C++ Pseudo Random Number Generators

random.h

/* Copyright (c) 2018 Arvid Gerstmann. */
/* This code is licensed under MIT license. */
#ifndef AG_RANDOM_H
#define AG_RANDOM_H

class splitmix
{
public:
    using result_type = uint32_t;
    static constexpr result_type (min)() { return 0; }
    static constexpr result_type (max)() { return UINT32_MAX; }
    friend bool operator==(splitmix const &, splitmix const &);
    friend bool operator!=(splitmix const &, splitmix const &);

    splitmix() : m_seed(1) {}
    explicit splitmix(std::random_device &rd)
    {
        seed(rd);
    }

    void seed(std::random_device &rd)
    {
        m_seed = uint64_t(rd()) << 31 | uint64_t(rd());
    }

    result_type operator()()
    {
        uint64_t z = (m_seed += UINT64_C(0x9E3779B97F4A7C15));
        z = (z ^ (z >> 30)) * UINT64_C(0xBF58476D1CE4E5B9);
        z = (z ^ (z >> 27)) * UINT64_C(0x94D049BB133111EB);
        return result_type((z ^ (z >> 31)) >> 31);
    }

    void discard(unsigned long long n)
    {
        for (unsigned long long i = 0; i < n; ++i)
            operator()();
    }

private:
    uint64_t m_seed;
};

bool operator==(splitmix const &lhs, splitmix const &rhs)
{
    return lhs.m_seed == rhs.m_seed;
}
bool operator!=(splitmix const &lhs, splitmix const &rhs)
{
    return lhs.m_seed != rhs.m_seed;
}

class xorshift
{
public:
    using result_type = uint32_t;
    static constexpr result_type (min)() { return 0; }
    static constexpr result_type (max)() { return UINT32_MAX; }
    friend bool operator==(xorshift const &, xorshift const &);
    friend bool operator!=(xorshift const &, xorshift const &);

    xorshift() : m_seed(0xc1f651c67c62c6e0ull) {}
    explicit xorshift(std::random_device &rd)
    {
        seed(rd);
    }

    void seed(std::random_device &rd)
    {
        m_seed = uint64_t(rd()) << 31 | uint64_t(rd());
    }

    result_type operator()()
    {
        uint64_t result = m_seed * 0xd989bcacc137dcd5ull;
        m_seed ^= m_seed >> 11;
        m_seed ^= m_seed << 31;
        m_seed ^= m_seed >> 18;
        return uint32_t(result >> 32ull);
    }

    void discard(unsigned long long n)
    {
        for (unsigned long long i = 0; i < n; ++i)
            operator()();
    }

private:
    uint64_t m_seed;
};

bool operator==(xorshift const &lhs, xorshift const &rhs)
{
    return lhs.m_seed == rhs.m_seed;
}
bool operator!=(xorshift const &lhs, xorshift const &rhs)
{
    return lhs.m_seed != rhs.m_seed;
}

class pcg
{
public:
    using result_type = uint32_t;
    static constexpr result_type (min)() { return 0; }
    static constexpr result_type (max)() { return UINT32_MAX; }
    friend bool operator==(pcg const &, pcg const &);
    friend bool operator!=(pcg const &, pcg const &);

    pcg()
        : m_state(0x853c49e6748fea9bULL)
        , m_inc(0xda3e39cb94b95bdbULL)
    {}
    explicit pcg(std::random_device &rd)
    {
        seed(rd);
    }

    void seed(std::random_device &rd)
    {
        uint64_t s0 = uint64_t(rd()) << 31 | uint64_t(rd());
        uint64_t s1 = uint64_t(rd()) << 31 | uint64_t(rd());

        m_state = 0;
        m_inc = (s1 << 1) | 1;
        (void)operator()();
        m_state += s0;
        (void)operator()();
    }

    result_type operator()()
    {
        uint64_t oldstate = m_state;
        m_state = oldstate * 6364136223846793005ULL + m_inc;
        uint32_t xorshifted = uint32_t(((oldstate >> 18u) ^ oldstate) >> 27u);
        int rot = oldstate >> 59u;
        return (xorshifted >> rot) | (xorshifted << ((-rot) & 31));
    }

    void discard(unsigned long long n)
    {
        for (unsigned long long i = 0; i < n; ++i)
            operator()();
    }

private:
    uint64_t m_state;
    uint64_t m_inc;
};

bool operator==(pcg const &lhs, pcg const &rhs)
{
    return lhs.m_state == rhs.m_state
        && lhs.m_inc == rhs.m_inc;
}
bool operator!=(pcg const &lhs, pcg const &rhs)
{
    return lhs.m_state != rhs.m_state
        || lhs.m_inc != rhs.m_inc;
}

#endif /* AG_RANDOM_H */

The free functions (operator== and operator!=) should be declared inline. Otherwise there will be a linking error if this header is included in multiple cpp files.

On line 134:

 m_state = oldstate * 6364136223846793005ULL + m_inc;

this diverges from the original PCG algorithm, which does m_inc | 1 no ?