304 lines
14 KiB
Java
304 lines
14 KiB
Java
/*
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* Copyright (c) 2021, Oracle and/or its affiliates. All rights reserved.
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* DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
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*
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* This code is free software; you can redistribute it and/or modify it
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* under the terms of the GNU General Public License version 2 only, as
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* published by the Free Software Foundation. Oracle designates this
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* particular file as subject to the "Classpath" exception as provided
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* by Oracle in the LICENSE file that accompanied this code.
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*
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* This code is distributed in the hope that it will be useful, but WITHOUT
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* ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
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* FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
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* version 2 for more details (a copy is included in the LICENSE file that
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* accompanied this code).
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*
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* You should have received a copy of the GNU General Public License version
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* 2 along with this work; if not, write to the Free Software Foundation,
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* Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
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*
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* Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
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* or visit www.oracle.com if you need additional information or have any
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* questions.
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*/
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package jdk.random;
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import java.util.concurrent.atomic.AtomicLong;
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import java.util.random.RandomGenerator;
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import jdk.internal.util.random.RandomSupport;
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import jdk.internal.util.random.RandomSupport.AbstractSplittableWithBrineGenerator;
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import jdk.internal.util.random.RandomSupport.RandomGeneratorProperties;
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/**
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* A "splittable" pseudorandom number generator (PRNG) whose period
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* is roughly 2<sup>384</sup>. Class {@link L128X256MixRandom} implements
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* interfaces {@link RandomGenerator} and {@link SplittableGenerator},
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* and therefore supports methods for producing pseudorandomly chosen
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* values of type {@code int}, {@code long}, {@code float}, {@code double},
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* and {@code boolean} (and for producing streams of pseudorandomly chosen
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* numbers of type {@code int}, {@code long}, and {@code double}),
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* as well as methods for creating new split-off {@link L128X256MixRandom}
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* objects or streams of such objects.
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*
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* <p>The {@link L128X256MixRandom} algorithm is a specific member of
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* the LXM family of algorithms for pseudorandom number generators;
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* for more information, see the documentation for package
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* {@link jdk.random}. Each instance of {@link L128X256MixRandom}
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* has 384 bits of state plus one 128-bit instance-specific parameter.
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*
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* <p>If two instances of {@link L128X256MixRandom} are created with
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* the same seed within the same program execution, and the same
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* sequence of method calls is made for each, they will generate and
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* return identical sequences of values.
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*
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* <p>As with {@link java.util.SplittableRandom}, instances of
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* {@link L128X256MixRandom} are <em>not</em> thread-safe. They are
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* designed to be split, not shared, across threads (see the {@link #split}
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* method). For example, a {@link java.util.concurrent.ForkJoinTask}
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* fork/join-style computation using random numbers might include a
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* construction of the form
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* {@code new Subtask(someL128X256MixRandom.split()).fork()}.
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*
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* <p>This class provides additional methods for generating random
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* streams, that employ the above techniques when used in
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* {@code stream.parallel()} mode.
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*
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* <p>Instances of {@link L128X256MixRandom} 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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* @since 17
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*
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*/
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@RandomGeneratorProperties(
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name = "L128X256MixRandom",
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group = "LXM",
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i = 256, j = 1, k = 128,
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equidistribution = 1
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)
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public final class L128X256MixRandom extends AbstractSplittableWithBrineGenerator {
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/*
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* Implementation Overview.
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*
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* The 128-bit parameter `a` is represented as two long fields `ah` and `al`.
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* The 128-bit state variable `s` is represented as two long fields `sh` and `sl`.
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*
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* The split operation uses the current generator to choose eight
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* new 64-bit long values that are then used to initialize the
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* parameters `ah` and `al` and the state variables `sh`, `sl`,
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* `x0`, `x1`, `x2`, and `x3` for a newly constructed generator.
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*
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* With extremely high probability, no two generators so chosen
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* will have the same `a` parameter, and testing has indicated
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* that the values generated by two instances of {@link L128X256MixRandom}
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* will be (approximately) independent if have different values for `a`.
