/* 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> 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]; ContextLut; /* typeof(MODE) == ContextType; returns ContextLut */ #define BROTLI_CONTEXT_LUT(MODE) … /* typeof(LUT) == ContextLut */ #define BROTLI_CONTEXT(P1, P2, LUT) … #endif /* BROTLI_COMMON_CONTEXT_H_ */
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