A C++ implementation of 128-bit Lehmer-style PRNGs

lehmer.hpp

#ifndef LEHMER_HPP_INCLUDED
#define LEHMER_HPP_INCLUDED 1

/*
 * A C++ implementation of fast, 128-bit, Lehmer-style PRNGs
 *
 * 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.
 */

#include <cstdint>

namespace lehmer_detail {

template <typename rtype, typename stype, auto multiplier>
class mcg {
    stype state_;
    static constexpr auto MCG_MULT = multiplier;

    static constexpr unsigned int STYPE_BITS = 8*sizeof(stype);
    static constexpr unsigned int RTYPE_BITS = 8*sizeof(rtype);
    
 
public:
    using result_type = rtype;
    static constexpr result_type min() { return result_type(0);  }
    static constexpr result_type max() { return ~result_type(0); }

    mcg(stype state = stype(0x9f57c403d06c42fcUL))
        : state_(state | 1)
    {
        // Nothing (else) to do.
    }

    void advance()
    {
        state_ *= MCG_MULT;
    }

    result_type operator()()
    {
        advance();
        return result_type(state_ >> (STYPE_BITS - RTYPE_BITS));
    }

    bool operator==(const mcg& rhs)
    {
        return (state_ == rhs.state_);
    }

    bool operator!=(const mcg& rhs)
    {
        return !operator==(rhs);
    }

    // Not (yet) implemented:
    //   - arbitrary jumpahead (see PCG code for an implementation)
    //   - I/O
    //   - Seeding from a seed_seq.
};

} // namespace lehmer_detail

using mcg128      = lehmer_detail::mcg<uint64_t,__uint128_t,
                                      (__uint128_t(5017888479014934897ULL) << 64)
                                      + 2747143273072462557ULL>;

using mcg128_fast = lehmer_detail::mcg<uint64_t,__uint128_t,
                                       0xda942042e4dd58b5ULL>;


#endif // LEHMER_HPP_INCLUDED