1 /* |
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2 * Copyright (c) 2013, 2019, Oracle and/or its affiliates. All rights reserved. |
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3 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER. |
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4 * |
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5 * This code is free software; you can redistribute it and/or modify it |
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6 * under the terms of the GNU General Public License version 2 only, as |
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7 * published by the Free Software Foundation. Oracle designates this |
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8 * particular file as subject to the "Classpath" exception as provided |
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9 * by Oracle in the LICENSE file that accompanied this code. |
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10 * |
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11 * This code is distributed in the hope that it will be useful, but WITHOUT |
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12 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or |
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13 * FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License |
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14 * version 2 for more details (a copy is included in the LICENSE file that |
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15 * accompanied this code). |
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16 * |
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17 * You should have received a copy of the GNU General Public License version |
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18 * 2 along with this work; if not, write to the Free Software Foundation, |
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19 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA. |
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20 * |
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21 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA |
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22 * or visit www.oracle.com if you need additional information or have any |
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23 * questions. |
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24 */ |
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25 package java.util; |
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26 |
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27 import java.math.BigInteger; |
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28 import java.util.concurrent.atomic.AtomicLong; |
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29 |
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30 /** |
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31 * A generator of uniform pseudorandom values applicable for use in |
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32 * (among other contexts) isolated parallel computations that may |
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33 * generate subtasks. Class {@code Xoroshiro128Plus} implements |
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34 * interfaces {@link java.util.Rng} and {@link java.util.LeapableRng}, |
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35 * and therefore supports methods for producing pseudorandomly chosen |
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36 * numbers of type {@code int}, {@code long}, {@code float}, and {@code double} |
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37 * as well as creating new {@code Xoroshiro128Plus} objects |
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38 * by "jumping" or "leaping". |
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39 * |
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40 * <p>Series of generated values pass the TestU01 BigCrush and PractRand test suites |
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41 * that measure independence and uniformity properties of random number generators, |
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42 * <em>except</em> that it does not pass the binary rank tests of PractRand, |
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43 * which fail due to the lowest bit being an LFSR; all other bits pass all tests. |
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44 * For this reason may be best for some purposes to use this generator to generate |
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45 * pseudorandom {@code int}, {@code float}, and {@code double} values but not |
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46 * {@code long} values. For the same reason, it may be best not to use the |
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47 * method {@code nextGaussian} or {@code nextExponential} with this generator. |
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48 * |
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49 * <p>The class {@code Xoroshiro128Plus} uses the {@code xoroshiro128} algorithm, |
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50 * version 1.0 (parameters 24, 16, 37), with the "+" scrambler |
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51 * (the returned value is the sum of the two state variables {@code x0} and {@code x1}). |
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52 * Its state consists of two {@code long} fields {@code x0} and {@code x1}, |
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53 * which can take on any values provided that they are not both zero. |
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54 * The period of this generator is 2<sup>128</sup>-1. |
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55 * |
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56 * <p>The 64-bit values produced by the {@code nextLong()} method are equidistributed. |
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57 * To be precise, over the course of the cycle of length 2<sup>128</sup>-1, |
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58 * each nonzero {@code long} value is generated 2<sup>64</sup> times, |
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59 * but the value 0 is generated only 2<sup>64</sup>-1 times. |
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60 * The values produced by the {@code nextInt()}, {@code nextFloat()}, and {@code nextDouble()} |
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61 * methods are likewise equidistributed. |
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62 * |
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63 * <p>Instances {@code Xoroshiro128Plus} are <em>not</em> thread-safe. |
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64 * They are designed to be used so that each thread as its own instance. |
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65 * The methods {@link #jump} and {@link #leap} and {@link #jumps} and {@link #leaps} |
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66 * can be used to construct new instances of {@code Xoroshiro128Plus} that traverse |
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67 * other parts of the state cycle. |
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68 * |
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69 * <p>Instances of {@code Xoroshiro128Plus} are not cryptographically |
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70 * secure. Consider instead using {@link java.security.SecureRandom} |
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71 * in security-sensitive applications. Additionally, |
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72 * default-constructed instances do not use a cryptographically random |
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73 * seed unless the {@linkplain System#getProperty system property} |
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74 * {@code java.util.secureRandomSeed} is set to {@code true}. |
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75 * |
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76 * @author Guy Steele |
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77 * @author Doug Lea |
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78 * @since 1.8 |
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79 */ |
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80 public final class Xoroshiro128Plus implements LeapableRng { |
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81 |
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82 /* |
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83 * Implementation Overview. |
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84 * |
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85 * This is an implementation of the xoroshiro128+ algorithm written |
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86 * in 2016 by David Blackman and Sebastiano Vigna (vigna@acm.org), |
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87 * and updated with improved parameters in 2018. |
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88 * See http://xoshiro.di.unimi.it and these two papers: |
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89 * |
