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 * <p><strong>Note:</strong> Although the seed size is 128 bits, only the first 64 are 29 * effective: in effect, two seeds that only differ by the last 64 bits may produce 30 * highly correlated sequences. 31 * 32 * @see <a href="http://www.pcg-random.org/"> 33 * PCG, A Family of Better Random Number Generators</a> 34 * @since 1.3 35 */ 36 public class PcgRxsMXs64 extends LongProvider { 37 /** Size of the seed array. */ 38 private static final int SEED_SIZE = 2; 39 /** The default increment. */ 40 private static final long DEFAULT_INCREMENT = 1442695040888963407L; 41 42 /** The state of the LCG. */ 43 private long state; 44 45 /** The increment of the LCG. */ 46 private long increment; 47 48 /** 49 * Creates a new instance using a default increment. 50 * 51 * @param seed Initial state. 52 * @since 1.4 53 */ 54 public PcgRxsMXs64(Long seed) { 55 increment = DEFAULT_INCREMENT; 56 state = bump(seed + this.increment); 57 } 58 59 /** 60 * Creates a new instance. 61 * 62 * <p><strong>Note:</strong> Although the seed size is 128 bits, only the first 64 are 63 * effective: in effect, two seeds that only differ by the last 64 bits may produce 64 * highly correlated sequences. 65 * 66 * @param seed Initial seed. 67 * If the length is larger than 2, only the first 2 elements will 68 * be used; if smaller, the remaining elements will be automatically set. 69 * 70 * <p>The 1st element is used to set the LCG state. The 2nd element is used 71 * to set the LCG increment; the most significant bit 72 * is discarded by left shift and the increment is set to odd.</p> 73 */ 74 public PcgRxsMXs64(long[] seed) { 75 if (seed.length < SEED_SIZE) { 76 final long[] tmp = new long[SEED_SIZE]; 77 fillState(tmp, seed); 78 setSeedInternal(tmp); 79 } else { 80 setSeedInternal(seed); 81 } 82 } 83 84 /** 85 * Seeds the RNG. 86 * 87 * @param seed Seed. 88 */ 89 private void setSeedInternal(long[] seed) { 90 // Ensure the increment is odd to provide a maximal period LCG. 91 this.increment = (seed[1] << 1) | 1; 92 this.state = bump(seed[0] + this.increment); 93 } 94 95 /** 96 * Provides the next state of the LCG. 97 * 98 * @param input Current state. 99 * @return next state 100 */ 101 private long bump(long input) { 102 return input * 6364136223846793005L + increment; 103 } 104 105 /** {@inheritDoc} */ 106 @Override 107 public long next() { 108 final long x = state; 109 state = bump(state); 110 final long word = ((x >>> ((x >>> 59) + 5)) ^ x) * -5840758589994634535L; 111 return (word >>> 43) ^ word; 112 } 113 114 /** {@inheritDoc} */ 115 @Override 116 protected byte[] getStateInternal() { 117 // The increment is divided by 2 before saving. 118 // This transform is used in the reference PCG code; it prevents restoring from 119 // a byte state a non-odd increment that results in a sub-maximal period generator. 120 return composeStateInternal(NumberFactory.makeByteArray( 121 new long[] {state, increment >>> 1}), 122 super.getStateInternal()); 123 } 124 125 /** {@inheritDoc} */ 126 @Override 127 protected void setStateInternal(byte[] s) { 128 final byte[][] c = splitStateInternal(s, SEED_SIZE * 8); 129 final long[] tempseed = NumberFactory.makeLongArray(c[0]); 130 state = tempseed[0]; 131 // Reverse the transform performed during getState to make the increment odd again. 132 increment = tempseed[1] << 1 | 1; 133 super.setStateInternal(c[1]); 134 } 135 }