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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
*/
/* Functions to map previous bytes into a context id. */
#ifndef BROTLI_ENC_CONTEXT_H_
#define BROTLI_ENC_CONTEXT_H_
#include "../common/types.h"
#include "../common/port.h"
#if defined(__cplusplus) || defined(c_plusplus)
extern "C" {
#endif
/* Second-order context lookup table for UTF8 byte streams.
If p1 and p2 are the previous two bytes, we calculate 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 uint8_t 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,
};
typedef enum ContextType {
CONTEXT_LSB6 = 0,
CONTEXT_MSB6 = 1,
CONTEXT_UTF8 = 2,
CONTEXT_SIGNED = 3
} ContextType;
static BROTLI_INLINE uint8_t Context(uint8_t p1, uint8_t p2, ContextType mode) {
switch (mode) {
case CONTEXT_LSB6:
return p1 & 0x3f;
case CONTEXT_MSB6:
return (uint8_t)(p1 >> 2);
case CONTEXT_UTF8:
return kUTF8ContextLookup[p1] | kUTF8ContextLookup[p2 + 256];
case CONTEXT_SIGNED:
return (uint8_t)((kSigned3BitContextLookup[p1] << 3) +
kSigned3BitContextLookup[p2]);
default:
return 0;
}
}
#if defined(__cplusplus) || defined(c_plusplus)
} /* extern "C" */
#endif
#endif /* BROTLI_ENC_CONTEXT_H_ */
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