1 package nl.tudelft.simulation.jstats.distributions;
2
3 import org.djutils.exceptions.Throw;
4
5 import nl.tudelft.simulation.jstats.math.ProbMath;
6 import nl.tudelft.simulation.jstats.streams.StreamInterface;
7
8 /**
9 * The Poisson distribution. For more information on this distribution see
10 * <a href="https://mathworld.wolfram.com/PoissonDistribution.html"> https://mathworld.wolfram.com/PoissonDistribution.html </a>
11 * <p>
12 * Copyright (c) 2002-2024 Delft University of Technology, Jaffalaan 5, 2628 BX Delft, the Netherlands. All rights reserved. See
13 * for project information <a href="https://simulation.tudelft.nl/" target="_blank"> https://simulation.tudelft.nl</a>. The DSOL
14 * project is distributed under a three-clause BSD-style license, which can be found at
15 * <a href="https://https://simulation.tudelft.nl/dsol/docs/latest/license.html" target="_blank">
16 * https://https://simulation.tudelft.nl/dsol/docs/latest/license.html</a>.
17 * </p>
18 * @author <a href="https://www.linkedin.com/in/peterhmjacobs">Peter Jacobs </a>
19 * @author <a href="https://www.tudelft.nl/averbraeck">Alexander Verbraeck</a>
20 */
21 public class DistPoisson extends DistDiscrete
22 {
23 /** */
24 private static final long serialVersionUID = 1L;
25
26 /** lambda is the lambda parameter. */
27 private final double lambda;
28
29 /** expl is a helper variable. */
30 private final double expl;
31
32 /**
33 * constructs a new Poisson distribution.
34 * @param stream StreamInterface; the random number stream
35 * @param lambda double; the lambda parameter
36 * @throws IllegalArgumentException when lambda <= 0
37 */
38 public DistPoisson(final StreamInterface stream, final double lambda)
39 {
40 super(stream);
41 Throw.when(lambda <= 0.0, IllegalArgumentException.class, "Error Poisson - lambda<=0");
42 this.lambda = lambda;
43 this.expl = Math.exp(-this.lambda);
44 }
45
46 /** {@inheritDoc} */
47 @Override
48 public long draw()
49 {
50 // Adapted from Fortran program in Shannon, Systems Simulation, 1975, p. 359
51 double s = 1.0;
52 long x = -1;
53 do
54 {
55 s = s * this.stream.nextDouble();
56 x++;
57 }
58 while (s > this.expl);
59 return x;
60 }
61
62 /** {@inheritDoc} */
63 @Override
64 public double probability(final long observation)
65 {
66 if (observation >= 0)
67 {
68 return (Math.exp(-this.lambda) * Math.pow(this.lambda, observation)) / ProbMath.factorial(observation);
69 }
70 return 0;
71 }
72
73 /**
74 * @return lambda
75 */
76 public double getLambda()
77 {
78 return this.lambda;
79 }
80
81 /** {@inheritDoc} */
82 @Override
83 public String toString()
84 {
85 return "Poisson(" + this.lambda + ")";
86 }
87 }