3.5. Complex prefix codes */ |
| #define BROTLI_REPEAT_PREVIOUS_CODE_LENGTH 16 |
| #define BROTLI_REPEAT_ZERO_CODE_LENGTH 17 |
| #define BROTLI_CODE_LENGTH_CODES (BROTLI_REPEAT_ZERO_CODE_LENGTH + 1) |
| /* "code length of 8 is repeated" */ |
| #define BROTLI_INITIAL_REPEATED_CODE_LENGTH 8 |
| |
| /* "Large Window Brotli" */ |
| |
| /** |
| * The theoretical maximum number of distance bits specified for large window |
| * brotli, for 64-bit encoders and decoders. Even when in practice 32-bit |
| * encoders and decoders only support up to 30 max distance bits, the value is |
| * set to 62 because it affects the large window brotli file format. |
| * Specifically, it affects the encoding of simple huffman tree for distances, |
| * see Specification RFC 7932 chapter 3.4. |
| */ |
| #define BROTLI_LARGE_MAX_DISTANCE_BITS 62U |
| #define BROTLI_LARGE_MIN_WBITS 10 |
| /** |
| * The maximum supported large brotli window bits by the encoder and decoder. |
| * Large window brotli allows up to 62 bits, however the current encoder and |
| * decoder, designed for 32-bit integers, only support up to 30 bits maximum. |
| */ |
| #define BROTLI_LARGE_MAX_WBITS 30 |
| |
| /* Specification: 4. Encoding of distances */ |
| #define BROTLI_NUM_DISTANCE_SHORT_CODES 16 |
| /** |
| * Maximal number of "postfix" bits. |
| * |
| * Number of "postfix" bits is stored as 2 bits in meta-block header. |
| */ |
| #define BROTLI_MAX_NPOSTFIX 3 |
| #define BROTLI_MAX_NDIRECT 120 |
| #define BROTLI_MAX_DISTANCE_BITS 24U |
| #define BROTLI_DISTANCE_ALPHABET_SIZE(NPOSTFIX, NDIRECT, MAXNBITS) ( \ |
| BROTLI_NUM_DISTANCE_SHORT_CODES + (NDIRECT) + \ |
| ((MAXNBITS) << ((NPOSTFIX) + 1))) |
| /* BROTLI_NUM_DISTANCE_SYMBOLS == 1128 */ |
| #define BROTLI_NUM_DISTANCE_SYMBOLS \ |
| BROTLI_DISTANCE_ALPHABET_SIZE( \ |
| BROTLI_MAX_NDIRECT, BROTLI_MAX_NPOSTFIX, BROTLI_LARGE_MAX_DISTANCE_BITS) |
| |
| /* ((1 << 26) - 4) is the maximal distance that can be expressed in RFC 7932 |
| brotli stream using NPOSTFIX = 0 and NDIRECT = 0. With other NPOSTFIX and |
| NDIRECT values distances up to ((1 << 29) + 88) could be expressed. */ |
| #define BROTLI_MAX_DISTANCE 0x3FFFFFC |
| |
| /* ((1 << 31) - 4) is the safe distance limit. Using this number as a limit |
| allows safe distance calculation without overflows, given the distance |
| alphabet size is limited to corresponding size |
| (see kLargeWindowDistanceCodeLimits). */ |
| #define BROTLI_MAX_ALLOWED_DISTANCE 0x7FFFFFFC |
| |
| |
| /* Specification: 4. Encoding of Literal Insertion Lengths and Copy Lengths */ |
| #define BROTLI_NUM_INS_COPY_CODES 24 |
| |
| /* 7.1. Context modes and context ID lookup for literals */ |
| /* "context IDs for literals are in the range of 0..63" */ |
| #define BROTLI_LITERAL_CONTEXT_BITS 6 |
| |
| /* 7.2. Context ID for distances */ |
| #define BROTLI_DISTANCE_CONTEXT_BITS 2 |
| |
| /* 9.1. Format of the Stream Header */ |
| /* Number of slack bytes for window size. Don't confuse |
| with BROTLI_NUM_DISTANCE_SHORT_CODES. */ |
| #define BROTLI_WINDOW_GAP 16 |
| #define BROTLI_MAX_BACKWARD_LIMIT(W) (((size_t)1 << (W)) - BROTLI_WINDOW_GAP) |
| |
| typedef struct BrotliDistanceCodeLimit { |
| uint32_t max_alphabet_size; |
| uint32_t max_distance; |
| } BrotliDistanceCodeLimit; |
| |
| /* This function calculates maximal size of distance alphabet, such that the |
| distances greater than the given values can not be represented. |
| |
| This limits are designed to support fast and safe 32-bit decoders. |
| "32-bit" means that signed integer values up to ((1 << 31) - 1) could be |
| safely expressed. |
| |
| Brotli distance alphabet symbols do not represent consecutive distance |
