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context.h
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1 /* Copyright 2013 Google Inc. All Rights Reserved.
2 
3  Distributed under MIT license.
4  See file LICENSE for detail or copy at https://opensource.org/licenses/MIT
5 */
6 
7 /* Lookup table to map the previous two bytes to a context id.
8 
9  There are four different context modeling modes defined here:
10  CONTEXT_LSB6: context id is the least significant 6 bits of the last byte,
11  CONTEXT_MSB6: context id is the most significant 6 bits of the last byte,
12  CONTEXT_UTF8: second-order context model tuned for UTF8-encoded text,
13  CONTEXT_SIGNED: second-order context model tuned for signed integers.
14 
15  The context id for the UTF8 context model is calculated as follows. If p1
16  and p2 are the previous two bytes, we calculate the context as
17 
18  context = kContextLookup[p1] | kContextLookup[p2 + 256].
19 
20  If the previous two bytes are ASCII characters (i.e. < 128), this will be
21  equivalent to
22 
23  context = 4 * context1(p1) + context2(p2),
24 
25  where context1 is based on the previous byte in the following way:
26 
27  0 : non-ASCII control
28  1 : \t, \n, \r
29  2 : space
30  3 : other punctuation
31  4 : " '
32  5 : %
33  6 : ( < [ {
34  7 : ) > ] }
35  8 : , ; :
36  9 : .
37  10 : =
38  11 : number
39  12 : upper-case vowel
40  13 : upper-case consonant
41  14 : lower-case vowel
42  15 : lower-case consonant
43 
44  and context2 is based on the second last byte:
45 
46  0 : control, space
47  1 : punctuation
48  2 : upper-case letter, number
49  3 : lower-case letter
50 
51  If the last byte is ASCII, and the second last byte is not (in a valid UTF8
52  stream it will be a continuation byte, value between 128 and 191), the
53  context is the same as if the second last byte was an ASCII control or space.
54 
55  If the last byte is a UTF8 lead byte (value >= 192), then the next byte will
56  be a continuation byte and the context id is 2 or 3 depending on the LSB of
57  the last byte and to a lesser extent on the second last byte if it is ASCII.
58 
59  If the last byte is a UTF8 continuation byte, the second last byte can be:
60  - continuation byte: the next byte is probably ASCII or lead byte (assuming
61  4-byte UTF8 characters are rare) and the context id is 0 or 1.
62  - lead byte (192 - 207): next byte is ASCII or lead byte, context is 0 or 1
63  - lead byte (208 - 255): next byte is continuation byte, context is 2 or 3
64 
65  The possible value combinations of the previous two bytes, the range of
66  context ids and the type of the next byte is summarized in the table below:
67 
68  |--------\-----------------------------------------------------------------|
69  | \ Last byte |
70  | Second \---------------------------------------------------------------|
71  | last byte \ ASCII | cont. byte | lead byte |
72  | \ (0-127) | (128-191) | (192-) |
73  |=============|===================|=====================|==================|
74  | ASCII | next: ASCII/lead | not valid | next: cont. |
75  | (0-127) | context: 4 - 63 | | context: 2 - 3 |
76  |-------------|-------------------|---------------------|------------------|
77  | cont. byte | next: ASCII/lead | next: ASCII/lead | next: cont. |
78  | (128-191) | context: 4 - 63 | context: 0 - 1 | context: 2 - 3 |
79  |-------------|-------------------|---------------------|------------------|
80  | lead byte | not valid | next: ASCII/lead | not valid |
81  | (192-207) | | context: 0 - 1 | |
82  |-------------|-------------------|---------------------|------------------|
83  | lead byte | not valid | next: cont. | not valid |
84  | (208-) | | context: 2 - 3 | |
85  |-------------|-------------------|---------------------|------------------|
86 
87  The context id for the signed context mode is calculated as:
88 
89  context = (kContextLookup[512 + p1] << 3) | kContextLookup[512 + p2].
90 
91  For any context modeling modes, the context ids can be calculated by |-ing
92  together two lookups from one table using context model dependent offsets:
93 
94  context = kContextLookup[offset1 + p1] | kContextLookup[offset2 + p2].
