L128X128Mix.java

  1. /*
  2.  * Licensed to the Apache Software Foundation (ASF) under one or more
  3.  * contributor license agreements.  See the NOTICE file distributed with
  4.  * this work for additional information regarding copyright ownership.
  5.  * The ASF licenses this file to You under the Apache License, Version 2.0
  6.  * (the "License"); you may not use this file except in compliance with
  7.  * the License.  You may obtain a copy of the License at
  8.  *
  9.  *      http://www.apache.org/licenses/LICENSE-2.0
  10.  *
  11.  * Unless required by applicable law or agreed to in writing, software
  12.  * distributed under the License is distributed on an "AS IS" BASIS,
  13.  * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
  14.  * See the License for the specific language governing permissions and
  15.  * limitations under the License.
  16.  */

  18. package org.apache.commons.rng.core.source64;

  20. import java.util.stream.Stream;
  21. import org.apache.commons.rng.JumpableUniformRandomProvider;
  22. import org.apache.commons.rng.SplittableUniformRandomProvider;
  23. import org.apache.commons.rng.UniformRandomProvider;
  24. import org.apache.commons.rng.core.util.NumberFactory;
  25. import org.apache.commons.rng.core.util.RandomStreams;

  27. /**
  28.  * A 64-bit all purpose generator.
  29.  *
  30.  * <p>This is a member of the LXM family of generators: L=Linear congruential generator;
  31.  * X=Xor based generator; and M=Mix. This member uses a 128-bit LCG and 128-bit Xor-based
  32.  * generator. It is named as {@code "L128X128MixRandom"} in the {@code java.util.random}
  33.  * package introduced in JDK 17; the LXM family is described in further detail in:
  34.  *
  35.  * <blockquote>Steele and Vigna (2021) LXM: better splittable pseudorandom number generators
  36.  * (and almost as fast). Proceedings of the ACM on Programming Languages, Volume 5,
  37.  * Article 148, pp 1–31.</blockquote>
  38.  *
  39.  * <p>Memory footprint is 384 bits and the period is 2<sup>128</sup> (2<sup>128</sup> - 1).
  40.  *
  41.  * <p>This generator implements
  42.  * {@link org.apache.commons.rng.LongJumpableUniformRandomProvider LongJumpableUniformRandomProvider}.
  43.  * In addition instances created with a different additive parameter for the LCG are robust
  44.  * against accidental correlation in a multi-threaded setting. The additive parameters must be
  45.  * different in the most significant 127-bits.
  46.  *
  47.  * <p>This generator implements
  48.  * {@link org.apache.commons.rng.SplittableUniformRandomProvider SplittableUniformRandomProvider}.
  49.  * The stream of generators created using the {@code splits} methods support parallelisation
  50.  * and are robust against accidental correlation by using unique values for the additive parameter
  51.  * for each instance in the same stream. The primitive streaming methods support parallelisation
  52.  * but with no assurances of accidental correlation; each thread uses a new instance with a
  53.  * randomly initialised state.
  54.  *
  55.  * @see <a href="https://doi.org/10.1145/3485525">Steele &amp; Vigna (2021) Proc. ACM Programming
  56.  *      Languages 5, 1-31</a>
  57.  * @see <a href="https://docs.oracle.com/en/java/javase/17/docs/api/java.base/java/util/random/package-summary.html">
  58.  *      JDK 17 java.util.random javadoc</a>
  59.  * @since 1.5
  60.  */
  61. public class L128X128Mix extends AbstractL128 implements SplittableUniformRandomProvider {
  62.     /** Size of the seed vector. */
  63.     private static final int SEED_SIZE = 6;
  64.     /** Size of the XBG state vector. */
  65.     private static final int XBG_STATE_SIZE = 2;
  66.     /** Low half of 128-bit LCG multiplier. */
  67.     private static final long ML = LXMSupport.M128L;

  69.     /** State 0 of the XBG. */
  70.     private long x0;
  71.     /** State 1 of the XBG. */
  72.     private long x1;

  74.     /**
  75.      * Creates a new instance.
  76.      *
  77.      * @param seed Initial seed.
  78.      * If the length is larger than 6, only the first 6 elements will
  79.      * be used; if smaller, the remaining elements will be automatically
  80.      * set. A seed containing all zeros in the last four elements
  81.      * will create a non-functional XBG sub-generator and a low
  82.      * quality output with a period of 2<sup>128</sup>.
  83.      *
  84.      * <p>The 1st and 2nd elements are used to set the LCG increment; the least significant bit
  85.      * is set to odd to ensure a full period LCG. The 3rd and 4th elements are used
  86.      * to set the LCG state.</p>
  87.      */
  88.     public L128X128Mix(long[] seed) {
  89.         super(seed = extendSeed(seed, SEED_SIZE));
  90.         x0 = seed[4];
  91.         x1 = seed[5];
  92.     }

  94.     /**
  95.      * Creates a new instance using a 6 element seed.
  96.      * A seed containing all zeros in the last four elements
  97.      * will create a non-functional XBG sub-generator and a low
  98.      * quality output with a period of 2<sup>128</sup>.
  99.      *
  100.      * <p>The 1st and 2nd elements are used to set the LCG increment; the least significant bit
  101.      * is set to odd to ensure a full period LCG. The 3rd and 4th elements are used
  102.      * to set the LCG state.</p>
  103.      *
  104.      * @param seed0 Initial seed element 0.
  105.      * @param seed1 Initial seed element 1.
  106.      * @param seed2 Initial seed element 2.
  107.      * @param seed3 Initial seed element 3.
  108.      * @param seed4 Initial seed element 4.
  109.      * @param seed5 Initial seed element 5.
  110.      */
  111.     public L128X128Mix(long seed0, long seed1, long seed2, long seed3,
  112.                        long seed4, long seed5) {
  113.         super(seed0, seed1, seed2, seed3);
  114.         x0 = seed4;
  115.         x1 = seed5;
  116.     }

