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/* 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_ */
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