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CLK/Numeric/LFSR.hpp
2020-01-19 23:14:35 -05:00

68 lines
1.7 KiB
C++

//
// LFSR.hpp
// Clock Signal
//
// Created by Thomas Harte on 19/01/2020.
// Copyright © 2020 Thomas Harte. All rights reserved.
//
#ifndef LFSR_h
#define LFSR_h
namespace Numeric {
template <typename IntType> struct LSFRPolynomial {};
// The following were taken 'at random' from https://users.ece.cmu.edu/~koopman/lfsr/index.html
template <> struct LSFRPolynomial<uint64_t> {
static constexpr uint64_t value = 0x80000000000019E2;
};
template <> struct LSFRPolynomial<uint32_t> {
static constexpr uint32_t value = 0x80000C34;
};
template <> struct LSFRPolynomial<uint16_t> {
static constexpr uint16_t value = 0x853E;
};
template <> struct LSFRPolynomial<uint8_t> {
static constexpr uint8_t value = 0xAF;
};
/*!
Provides a linear-feedback shift register with a random initial state; if no polynomial is supplied
then one will be picked that is guaranteed to give the maximal number of LFSR states that can fit
in the specified int type.
*/
template <typename IntType = uint64_t, IntType polynomial = LSFRPolynomial<IntType>::value> class LFSR {
public:
LFSR() {
// Randomise the value, ensuring it doesn't end up being 0.
while(!value_) {
uint8_t *value_byte = reinterpret_cast<uint8_t *>(&value_);
for(size_t c = 0; c < sizeof(IntType); ++c) {
*value_byte = uint8_t(uint64_t(rand()) * 255 / RAND_MAX);
++value_byte;
}
}
}
/*!
Advances the LSFR, returning either an @c IntType of value @c 1 or @c 0,
determining the bit that was just shifted out.
*/
IntType next() {
const auto result = value_ & 1;
value_ = (value_ >> 1) ^ (result * polynomial);
return result;
}
private:
IntType value_ = 0;
};
}
#endif /* LFSR_h */