99 lines
3.6 KiB
C++
99 lines
3.6 KiB
C++
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// Copyright 2020 The Pigweed Authors
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//
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// Licensed under the Apache License, Version 2.0 (the "License"); you may not
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// use this file except in compliance with the License. You may obtain a copy of
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// the License at
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//
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// https://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, WITHOUT
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// WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
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// License for the specific language governing permissions and limitations under
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// the License.
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#include "pw_checksum/crc32.h"
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namespace pw::checksum {
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namespace {
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// Calculates the partial CRC32 of the low order kBits of value using
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// the reversed polynomial kPolynomial. This is a building block for
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// both implementing a tableless CRC32 implementation as well as generating
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// look up tables for tables based implementations.
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//
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// Information on CRC32 can be found at:
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// https://en.wikipedia.org/wiki/Cyclic_redundancy_check
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template <std::size_t kBits, uint32_t kPolynomial>
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constexpr uint32_t Crc32ProcessDataChunk(uint32_t value) {
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for (uint32_t j = 0; j < kBits; j++) {
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value = (value >> 1u) ^
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(static_cast<uint32_t>(-static_cast<int32_t>(value & 1u)) &
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kPolynomial);
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}
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return value;
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}
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// Generates a lookup table for a table based CRC32 implementation.
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// The table pre-computes the CRC for every value representable by
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// kBits of data. kPolynomial is used as the reversed polynomial
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// for the computation. The returned table will have 2^kBits entries.
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template <std::size_t kBits, uint32_t kPolynomial>
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constexpr std::array<uint32_t, (1 << kBits)> GenerateCrc32Table() {
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std::array<uint32_t, (1 << kBits)> table{};
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for (uint32_t i = 0; i < (1 << kBits); i++) {
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table[i] = Crc32ProcessDataChunk<kBits, kPolynomial>(i);
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}
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return table;
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}
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// Reversed polynomial for the commonly used CRC32 variant. See:
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// https://en.wikipedia.org/wiki/Cyclic_redundancy_check#Polynomial_representations_of_cyclic_redundancy_checks
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constexpr uint32_t kCrc32Polynomial = 0xEDB88320;
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} // namespace
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extern "C" uint32_t _pw_checksum_InternalCrc32EightBit(const void* data,
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size_t size_bytes,
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uint32_t state) {
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static constexpr std::array<uint32_t, 256> kCrc32Table =
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GenerateCrc32Table<8, kCrc32Polynomial>();
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const uint8_t* data_bytes = static_cast<const uint8_t*>(data);
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for (size_t i = 0; i < size_bytes; ++i) {
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state = kCrc32Table[(state ^ data_bytes[i]) & 0xFFu] ^ (state >> 8);
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}
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return state;
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}
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extern "C" uint32_t _pw_checksum_InternalCrc32FourBit(const void* data,
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size_t size_bytes,
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uint32_t state) {
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static constexpr std::array<uint32_t, 16> kCrc32Table =
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GenerateCrc32Table<4, kCrc32Polynomial>();
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const uint8_t* data_bytes = static_cast<const uint8_t*>(data);
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for (size_t i = 0; i < size_bytes; ++i) {
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state ^= data_bytes[i];
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state = kCrc32Table[state & 0x0f] ^ (state >> 4);
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state = kCrc32Table[state & 0x0f] ^ (state >> 4);
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}
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return state;
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}
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extern "C" uint32_t _pw_checksum_InternalCrc32OneBit(const void* data,
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size_t size_bytes,
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uint32_t state) {
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const uint8_t* data_bytes = static_cast<const uint8_t*>(data);
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for (size_t i = 0; i < size_bytes; ++i) {
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state = Crc32ProcessDataChunk<8, kCrc32Polynomial>(state ^ data_bytes[i]);
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}
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return state;
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}
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} // namespace pw::checksum
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