| // Copyright 2013 Google Inc. All Rights Reserved. |
| // |
| // Licensed under the Apache License, Version 2.0 (the "License"); |
| // you may not use this file except in compliance with the License. |
| // You may obtain a copy of the License at |
| // |
| // http://www.apache.org/licenses/LICENSE-2.0 |
| // |
| // Unless required by applicable law or agreed to in writing, software |
| // distributed under the License is distributed on an "AS IS" BASIS, |
| // WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. |
| // See the License for the specific language governing permissions and |
| // limitations under the License. |
| // |
| // Functions to map previous bytes into a context id. |
| |
| #ifndef BROTLI_ENC_CONTEXT_H_ |
| #define BROTLI_ENC_CONTEXT_H_ |
| |
| #include <stdint.h> |
| |
| namespace brotli { |
| |
| // Second-order context lookup table for UTF8 byte streams. |
| // |
| // If p1 and p2 are the previous two bytes, we calcualte the context as |
| // |
| // context = kUTF8ContextLookup[p1] | kUTF8ContextLookup[p2 + 256]. |
| // |
| // 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 | | |
| // |-------------|-------------------|---------------------|------------------| |
| static const uint8_t kUTF8ContextLookup[512] = { |
| // Last byte. |
| // |
| // ASCII range. |
| 0, 0, 0, 0, 0, 0, 0, 0, 0, 4, 4, 0, 0, 4, 0, 0, |
| 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, |
| 8, 12, 16, 12, 12, 20, 12, 16, 24, 28, 12, 12, 32, 12, 36, 12, |
| 44, 44, 44, 44, 44, 44, 44, 44, 44, 44, 32, 32, 24, 40, 28, 12, |
| 12, 48, 52, 52, 52, 48, 52, 52, 52, 48, 52, 52, 52, 52, 52, 48, |
| 52, 52, 52, 52, 52, 48, 52, 52, 52, 52, 52, 24, 12, 28, 12, 12, |
| 12, 56, 60, 60, 60, 56, 60, 60, 60, 56, 60, 60, 60, 60, 60, 56, |
| 60, 60, 60, 60, 60, 56, 60, 60, 60, 60, 60, 24, 12, 28, 12, 0, |
| // UTF8 continuation byte range. |
| 0, 1, 0, 1, 0, 1, 0, 1, 0, 1, 0, 1, 0, 1, 0, 1, |
| 0, 1, 0, 1, 0, 1, 0, 1, 0, 1, 0, 1, 0, 1, 0, 1, |
| 0, 1, 0, 1, 0, 1, 0, 1, 0, 1, 0, 1, 0, 1, 0, 1, |
| 0, 1, 0, 1, 0, 1, 0, 1, 0, 1, 0, 1, 0, 1, 0, 1, |
| // UTF8 lead byte range. |
| 2, 3, 2, 3, 2, 3, 2, 3, 2, 3, 2, 3, 2, 3, 2, 3, |
| 2, 3, 2, 3, 2, 3, 2, 3, 2, 3, 2, 3, 2, 3, 2, 3, |
| 2, 3, 2, 3, 2, 3, 2, 3, 2, 3, 2, 3, 2, 3, 2, 3, |
| 2, 3, 2, 3, 2, 3, 2, 3, 2, 3, 2, 3, 2, 3, 2, 3, |
| // Second last byte. |
| // |
| // ASCII range. |
| 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, |
| 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, |
| 0, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, |
| 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 1, 1, 1, 1, 1, 1, |
| 1, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, |
| 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 1, 1, 1, 1, 1, |
| 1, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, |
| 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 1, 1, 1, 1, 0, |
| // UTF8 continuation byte range. |
| 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, |
| 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, |
| 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, |
| 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, |
| 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, |
| // UTF8 lead byte range. |
| 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, |
| 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, |
| 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, |
| }; |
| |
| // Context lookup table for small signed integers. |
| static const int kSigned3BitContextLookup[] = { |
| 0, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, |
| 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, |
| 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, |
| 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, |
| 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, |
| 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, |
| 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, |
| 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, |
| 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, |
| 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, |
| 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, |
| 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, |
| 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, |
| 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, |
| 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, |
| 6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 7, |
| }; |
| |
| enum ContextType { |
| CONTEXT_LSB6 = 0, |
| CONTEXT_MSB6 = 1, |
| CONTEXT_UTF8 = 2, |
| CONTEXT_SIGNED = 3 |
| }; |
| |
| static inline uint8_t Context(uint8_t p1, uint8_t p2, int mode) { |
| switch (mode) { |
| case CONTEXT_LSB6: |
| return p1 & 0x3f; |
| case CONTEXT_MSB6: |
| return p1 >> 2; |
| case CONTEXT_UTF8: |
| return kUTF8ContextLookup[p1] | kUTF8ContextLookup[p2 + 256]; |
| case CONTEXT_SIGNED: |
| return (kSigned3BitContextLookup[p1] << 3) + kSigned3BitContextLookup[p2]; |
| default: |
| return 0; |
| } |
| } |
| |
| } // namespace brotli |
| |
| #endif // BROTLI_ENC_CONTEXT_H_ |