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82 lines
2.0 KiB
C++
82 lines
2.0 KiB
C++
//
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// LFSR.hpp
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// Clock Signal
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//
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// Created by Thomas Harte on 19/01/2020.
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// Copyright © 2020 Thomas Harte. All rights reserved.
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//
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#ifndef LFSR_h
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#define LFSR_h
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#include <cstdint>
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#include <cstdlib>
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namespace Numeric {
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template <typename IntType> struct LSFRPolynomial {};
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// The following were taken 'at random' from https://users.ece.cmu.edu/~koopman/lfsr/index.html
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template <> struct LSFRPolynomial<uint64_t> {
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static constexpr uint64_t value = 0x80000000000019E2;
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};
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template <> struct LSFRPolynomial<uint32_t> {
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static constexpr uint32_t value = 0x80000C34;
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};
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template <> struct LSFRPolynomial<uint16_t> {
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static constexpr uint16_t value = 0x853E;
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};
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template <> struct LSFRPolynomial<uint8_t> {
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static constexpr uint8_t value = 0xAF;
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};
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/*!
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Provides a linear-feedback shift register with a random initial state; if no polynomial is supplied
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then one will be picked that is guaranteed to give the maximal number of LFSR states that can fit
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in the specified int type.
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*/
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template <typename IntType = uint64_t, IntType polynomial = LSFRPolynomial<IntType>::value> class LFSR {
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public:
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/*!
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Constructs an LFSR with a random initial value.
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*/
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constexpr LFSR() noexcept {
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// Randomise the value, ensuring it doesn't end up being 0;
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// don't set any top bits, in case this is a signed type.
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while(!value_) {
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uint8_t *value_byte = reinterpret_cast<uint8_t *>(&value_);
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for(size_t c = 0; c < sizeof(IntType); ++c) {
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*value_byte = uint8_t(uint64_t(rand()) * 127 / RAND_MAX);
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++value_byte;
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}
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}
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}
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/*!
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Constructs an LFSR with the specified initial value.
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An initial value of 0 is invalid.
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*/
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LFSR(IntType initial_value) : value_(initial_value) {}
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/*!
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Advances the LSFR, returning either an @c IntType of value @c 1 or @c 0,
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determining the bit that was just shifted out.
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*/
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IntType next() {
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const auto result = value_ & 1;
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value_ = (value_ >> 1) ^ (result * polynomial);
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return result;
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}
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private:
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IntType value_ = 0;
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};
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}
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#endif /* LFSR_h */
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