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187 lines
7.6 KiB
C++
187 lines
7.6 KiB
C++
// Copyright 2013 Google Inc. All Rights Reserved.
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//
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// Licensed under the Apache License, Version 2.0 (the "License");
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// you may not use this file except in compliance with the License.
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// You may obtain a copy of the License at
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//
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// http://www.apache.org/licenses/LICENSE-2.0
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//
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// Unless required by applicable law or agreed to in writing, software
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// distributed under the License is distributed on an "AS IS" BASIS,
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// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
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// See the License for the specific language governing permissions and
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// limitations under the License.
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//
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// Functions to map previous bytes into a context id.
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#ifndef BROTLI_ENC_CONTEXT_H_
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#define BROTLI_ENC_CONTEXT_H_
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#include "./types.h"
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namespace brotli {
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// Second-order context lookup table for UTF8 byte streams.
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//
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// If p1 and p2 are the previous two bytes, we calculate the context as
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//
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// context = kUTF8ContextLookup[p1] | kUTF8ContextLookup[p2 + 256].
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//
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// If the previous two bytes are ASCII characters (i.e. < 128), this will be
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// equivalent to
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//
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// context = 4 * context1(p1) + context2(p2),
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//
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// where context1 is based on the previous byte in the following way:
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//
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// 0 : non-ASCII control
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// 1 : \t, \n, \r
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// 2 : space
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// 3 : other punctuation
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// 4 : " '
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// 5 : %
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// 6 : ( < [ {
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// 7 : ) > ] }
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// 8 : , ; :
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// 9 : .
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// 10 : =
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// 11 : number
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// 12 : upper-case vowel
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// 13 : upper-case consonant
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// 14 : lower-case vowel
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// 15 : lower-case consonant
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//
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// and context2 is based on the second last byte:
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//
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// 0 : control, space
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// 1 : punctuation
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// 2 : upper-case letter, number
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// 3 : lower-case letter
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//
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// If the last byte is ASCII, and the second last byte is not (in a valid UTF8
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// stream it will be a continuation byte, value between 128 and 191), the
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// context is the same as if the second last byte was an ASCII control or space.
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//
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// If the last byte is a UTF8 lead byte (value >= 192), then the next byte will
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// be a continuation byte and the context id is 2 or 3 depending on the LSB of
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// the last byte and to a lesser extent on the second last byte if it is ASCII.
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//
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// If the last byte is a UTF8 continuation byte, the second last byte can be:
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// - continuation byte: the next byte is probably ASCII or lead byte (assuming
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// 4-byte UTF8 characters are rare) and the context id is 0 or 1.
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// - lead byte (192 - 207): next byte is ASCII or lead byte, context is 0 or 1
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// - lead byte (208 - 255): next byte is continuation byte, context is 2 or 3
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//
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// The possible value combinations of the previous two bytes, the range of
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// context ids and the type of the next byte is summarized in the table below:
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//
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// |--------\-----------------------------------------------------------------|
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// | \ Last byte |
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// | Second \---------------------------------------------------------------|
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// | last byte \ ASCII | cont. byte | lead byte |
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// | \ (0-127) | (128-191) | (192-) |
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// |=============|===================|=====================|==================|
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// | ASCII | next: ASCII/lead | not valid | next: cont. |
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// | (0-127) | context: 4 - 63 | | context: 2 - 3 |
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// |-------------|-------------------|---------------------|------------------|
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// | cont. byte | next: ASCII/lead | next: ASCII/lead | next: cont. |
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// | (128-191) | context: 4 - 63 | context: 0 - 1 | context: 2 - 3 |
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// |-------------|-------------------|---------------------|------------------|
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// | lead byte | not valid | next: ASCII/lead | not valid |
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// | (192-207) | | context: 0 - 1 | |
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// |-------------|-------------------|---------------------|------------------|
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// | lead byte | not valid | next: cont. | not valid |
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// | (208-) | | context: 2 - 3 | |
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// |-------------|-------------------|---------------------|------------------|
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static const uint8_t kUTF8ContextLookup[512] = {
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// Last byte.