95 
96  where offset1 and offset2 are dependent on the context mode.
97 */
98 
99 #ifndef BROTLI_DEC_CONTEXT_H_
100 #define BROTLI_DEC_CONTEXT_H_
101 
102 #include "./types.h"
103 
104 enum ContextType {
105  CONTEXT_LSB6 = 0,
106  CONTEXT_MSB6 = 1,
107  CONTEXT_UTF8 = 2,
108  CONTEXT_SIGNED = 3
109 };
110 
111 /* Common context lookup table for all context modes. */
112 static const uint8_t kContextLookup[1792] = {
113  /* CONTEXT_UTF8, last byte. */
114  /* ASCII range. */
115  0, 0, 0, 0, 0, 0, 0, 0, 0, 4, 4, 0, 0, 4, 0, 0,
116  0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
117  8, 12, 16, 12, 12, 20, 12, 16, 24, 28, 12, 12, 32, 12, 36, 12,
118  44, 44, 44, 44, 44, 44, 44, 44, 44, 44, 32, 32, 24, 40, 28, 12,
119  12, 48, 52, 52, 52, 48, 52, 52, 52, 48, 52, 52, 52, 52, 52, 48,
120  52, 52, 52, 52, 52, 48, 52, 52, 52, 52, 52, 24, 12, 28, 12, 12,
121  12, 56, 60, 60, 60, 56, 60, 60, 60, 56, 60, 60, 60, 60, 60, 56,
122  60, 60, 60, 60, 60, 56, 60, 60, 60, 60, 60, 24, 12, 28, 12, 0,
123  /* UTF8 continuation byte range. */
124  0, 1, 0, 1, 0, 1, 0, 1, 0, 1, 0, 1, 0, 1, 0, 1,
125  0, 1, 0, 1, 0, 1, 0, 1, 0, 1, 0, 1, 0, 1, 0, 1,
126  0, 1, 0, 1, 0, 1, 0, 1, 0, 1, 0, 1, 0, 1, 0, 1,
127  0, 1, 0, 1, 0, 1, 0, 1, 0, 1, 0, 1, 0, 1, 0, 1,
128  /* UTF8 lead byte range. */
129  2, 3, 2, 3, 2, 3, 2, 3, 2, 3, 2, 3, 2, 3, 2, 3,
130  2, 3, 2, 3, 2, 3, 2, 3, 2, 3, 2, 3, 2, 3, 2, 3,
131  2, 3, 2, 3, 2, 3, 2, 3, 2, 3, 2, 3, 2, 3, 2, 3,
132  2, 3, 2, 3, 2, 3, 2, 3, 2, 3, 2, 3, 2, 3, 2, 3,
133  /* CONTEXT_UTF8 second last byte. */
134  /* ASCII range. */
135  0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
136  0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
137  0, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1,
138  2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 1, 1, 1, 1, 1, 1,
139  1, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2,
140  2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 1, 1, 1, 1, 1,
141  1, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3,
142  3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 1, 1, 1, 1, 0,
143  /* UTF8 continuation byte range. */
144  0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
145  0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
146  0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
147  0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
148  0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
149  /* UTF8 lead byte range. */
150  0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
151  2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2,
152  2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2,
153  /* CONTEXT_SIGNED, second last byte. */
154  0, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1,
155  2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2,
156  2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2,
157  2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2,
158  3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3,
159  3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3,
160  3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3,
161  3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3,
162  4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4,
163  4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4,
164  4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4,
165  4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4,
166  5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5,
167  5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5,
168  5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5,
169  6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 7,
170  /* CONTEXT_SIGNED, last byte, same as the above values shifted by 3 bits. */
171  0, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8,
172  16, 16, 16, 16, 16, 16, 16, 16, 16, 16, 16, 16, 16, 16, 16, 16,
173  16, 16, 16, 16, 16, 16, 16, 16, 16, 16, 16, 16, 16, 16, 16, 16,
174  16, 16, 16, 16, 16, 16, 16, 16, 16, 16, 16, 16, 16, 16, 16, 16,
175  24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24,
176  24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24,
177  24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24,
178  24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24,