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*
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* The default (no-argument) constructor, in essence, uses
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* "defaultGen" to generate eight new 64-bit values for the same
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* purpose. Multiple generators created in this way will certainly
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* differ in their `a` parameters. The defaultGen state must be accessed
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* in a thread-safe manner, so we use an AtomicLong to represent
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* this state. To bootstrap the defaultGen, we start off using a
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* seed based on current time unless the
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* java.util.secureRandomSeed property is set. This serves as a
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* slimmed-down (and insecure) variant of SecureRandom that also
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* avoids stalls that may occur when using /dev/random.
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*
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* File organization: First static fields, then instance
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* fields, then constructors, then instance methods.
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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(RandomSupport.initialSeed());
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/*
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* The equidistribution of the algorithm.
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*/
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private static final int EQUIDISTRIBUTION = 1;
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/*
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* Low half of multiplier used in the LCG portion of the algorithm;
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* the overall multiplier is (2**64 + ML).
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* Chosen based on research by Sebastiano Vigna and Guy Steele (2019).
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* The spectral scores for dimensions 2 through 8 for the multiplier 0x1d605bbb58c8abbfdLL
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* are [0.991889, 0.907938, 0.830964, 0.837980, 0.780378, 0.797464, 0.761493].
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*/
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private static final long ML = 0xd605bbb58c8abbfdL;
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/* ---------------- instance fields ---------------- */
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/**
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* The parameter that is used as an additive constant for the LCG.
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* Must be odd (therefore al must be odd).
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*/
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private final long ah, al;
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/**
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* The per-instance state: sh and sl for the LCG; x0, x1, x2, and x3 for the XBG.
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* At least one of the four fields x0, x1, x2, and x3 must be nonzero.
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*/
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private long sh, sl, x0, x1, x2, x3;
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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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* @param ah high half of the additive parameter for the LCG
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* @param al low half of the additive parameter for the LCG
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* @param sh high half of the initial state for the LCG
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* @param sl low half of the initial state for the LCG
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* @param x0 first word of the initial state for the XBG
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* @param x1 second word of the initial state for the XBG
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* @param x2 third word of the initial state for the XBG
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* @param x3 fourth word of the initial state for the XBG
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*/
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public L128X256MixRandom(long ah, long al, long sh, long sl, long x0, long x1, long x2, long x3) {
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// Force a to be odd.
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this.ah = ah;
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this.al = al | 1;
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this.sh = sh;
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this.sl = sl;
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this.x0 = x0;
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this.x1 = x1;
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this.x2 = x2;
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this.x3 = x3;
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// If x0, x1, x2, and x3 are all zero, we must choose nonzero values.
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if ((x0 | x1 | x2 | x3) == 0) {
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long v = sh;
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// At least three of the four values generated here will be nonzero.
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this.x0 = RandomSupport.mixStafford13(v += RandomSupport.GOLDEN_RATIO_64);
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this.x1 = RandomSupport.mixStafford13(v += RandomSupport.GOLDEN_RATIO_64);
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this.x2 = RandomSupport.mixStafford13(v += RandomSupport.GOLDEN_RATIO_64);
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this.x3 = RandomSupport.mixStafford13(v + RandomSupport.GOLDEN_RATIO_64);
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}
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}
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/**
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* Creates a new instance of {@link L128X256MixRandom} using the
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* specified {@code long} value as the initial seed. Instances of
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* {@link L128X256MixRandom} 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 L128X256MixRandom(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 seed is hashed by mixMurmur64 to produce the `a` parameter.
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// The seed is hashed by mixStafford13 to produce the initial `x0`,
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// which will then be used to produce the first generated value.
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// The other x values are filled in as if by a SplitMix PRNG with
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// GOLDEN_RATIO_64 as the gamma value and mixStafford13 as the mixer.
