1 /* Copyright 2013 Google Inc. All Rights Reserved. 2 3 Distributed under MIT license. 4 See file LICENSE for detail or copy at https://opensource.org/licenses/MIT 5 */ 6 7 /* Lookup table to map the previous two bytes to a context id. 8 9 There are four different context modeling modes defined here: 10 CONTEXT_LSB6: context id is the least significant 6 bits of the last byte, 11 CONTEXT_MSB6: context id is the most significant 6 bits of the last byte, 12 CONTEXT_UTF8: second-order context model tuned for UTF8-encoded text, 13 CONTEXT_SIGNED: second-order context model tuned for signed integers. 14 15 If |p1| and |p2| are the previous two bytes, and |mode| is current context 16 mode, we calculate the context as: 17 18 context = ContextLut(mode)[p1] | ContextLut(mode)[p2 + 256]. 19 20 For CONTEXT_UTF8 mode, if the previous two bytes are ASCII characters 21 (i.e. < 128), this will be equivalent to 22 23 context = 4 * context1(p1) + context2(p2), 24 25 where context1 is based on the previous byte in the following way: 26 27 0 : non-ASCII control 28 1 : \t, \n, \r 29 2 : space 30 3 : other punctuation 31 4 : " ' 32 5 : % 33 6 : ( < [ { 34 7 : ) > ] } 35 8 : , ; : 36 9 : . 37 10 : = 38 11 : number 39 12 : upper-case vowel 40 13 : upper-case consonant 41 14 : lower-case vowel 42 15 : lower-case consonant 43 44 and context2 is based on the second last byte: 45 46 0 : control, space 47 1 : punctuation 48 2 : upper-case letter, number 49 3 : lower-case letter 50 51 If the last byte is ASCII, and the second last byte is not (in a valid UTF8 52 stream it will be a continuation byte, value between 128 and 191), the 53 context is the same as if the second last byte was an ASCII control or space. 54 55 If the last byte is a UTF8 lead byte (value >= 192), then the next byte will 56 be a continuation byte and the context id is 2 or 3 depending on the LSB of 57 the last byte and to a lesser extent on the second last byte if it is ASCII. 58 59 If the last byte is a UTF8 continuation byte, the second last byte can be: 60 - continuation byte: the next byte is probably ASCII or lead byte (assuming 61 4-byte UTF8 characters are rare) and the context id is 0 or 1. 62 - lead byte (192 - 207): next byte is ASCII or lead byte, context is 0 or 1 63 - lead byte (208 - 255): next byte is continuation byte, context is 2 or 3 64 65 The possible value combinations of the previous two bytes, the range of 66 context ids and the type of the next byte is summarized in the table below: 67 68 |--------\-----------------------------------------------------------------| 69 | \ Last byte | 70 | Second \---------------------------------------------------------------| 71 | last byte \ ASCII | cont. byte | lead byte | 72 | \ (0-127) | (128-191) | (192-) | 73 |=============|===================|=====================|==================| 74 | ASCII | next: ASCII/lead | not valid | next: cont. | 75 | (0-127) | context: 4 - 63 | | context: 2 - 3 | 76 |-------------|-------------------|---------------------|------------------| 77 | cont. byte | next: ASCII/lead | next: ASCII/lead | next: cont. | 78 | (128-191) | context: 4 - 63 | context: 0 - 1 | context: 2 - 3 | 79 |-------------|-------------------|---------------------|------------------| 80 | lead byte | not valid | next: ASCII/lead | not valid | 81 | (192-207) | | context: 0 - 1 | | 82 |-------------|-------------------|---------------------|------------------| 83 | lead byte | not valid | next: cont. | not valid | 84 | (208-) | | context: 2 - 3 | | 85 |-------------|-------------------|---------------------|------------------| 86 */ 87 88 #ifndef BROTLI_COMMON_CONTEXT_H_ 89 #define BROTLI_COMMON_CONTEXT_H_ 90 91 #include <brotli/port.h> 92 #include <brotli/types.h> 93 94 typedef enum ContextType { 95 CONTEXT_LSB6 = 0, 96 CONTEXT_MSB6 = 1, 97 CONTEXT_UTF8 = 2, 98 CONTEXT_SIGNED = 3 99 } ContextType; 100 101 /* "Soft-private", it is exported, but not "advertised" as API. */ 102 /* Common context lookup table for all context modes. */ 103 BROTLI_COMMON_API extern const uint8_t _kBrotliContextLookupTable[2048]; 104 105 typedef const uint8_t* ContextLut; 106 107 /* typeof(MODE) == ContextType; returns ContextLut */ 108 #define BROTLI_CONTEXT_LUT(MODE) (&_kBrotliContextLookupTable[(MODE) << 9]) 109 110 /* typeof(LUT) == ContextLut */ 111 #define BROTLI_CONTEXT(P1, P2, LUT) ((LUT)[P1] | ((LUT) + 256)[P2]) 112 113 #endif /* BROTLI_COMMON_CONTEXT_H_ */ 114