mirror of
https://github.com/google/brotli.git
synced 2024-11-22 19:50:06 +00:00
179 lines
7.1 KiB
C++
179 lines
7.1 KiB
C++
/* 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 "./types.h"
|
|
|
|
namespace brotli {
|
|
|
|
// 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,
|
|
};
|
|
|
|
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, ContextType mode) {
|
|
switch (mode) {
|
|
case CONTEXT_LSB6:
|
|
return p1 & 0x3f;
|
|
case CONTEXT_MSB6:
|
|
return static_cast<uint8_t>(p1 >> 2);
|
|
case CONTEXT_UTF8:
|
|
return kUTF8ContextLookup[p1] | kUTF8ContextLookup[p2 + 256];
|
|
case CONTEXT_SIGNED:
|
|
return static_cast<uint8_t>((kSigned3BitContextLookup[p1] << 3) +
|
|
kSigned3BitContextLookup[p2]);
|
|
default:
|
|
return 0;
|
|
}
|
|
}
|
|
|
|
} // namespace brotli
|
|
|
|
#endif // BROTLI_ENC_CONTEXT_H_
|