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