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