mirror of
https://github.com/claunia/findcrcs.git
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305 lines
8.5 KiB
C++
305 lines
8.5 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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// Defines GfUtil template class which implements
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// 1. some useful operations in GF(2^n),
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// 2. CRC helper function (e.g. concatenation of CRCs) which are
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// not affected by specific implemenation of CRC computation per se.
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//
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// Please read crc.pdf to understand how it all works.
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#ifndef CRCUTIL_GF_UTIL_H_
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#define CRCUTIL_GF_UTIL_H_
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#include "base_types.h" // uint8, uint64
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#include "crc_casts.h" // TO_BYTE()
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#include "platform.h" // GCC_ALIGN_ATTRIBUTE(16), SHIFT_*_SAFE
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namespace crcutil {
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#pragma pack(push, 16)
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// "Crc" is the type used internally and to return values of N-bit CRC.
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template<typename Crc> class GfUtil {
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public:
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// Initializes the tables given generating polynomial of degree (degree).
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// If "canonical" is true, starting CRC value and computed CRC value will be
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// XOR-ed with 111...111.
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GfUtil() {}
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GfUtil(const Crc &generating_polynomial, size_t degree, bool canonical) {
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Init(generating_polynomial, degree, canonical);
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}
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void Init(const Crc &generating_polynomial, size_t degree, bool canonical) {
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Crc one = 1;
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one <<= degree - 1;
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this->generating_polynomial_ = generating_polynomial;
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this->crc_bytes_ = (degree + 7) >> 3;
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this->degree_ = degree;
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this->one_ = one;
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if (canonical) {
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this->canonize_ = one | (one - 1);
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} else {
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this->canonize_ = 0;
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}
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this->normalize_[0] = 0;
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this->normalize_[1] = generating_polynomial;
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Crc k = one >> 1;
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for (size_t i = 0; i < sizeof(uint64) * 8; ++i) {
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this->x_pow_2n_[i] = k;
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k = Multiply(k, k);
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}
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this->crc_of_crc_ = Multiply(this->canonize_,
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this->one_ ^ Xpow8N((degree + 7) >> 3));
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FindLCD(Xpow8N(this->crc_bytes_), &this->x_pow_minus_W_);
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}
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// Returns generating polynomial.
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Crc GeneratingPolynomial() const {
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return this->generating_polynomial_;
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}
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// Returns number of bits in CRC (degree of generating polynomial).
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size_t Degree() const {
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return this->degree_;
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}
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// Returns start/finish adjustment constant.
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Crc Canonize() const {
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return this->canonize_;
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}
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// Returns normalized value of 1.
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Crc One() const {
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return this->one_;
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}
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// Returns value of CRC(A, |A|, start_new) given known
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// crc=CRC(A, |A|, start_old) -- without touching the data.
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Crc ChangeStartValue(const Crc &crc, uint64 bytes,
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const Crc &start_old,
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const Crc &start_new) const {
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return (crc ^ Multiply(start_new ^ start_old, Xpow8N(bytes)));
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}
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// Returns CRC of concatenation of blocks A and B when CRCs
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// of blocks A and B are known -- without touching the data.
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//
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// To be precise, given CRC(A, |A|, startA) and CRC(B, |B|, 0),
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// returns CRC(AB, |AB|, startA).
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Crc Concatenate(const Crc &crc_A, const Crc &crc_B, uint64 bytes_B) const {
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return ChangeStartValue(crc_B, bytes_B, 0 /* start_B */, crc_A);
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}
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// Returns CRC of sequence of zeroes -- without touching the data.
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Crc CrcOfZeroes(uint64 bytes, const Crc &start) const {
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Crc tmp = Multiply(start ^ this->canonize_, Xpow8N(bytes));
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return (tmp ^ this->canonize_);
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}
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// Given CRC of a message, stores extra (degree + 7)/8 bytes after
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// the message so that CRC(message+extra, start) = result.
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// Does not change CRC start value (use ChangeStartValue for that).
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// Returns number of stored bytes.
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size_t StoreComplementaryCrc(void *dst,
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const Crc &message_crc,
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const Crc &result) const {
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Crc crc0 = Multiply(result ^ this->canonize_, this->x_pow_minus_W_);
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crc0 ^= message_crc ^ this->canonize_;
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uint8 *d = reinterpret_cast<uint8 *>(dst);
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for (size_t i = 0; i < this->crc_bytes_; ++i) {
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d[i] = TO_BYTE(crc0);
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crc0 >>= 8;
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}
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return this->crc_bytes_;
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}
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// Stores given CRC of a message as (degree + 7)/8 bytes filled
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// with 0s to the right. Returns number of stored bytes.
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// CRC of the message and stored CRC is a constant value returned
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// by CrcOfCrc() -- it does not depend on contents of the message.
