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 */ 17 package org.apache.commons.rng.core.source64; 18 19 import org.apache.commons.rng.core.util.NumberFactory; 20 21 /** 22 * A Permuted Congruential Generator (PCG) that is composed of a 64-bit Linear Congruential 23 * Generator (LCG) combined with the RXS-M-XS (random xorshift; multiply; xorshift) output 24 * transformation to create 64-bit output. 25 * 26 * <p>State size is 128 bits and the period is 2<sup>64</sup>.</p> 27 * 28 * @see <a href="http://www.pcg-random.org/"> 29 * PCG, A Family of Better Random Number Generators</a> 30 * @since 1.3 31 */ 32 public class PcgRxsMXs64 extends LongProvider { 33 /** Size of the seed array. */ 34 private static final int SEED_SIZE = 2; 35 36 /** The state of the LCG. */ 37 private long state; 38 39 /** The increment of the LCG. */ 40 private long increment; 41 42 /** 43 * Creates a new instance. 44 * 45 * @param seed Initial seed. 46 * If the length is larger than 2, only the first 2 elements will 47 * be used; if smaller, the remaining elements will be automatically set. 48 */ 49 public PcgRxsMXs64(long[] seed) { 50 if (seed.length < SEED_SIZE) { 51 final long[] tmp = new long[SEED_SIZE]; 52 fillState(tmp, seed); 53 setSeedInternal(tmp); 54 } else { 55 setSeedInternal(seed); 56 } 57 } 58 59 /** 60 * Seeds the RNG. 61 * 62 * @param seed Seed. 63 */ 64 private void setSeedInternal(long[] seed) { 65 // Ensure the increment is odd to provide a maximal period LCG. 66 this.increment = (seed[1] << 1) | 1; 67 this.state = bump(seed[0] + this.increment); 68 } 69 70 /** 71 * Provides the next state of the LCG. 72 * 73 * @param input Current state. 74 * @return next state 75 */ 76 private long bump(long input) { 77 return input * 6364136223846793005L + increment; 78 } 79 80 /** {@inheritDoc} */ 81 @Override 82 public long next() { 83 final long x = state; 84 state = bump(state); 85 final long word = ((x >>> ((x >>> 59) + 5)) ^ x) * -5840758589994634535L; 86 return (word >>> 43) ^ word; 87 } 88 89 /** {@inheritDoc} */ 90 @Override 91 protected byte[] getStateInternal() { 92 // The increment is divided by 2 before saving. 93 // This transform is used in the reference PCG code; it prevents restoring from 94 // a byte state a non-odd increment that results in a sub-maximal period generator. 95 return composeStateInternal(NumberFactory.makeByteArray( 96 new long[] {state, increment >>> 1}), 97 super.getStateInternal()); 98 } 99 100 /** {@inheritDoc} */ 101 @Override 102 protected void setStateInternal(byte[] s) { 103 final byte[][] c = splitStateInternal(s, SEED_SIZE * 8); 104 final long[] tempseed = NumberFactory.makeLongArray(c[0]); 105 state = tempseed[0]; 106 // Reverse the transform performed during getState to make the increment odd again. 107 increment = tempseed[1] << 1 | 1; 108 super.setStateInternal(c[1]); 109 } 110 }