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/*
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* Copyright (c) 2016, 2019, Oracle and/or its affiliates. All rights reserved.
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* ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
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*
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*
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*
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*
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*
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*
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*
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*
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*
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*
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*
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*
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*
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*
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*
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*
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*
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*
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*
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*
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*/
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// package java.util;
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import java.math.BigInteger;
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import java.util.concurrent.atomic.AtomicLong;
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/**
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* A generator of uniform pseudorandom values applicable for use in
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* (among other contexts) isolated parallel computations that may
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* generate subtasks. Class {@code Xoroshiro128Plus} implements
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* interfaces {@link java.util.Rng} and {@link java.util.LeapableRng},
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* and therefore supports methods for producing pseudorandomly chosen
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* numbers of type {@code int}, {@code long}, {@code float}, and {@code double}
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* as well as creating new {@code Xoroshiro128Plus} objects
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* by "jumping" or "leaping".
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*
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* <p>Series of generated values pass the TestU01 BigCrush and PractRand test suites
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* that measure independence and uniformity properties of random number generators,
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* <em>except</em> that it does not pass the binary rank tests of PractRand,
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* which fail due to the lowest bit being an LFSR; all other bits pass all tests.
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* For this reason may be best for some purposes to use this generator to generate
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* pseudorandom {@code int}, {@code float}, and {@code double} values but not
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* {@code long} values. For the same reason, it may be best not to use the
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* method {@code nextGaussian} or {@code nextExponential} with this generator.
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*
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* <p>The class {@code Xoroshiro128Plus} uses the {@code xoroshiro128} algorithm,
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* version 1.0 (parameters 24, 16, 37), with the "+" scrambler
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* (the returned value is the sum of the two state variables {@code x0} and {@code x1}).
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* Its state consists of two {@code long} fields {@code x0} and {@code x1},
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* which can take on any values provided that they are not both zero.
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* The period of this generator is 2<sup>128</sup>-1.
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*
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* <p>The 64-bit values produced by the {@code nextLong()} method are equidistributed.
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* To be precise, over the course of the cycle of length 2<sup>128</sup>-1,
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* each nonzero {@code long} value is generated 2<sup>64</sup> times,
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* but the value 0 is generated only 2<sup>64</sup>-1 times.
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* The values produced by the {@code nextInt()}, {@code nextFloat()}, and {@code nextDouble()}
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* methods are likewise equidistributed.
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*
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* <p>Instances {@code Xoroshiro128Plus} are <em>not</em> thread-safe.
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* They are designed to be used so that each thread as its own instance.
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* The methods {@link #jump} and {@link #leap} and {@link #jumps} and {@link #leaps}
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* can be used to construct new instances of {@code Xoroshiro128Plus} that traverse
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* other parts of the state cycle.
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*
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* <p>Instances of {@code Xoroshiro128Plus} are not cryptographically
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* secure. Consider instead using {@link java.security.SecureRandom}
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* in security-sensitive applications. Additionally,
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* default-constructed instances do not use a cryptographically random
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* seed unless the {@linkplain System#getProperty system property}
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* {@code java.util.secureRandomSeed} is set to {@code true}.
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*
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* @author Guy Steele
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* @author Doug Lea
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* @since 1.8
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*/
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public final class Xoroshiro128Plus implements LeapableRng {
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/*
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* Implementation Overview.
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*
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* This is an implementation of the xoroshiro128+ algorithm written
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* in 2016 by David Blackman and Sebastiano Vigna (vigna@acm.org),
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* and updated with improved parameters in 2018.
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* See http://xoshiro.di.unimi.it and these two papers:
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*
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* Sebastiano Vigna. 2016. An Experimental Exploration of Marsaglia's
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* xorshift Generators, Scrambled. ACM Transactions on Mathematical
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* Software 42, 4, Article 30 (June 2016), 23 pages.
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* https://doi.org/10.1145/2845077
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*
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* David Blackman and Sebastiano Vigna. 2018. Scrambled Linear
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* Pseudorandom Number Generators. Computing Research Repository (CoRR).
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* http://arxiv.org/abs/1805.01407
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*
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* The jump operation moves the current generator forward by 2*64
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* steps; this has the same effect as calling nextLong() 2**64
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* times, but is much faster. Similarly, the leap operation moves
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* the current generator forward by 2*96 steps; this has the same
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* effect as calling nextLong() 2**96 times, but is much faster.
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* The copy method may be used to make a copy of the current
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* generator. Thus one may repeatedly and cumulatively copy and
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* jump to produce a sequence of generators whose states are well
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* spaced apart along the overall state cycle (indeed, the jumps()
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* and leaps() methods each produce a stream of such generators).
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* The generators can then be parceled out to other threads.
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*
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* File organization: First the non-public methods that constitute the
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* main algorithm, then the public methods. Note that many methods are
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* defined by classes {@code AbstractJumpableRng} and {@code AbstractRng}.
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*/
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/* ---------------- static fields ---------------- */
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/**
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* The seed generator for default constructors.
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*/
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private static final AtomicLong defaultGen = new AtomicLong(RngSupport.initialSeed());
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/*
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* The period of this generator, which is 2**128 - 1.
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*/
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private static final BigInteger thePeriod =
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BigInteger.ONE.shiftLeft(128).subtract(BigInteger.ONE);
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/* ---------------- instance fields ---------------- */
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/**
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* The per-instance state.
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* At least one of the two fields x0 and x1 must be nonzero.
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*/
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private long x0, x1;
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/* ---------------- constructors ---------------- */
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/**
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* Basic constructor that initializes all fields from parameters.
