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3862fdb44c
With the side effect of allowing every machine to try to load only the ROMs that it needs.
111 lines
2.8 KiB
C++
111 lines
2.8 KiB
C++
//
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// CRC.hpp
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// Clock Signal
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//
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// Created by Thomas Harte on 18/09/2016.
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// Copyright 2016 Thomas Harte. All rights reserved.
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//
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#ifndef CRC_hpp
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#define CRC_hpp
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#include <cstdint>
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#include <vector>
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namespace CRC {
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/*! Provides a class capable of generating a CRC from source data. */
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template <typename T, T reset_value, T xor_output, bool reflect_input, bool reflect_output> class Generator {
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public:
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/*!
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Instantiates a CRC16 that will compute the CRC16 specified by the supplied
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@c polynomial and @c reset_value.
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*/
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Generator(T polynomial): value_(reset_value) {
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const T top_bit = T(~(T(~0) >> 1));
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for(int c = 0; c < 256; c++) {
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T shift_value = static_cast<T>(c << multibyte_shift);
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for(int b = 0; b < 8; b++) {
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T exclusive_or = (shift_value&top_bit) ? polynomial : 0;
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shift_value = static_cast<T>(shift_value << 1) ^ exclusive_or;
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}
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xor_table[c] = shift_value;
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}
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}
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/// Resets the CRC to the reset value.
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void reset() { value_ = reset_value; }
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/// Updates the CRC to include @c byte.
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void add(uint8_t byte) {
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if(reflect_input) byte = reverse_byte(byte);
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value_ = static_cast<T>((value_ << 8) ^ xor_table[(value_ >> multibyte_shift) ^ byte]);
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}
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/// @returns The current value of the CRC.
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inline T get_value() const {
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T result = value_^xor_output;
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if(reflect_output) {
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T reflected_output = 0;
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for(std::size_t c = 0; c < sizeof(T); ++c) {
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reflected_output = T(reflected_output << 8) | T(reverse_byte(result & 0xff));
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result >>= 8;
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}
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return reflected_output;
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}
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return result;
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}
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/// Sets the current value of the CRC.
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inline void set_value(T value) { value_ = value; }
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/*!
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A compound for:
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reset()
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[add all data from @c data]
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get_value()
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*/
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T compute_crc(const std::vector<uint8_t> &data) {
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reset();
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for(const auto &byte: data) add(byte);
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return get_value();
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}
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private:
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static constexpr int multibyte_shift = (sizeof(T) * 8) - 8;
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T xor_table[256];
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T value_;
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constexpr uint8_t reverse_byte(uint8_t byte) const {
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return
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((byte & 0x80) ? 0x01 : 0x00) |
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((byte & 0x40) ? 0x02 : 0x00) |
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((byte & 0x20) ? 0x04 : 0x00) |
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((byte & 0x10) ? 0x08 : 0x00) |
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((byte & 0x08) ? 0x10 : 0x00) |
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((byte & 0x04) ? 0x20 : 0x00) |
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((byte & 0x02) ? 0x40 : 0x00) |
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((byte & 0x01) ? 0x80 : 0x00);
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}
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};
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/*!
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Provides a generator of 16-bit CCITT CRCs, which amongst other uses are
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those used by the FM and MFM disk encodings.
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*/
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struct CCITT: public Generator<uint16_t, 0xffff, 0x0000, false, false> {
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CCITT(): Generator(0x1021) {}
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};
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/*!
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Provides a generator of "standard 32-bit" CRCs.
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*/
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struct CRC32: public Generator<uint32_t, 0xffffffff, 0xffffffff, true, true> {
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CRC32(): Generator(0x04c11db7) {}
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};
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}
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#endif /* CRC_hpp */
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