package umontreal.iro.lecuyer.randvar;
import umontreal.iro.lecuyer.probdist.*;
import umontreal.iro.lecuyer.rng.*;
   
/**
 * This class implements random variate generators having the Poisson 
 * distribution.  Its mass function is
 * 
 * 

 * 

 * p(x) = e-λλx/(x!) for x = 0, 1,...
 * 

 * where 
 * λ > 0 is a real valued parameter equal to the mean.
 * 
 * 

 * No local copy of the parameter λ = lambda
 * is maintained in this class.
 * The (non-static) nextInt method simply calls inverseF on the
 * distribution.
 * 
 */
public class PoissonGen extends RandomVariateGenInt  {
   protected double lambda; 



   /**
    * Creates a Poisson random variate generator with 
    *   parameter λ = lambda, using stream s.
    * 
    */
   public PoissonGen (RandomStream s, double lambda)  {
      super (s, new PoissonDist (lambda));
      setParams (lambda);
   }


   /**
    * Creates a new random variate generator using the Poisson 
    *     distribution dist and stream s.
    * 
    */
   public PoissonGen (RandomStream s, PoissonDist dist)  {
      super (s, dist);
      if (dist != null)
         setParams (dist.getLambda());
   }


   /**
    * A static method for generating a random variate from a 
    *   Poisson distribution with parameter λ = lambda.
    * 
    */
   public static int nextInt (RandomStream s, double lambda)  {
      return PoissonDist.inverseF (lambda, s.nextDouble());
   }


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



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

}