libcruft-util/rand/lcg.cpp

/*
 * 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 "lcg.hpp"

#include "../maths.hpp"

using cruft::rand::lcg;


///////////////////////////////////////////////////////////////////////////////
template <typename T>
static constexpr
bool is_coprime (T M, T C)
{
    if (M == 0)
        return true;
    if (std::gcd (M, C) == 1u)
        return true;
    return false;
}


///////////////////////////////////////////////////////////////////////////////
template <typename T, T M, T A, T C>
lcg<T,M,A,C>::lcg (T seed):
    m_x (seed)
{
    // ensure this assertion isn't in a header, it could be pretty expensive
    // to evaluate often.
    static_assert (is_coprime (M, C),
                   "multiplier and increment must be coprime");
}


///////////////////////////////////////////////////////////////////////////////
template <typename T, T M, T A, T C>
T
lcg<T,M,A,C>::operator() (void)
{
    m_x = (A * m_x + C);
    if (M != 0)
        m_x %= M;
    return m_x;
}


///////////////////////////////////////////////////////////////////////////////
template <typename T, T M, T A, T C>
void
lcg<T,M,A,C>::discard (unsigned count)
{
    while (count--)
        (*this)();
}


///////////////////////////////////////////////////////////////////////////////
template struct cruft::rand::lcg<uint32_t, cruft::pow(31,2), 1103515245, 12345>;
template struct cruft::rand::lcg<uint64_t, 0ul, 6364136223846793005ul, 1ul>;