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


/**
 * This class implements random variate generators for the gamma 
 * distribution. Its parameters are α > 0 and λ > 0. 
 * Its density function is
 * 
 * 

 * 

 * f (x) = λαxα-1e-λx/Γ(α)         for x > 0,
 * 

 * where Γ is the gamma function defined by
 * 
 * 

 * 

 * Γ(α) = ∫0∞xα-1e-x dx.
 * 

 * 
 * 

 * The (non-static) nextDouble method simply calls inverseF on the
 * distribution. 
 * 
 */
public class GammaGen extends RandomVariateGen  {
   protected double alpha = -1.0;
   protected double lambda = -1.0;




   /**
    * Creates a gamma random variate generator with parameters
    *  α = alpha and λ = lambda,
    *   using stream s.
    * 
    */
   public GammaGen (RandomStream s, double alpha, double lambda)  {
      super (s, new GammaDist(alpha, lambda));
      setParams (alpha, lambda);
   }


   /**
    * Creates a gamma random variate generator with parameters
    *  α = alpha and 
    * λ = 1, using stream s.
    * 
    */
   public GammaGen (RandomStream s, double alpha)  {
      this (s, alpha, 1.0);
   }

   
   /**
    * Creates a new generator object for the gamma
    *     distribution dist and stream s.
    * 
    */
   public GammaGen (RandomStream s, GammaDist dist)  {
      super (s, dist);
      if (dist != null)
         setParams (dist.getAlpha(), dist.getLambda());
   }


   /**
    * Generates a new gamma random variate 
    *   with parameters α = alpha and λ = lambda, 
    *   using stream s.
    * 
    */
   public static double nextDouble (RandomStream s, 
                                    double alpha, double lambda) {
      return GammaDist.inverseF (alpha, lambda, 15, s.nextDouble());
   }


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


   /**
    * Returns the parameter λ of this object.
    * 
    * 
    */
   public double getLambda() {
      return lambda;
   }


   /**
    * Sets the parameter α and λ of this object.
    * 
    */
   protected void setParams (double alpha, double lambda) {
      if (lambda <= 0.0)
         throw new IllegalArgumentException ("lambda <= 0");
      if (alpha <= 0.0)
         throw new IllegalArgumentException ("alpha <= 0");
      this.lambda = lambda;
      this.alpha = alpha;
   }

}