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//
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// ASCII range.
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0, 0, 0, 0, 0, 0, 0, 0, 0, 4, 4, 0, 0, 4, 0, 0,
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0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
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8, 12, 16, 12, 12, 20, 12, 16, 24, 28, 12, 12, 32, 12, 36, 12,
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44, 44, 44, 44, 44, 44, 44, 44, 44, 44, 32, 32, 24, 40, 28, 12,
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12, 48, 52, 52, 52, 48, 52, 52, 52, 48, 52, 52, 52, 52, 52, 48,
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52, 52, 52, 52, 52, 48, 52, 52, 52, 52, 52, 24, 12, 28, 12, 12,
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12, 56, 60, 60, 60, 56, 60, 60, 60, 56, 60, 60, 60, 60, 60, 56,
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60, 60, 60, 60, 60, 56, 60, 60, 60, 60, 60, 24, 12, 28, 12, 0,
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// UTF8 continuation byte range.
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0, 1, 0, 1, 0, 1, 0, 1, 0, 1, 0, 1, 0, 1, 0, 1,
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0, 1, 0, 1, 0, 1, 0, 1, 0, 1, 0, 1, 0, 1, 0, 1,
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0, 1, 0, 1, 0, 1, 0, 1, 0, 1, 0, 1, 0, 1, 0, 1,
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0, 1, 0, 1, 0, 1, 0, 1, 0, 1, 0, 1, 0, 1, 0, 1,
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// UTF8 lead byte range.
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2, 3, 2, 3, 2, 3, 2, 3, 2, 3, 2, 3, 2, 3, 2, 3,
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2, 3, 2, 3, 2, 3, 2, 3, 2, 3, 2, 3, 2, 3, 2, 3,
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2, 3, 2, 3, 2, 3, 2, 3, 2, 3, 2, 3, 2, 3, 2, 3,
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2, 3, 2, 3, 2, 3, 2, 3, 2, 3, 2, 3, 2, 3, 2, 3,
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// Second last byte.
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//
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// ASCII range.
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0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
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0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
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0, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1,
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2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 1, 1, 1, 1, 1, 1,
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1, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2,
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2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 1, 1, 1, 1, 1,
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1, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3,
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3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 1, 1, 1, 1, 0,
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// UTF8 continuation byte range.
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0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
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0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
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0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
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0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
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0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
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// UTF8 lead byte range.
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0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
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2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2,
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2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2,
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};
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// Context lookup table for small signed integers.
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static const uint8_t kSigned3BitContextLookup[] = {
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0, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1,
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2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2,
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2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2,
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2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2,
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3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3,
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3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3,
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3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3,
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3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3,
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4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4,
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4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4,
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4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4,
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4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4,
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5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5,
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5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5,
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5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5,
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6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 7,
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};
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enum ContextType {
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CONTEXT_LSB6 = 0,
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CONTEXT_MSB6 = 1,
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CONTEXT_UTF8 = 2,
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CONTEXT_SIGNED = 3
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};
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static inline uint8_t Context(uint8_t p1, uint8_t p2, int mode) {
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switch (mode) {
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case CONTEXT_LSB6:
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return p1 & 0x3f;
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case CONTEXT_MSB6:
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return static_cast<uint8_t>(p1 >> 2);
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case CONTEXT_UTF8:
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return kUTF8ContextLookup[p1] | kUTF8ContextLookup[p2 + 256];
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case CONTEXT_SIGNED:
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return static_cast<uint8_t>((kSigned3BitContextLookup[p1] << 3) +
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kSigned3BitContextLookup[p2]);
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default:
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return 0;
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}
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}
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} // namespace brotli
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#endif // BROTLI_ENC_CONTEXT_H_
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