79 lines
2.0 KiB
C++
79 lines
2.0 KiB
C++
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#pragma once
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#include <climits> // CHAR_BIT
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#include <cstddef>
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#include <cstdint>
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#include <random>
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namespace base {
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namespace detail {
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template <class T>
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constexpr size_t BitSizeOf() {
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return sizeof(T) * CHAR_BIT;
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}
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constexpr std::uint64_t splitmix64(std::uint64_t x) {
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std::uint64_t z = (x += 0x9e3779b97f4a7c15uLL);
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z = (z ^ (z >> 30)) * 0xbf58476d1ce4e5b9uLL;
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z = (z ^ (z >> 27)) * 0x94d049bb133111ebuLL;
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return z ^ (z >> 31);
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}
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constexpr std::uint64_t rotl(std::uint64_t x, int k) {
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return (x << k) | (x >> (BitSizeOf<std::uint64_t>() - k));
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}
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struct Xoshiro256ss {
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using result_type = std::uint64_t;
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std::uint64_t s[4] {};
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constexpr explicit Xoshiro256ss() : Xoshiro256ss(0) {}
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constexpr explicit Xoshiro256ss(std::uint64_t seed) {
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s[0] = splitmix64(seed);
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s[1] = splitmix64(seed);
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s[2] = splitmix64(seed);
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s[3] = splitmix64(seed);
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}
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constexpr explicit Xoshiro256ss(std::random_device& rd) {
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// Get 64 bits out of the random device.
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//
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// This lambda is quite literal, as it fetches
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// 2 iterations of the random engine, and
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// shifts + OR's them into a 64bit value.
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auto get_u64 = [&rd] {
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std::uint64_t the_thing = rd();
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return (the_thing << 32) | rd();
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};
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// seed with 256 bits of entropy from the random device + splitmix64
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// to ensure we seed it well, as per recommendation.
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s[0] = splitmix64(get_u64());
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s[1] = splitmix64(get_u64());
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s[2] = splitmix64(get_u64());
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s[3] = splitmix64(get_u64());
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}
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static constexpr result_type min() { return 0; }
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static constexpr result_type max() { return std::uint64_t(-1); }
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constexpr result_type operator()() {
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result_type result = rotl(s[1] * 5, 7) * 9;
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result_type t = s[1] << 17;
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s[2] ^= s[0];
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s[3] ^= s[1];
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s[1] ^= s[2];
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s[0] ^= s[3];
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s[2] ^= t;
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s[3] = rotl(s[3], 45);
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return result;
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}
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};
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} // namespace detail
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using detail::Xoshiro256ss;
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} // namespace base
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