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https://github.com/RGBCube/serenity
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daf50ed885
The implementation of this is naive enough so it can handle all 8-bit CRC polynomials, of which there are quite a few. The table generation and update procedure is MSB first, which is backwards from the LSB first method of CRC32.
76 lines
1.9 KiB
C++
76 lines
1.9 KiB
C++
/*
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* Copyright (c) 2023, kleines Filmröllchen <filmroellchen@serenityos.org>
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*
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* SPDX-License-Identifier: BSD-2-Clause
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*/
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#pragma once
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#include <AK/Types.h>
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#include <LibCrypto/Checksum/ChecksumFunction.h>
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namespace Crypto::Checksum {
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// A generic 8-bit Cyclic Redundancy Check.
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// Note that as opposed to CRC32, this class operates with MSB first, so the polynomial must not be reversed.
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// For example, the polynomial x⁸ + x² + x + 1 is represented as 0x07 and not 0xE0.
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template<u8 polynomial>
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class CRC8 : public ChecksumFunction<u8> {
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public:
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// This is a big endian table, while CRC-32 uses a little endian table.
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static constexpr auto generate_table()
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{
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Array<u8, 256> data {};
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u8 value = 0x80;
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auto i = 1u;
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do {
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if ((value & 0x80) != 0) {
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value = polynomial ^ (value << 1);
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} else {
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value = value << 1;
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}
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for (auto j = 0u; j < i; ++j) {
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data[i + j] = value ^ data[j];
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}
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i <<= 1;
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} while (i < 256);
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return data;
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}
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static constexpr auto table = generate_table();
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virtual ~CRC8() = default;
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CRC8() = default;
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CRC8(ReadonlyBytes data)
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{
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update(data);
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}
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CRC8(u8 initial_state, ReadonlyBytes data)
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: m_state(initial_state)
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{
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update(data);
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}
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// FIXME: This implementation is naive and slow.
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// Figure out how to adopt the slicing-by-8 algorithm (see CRC32) for 8 bit polynomials.
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virtual void update(ReadonlyBytes data) override
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{
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for (size_t i = 0; i < data.size(); i++) {
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size_t table_index = (m_state ^ data.at(i)) & 0xFF;
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m_state = (table[table_index] ^ (static_cast<u32>(m_state) << 8)) & 0xFF;
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}
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}
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virtual u8 digest() override
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{
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return m_state;
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}
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private:
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u8 m_state { 0 };
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};
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}
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