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1   package nl.tudelft.simulation.jstats.distributions;
2   
3   import org.djutils.exceptions.Throw;
4   
5   import nl.tudelft.simulation.jstats.streams.StreamInterface;
6   
7   /**
8    * The Weibull distribution with a shape parameter α and a scale parameter β. For more information on this
9    * distribution see <a href="https://mathworld.wolfram.com/WeibullDistribution.html">
10   * https://mathworld.wolfram.com/WeibullDistribution.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 DistWeibull extends DistContinuous
22  {
23      /** */
24      private static final long serialVersionUID = 1L;
25  
26      /** alpha is the shape parameter &alpha;. */
27      private final double alpha;
28  
29      /** beta is the scale parameter &beta;. */
30      private final double beta;
31  
32      /**
33       * constructs a new Weibull distribution.
34       * @param stream StreamInterface; the random number stream
35       * @param alpha double; the shape parameter &alpha;
36       * @param beta double; the scale parameter &beta;
37       */
38      public DistWeibull(final StreamInterface stream, final double alpha, final double beta)
39      {
40          super(stream);
41          Throw.when(alpha <= 0.0 || beta <= 0.0, IllegalArgumentException.class, "Error Weibull - alpha <= 0.0 or beta <= 0.0");
42          this.alpha = alpha;
43          this.beta = beta;
44      }
45  
46      /** {@inheritDoc} */
47      @Override
48      public double draw()
49      {
50          return this.beta * Math.pow(-Math.log(this.stream.nextDouble()), 1.0d / this.alpha);
51      }
52  
53      /** {@inheritDoc} */
54      @Override
55      public double getProbabilityDensity(final double x)
56      {
57          if (x > 0)
58          {
59              return this.alpha * Math.pow(this.beta, -this.alpha) * Math.pow(x, this.alpha - 1)
60                      * Math.exp(-Math.pow(x / this.beta, this.alpha));
61          }
62          return 0.0;
63      }
64  
65      /**
66       * Return the shape parameter &alpha;.
67       * @return double; the shape parameter &alpha;
68       */
69      public double getAlpha()
70      {
71          return this.alpha;
72      }
73  
74      /**
75       * Return the scale parameter &beta;.
76       * @return double; the scale parameter &beta;
77       */
78      public double getBeta()
79      {
80          return this.beta;
81      }
82  
83      /** {@inheritDoc} */
84      @Override
85      public String toString()
86      {
87          return "Weibull(" + this.alpha + "," + this.beta + ")";
88      }
89  }