biski64 C++ implementation

biski64.hpp

struct biski64 {
  uint64_t fast_loop{};
  uint64_t mix{};
  uint64_t loop_mix{};

  static constexpr uint64_t splitmix64(uint64_t& x) {  // NOLINT
    uint64_t z = (x += 0x9e3779b97f4a7c15uLL);
    z = (z ^ (z >> 30)) * 0xbf58476d1ce4e5b9uLL;
    z = (z ^ (z >> 27)) * 0x94d049bb133111ebuLL;
    return z ^ (z >> 31);
  }

  constexpr biski64() : biski64(0) {}

  constexpr explicit biski64(uint64_t seed) {
    // It is the caller's responsibility to ensure 'state' is not NULL.
    uint64_t seeder_state = seed;

    // Derive initial values for each biski64 state variable.
    mix = splitmix64(seeder_state);
    loop_mix = splitmix64(seeder_state);
    fast_loop = splitmix64(seeder_state);

    for (int i = 0; i < 16; ++i) {
      operator()();
    }
  }

  using result_type = uint64_t;
  static constexpr uint64_t min() { return std::numeric_limits<uint64_t>::min(); }
  static constexpr uint64_t max() { return std::numeric_limits<uint64_t>::max(); }

  static constexpr uint64_t rotate_left(uint64_t x, int k) { return (x << k) | (x >> (-k & 63)); }

  constexpr uint64_t operator()() {
    const uint64_t output = mix + loop_mix;
    const uint64_t old_loop_mix = loop_mix;

    loop_mix = fast_loop ^ mix;
    mix = rotate_left(mix, 16) + rotate_left(old_loop_mix, 40);
    fast_loop += 0x9999999999999999ULL;  // Additive constant for the Weyl sequence.

    return output;
  }
};