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) = xn/2-1e-x/2/(2n/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;
   }
}