tor-browser

The Tor Browser
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context.h (4637B)

      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  If |p1| and |p2| are the previous two bytes, and |mode| is current context
     16  mode, we calculate the context as:
     17 
     18    context = ContextLut(mode)[p1] | ContextLut(mode)[p2 + 256].
     19 
     20  For CONTEXT_UTF8 mode, if the previous two bytes are ASCII characters
     21  (i.e. < 128), this will be 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 
     88 #ifndef BROTLI_COMMON_CONTEXT_H_
     89 #define BROTLI_COMMON_CONTEXT_H_
     90 
     91 #include "platform.h"
     92 
     93 typedef enum ContextType {
     94  CONTEXT_LSB6 = 0,
     95  CONTEXT_MSB6 = 1,
     96  CONTEXT_UTF8 = 2,
     97  CONTEXT_SIGNED = 3
     98 } ContextType;
     99 
    100 /* "Soft-private", it is exported, but not "advertised" as API. */
    101 /* Common context lookup table for all context modes. */
    102 BROTLI_COMMON_API extern const uint8_t _kBrotliContextLookupTable[2048];
    103 
    104 typedef const uint8_t* ContextLut;
    105 
    106 /* typeof(MODE) == ContextType; returns ContextLut */
    107 #define BROTLI_CONTEXT_LUT(MODE) (&_kBrotliContextLookupTable[(MODE) << 9])
    108 
    109 /* typeof(LUT) == ContextLut */
    110 #define BROTLI_CONTEXT(P1, P2, LUT) ((LUT)[P1] | ((LUT) + 256)[P2])
    111 
    112 #endif  /* BROTLI_COMMON_CONTEXT_H_ */