107 lines
3.3 KiB
C++
107 lines
3.3 KiB
C++
// Copyright 2010 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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// Implements rolling CRC (e.g. for Rabin fingerprinting).
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#ifndef CRCUTIL_ROLLING_CRC_H_
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#define CRCUTIL_ROLLING_CRC_H_
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#include "base_types.h" // size_t, uint8
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#include "crc_casts.h" // TO_BYTE
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namespace crcutil {
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#pragma pack(push, 16)
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// CrcImplementation should provide:
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// - typename Crc
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// - typename TableEntry
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// - typename Word
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// - Crc CrcDefault(const void *data, size_t bytes, const Crc &start)
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// - const GfUtil<Crc> &Base() const
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template<typename CrcImplementation> class RollingCrc {
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public:
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typedef typename CrcImplementation::Crc Crc;
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typedef typename CrcImplementation::TableEntry TableEntry;
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typedef typename CrcImplementation::Word Word;
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RollingCrc() {}
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// Initializes internal data structures.
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// Retains reference to "crc" instance -- it is used by Start().
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RollingCrc(const CrcImplementation &crc,
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size_t roll_window_bytes,
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const Crc &start_value) {
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Init(crc, roll_window_bytes, start_value);
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}
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// Computes crc of "roll_window_bytes" using
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// "start_value" of "crc" (see Init()).
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Crc Start(const void *data) const {
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return crc_->CrcDefault(data, roll_window_bytes_, start_value_);
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}
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// Computes CRC of "roll_window_bytes" starting in next position.
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Crc Roll(const Crc &old_crc, size_t byte_out, size_t byte_in) const {
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return (old_crc >> 8) ^ in_[TO_BYTE(old_crc) ^ byte_in] ^ out_[byte_out];
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}
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// Initializes internal data structures.
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// Retains reference to "crc" instance -- it is used by Start().
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void Init(const CrcImplementation &crc,
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size_t roll_window_bytes,
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const Crc &start_value) {
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crc_ = &crc;
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roll_window_bytes_ = roll_window_bytes;
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start_value_ = start_value;
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Crc add = crc.Base().Canonize() ^ start_value;
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add = crc.Base().Multiply(add, crc.Base().Xpow8N(roll_window_bytes));
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add ^= crc.Base().Canonize();
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Crc mul = crc.Base().One() ^ crc.Base().Xpow8N(1);
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add = crc.Base().Multiply(add, mul);
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mul = crc.Base().XpowN(8 * roll_window_bytes + crc.Base().Degree());
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for (size_t i = 0; i < 256; ++i) {
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out_[i] = static_cast<TableEntry>(
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crc.Base().MultiplyUnnormalized(
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static_cast<Crc>(i), 8, mul) ^ add);
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}
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for (size_t i = 0; i < 256; ++i) {
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in_[i] = crc.crc_word_[sizeof(Word) - 1][i];
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}
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}
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// Returns start value.
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Crc StartValue() const { return start_value_; }
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// Returns length of roll window.
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size_t WindowBytes() const { return roll_window_bytes_; }
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protected:
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TableEntry in_[256];
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TableEntry out_[256];
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// Used only by Start().
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Crc start_value_;
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const CrcImplementation *crc_;
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size_t roll_window_bytes_;
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} GCC_ALIGN_ATTRIBUTE(16);
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#pragma pack(pop)
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} // namespace crcutil
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#endif // CRCUTIL_ROLLING_CRC_H_
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