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gitea/vendor/github.com/andybalholm/brotli/context.go

2176 lines
13 KiB
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package brotli
/* Lookup table to map the previous two bytes to a context id.
There are four different context modeling modes defined here:
contextLSB6: context id is the least significant 6 bits of the last byte,
contextMSB6: context id is the most significant 6 bits of the last byte,
contextUTF8: second-order context model tuned for UTF8-encoded text,
contextSigned: 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 contextUTF8 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 | |
|-------------|-------------------|---------------------|------------------|
*/
const (
contextLSB6 = 0
contextMSB6 = 1
contextUTF8 = 2
contextSigned = 3
)
/* Common context lookup table for all context modes. */
var kContextLookup = [2048]byte{
/* CONTEXT_LSB6, last byte. */
0,
1,
2,
3,
4,
5,
6,
7,
8,
9,
10,
11,
12,
13,
14,
15,
16,
17,
18,
19,
20,
21,
22,
23,
24,
25,
26,
27,
28,
29,
30,
31,
32,
33,
34,
35,
36,
37,
38,
39,
40,
41,
42,
43,
44,
45,
46,
47,
48,
49,
50,
51,
52,
53,
54,
55,
56,
57,
58,
59,
60,
61,
62,
63,
0,
1,
2,
3,
4,
5,
6,
7,
8,
9,
10,
11,
12,
13,
14,
15,
16,
17,
18,
19,
20,
21,
22,
23,
24,
25,
26,
27,
28,
29,
30,
31,
32,
33,
34,
35,
36,
37,
38,
39,
40,
41,
42,
43,
44,
45,
46,
47,
48,
49,
50,
51,
52,
53,
54,
55,
56,
57,
58,
59,
60,
61,
62,
63,
0,
1,
2,
3,
4,
5,
6,
7,
8,
9,
10,
11,
12,
13,
14,
15,
16,
17,
18,
19,
20,
21,
22,
23,
24,
25,
26,
27,
28,
29,
30,
31,
32,
33,
34,
35,
36,
37,
38,
39,
40,
41,
42,
43,
44,
45,
46,
47,
48,
49,
50,
51,
52,
53,
54,
55,
56,
57,
58,
59,
60,
61,
62,
63,
0,
1,
2,
3,
4,
5,
6,
7,
8,
9,
10,
11,
12,
13,
14,
15,
16,
17,
18,
19,
20,
21,
22,
23,
24,
25,
26,
27,
28,
29,
30,
31,
32,
33,
34,
35,
36,
37,
38,
39,
40,
41,
42,
43,
44,
45,
46,
47,
48,
49,
50,
51,
52,
53,
54,
55,
56,
57,
58,
59,
60,
61,
62,
63,
/* CONTEXT_LSB6, second last byte, */
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,
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,
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,
0,
0,
0,
0,
0,
0,
0,
0,
0,
0,
0,
0,
0,
0,
0,
0,
/* CONTEXT_MSB6, last byte. */
0,
0,
0,
0,
1,
1,
1,
1,
2,
2,
2,
2,
3,
3,
3,
3,
4,
4,
4,
4,
5,
5,
5,
5,
6,
6,
6,
6,
7,
7,
7,
7,
8,
8,
8,
8,
9,
9,
9,
9,
10,
10,
10,
10,
11,
11,
11,
11,
12,
12,
12,
12,
13,
13,
13,
13,
14,
14,
14,
14,
15,
15,
15,
15,
16,
16,
16,
16,
17,
17,
17,
17,
18,
18,
18,
18,
19,
19,
19,
19,
20,
20,
20,
20,
21,
21,
21,
21,
22,
22,
22,
22,
23,
23,
23,
23,
24,
24,
24,
24,
25,
25,
25,
25,
26,
26,
26,
26,
27,
27,
27,
27,
28,
28,
28,
28,
29,
29,
29,
29,
30,
30,
30,
30,
31,
31,
31,
31,
32,
32,
32,
32,
33,
33,
33,
33,
34,
34,
34,
34,
35,
35,
35,
35,
36,
36,
36,
36,
37,
37,
37,
37,
38,
38,
38,
38,
39,
39,
39,
39,
40,
40,
40,
40,
41,
41,
41,
41,
42,
42,
42,
42,
43,
43,
43,
43,
44,
44,
44,
44,
45,
45,
45,
45,
46,
46,
46,
46,
47,
47,
47,
47,
48,
48,
48,
48,
49,
49,
49,
49,
50,
50,
50,
50,
51,
51,
51,
51,
52,
52,
52,
52,
53,
53,
53,
53,
54,
54,
54,
54,
55,
55,
55,
55,
56,
56,
56,
56,
57,
57,
57,
57,
58,
58,
58,
58,
59,
59,
59,
59,
60,
60,
60,
60,
61,
61,
61,
61,
62,
62,
62,
62,
63,
63,
63,
63,
/* CONTEXT_MSB6, second last byte, */
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,
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,
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,
0,
0,
0,
0,
0,
0,
0,
0,
0,
0,
0,
0,
0,
0,
0,
0,
/* CONTEXT_UTF8, 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,
/* CONTEXT_UTF8 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_SIGNED, last byte, same as the above values shifted by 3 bits. */
0,
8,
8,
8,
8,
8,
8,
8,
8,
8,
8,
8,
8,
8,
8,
8,
16,
16,
16,
16,
16,
16,
16,
16,
16,
16,
16,
16,
16,
16,
16,
16,
16,
16,
16,
16,
16,
16,
16,
16,
16,
16,
16,
16,
16,
16,
16,
16,
16,
16,
16,
16,
16,
16,
16,
16,
16,
16,
16,
16,
16,
16,
16,
16,
24,
24,
24,
24,
24,
24,
24,
24,
24,
24,
24,
24,
24,
24,
24,
24,
24,
24,
24,
24,
24,
24,
24,
24,
24,
24,
24,
24,
24,
24,
24,
24,
24,
24,
24,
24,
24,
24,
24,
24,
24,
24,
24,
24,
24,
24,
24,
24,
24,
24,
24,
24,
24,
24,
24,
24,
24,
24,
24,
24,
24,
24,
24,
24,
32,
32,
32,
32,
32,
32,
32,
32,
32,
32,
32,
32,
32,
32,
32,
32,
32,
32,
32,
32,
32,
32,
32,
32,
32,
32,
32,
32,
32,
32,
32,
32,
32,
32,
32,
32,
32,
32,
32,
32,
32,
32,
32,
32,
32,
32,
32,
32,
32,
32,
32,
32,
32,
32,
32,
32,
32,
32,
32,
32,
32,
32,
32,
32,
40,
40,
40,
40,
40,
40,
40,
40,
40,
40,
40,
40,
40,
40,
40,
40,
40,
40,
40,
40,
40,
40,
40,
40,
40,
40,
40,
40,
40,
40,
40,
40,
40,
40,
40,
40,
40,
40,
40,
40,
40,
40,
40,
40,
40,
40,
40,
40,
48,
48,
48,
48,
48,
48,
48,
48,
48,
48,
48,
48,
48,
48,
48,
56,
/* CONTEXT_SIGNED, second last byte. */
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,
}
type contextLUT []byte
func getContextLUT(mode int) contextLUT {
return kContextLookup[mode<<9:]
}
func getContext(p1 byte, p2 byte, lut contextLUT) byte {
return lut[p1] | lut[256+int(p2)]
}