| ranges. Each distance alphabet symbol (excluding direct distances and short |
| codes), represent interleaved (for NPOSTFIX > 0) range of distances. |
| A "group" of consecutive (1 << NPOSTFIX) symbols represent non-interleaved |
| range. Two consecutive groups require the same amount of "extra bits". |
| |
| It is important that distance alphabet represents complete "groups". |
| To avoid complex logic on encoder side about interleaved ranges |
| it was decided to restrict both sides to complete distance code "groups". |
| */ |
| BROTLI_UNUSED_FUNCTION BrotliDistanceCodeLimit BrotliCalculateDistanceCodeLimit( |
| uint32_t max_distance, uint32_t npostfix, uint32_t ndirect) { |
| BrotliDistanceCodeLimit result; |
| /* Marking this function as unused, because not all files |
| including "constants.h" use it -> compiler warns about that. */ |
| BROTLI_UNUSED(&BrotliCalculateDistanceCodeLimit); |
| if (max_distance <= ndirect) { |
| /* This case never happens / exists only for the sake of completeness. */ |
| result.max_alphabet_size = max_distance + BROTLI_NUM_DISTANCE_SHORT_CODES; |
| result.max_distance = max_distance; |
| return result; |
| } else { |
| /* The first prohibited value. */ |
| uint32_t forbidden_distance = max_distance + 1; |
| /* Subtract "directly" encoded region. */ |
| uint32_t offset = forbidden_distance - ndirect - 1; |
| uint32_t ndistbits = 0; |
| uint32_t tmp; |
| uint32_t half; |
| uint32_t group; |
| /* Postfix for the last dcode in the group. */ |
| uint32_t postfix = (1u << npostfix) - 1; |
| uint32_t extra; |
| uint32_t start; |
| /* Remove postfix and "head-start". */ |
| offset = (offset >> npostfix) + 4; |
| /* Calculate the number of distance bits. */ |
| tmp = offset / 2; |
| /* Poor-man's log2floor, to avoid extra dependencies. */ |
| while (tmp != 0) {ndistbits++; tmp = tmp >> 1;} |
| /* One bit is covered with subrange addressing ("half"). */ |
| ndistbits--; |
| /* Find subrange. */ |
| half = (offset >> ndistbits) & 1; |
| /* Calculate the "group" part of dcode. */ |
| group = ((ndistbits - 1) << 1) | half; |
| /* Calculated "group" covers the prohibited distance value. */ |
| if (group == 0) { |
| /* This case is added for correctness; does not occur for limit > 128. */ |
| result.max_alphabet_size = ndirect + BROTLI_NUM_DISTANCE_SHORT_CODES; |
| result.max_distance = ndirect; |
| return result; |
| } |
| /* Decrement "group", so it is the last permitted "group". */ |
| group--; |
| /* After group was decremented, ndistbits and half must be recalculated. */ |
| ndistbits = (group >> 1) + 1; |
| /* The last available distance in the subrange has all extra bits set. */ |
| extra = (1u << ndistbits) - 1; |
| /* Calculate region start. NB: ndistbits >= 1. */ |
| start = (1u << (ndistbits + 1)) - 4; |
| /* Move to subregion. */ |
| start += (group & 1) << ndistbits; |
| /* Calculate the alphabet size. */ |
| result.max_alphabet_size = ((group << npostfix) | postfix) + ndirect + |
| BROTLI_NUM_DISTANCE_SHORT_CODES + 1; |
| /* Calculate the maximal distance representable by alphabet. */ |
| result.max_distance = ((start + extra) << npostfix) + postfix + ndirect + 1; |
| return result; |
| } |
| } |
| |
| /* Represents the range of values belonging to a prefix code: |
| [offset, offset + 2^nbits) */ |
| typedef struct { |
| uint16_t offset; |
| uint8_t nbits; |
| } BrotliPrefixCodeRange; |
| |
| /* "Soft-private", it is exported, but not "advertised" as API. */ |
| BROTLI_COMMON_API extern const BrotliPrefixCodeRange |
| _kBrotliPrefixCodeRanges[BROTLI_NUM_BLOCK_LEN_SYMBOLS]; |
| |
| #endif /* BROTLI_COMMON_CONSTANTS_H_ */ |