mirror of
https://github.com/TomHarte/CLK.git
synced 2024-11-30 04:50:08 +00:00
82 lines
2.0 KiB
C++
82 lines
2.0 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
|
|
|
|
#include <cstdint>
|
|
#include <cstdlib>
|
|
|
|
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:
|
|
/*!
|
|
Constructs an LFSR with a random initial value.
|
|
*/
|
|
constexpr LFSR() noexcept {
|
|
// Randomise the value, ensuring it doesn't end up being 0;
|
|
// don't set any top bits, in case this is a signed type.
|
|
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()) * 127 / RAND_MAX);
|
|
++value_byte;
|
|
}
|
|
}
|
|
}
|
|
|
|
/*!
|
|
Constructs an LFSR with the specified initial value.
|
|
|
|
An initial value of 0 is invalid.
|
|
*/
|
|
LFSR(IntType initial_value) : value_(initial_value) {}
|
|
|
|
/*!
|
|
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 */
|