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size_t StoreCrc(void *dst, const Crc &crc) const {
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uint8 *d = reinterpret_cast<uint8 *>(dst);
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Crc crc0 = crc;
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for (size_t i = 0; i < this->crc_bytes_; ++i) {
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d[i] = TO_BYTE(crc0);
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crc0 >>= 8;
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}
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return this->crc_bytes_;
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}
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// Returns expected CRC value of CRC(Message,CRC(Message))
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// when CRC is stored after the message. This value is fixed
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// and does not depend on the message or CRC start value.
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Crc CrcOfCrc() const {
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return this->crc_of_crc_;
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}
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// Returns ((a * b) mod P) where "a" and "b" are of degree <= (D-1).
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Crc Multiply(const Crc &aa, const Crc &bb) const {
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Crc a = aa;
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Crc b = bb;
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if ((a ^ (a - 1)) < (b ^ (b - 1))) {
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Crc temp = a;
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a = b;
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b = temp;
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}
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if (a == 0) {
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return a;
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}
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Crc product = 0;
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Crc one = this->one_;
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for (; a != 0; a <<= 1) {
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if ((a & one) != 0) {
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product ^= b;
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a ^= one;
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}
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b = (b >> 1) ^ this->normalize_[Downcast<Crc, size_t>(b & 1)];
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}
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return product;
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}
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// Returns ((unnorm * m) mod P) where degree of m is <= (D-1)
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// and degree of value "unnorm" is provided explicitly.
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Crc MultiplyUnnormalized(const Crc &unnorm, size_t degree,
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const Crc &m) const {
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Crc v = unnorm;
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Crc result = 0;
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while (degree > this->degree_) {
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degree -= this->degree_;
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Crc value = v & (this->one_ | (this->one_ - 1));
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result ^= Multiply(value, Multiply(m, XpowN(degree)));
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v >>= this->degree_;
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}
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result ^= Multiply(v << (this->degree_ - degree), m);
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return result;
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}
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// returns ((x ** n) mod P).
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Crc XpowN(uint64 n) const {
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Crc one = this->one_;
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Crc result = one;
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for (size_t i = 0; n != 0; ++i, n >>= 1) {
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if (n & 1) {
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result = Multiply(result, this->x_pow_2n_[i]);
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}
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}
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return result;
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}
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// Returns (x ** (8 * n) mod P).
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Crc Xpow8N(uint64 n) const {
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return XpowN(n << 3);
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}
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// Returns remainder (A mod B) and sets *q = (A/B) of division
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// of two polynomials:
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// A = dividend + dividend_x_pow_D_coef * x**degree,
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// B = divisor.
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Crc Divide(const Crc ÷nd0, int dividend_x_pow_D_coef,
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const Crc &divisor0, Crc *q) const {
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Crc divisor = divisor0;
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Crc dividend = dividend0;
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Crc quotient = 0;
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Crc coef = this->one_;
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while ((divisor & 1) == 0) {
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divisor >>= 1;
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coef >>= 1;
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}
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if (dividend_x_pow_D_coef) {
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quotient = coef >> 1;
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dividend ^= divisor >> 1;
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}
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Crc x_pow_degree_b = 1;
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for (;;) {
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if ((dividend & x_pow_degree_b) != 0) {
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dividend ^= divisor;
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quotient ^= coef;
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}
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if (coef == this->one_) {
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break;
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}
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coef <<= 1;
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x_pow_degree_b <<= 1;
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divisor <<= 1;
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}
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*q = quotient;
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return dividend;
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}
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// Extended Euclid's algorith -- for given A finds LCD(A, P) and
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// value B such that (A * B) mod P = LCD(A, P).
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Crc FindLCD(const Crc &A, Crc *B) const {
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if (A == 0 || A == this->one_) {
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*B = A;
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return A;
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}
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// Actually, generating polynomial is
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// (generating_polynomial_ + x**degree).
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int r0_x_pow_D_coef = 1;
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Crc r0 = this->generating_polynomial_;
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Crc b0 = 0;
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Crc r1 = A;
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Crc b1 = this->one_;
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for (;;) {
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Crc q;
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Crc r = Divide(r0, r0_x_pow_D_coef, r1, &q);
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if (r == 0) {
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break;
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}
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r0_x_pow_D_coef = 0;
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r0 = r1;
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r1 = r;
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Crc b = b0 ^ Multiply(q, b1);
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b0 = b1;
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b1 = b;
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}
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*B = b1;
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return r1;
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}
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protected:
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Crc canonize_;
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Crc x_pow_2n_[sizeof(uint64) * 8];
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Crc generating_polynomial_;
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Crc one_;
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Crc x_pow_minus_W_;
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Crc crc_of_crc_;
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Crc normalize_[2];
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size_t crc_bytes_;
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size_t degree_;
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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_GF_UTIL_H_
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