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this(RandomSupport.mixMurmur64(seed ^= RandomSupport.SILVER_RATIO_64),
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RandomSupport.mixMurmur64(seed += RandomSupport.GOLDEN_RATIO_64),
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0,
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1,
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RandomSupport.mixStafford13(seed),
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RandomSupport.mixStafford13(seed += RandomSupport.GOLDEN_RATIO_64),
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RandomSupport.mixStafford13(seed += RandomSupport.GOLDEN_RATIO_64),
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RandomSupport.mixStafford13(seed + RandomSupport.GOLDEN_RATIO_64));
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}
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/**
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* Creates a new instance of {@link L128X256MixRandom} 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 L128X256MixRandom() {
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// Using GOLDEN_RATIO_64 here gives us a good Weyl sequence of values.
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this(defaultGen.getAndAdd(RandomSupport.GOLDEN_RATIO_64));
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}
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/**
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* Creates a new instance of {@link L128X256MixRandom} using the specified array of
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* initial seed bytes. Instances of {@link L128X256MixRandom} 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 L128X256MixRandom(byte[] seed) {
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// Android-changed: backport https://bugs.openjdk.org/browse/JDK-8283083.
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// Convert the seed to 8 long values, of which the last 4 are not all zero.
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// long[] data = RandomSupport.convertSeedBytesToLongs(seed, 6, 4);
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long[] data = RandomSupport.convertSeedBytesToLongs(seed, 8, 4);
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long ah = data[0], al = data[1], sh = data[2], sl = data[3],
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x0 = data[4], x1 = data[5], x2 = data[6], x3 = data[7];
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// Force a to be odd.
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this.ah = ah;
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this.al = al | 1;
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this.sh = sh;
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this.sl = sl;
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this.x0 = x0;
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this.x1 = x1;
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this.x2 = x2;
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this.x3 = x3;
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}
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/* ---------------- public methods ---------------- */
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@Override
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public SplittableGenerator split(SplittableGenerator source, long brine) {
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// Pick a new instance "at random", but use the brine for (the low half of) `a`.
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return new L128X256MixRandom(source.nextLong(), brine << 1,
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source.nextLong(), source.nextLong(),
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source.nextLong(), source.nextLong(),
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source.nextLong(), source.nextLong());
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}
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@Override
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public long nextLong() {
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// Compute the result based on current state information
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// (this allows the computation to be overlapped with state update).
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final long result = RandomSupport.mixLea64(sh + x0);
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// Update the LCG subgenerator
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// The LCG is, in effect, s = ((1LL << 64) + ML) * s + a, if only we had 128-bit arithmetic.
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final long u = ML * sl;
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// Note that Math.multiplyHigh computes the high half of the product of signed values,
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// but what we need is the high half of the product of unsigned values; for this we use the
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// formula "unsignedMultiplyHigh(a, b) = multiplyHigh(a, b) + ((a >> 63) & b) + ((b >> 63) & a)";
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// in effect, each operand is added to the result iff the sign bit of the other operand is 1.
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// (See Henry S. Warren, Jr., _Hacker's Delight_ (Second Edition), Addison-Wesley (2013),
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// Section 8-3, p. 175; or see the First Edition, Addison-Wesley (2003), Section 8-3, p. 133.)
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// If Math.unsignedMultiplyHigh(long, long) is ever implemented, the following line can become:
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// sh = (ML * sh) + Math.unsignedMultiplyHigh(ML, sl) + sl + ah;
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// and this entire comment can be deleted.
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sh = (ML * sh) + (Math.multiplyHigh(ML, sl) + ((ML >> 63) & sl) + ((sl >> 63) & ML)) + sl + ah;
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sl = u + al;
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if (Long.compareUnsigned(sl, u) < 0) ++sh; // Handle the carry propagation from low half to high half.
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// Update the XBG subgenerator
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long q0 = x0, q1 = x1, q2 = x2, q3 = x3;
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{ // xoshiro256 1.0
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long t = q1 << 17;
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q2 ^= q0;
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q3 ^= q1;
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q1 ^= q2;
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q0 ^= q3;
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q2 ^= t;
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q3 = Long.rotateLeft(q3, 45);
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}
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x0 = q0; x1 = q1; x2 = q2; x3 = q3;
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return result;
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}
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}
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