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SSJ is a Java library for stochastic simulation, developed under the direction of Pierre L'Ecuyer, in the Département d'Informatique et de Recherche Opérationnelle (DIRO), at the Université de Montréal. It provides facilities for generating uniform and nonuniform random variates, computing different measures related to probability distributions, performing goodness-of-fit tests, applying quasi-Monte Carlo methods, collecting (elementary) statistics, and programming discrete-event simulations with both events and processes.

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/*
 * Class:        ChiSquareGen
 * Description:  random variate generators with the chi square distribution
 * Environment:  Java
 * Software:     SSJ 
 * Copyright (C) 2001  Pierre L'Ecuyer and Université de Montréal
 * Organization: DIRO, Université de Montréal
 * @author       
 * @since

 * SSJ is free software: you can redistribute it and/or modify it under
 * the terms of the GNU General Public License (GPL) as published by the
 * Free Software Foundation, either version 3 of the License, or
 * any later version.

 * SSJ is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 * GNU General Public License for more details.

 * A copy of the GNU General Public License is available at
   GPL licence site.
 */

package umontreal.iro.lecuyer.randvar;
import umontreal.iro.lecuyer.rng.*;
import umontreal.iro.lecuyer.probdist.*;


/**
 * This class implements random variate generators with the
 * chi square distribution with n > 0 degrees of freedom.
 * Its density function is 
 * 
 * 
 * 
 * f (x) = x^n/2-1e^-x/2/(2^n/2Γ(n/2)) for x > 0, 0 elsewhere
 * 

 * where 
 * Γ(x) is the gamma function defined
 * in {@link GammaGen}.
 * 
 * 
 * The (non-static) nextDouble method simply calls inverseF on the
 * distribution.
 * 
 */
public class ChiSquareGen extends RandomVariateGen  {
   protected int n = -1;
    



   /**
    * Creates a chi square  random variate generator with 
    *  n degrees of freedom, using stream s.
    * 
    */
   public ChiSquareGen (RandomStream s, int n)  {
      super (s, new ChiSquareDist(n));
      setParams (n);
   }

 
   /**
    * Create a new generator for the distribution dist
    *     and stream s.
    * 
    */
   public ChiSquareGen (RandomStream s, ChiSquareDist dist)  {
      super (s, dist);
      if (dist != null)
         setParams (dist.getN ());
   }


   /**
    * Generates a new variate from the chi square distribution 
    *    with n degrees of freedom, using stream s.
    * 
    */
   public static double nextDouble (RandomStream s, int n)  {
      return ChiSquareDist.inverseF (n, s.nextDouble());
   }

 
     
   /**
    * Returns the value of n for this object.
    * 
    * 
    */
   public int getN() {
      return n;
   }

 
     
   protected void setParams (int n) {
      if (n <= 0)
         throw new IllegalArgumentException ("n <= 0");
      this.n = n;
   }
}