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.sampling.distribution; 18 19 import org.apache.commons.rng.UniformRandomProvider; 20 21 /** 22 * Sampler for the <a href="http://mathworld.wolfram.com/PoissonDistribution.html">Poisson distribution</a>. 23 * 24 * <ul> 25 * <li> 26 * For small means, a Poisson process is simulated using uniform deviates, as 27 * described <a href="http://mathaa.epfl.ch/cours/PMMI2001/interactive/rng7.htm">here</a>. 28 * The Poisson process (and hence, the returned value) is bounded by 1000 * mean. 29 * </li> 30 * <li> 31 * For large means, we use the rejection algorithm described in 32 * <blockquote> 33 * Devroye, Luc. (1981). <i>The Computer Generation of Poisson Random Variables</i><br> 34 * <strong>Computing</strong> vol. 26 pp. 197-207. 35 * </blockquote> 36 * </li> 37 * </ul> 38 * 39 * @since 1.0 40 */ 41 public class PoissonSampler 42 extends SamplerBase 43 implements DiscreteSampler { 44 45 /** Value for switching sampling algorithm. */ 46 private static final double PIVOT = 40; 47 /** The internal Poisson sampler. */ 48 private final DiscreteSampler poissonSampler; 49 50 /** 51 * @param rng Generator of uniformly distributed random numbers. 52 * @param mean Mean. 53 * @throws IllegalArgumentException if {@code mean <= 0}. 54 */ 55 public PoissonSampler(UniformRandomProvider rng, 56 double mean) { 57 super(null); 58 59 // Delegate all work to specialised samplers. 60 // These should check the input arguments. 61 poissonSampler = mean < PIVOT ? 62 new SmallMeanPoissonSampler(rng, mean) : 63 new LargeMeanPoissonSampler(rng, mean); 64 } 65 66 /** {@inheritDoc} */ 67 @Override 68 public int sample() { 69 return poissonSampler.sample(); 70 } 71 72 /** {@inheritDoc} */ 73 @Override 74 public String toString() { 75 return poissonSampler.toString(); 76 } 77 }