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* It then adjusts the field values if necessary to ensure that
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* all constraints on the values of fields are met.
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*/
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public Xoroshiro128Plus(long x0, long x1) {
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this.x0 = x0;
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this.x1 = x1;
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// If x0 and x1 are both zero, we must choose nonzero values.
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if ((x0 | x1) == 0) {
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// At least one of the two values generated here will be nonzero.
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this.x0 = RngSupport.mixStafford13(x0 += RngSupport.GOLDEN_RATIO_64);
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this.x1 = (x0 += RngSupport.GOLDEN_RATIO_64);
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}
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}
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/**
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* Creates a new instance of {@code Xoroshiro128Plus} using the
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* specified {@code long} value as the initial seed. Instances of
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* {@code Xoroshiro128Plus} created with the same seed in the same
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* program generate identical sequences of values.
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*
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* @param seed the initial seed
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*/
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public Xoroshiro128Plus(long seed) {
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// Using a value with irregularly spaced 1-bits to xor the seed
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// argument tends to improve "pedestrian" seeds such as 0 or
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// other small integers. We may as well use SILVER_RATIO_64.
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//
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// The x values are then filled in as if by a SplitMix PRNG with
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// GOLDEN_RATIO_64 as the gamma value and Stafford13 as the mixer.
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this(RngSupport.mixStafford13(seed ^= RngSupport.SILVER_RATIO_64),
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RngSupport.mixStafford13(seed + RngSupport.GOLDEN_RATIO_64));
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}
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/**
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* Creates a new instance of {@code Xoroshiro128Plus} that is likely to
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* generate sequences of values that are statistically independent
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* of those of any other instances in the current program execution,
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* but may, and typically does, vary across program invocations.
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*/
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public Xoroshiro128Plus() {
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// Using GOLDEN_RATIO_64 here gives us a good Weyl sequence of values.
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this(defaultGen.getAndAdd(RngSupport.GOLDEN_RATIO_64));
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}
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/**
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* Creates a new instance of {@code Xoroshiro128Plus} using the specified array of
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* initial seed bytes. Instances of {@code Xoroshiro128Plus} created with the same
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* seed array in the same program execution generate identical sequences of values.
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*
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* @param seed the initial seed
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*/
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public Xoroshiro128Plus(byte[] seed) {
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// Convert the seed to 2 long values, which are not both zero.
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long[] data = RngSupport.convertSeedBytesToLongs(seed, 2, 2);
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long x0 = data[0], x1 = data[1];
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this.x0 = x0;
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this.x1 = x1;
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}
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/* ---------------- public methods ---------------- */
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public Xoroshiro128Plus copy() { return new Xoroshiro128Plus(x0, x1); }
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/*
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To the extent possible under law, the author has dedicated all copyright
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and related and neighboring rights to this software to the public domain
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worldwide. This software is distributed without any warranty.
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See <http://creativecommons.org/publicdomain/zero/1.0/>. */
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/* This is the successor to xorshift128+. It is the fastest full-period
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generator passing BigCrush without systematic failures, but due to the
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relatively short period it is acceptable only for applications with a
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mild amount of parallelism; otherwise, use a xorshift1024* generator.
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Beside passing BigCrush, this generator passes the PractRand test suite
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up to (and included) 16TB, with the exception of binary rank tests,
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which fail due to the lowest bit being an LFSR; all other bits pass all
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tests. We suggest to use a sign test to extract a random Boolean value.
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Note that the generator uses a simulated rotate operation, which most C
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compilers will turn into a single instruction. In Java, you can use
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Long.rotateLeft(). In languages that do not make low-level rotation
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instructions accessible xorshift128+ could be faster.
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The state must be seeded so that it is not everywhere zero. If you have
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a 64-bit seed, we suggest to seed a splitmix64 generator and use its
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output to fill s. */
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/**
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* Returns a pseudorandom {@code long} value.
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*
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* @return a pseudorandom {@code long} value
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*/
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public long nextLong() {
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final long s0 = x0;
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long s1 = x1;
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final long z = s0 + s1;
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s1 ^= s0;
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x0 = Long.rotateLeft(s0, 24) ^ s1 ^ (s1 << 16); // a, b
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x1 = Long.rotateLeft(s1, 37); // c
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return z;
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}
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public BigInteger period() { return thePeriod; }
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public double defaultJumpDistance() { return 0x1.0p64; }
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public double defaultLeapDistance() { return 0x1.0p96; }
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private static final long[] JUMP_TABLE = { 0xdf900294d8f554a5L, 0x170865df4b3201fcL };
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private static final long[] LEAP_TABLE = { 0xd2a98b26625eee7bL, 0xdddf9b1090aa7ac1L };
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/* This is the jump function for the generator. It is equivalent
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to 2**64 calls to nextLong(); it can be used to generate 2**64
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non-overlapping subsequences for parallel computations. */
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public void jump() { jumpAlgorithm(JUMP_TABLE); }
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/* This is the long-jump function for the generator. It is equivalent to
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2**96 calls to next(); it can be used to generate 2**32 starting points,
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from each of which jump() will generate 2**32 non-overlapping
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subsequences for parallel distributed computations. */
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public void leap() { jumpAlgorithm(LEAP_TABLE); }
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private void jumpAlgorithm(long[] table) {
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long s0 = 0, s1 = 0;
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for (int i = 0; i < table.length; i++) {
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for (int b = 0; b < 64; b++) {
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if ((table[i] & (1L << b)) != 0) {
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s0 ^= x0;
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s1 ^= x1;
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}
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nextLong();
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}
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x0 = s0;
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x1 = s1;
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}
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}
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}
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