brotli/enc/context.h

/* 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"

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_ */