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| -rw-r--r-- | modules/brotli/common/context.h | 113 |
1 files changed, 113 insertions, 0 deletions
diff --git a/modules/brotli/common/context.h b/modules/brotli/common/context.h new file mode 100644 index 0000000000..685a279dc0 --- /dev/null +++ b/modules/brotli/common/context.h @@ -0,0 +1,113 @@ +/* Copyright 2013 Google Inc. All Rights Reserved. + + Distributed under MIT license. + See file LICENSE for detail or copy at https://opensource.org/licenses/MIT +*/ + +/* Lookup table to map the previous two bytes to a context id. + + There are four different context modeling modes defined here: + CONTEXT_LSB6: context id is the least significant 6 bits of the last byte, + CONTEXT_MSB6: context id is the most significant 6 bits of the last byte, + CONTEXT_UTF8: second-order context model tuned for UTF8-encoded text, + CONTEXT_SIGNED: second-order context model tuned for signed integers. + + If |p1| and |p2| are the previous two bytes, and |mode| is current context + mode, we calculate the context as: + + context = ContextLut(mode)[p1] | ContextLut(mode)[p2 + 256]. + + For CONTEXT_UTF8 mode, if the previous two bytes are ASCII characters + (i.e. < 128), this will be equivalent to + + context = 4 * context1(p1) + context2(p2), + + where context1 is based on the previous byte in the following way: + + 0 : non-ASCII control + 1 : \t, \n, \r + 2 : space + 3 : other punctuation + 4 : " ' + 5 : % + 6 : ( < [ { + 7 : ) > ] } + 8 : , ; : + 9 : . + 10 : = + 11 : number + 12 : upper-case vowel + 13 : upper-case consonant + 14 : lower-case vowel + 15 : lower-case consonant + + and context2 is based on the second last byte: + + 0 : control, space + 1 : punctuation + 2 : upper-case letter, number + 3 : lower-case letter + + If the last byte is ASCII, and the second last byte is not (in a valid UTF8 + stream it will be a continuation byte, value between 128 and 191), the + context is the same as if the second last byte was an ASCII control or space. + + If the last byte is a UTF8 lead byte (value >= 192), then the next byte will + be a continuation byte and the context id is 2 or 3 depending on the LSB of + the last byte and to a lesser extent on the second last byte if it is ASCII. + + If the last byte is a UTF8 continuation byte, the second last byte can be: + - continuation byte: the next byte is probably ASCII or lead byte (assuming + 4-byte UTF8 characters are rare) and the context id is 0 or 1. + - lead byte (192 - 207): next byte is ASCII or lead byte, context is 0 or 1 + - lead byte (208 - 255): next byte is continuation byte, context is 2 or 3 + + The possible value combinations of the previous two bytes, the range of + context ids and the type of the next byte is summarized in the table below: + + |--------\-----------------------------------------------------------------| + | \ Last byte | + | Second \---------------------------------------------------------------| + | last byte \ ASCII | cont. byte | lead byte | + | \ (0-127) | (128-191) | (192-) | + |=============|===================|=====================|==================| + | ASCII | next: ASCII/lead | not valid | next: cont. | + | (0-127) | context: 4 - 63 | | context: 2 - 3 | + |-------------|-------------------|---------------------|------------------| + | cont. byte | next: ASCII/lead | next: ASCII/lead | next: cont. | + | (128-191) | context: 4 - 63 | context: 0 - 1 | context: 2 - 3 | + |-------------|-------------------|---------------------|------------------| + | lead byte | not valid | next: ASCII/lead | not valid | + | (192-207) | | context: 0 - 1 | | + |-------------|-------------------|---------------------|------------------| + | lead byte | not valid | next: cont. | not valid | + | (208-) | | context: 2 - 3 | | + |-------------|-------------------|---------------------|------------------| +*/ + +#ifndef BROTLI_COMMON_CONTEXT_H_ +#define BROTLI_COMMON_CONTEXT_H_ + +#include <brotli/port.h> +#include <brotli/types.h> + +typedef enum ContextType { + CONTEXT_LSB6 = 0, + CONTEXT_MSB6 = 1, + CONTEXT_UTF8 = 2, + CONTEXT_SIGNED = 3 +} ContextType; + +/* "Soft-private", it is exported, but not "advertised" as API. */ +/* Common context lookup table for all context modes. */ +BROTLI_COMMON_API extern const uint8_t _kBrotliContextLookupTable[2048]; + +typedef const uint8_t* ContextLut; + +/* typeof(MODE) == ContextType; returns ContextLut */ +#define BROTLI_CONTEXT_LUT(MODE) (&_kBrotliContextLookupTable[(MODE) << 9]) + +/* typeof(LUT) == ContextLut */ +#define BROTLI_CONTEXT(P1, P2, LUT) ((LUT)[P1] | ((LUT) + 256)[P2]) + +#endif /* BROTLI_COMMON_CONTEXT_H_ */ |