xoshiro256ss.h

#pragma once

// The "xoshiro256** 1.0" generator.
// C++ port by Arthur O'Dwyer (2021).
// Based on the C version by David Blackman and Sebastiano Vigna (2018),
// https://prng.di.unimi.it/xoshiro256starstar.c

static_assert(sizeof(long long) == 8, "64-bit machines only");

struct xoshiro256ss {
    using u64 = unsigned long long;
    u64 s[4] {};

    static constexpr u64 splitmix64(u64& x) {
        u64 z = (x += 0x9e3779b97f4a7c15uLL);
        z = (z ^ (z >> 30)) * 0xbf58476d1ce4e5b9uLL;
        z = (z ^ (z >> 27)) * 0x94d049bb133111ebuLL;
        return z ^ (z >> 31);
    }

    constexpr explicit xoshiro256ss() : xoshiro256ss(0) {}

    constexpr explicit xoshiro256ss(u64 seed) {
        s[0] = splitmix64(seed);
        s[1] = splitmix64(seed);
        s[2] = splitmix64(seed);
        s[3] = splitmix64(seed);
    }

    using result_type = u64;
    static constexpr u64 min() { return 0; }
    static constexpr u64 max() { return u64(-1); }

    static constexpr u64 rotl(u64 x, int k) {
        return (x << k) | (x >> (64 - k));
    }

    constexpr u64 operator()() {
        u64 result = rotl(s[1] * 5, 7) * 9;
        u64 t = s[1] << 17;
        s[2] ^= s[0];
        s[3] ^= s[1];
        s[1] ^= s[2];
        s[0] ^= s[3];
        s[2] ^= t;
        s[3] = rotl(s[3], 45);
        return result;
    }
};