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90 * Sebastiano Vigna. 2016. An Experimental Exploration of Marsaglia's |
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91 * xorshift Generators, Scrambled. ACM Transactions on Mathematical |
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92 * Software 42, 4, Article 30 (June 2016), 23 pages. |
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93 * https://doi.org/10.1145/2845077 |
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94 * |
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95 * David Blackman and Sebastiano Vigna. 2018. Scrambled Linear |
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96 * Pseudorandom Number Generators. Computing Research Repository (CoRR). |
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97 * http://arxiv.org/abs/1805.01407 |
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98 * |
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99 * The jump operation moves the current generator forward by 2*64 |
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100 * steps; this has the same effect as calling nextLong() 2**64 |
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101 * times, but is much faster. Similarly, the leap operation moves |
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102 * the current generator forward by 2*96 steps; this has the same |
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103 * effect as calling nextLong() 2**96 times, but is much faster. |
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104 * The copy method may be used to make a copy of the current |
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105 * generator. Thus one may repeatedly and cumulatively copy and |
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106 * jump to produce a sequence of generators whose states are well |
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107 * spaced apart along the overall state cycle (indeed, the jumps() |
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108 * and leaps() methods each produce a stream of such generators). |
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109 * The generators can then be parceled out to other threads. |
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110 * |
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111 * File organization: First the non-public methods that constitute the |
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112 * main algorithm, then the public methods. Note that many methods are |
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113 * defined by classes {@code AbstractJumpableRng} and {@code AbstractRng}. |
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114 */ |
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115 |
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116 /* ---------------- static fields ---------------- */ |
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117 |
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118 /** |
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119 * The seed generator for default constructors. |
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120 */ |
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121 private static final AtomicLong defaultGen = new AtomicLong(RngSupport.initialSeed()); |
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122 |
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123 /* |
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124 * The period of this generator, which is 2**128 - 1. |
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125 */ |
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126 private static final BigInteger thePeriod = |
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127 BigInteger.ONE.shiftLeft(128).subtract(BigInteger.ONE); |
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128 |
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129 /* ---------------- instance fields ---------------- */ |
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130 |
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131 /** |
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132 * The per-instance state. |
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133 * At least one of the two fields x0 and x1 must be nonzero. |
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134 */ |
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135 private long x0, x1; |
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136 |
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137 /* ---------------- constructors ---------------- */ |
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138 |
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139 /** |
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140 * Basic constructor that initializes all fields from parameters. |
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141 * It then adjusts the field values if necessary to ensure that |
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142 * all constraints on the values of fields are met. |
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143 * |
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144 * @param x0 first word of the initial state |
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145 * @param x1 second word of the initial state |
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146 */ |
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147 public Xoroshiro128Plus(long x0, long x1) { |
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148 this.x0 = x0; |
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149 this.x1 = x1; |
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150 // If x0 and x1 are both zero, we must choose nonzero values. |
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151 if ((x0 | x1) == 0) { |
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152 // At least one of the two values generated here will be nonzero. |
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153 this.x0 = RngSupport.mixStafford13(x0 += RngSupport.GOLDEN_RATIO_64); |
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154 this.x1 = (x0 += RngSupport.GOLDEN_RATIO_64); |
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155 } |
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156 } |
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157 |
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158 /** |
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159 * Creates a new instance of {@code Xoroshiro128Plus} using the |
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160 * specified {@code long} value as the initial seed. Instances of |
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161 * {@code Xoroshiro128Plus} created with the same seed in the same |
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162 * program generate identical sequences of values. |
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163 * |
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164 * @param seed the initial seed |
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165 */ |
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166 public Xoroshiro128Plus(long seed) { |
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167 // Using a value with irregularly spaced 1-bits to xor the seed |
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168 // argument tends to improve "pedestrian" seeds such as 0 or |
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169 // other small integers. We may as well use SILVER_RATIO_64. |
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170 // |
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171 // The x values are then filled in as if by a SplitMix PRNG with |
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172 // GOLDEN_RATIO_64 as the gamma value and Stafford13 as the mixer. |
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173 this(RngSupport.mixStafford13(seed ^= RngSupport.SILVER_RATIO_64), |
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174 RngSupport.mixStafford13(seed + RngSupport.GOLDEN_RATIO_64)); |
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175 } |
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176 |
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177 /** |
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178 * Creates a new instance of {@code Xoroshiro128Plus} that is likely to |
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179 * generate sequences of values that are statistically independent |
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180 * of those of any other instances in the current program execution, |
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181 * but may, and typically does, vary across program invocations. |
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182 */ |
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183 public Xoroshiro128Plus() { |
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184 // Using GOLDEN_RATIO_64 here gives us a good Weyl sequence of values. |
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185 this(defaultGen.getAndAdd(RngSupport.GOLDEN_RATIO_64)); |
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186 } |