179  32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32,
180  32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32,
181  32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32,
182  32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32,
183  40, 40, 40, 40, 40, 40, 40, 40, 40, 40, 40, 40, 40, 40, 40, 40,
184  40, 40, 40, 40, 40, 40, 40, 40, 40, 40, 40, 40, 40, 40, 40, 40,
185  40, 40, 40, 40, 40, 40, 40, 40, 40, 40, 40, 40, 40, 40, 40, 40,
186  48, 48, 48, 48, 48, 48, 48, 48, 48, 48, 48, 48, 48, 48, 48, 56,
187  /* CONTEXT_LSB6, last byte. */
188  0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15,
189  16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31,
190  32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47,
191  48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62, 63,
192  0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15,
193  16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31,
194  32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47,
195  48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62, 63,
196  0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15,
197  16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31,
198  32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47,
199  48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62, 63,
200  0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15,
201  16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31,
202  32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47,
203  48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62, 63,
204  /* CONTEXT_MSB6, last byte. */
205  0, 0, 0, 0, 1, 1, 1, 1, 2, 2, 2, 2, 3, 3, 3, 3,
206  4, 4, 4, 4, 5, 5, 5, 5, 6, 6, 6, 6, 7, 7, 7, 7,
207  8, 8, 8, 8, 9, 9, 9, 9, 10, 10, 10, 10, 11, 11, 11, 11,
208  12, 12, 12, 12, 13, 13, 13, 13, 14, 14, 14, 14, 15, 15, 15, 15,
209  16, 16, 16, 16, 17, 17, 17, 17, 18, 18, 18, 18, 19, 19, 19, 19,
210  20, 20, 20, 20, 21, 21, 21, 21, 22, 22, 22, 22, 23, 23, 23, 23,
211  24, 24, 24, 24, 25, 25, 25, 25, 26, 26, 26, 26, 27, 27, 27, 27,
212  28, 28, 28, 28, 29, 29, 29, 29, 30, 30, 30, 30, 31, 31, 31, 31,
213  32, 32, 32, 32, 33, 33, 33, 33, 34, 34, 34, 34, 35, 35, 35, 35,
214  36, 36, 36, 36, 37, 37, 37, 37, 38, 38, 38, 38, 39, 39, 39, 39,
215  40, 40, 40, 40, 41, 41, 41, 41, 42, 42, 42, 42, 43, 43, 43, 43,
216  44, 44, 44, 44, 45, 45, 45, 45, 46, 46, 46, 46, 47, 47, 47, 47,
217  48, 48, 48, 48, 49, 49, 49, 49, 50, 50, 50, 50, 51, 51, 51, 51,
218  52, 52, 52, 52, 53, 53, 53, 53, 54, 54, 54, 54, 55, 55, 55, 55,
219  56, 56, 56, 56, 57, 57, 57, 57, 58, 58, 58, 58, 59, 59, 59, 59,
220  60, 60, 60, 60, 61, 61, 61, 61, 62, 62, 62, 62, 63, 63, 63, 63,
221  /* CONTEXT_{M,L}SB6, second last byte, */
222  0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
223  0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
224  0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
225  0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
226  0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
227  0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
228  0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
229  0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
230  0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
231  0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
232  0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
233  0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
234  0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
235  0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
236  0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
237  0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
238 };
239 
240 static const int kContextLookupOffsets[8] = {
241  /* CONTEXT_LSB6 */
242  1024, 1536,
243  /* CONTEXT_MSB6 */
244  1280, 1536,
245  /* CONTEXT_UTF8 */
246  0, 256,
247  /* CONTEXT_SIGNED */
248  768, 512,
249 };
250 
251 #endif /* BROTLI_DEC_CONTEXT_H_ */
CONTEXT_SIGNED
Definition: context.h:108
CONTEXT_UTF8
Definition: context.h:107
CONTEXT_MSB6
Definition: context.h:106
CONTEXT_LSB6
Definition: context.h:105
uint8_t
unsigned char uint8_t
Definition: ptypes.h:89
ContextType
ContextType
Definition: context.h:104
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