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80 lines
2.8 KiB
C++
80 lines
2.8 KiB
C++
/* Copyright 2013 Google Inc. All Rights Reserved.
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Distributed under MIT license.
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See file LICENSE for detail or copy at https://opensource.org/licenses/MIT
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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 "../common/types.h"
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#include "./fast_log.h"
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namespace brotli {
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static const uint32_t kNumInsertLenPrefixes = 24;
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static const uint32_t kNumCopyLenPrefixes = 24;
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static const uint32_t kNumCommandPrefixes = 704;
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static const uint32_t kNumBlockLenPrefixes = 26;
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static const uint32_t kNumDistanceShortCodes = 16;
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static const uint32_t 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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uint32_t offset;
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uint32_t 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(uint32_t len, uint32_t* code,
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uint32_t* n_extra, uint32_t* 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(size_t distance_code,
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size_t num_direct_codes,
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size_t 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 = static_cast<uint16_t>(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; /* >= 0 */
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distance_code += (1u << (postfix_bits + 2u)); /* > 0 */
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size_t bucket = Log2FloorNonZero(distance_code) - 1;
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size_t postfix_mask = (1 << postfix_bits) - 1;
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size_t postfix = distance_code & postfix_mask;
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size_t prefix = (distance_code >> bucket) & 1;
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size_t offset = (2 + prefix) << bucket;
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size_t nbits = bucket - postfix_bits;
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*code = static_cast<uint16_t>(
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(kNumDistanceShortCodes + num_direct_codes +
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((2 * (nbits - 1) + prefix) << postfix_bits) + postfix));
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*extra_bits = static_cast<uint32_t>(
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(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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