  118.     /**
  119.      * Creates a copy instance.
  120.      *
  121.      * @param source Source to copy.
  122.      */
  123.     protected L128X128Mix(L128X128Mix source) {
  124.         super(source);
  125.         x0 = source.x0;
  126.         x1 = source.x1;
  127.     }

  129.     /** {@inheritDoc} */
  130.     @Override
  131.     protected byte[] getStateInternal() {
  132.         return composeStateInternal(NumberFactory.makeByteArray(
  133.                                         new long[] {x0, x1}),
  134.                                     super.getStateInternal());
  135.     }

  137.     /** {@inheritDoc} */
  138.     @Override
  139.     protected void setStateInternal(byte[] s) {
  140.         final byte[][] c = splitStateInternal(s, XBG_STATE_SIZE * Long.BYTES);
  141.         final long[] tmp = NumberFactory.makeLongArray(c[0]);
  142.         x0 = tmp[0];
  143.         x1 = tmp[1];
  144.         super.setStateInternal(c[1]);
  145.     }

  147.     /** {@inheritDoc} */
  148.     @Override
  149.     public long next() {
  150.         // LXM generate.
  151.         // Old state is used for the output allowing parallel pipelining
  152.         // on processors that support multiple concurrent instructions.

  154.         final long s0 = x0;
  155.         final long sh = lsh;

  157.         // Mix
  158.         final long z = LXMSupport.lea64(sh + s0);

  160.         // LCG update
  161.         // The LCG is, in effect, "s = m * s + a" where m = ((1LL << 64) + ML)
  162.         final long sl = lsl;
  163.         final long al = lal;
  164.         final long u = ML * sl;
  165.         // High half
  166.         lsh = ML * sh + LXMSupport.unsignedMultiplyHigh(ML, sl) + sl + lah +
  167.               // Carry propagation
  168.               LXMSupport.unsignedAddHigh(u, al);
  169.         // Low half
  170.         lsl = u + al;

  172.         // XBG update
  173.         long s1 = x1;

  175.         s1 ^= s0;
  176.         x0 = Long.rotateLeft(s0, 24) ^ s1 ^ (s1 << 16); // a, b
  177.         x1 = Long.rotateLeft(s1, 37); // c

  179.         return z;
  180.     }

  182.     /**
  183.      * {@inheritDoc}
  184.      *
  185.      * <p>The jump size is the equivalent of moving the state <em>backwards</em> by
  186.      * (2<sup>128</sup> - 1) positions. It can provide up to 2<sup>128</sup>
  187.      * non-overlapping subsequences.
  188.      */
  189.     @Override
  190.     public UniformRandomProvider jump() {
  191.         return super.jump();
  192.     }

  194.     /**
  195.      * {@inheritDoc}
  196.      *
  197.      * <p>The jump size is the equivalent of moving the state <em>backwards</em> by
  198.      * 2<sup>64</sup> (2<sup>128</sup> - 1) positions. It can provide up to
  199.      * 2<sup>64</sup> non-overlapping subsequences of length 2<sup>64</sup>
  200.      * (2<sup>128</sup> - 1); each subsequence can provide up to 2<sup>64</sup>
  201.      * non-overlapping subsequences of length (2<sup>128</sup> - 1) using the
  202.      * {@link #jump()} method.
  203.      */
  204.     @Override
  205.     public JumpableUniformRandomProvider longJump() {
  206.         return super.longJump();
  207.     }

  209.     /** {@inheritDoc} */
  210.     @Override
  211.     AbstractL128 copy() {
  212.         // This exists to ensure the jump function performed in the super class returns
  213.         // the correct class type. It should not be public.
  214.         return new L128X128Mix(this);
  215.     }

  217.     /** {@inheritDoc} */
  218.     @Override
  219.     public SplittableUniformRandomProvider split(UniformRandomProvider source) {
  220.         return create(source.nextLong(), source);
  221.     }

  223.     /** {@inheritDoc} */
  224.     @Override
  225.     public Stream<SplittableUniformRandomProvider> splits(long streamSize, SplittableUniformRandomProvider source) {
  226.         return RandomStreams.generateWithSeed(streamSize, source, L128X128Mix::create);
  227.     }

  229.     /**
  230.      * Create a new instance using the given {@code seed} and {@code source} of randomness
  231.      * to initialise the instance.
  232.      *
  233.      * @param seed Seed used to initialise the instance.
  234.      * @param source Source of randomness used to initialise the instance.
  235.      * @return A new instance.
  236.      */
  237.     private static SplittableUniformRandomProvider create(long seed, UniformRandomProvider source) {
  238.         // LCG state. The addition lower-half uses the input seed.
  239.         // The LCG addition parameter is set to odd so left-shift the seed.
  240.         final long s0 = source.nextLong();
  241.         final long s1 = seed << 1;
  242.         final long s2 = source.nextLong();
  243.         final long s3 = source.nextLong();
  244.         // XBG state must not be all zero
  245.         long x0 = source.nextLong();
  246.         long x1 = source.nextLong();
  247.         if ((x0 | x1) == 0) {
  248.             // SplitMix style seed ensures at least one non-zero value
  249.             final long z = s3;
  250.             x0 = LXMSupport.lea64(z);
  251.             x1 = LXMSupport.lea64(z + LXMSupport.GOLDEN_RATIO_64);
  252.         }
  253.         return new L128X128Mix(s0, s1, s2, s3, x0, x1);
  254.     }
  255. }
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