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


/**
 * This class implements random variate generators for the 
 * chi distribution. It has  ν > 0 degrees of freedom and
 * its density function is
 * 
 * 

 * 

 * f (x) = e-x2/2xν-1/(2(ν/2)-1Γ(ν/2))        for x > 0
 * 

 * where 
 * Γ(x) is the gamma function defined
 * in {@link GammaGen}.
 * 
 * 

 * The (non-static) nextDouble method simply calls inverseF on the
 * distribution (slow).
 * 
 */
public class ChiGen extends RandomVariateGen  {
   protected int nu = -1;



   /**
    * Creates a chi  random variate generator with 
    *  ν = nu degrees of freedom, using stream s.
    * 
    */
   public ChiGen (RandomStream s, int nu)  {
      super (s, new ChiDist(nu));
      setParams (nu);
   }


   /**
    * Create a new generator for the distribution dist,
    *     using stream s.
    * 
    */
   public ChiGen (RandomStream s, ChiDist dist)  {
      super (s, dist);
      if (dist != null)
         setParams (dist.getNu ());
   }


   /**
    * Generates a random variate from the chi distribution with ν = nu
    *    degrees of freedom, using stream s.
    * 
    */
   public static double nextDouble (RandomStream s, int nu) {
      if (nu <= 0)
         throw new IllegalArgumentException ("nu <= 0");
      return ChiDist.inverseF (nu, s.nextDouble());
   }
 
     
   /**
    * Returns the value of ν for this object.
    * 
    * 
    */
   public int getNu() {
      return nu;
   }

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