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-rw-r--r--modules/brotli/common/constants.h136
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_ */