A C++ implementation of a set of PRNG adapters

rng_adapters.hpp

#ifndef RND_ADAPTERS_HPP_INCLUDED
#define RND_ADAPTERS_HPP_INCLUDED 1

/*
 * A C++ implementation of a set of PRNG adapters.
 *
 * The MIT License (MIT)
 *
 * Copyright (c) 2018 Melissa E. O'Neill
 *
 * Permission is hereby granted, free of charge, to any person obtaining a
 * copy of this software and associated documentation files (the "Software"),
 * to deal in the Software without restriction, including without limitation
 * the rights to use, copy, modify, merge, publish, distribute, sublicense,
 * and/or sell copies of the Software, and to permit persons to whom the
 * Software is furnished to do so, subject to the following conditions:
 *
 * The above copyright notice and this permission notice shall be included in
 * all copies or substantial portions of the Software.
 *
 * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
 * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
 * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
 * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
 * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING
 * FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER
 * DEALINGS IN THE SOFTWARE.
 */

// regularized_rng -- set min() to always be zero, use an appropriate type
//
// (the other adapters require sensible generators with a integer output and a
// min of zero, this adapter allows whacky generators to be used with those
// adapters)

template <typename RNG, typename rtype = typename RNG::result_type,
          auto SCALE = 1>
struct regularized_rng : public RNG {
    using RNG::RNG;
    using result_type = rtype;
    
    static constexpr result_type min() { return result_type(0); }
    static constexpr result_type max()
    {
        return result_type((RNG::max()*SCALE) - result_type(RNG::min()*SCALE));
    }

    result_type operator()()
    {
        return result_type(RNG::operator()()*SCALE)
               - result_type(RNG::min()*SCALE);
    }
};

// divided_rng -- divide the output by a constant

template <typename RNG, unsigned int DIVISOR,
          typename rtype = typename RNG::result_type>
struct divided_rng : public RNG {
    using RNG::RNG;
    using result_type = rtype;
    
    static_assert(RNG::min() == 0, "Requires sane PRNG");

    static constexpr result_type min() { return RNG::min() / DIVISOR; }
    static constexpr result_type max() { return RNG::max() / DIVISOR; }

    result_type operator()() { return RNG::operator()() / DIVISOR; }
};

// skipping_rng - divide the output by a constant, skipping all outputs
// that don't have a desired remainder (reduces the period of the PRNG by
// discarding output)

template <typename RNG, typename RNG::result_type DIVISOR,
          typename RNG::result_type LOW_ORDER_MATCH = 0,
          typename rtype = typename RNG::result_type>
struct skipping_rng : public RNG {
    using RNG::RNG;
    using result_type = rtype;
    
    static_assert(RNG::min() == 0, "Requires sane PRNG");

    static constexpr result_type min() { return RNG::min() / DIVISOR; }
    static constexpr result_type max() { return RNG::max() / DIVISOR; }

    result_type operator()() {
        for(;;) {
            result_type rval = RNG::operator()();
            if ((rval % result_type(DIVISOR)) == result_type(LOW_ORDER_MATCH)) {
                return rval / result_type(DIVISOR);
            }
        }
    }
};

// doubled_rng - group pairs of outputs into a single output

template <typename RNG, typename rtype>
struct doubled_rng : public RNG {
public:
    using RNG::RNG;
    using result_type = rtype;

    static_assert(RNG::min() == 0, "Requires sane PRNG");

    static constexpr result_type min()
    {
        return combine2(RNG::min(), RNG::min());
    }
    
    static constexpr result_type max()
    {
        return combine2(RNG::max(), RNG::max());
    }

    result_type operator()()
    {
        auto first  = RNG::operator()();
        auto second = RNG::operator()();
        return combine2(first,second);
    }

protected:
    static constexpr result_type combine2(typename RNG::result_type first,
                                          typename RNG::result_type second)
    {
        return (result_type(second) * (result_type(RNG::max())+1))
                + result_type(first);
    }
};

#endif // RND_ADAPTERS_HPP_INCLUDED