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187 |
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188 /** |
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189 * Creates a new instance of {@code Xoroshiro128Plus} using the specified array of |
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190 * initial seed bytes. Instances of {@code Xoroshiro128Plus} created with the same |
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191 * seed array in the same program execution generate identical sequences of values. |
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192 * |
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193 * @param seed the initial seed |
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194 */ |
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195 public Xoroshiro128Plus(byte[] seed) { |
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196 // Convert the seed to 2 long values, which are not both zero. |
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197 long[] data = RngSupport.convertSeedBytesToLongs(seed, 2, 2); |
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198 long x0 = data[0], x1 = data[1]; |
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199 this.x0 = x0; |
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200 this.x1 = x1; |
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201 } |
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202 |
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203 /* ---------------- public methods ---------------- */ |
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204 |
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205 public Xoroshiro128Plus copy() { return new Xoroshiro128Plus(x0, x1); } |
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206 |
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207 /* |
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208 |
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209 To the extent possible under law, the author has dedicated all copyright |
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210 and related and neighboring rights to this software to the public domain |
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211 worldwide. This software is distributed without any warranty. |
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212 |
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213 See <http://creativecommons.org/publicdomain/zero/1.0/>. */ |
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214 |
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215 /* This is the successor to xorshift128+. It is the fastest full-period |
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216 generator passing BigCrush without systematic failures, but due to the |
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217 relatively short period it is acceptable only for applications with a |
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218 mild amount of parallelism; otherwise, use a xorshift1024* generator. |
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219 |
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220 Beside passing BigCrush, this generator passes the PractRand test suite |
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221 up to (and included) 16TB, with the exception of binary rank tests, |
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222 which fail due to the lowest bit being an LFSR; all other bits pass all |
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223 tests. We suggest to use a sign test to extract a random Boolean value. |
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224 |
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225 Note that the generator uses a simulated rotate operation, which most C |
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226 compilers will turn into a single instruction. In Java, you can use |
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227 Long.rotateLeft(). In languages that do not make low-level rotation |
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228 instructions accessible xorshift128+ could be faster. |
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229 |
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230 The state must be seeded so that it is not everywhere zero. If you have |
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231 a 64-bit seed, we suggest to seed a splitmix64 generator and use its |
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232 output to fill s. */ |
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233 |
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234 |
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235 /** |
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236 * Returns a pseudorandom {@code long} value. |
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237 * |
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238 * @return a pseudorandom {@code long} value |
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239 */ |
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240 public long nextLong() { |
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241 final long s0 = x0; |
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242 long s1 = x1; |
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243 final long z = s0 + s1; |
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244 |
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245 s1 ^= s0; |
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246 x0 = Long.rotateLeft(s0, 24) ^ s1 ^ (s1 << 16); // a, b |
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247 x1 = Long.rotateLeft(s1, 37); // c |
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248 |
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249 return z; |
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250 } |
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251 |
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252 public BigInteger period() { return thePeriod; } |
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253 |
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254 public double defaultJumpDistance() { return 0x1.0p64; } |
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255 |
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256 public double defaultLeapDistance() { return 0x1.0p96; } |
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257 |
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258 private static final long[] JUMP_TABLE = { 0xdf900294d8f554a5L, 0x170865df4b3201fcL }; |
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259 |
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260 private static final long[] LEAP_TABLE = { 0xd2a98b26625eee7bL, 0xdddf9b1090aa7ac1L }; |
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261 |
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262 /* This is the jump function for the generator. It is equivalent |
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263 to 2**64 calls to nextLong(); it can be used to generate 2**64 |
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264 non-overlapping subsequences for parallel computations. */ |
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265 |
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266 public void jump() { jumpAlgorithm(JUMP_TABLE); } |
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267 |
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268 /* This is the long-jump function for the generator. It is equivalent to |
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269 2**96 calls to next(); it can be used to generate 2**32 starting points, |
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270 from each of which jump() will generate 2**32 non-overlapping |
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271 subsequences for parallel distributed computations. */ |
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272 |
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273 public void leap() { jumpAlgorithm(LEAP_TABLE); } |
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274 |
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275 private void jumpAlgorithm(long[] table) { |
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276 long s0 = 0, s1 = 0; |
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277 for (int i = 0; i < table.length; i++) { |
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278 for (int b = 0; b < 64; b++) { |
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279 if ((table[i] & (1L << b)) != 0) { |
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280 s0 ^= x0; |
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281 s1 ^= x1; |
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282 } |
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283 nextLong(); |
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284 } |
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285 x0 = s0; |
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286 x1 = s1; |
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287 } |
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288 } |
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289 } |
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