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86 lines
3.0 KiB
C++
86 lines
3.0 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 for encoding of integers into prefix codes the amount of extra
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// bits, and the actual values of the extra bits.
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#ifndef BROTLI_ENC_PREFIX_H_
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#define BROTLI_ENC_PREFIX_H_
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#include "./fast_log.h"
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#include "./types.h"
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namespace brotli {
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static const int kNumInsertLenPrefixes = 24;
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static const int kNumCopyLenPrefixes = 24;
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static const int kNumCommandPrefixes = 704;
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static const int kNumBlockLenPrefixes = 26;
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static const int kNumDistanceShortCodes = 16;
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static const int kNumDistancePrefixes = 520;
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// Represents the range of values belonging to a prefix code:
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// [offset, offset + 2^nbits)
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struct PrefixCodeRange {
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int offset;
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int nbits;
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};
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static const PrefixCodeRange kBlockLengthPrefixCode[kNumBlockLenPrefixes] = {
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{ 1, 2}, { 5, 2}, { 9, 2}, { 13, 2},
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{ 17, 3}, { 25, 3}, { 33, 3}, { 41, 3},
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{ 49, 4}, { 65, 4}, { 81, 4}, { 97, 4},
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{ 113, 5}, { 145, 5}, { 177, 5}, { 209, 5},
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{ 241, 6}, { 305, 6}, { 369, 7}, { 497, 8},
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{ 753, 9}, { 1265, 10}, {2289, 11}, {4337, 12},
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{8433, 13}, {16625, 24}
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};
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inline void GetBlockLengthPrefixCode(int len,
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int* code, int* n_extra, int* extra) {
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*code = 0;
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while (*code < 25 && len >= kBlockLengthPrefixCode[*code + 1].offset) {
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++(*code);
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}
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*n_extra = kBlockLengthPrefixCode[*code].nbits;
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*extra = len - kBlockLengthPrefixCode[*code].offset;
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}
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inline void PrefixEncodeCopyDistance(int distance_code,
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int num_direct_codes,
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int postfix_bits,
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uint16_t* code,
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uint32_t* extra_bits) {
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if (distance_code < kNumDistanceShortCodes + num_direct_codes) {
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*code = distance_code;
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*extra_bits = 0;
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return;
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}
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distance_code -= kNumDistanceShortCodes + num_direct_codes;
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distance_code += (1 << (postfix_bits + 2));
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int bucket = Log2Floor(distance_code) - 1;
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int postfix_mask = (1 << postfix_bits) - 1;
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int postfix = distance_code & postfix_mask;
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int prefix = (distance_code >> bucket) & 1;
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int offset = (2 + prefix) << bucket;
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int nbits = bucket - postfix_bits;
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*code = kNumDistanceShortCodes + num_direct_codes +
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((2 * (nbits - 1) + prefix) << postfix_bits) + postfix;
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*extra_bits = (nbits << 24) | ((distance_code - offset) >> postfix_bits);
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}
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} // namespace brotli
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#endif // BROTLI_ENC_PREFIX_H_
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