brotli/enc/context.h

// Copyright 2013 Google Inc. All Rights Reserved.
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
// http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.
//
// Functions to map previous bytes into a context id.

#ifndef BROTLI_ENC_CONTEXT_H_
#define BROTLI_ENC_CONTEXT_H_

#include <stdint.h>

namespace brotli {

// Second-order context lookup table for UTF8 byte streams.
//
// If p1 and p2 are the previous two bytes, we calcualte 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 int 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, int mode) {
  switch (mode) {
    case CONTEXT_LSB6:
      return p1 & 0x3f;
    case CONTEXT_MSB6:
      return p1 >> 2;
    case CONTEXT_UTF8:
      return kUTF8ContextLookup[p1] | kUTF8ContextLookup[p2 + 256];
    case CONTEXT_SIGNED:
      return (kSigned3BitContextLookup[p1] << 3) + kSigned3BitContextLookup[p2];
    default:
      return 0;
  }
}

}  // namespace brotli

#endif  // BROTLI_ENC_CONTEXT_H_