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123 lines
4.3 KiB
C++
123 lines
4.3 KiB
C++
/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
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/* vim: set ts=8 sts=2 et sw=2 tw=80: */
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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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/* The xorshift128+ pseudo-random number generator. */
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#ifndef mozilla_XorShift128Plus_h
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#define mozilla_XorShift128Plus_h
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#include "mozilla/Assertions.h"
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#include "mozilla/Attributes.h"
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#include "mozilla/FloatingPoint.h"
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#include <inttypes.h>
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namespace mozilla {
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namespace non_crypto {
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/*
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* A stream of pseudo-random numbers generated using the xorshift+ technique
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* described here:
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*
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* Vigna, Sebastiano (2014). "Further scramblings of Marsaglia's xorshift
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* generators". arXiv:1404.0390 (http://arxiv.org/abs/1404.0390)
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*
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* That paper says:
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*
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* In particular, we propose a tightly coded xorshift128+ generator that
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* does not fail systematically any test from the BigCrush suite of TestU01
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* (even reversed) and generates 64 pseudorandom bits in 1.10 ns on an
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* Intel(R) Core(TM) i7-4770 CPU @3.40GHz (Haswell). It is the fastest
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* generator we are aware of with such empirical statistical properties.
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*
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* The stream of numbers produced by this method repeats every 2**128 - 1 calls
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* (i.e. never, for all practical purposes). Zero appears 2**64 - 1 times in
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* this period; all other numbers appear 2**64 times. Additionally, each *bit*
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* in the produced numbers repeats every 2**128 - 1 calls.
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*
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* This generator is not suitable as a cryptographically secure random number
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* generator.
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*/
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class XorShift128PlusRNG {
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uint64_t mState[2];
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public:
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/*
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* Construct a xorshift128+ pseudo-random number stream using |aInitial0| and
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* |aInitial1| as the initial state. These MUST NOT both be zero.
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*
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* If the initial states contain many zeros, for a few iterations you'll see
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* many zeroes in the generated numbers. It's suggested to seed a SplitMix64
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* generator <http://xorshift.di.unimi.it/splitmix64.c> and use its first two
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* outputs to seed xorshift128+.
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*/
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XorShift128PlusRNG(uint64_t aInitial0, uint64_t aInitial1) {
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setState(aInitial0, aInitial1);
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}
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/**
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* Return a pseudo-random 64-bit number.
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*/
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MOZ_NO_SANITIZE_UNSIGNED_OVERFLOW
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uint64_t next() {
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/*
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* The offsetOfState*() methods below are provided so that exceedingly-rare
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* callers that want to observe or poke at RNG state in C++ type-system-
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* ignoring means can do so. Don't change the next() or nextDouble()
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* algorithms without altering code that uses offsetOfState*()!
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*/
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uint64_t s1 = mState[0];
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const uint64_t s0 = mState[1];
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mState[0] = s0;
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s1 ^= s1 << 23;
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mState[1] = s1 ^ s0 ^ (s1 >> 17) ^ (s0 >> 26);
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return mState[1] + s0;
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}
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/*
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* Return a pseudo-random floating-point value in the range [0, 1). More
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* precisely, choose an integer in the range [0, 2**53) and divide it by
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* 2**53. Given the 2**128 - 1 period noted above, the produced doubles are
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* all but uniformly distributed in this range.
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*/
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double nextDouble() {
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/*
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* Because the IEEE 64-bit floating point format stores the leading '1' bit
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* of the mantissa implicitly, it effectively represents a mantissa in the
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* range [0, 2**53) in only 52 bits. FloatingPoint<double>::kExponentShift
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* is the width of the bitfield in the in-memory format, so we must add one
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* to get the mantissa's range.
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*/
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static constexpr int kMantissaBits =
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mozilla::FloatingPoint<double>::kExponentShift + 1;
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uint64_t mantissa = next() & ((UINT64_C(1) << kMantissaBits) - 1);
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return double(mantissa) / (UINT64_C(1) << kMantissaBits);
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}
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/*
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* Set the stream's current state to |aState0| and |aState1|. These must not
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* both be zero; ideally, they should have an almost even mix of zero and one
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* bits.
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*/
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void setState(uint64_t aState0, uint64_t aState1) {
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MOZ_ASSERT(aState0 || aState1);
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mState[0] = aState0;
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mState[1] = aState1;
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}
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static size_t offsetOfState0() {
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return offsetof(XorShift128PlusRNG, mState[0]);
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}
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static size_t offsetOfState1() {
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return offsetof(XorShift128PlusRNG, mState[1]);
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}
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};
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} // namespace non_crypto
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} // namespace mozilla
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#endif // mozilla_XorShift128Plus_h
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