87 lines
2.8 KiB
C++
87 lines
2.8 KiB
C++
/*
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* This Source Code Form is subject to the terms of the Mozilla Public
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* License, v. 2.0. If a copy of the MPL was not distributed with this
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* file, You can obtain one at http://mozilla.org/MPL/2.0/.
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*
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* Copyright 2019 Danny Robson <danny@nerdcruft.net>
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*/
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#pragma once
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#include "../bitwise.hpp"
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#include "../std.hpp"
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#include <limits>
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#include <random>
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// Implementations of algorithms from the paper: "PCG: A Family of Simple
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// Fast Space-Efficient Statistically Good Algorithms for Random Number
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// Generation"
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namespace cruft::rand {
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// Implements the PCG-XSH-RR algorithm; xorshift-high, random rotation.
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template <
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typename StateT = u64,
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typename OutputT = u32
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>
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class pcg_xsh_rr {
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public:
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using result_type = OutputT;
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explicit pcg_xsh_rr (std::seed_seq seed)
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: state (increment)
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{
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// Overlap a series of unsigned words with state words. We might
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// over-provision but that shouldn't be an issue with the sizes we
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// support.
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union {
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unsigned u[sizeof (StateT) / sizeof (unsigned) + 1];
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StateT s;
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} words;
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seed.generate (std::begin (words.u), std::end (words.u));
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state += words.s;
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}
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explicit pcg_xsh_rr (StateT seed) noexcept
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: state (increment + seed)
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{ (*this)(); }
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static constexpr auto ShiftV = 5u;
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static constexpr auto state_bits = sizeof (StateT ) * 8u;
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static constexpr auto state_shift = sizeof (StateT ) * 8u - ShiftV;
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static constexpr auto output_shift = sizeof (OutputT) * 8u - ShiftV;
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static constexpr auto xshift = (state_bits - output_shift) / 2u;
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result_type operator() (void) noexcept
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{
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StateT x = state;
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StateT const rrotate = x >> state_shift;
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state = x * multiplier + increment;
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x ^= x >> xshift;
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// Be very careful to cast to result_type before doing the
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// rotation otherwise we'll rotate the zeroed upper bits into the
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// returned value.
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return rotater (result_type (x >> output_shift), rrotate);
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}
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static constexpr auto max () noexcept { return std::numeric_limits<result_type>::max (); }
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static constexpr auto min () noexcept { return std::numeric_limits<result_type>::min (); }
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private:
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// Initialiser from the PCG example code.
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StateT state = 0x4d595df4d0f33173;
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// Using multiplier and increment from the PCG paper, which appear to
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// also correspond to constants chosen by Knuth.
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static constexpr StateT const multiplier = 6364136223846793005u;
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static constexpr StateT const increment = 1442695040888963407u;
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};
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}
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