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Diffstat (limited to 'modules/brotli/common/constants.h')
| -rw-r--r-- | modules/brotli/common/constants.h | 136 |
1 files changed, 136 insertions, 0 deletions
diff --git a/modules/brotli/common/constants.h b/modules/brotli/common/constants.h index d1b88d12a..e848195a0 100644 --- a/modules/brotli/common/constants.h +++ b/modules/brotli/common/constants.h @@ -4,9 +4,18 @@ 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 @@ -29,12 +38,31 @@ #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 @@ -45,9 +73,22 @@ #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 @@ -61,4 +102,99 @@ #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_ */ |