libcruft-util/rand/xorshift.cpp

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/*
2018-08-04 15:14:06 +10:00
* This Source Code Form is subject to the terms of the Mozilla Public
* License, v. 2.0. If a copy of the MPL was not distributed with this
* file, You can obtain one at http://mozilla.org/MPL/2.0/.
*
* Copyright 2015-2019 Danny Robson <danny@nerdcruft.net>
*/
#include "xorshift.hpp"
#include "../debug/assert.hpp"
#include <cstdint>
using cruft::rand::xorshift;
///////////////////////////////////////////////////////////////////////////////
template <typename T>
xorshift<T>::xorshift (T seed):
m_state (seed)
{
if (!m_state)
throw std::invalid_argument ("xorshift seed must not be zero");
}
///////////////////////////////////////////////////////////////////////////////
// setup the constants a, b, and c. we use the values from the example
// generators provided in Marsaglia's paper.
template <typename T>
struct constants;
//-----------------------------------------------------------------------------
template <>
struct constants<uint32_t>
{
static constexpr uint32_t a = 13u;
static constexpr uint32_t b = 17u;
static constexpr uint32_t c = 5u;
};
//-----------------------------------------------------------------------------
template <>
struct constants<uint64_t>
{
static constexpr uint64_t a = 13u;
static constexpr uint64_t b = 7u;
static constexpr uint64_t c = 17u;
};
///////////////////////////////////////////////////////////////////////////////
template <typename T>
typename xorshift<T>::result_type
xorshift<T>::operator() (void)
{
m_state ^= m_state >> constants<T>::a;
m_state ^= m_state << constants<T>::b;
m_state ^= m_state >> constants<T>::c;
CHECK_NEZ (m_state);
return m_state;
}
///////////////////////////////////////////////////////////////////////////////
template <typename T>
void
xorshift<T>::discard (unsigned count)
{
while (count--)
(*this)();
}
///////////////////////////////////////////////////////////////////////////////
template struct cruft::rand::xorshift<uint32_t>;
template struct cruft::rand::xorshift<uint64_t>;