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


/**
 * This class implements methods for generating random variates from the
 * Fréchet distribution, with location parameter δ, scale
 *  parameter β > 0, and shape parameter 
 * α > 0, where we use
 *  the notation 
 * z = (x - δ)/β. It has density
 * 

 * 
 * 

 * f (x) = αe-z-α/(βzα+1),        for x > δ
 * 

 * The density is  0 for 
 * x <= δ.
 * 
 */
public class FrechetGen extends RandomVariateGen  {
   private double delta;
   private double beta;
   private double alpha;


   /**
    * Creates a Fréchet random number generator with α =
    *  alpha, β = 1 and 
    * δ = 0 using stream s.
    * 
    */
   public FrechetGen (RandomStream s, double alpha)  {
      this (s, alpha, 1.0, 0.0);
   }


   /**
    * Creates a Fréchet random number generator with parameters
    *  α = alpha, β = beta and δ = delta using stream s.
    * 
    */
   public FrechetGen (RandomStream s, double alpha, double beta,
                      double delta)  {
      super (s, new FrechetDist (alpha, beta, delta));
      setParams (alpha, beta, delta);
   }


   /**
    * Creates a new generator for the Fréchet distribution dist
    *    and stream s.
    * 
    */
   public FrechetGen (RandomStream s, FrechetDist dist)  {
      super (s, dist);
      if (dist != null)
         setParams (dist.getAlpha(), dist.getBeta(), dist.getDelta());
   } 


   /**
    * Generates a new variate from the Fréchet distribution with parameters
    * α = alpha,   β = beta and δ = delta using stream s.
    * 
    */
   public static double nextDouble (RandomStream s, double alpha,
                                    double beta, double delta) {
      return FrechetDist.inverseF (alpha, beta, delta, s.nextDouble());
   }


   /**
    * Returns the parameter α.
    * 
    */
   public double getAlpha() {
      return alpha;
   }


   /**
    * Returns the parameter β.
    * 
    */
   public double getBeta() {
      return beta;
   }


   /**
    * Returns the parameter δ.
    * 
    * 
    */
   public double getDelta() {
      return delta;
   }


   /**
    * Sets the parameters  α, β and δ of this object.
    * 
    */
   protected void setParams (double alpha, double beta, double delta) {
     if (beta <= 0.0)
         throw new IllegalArgumentException ("beta <= 0");
      if (alpha <= 0.0)
         throw new IllegalArgumentException ("alpha <= 0");
      this.delta = delta;
      this.beta = beta;
      this.alpha = alpha;
   }

}