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


/**
 * This class implements random variate generators having the 
 * negative binomial distribution. Its mass function is
 * 
 * 

 * 

 * p(x) = Γ(γ + x)/(x! Γ(γ)) pγ(1 - p)x,        for x = 0, 1, 2,…
 * 

 * where Γ is the gamma function, 
 * 
 * γ > 0 and 
 * 0 <= p <= 1.
 * No local copy of the parameters γ and p is maintained in this class.
 * The (non-static) nextInt method simply calls inverseF on the
 * distribution.
 * 
 */
public class NegativeBinomialGen extends RandomVariateGenInt  {
   protected double gamma;
   protected double p; 



   /**
    * Creates a negative binomial random variate generator with parameters
    *   γ = gamma and p,  using stream s.
    * 
    */
   public NegativeBinomialGen (RandomStream s, double gamma, double p)  {
      super (s, new NegativeBinomialDist (gamma, p));
      setParams (gamma, p);
   }


   /**
    * Creates a new generator for the distribution dist, using 
    *   stream s.
    * 
    */
   public NegativeBinomialGen (RandomStream s, NegativeBinomialDist dist)  {
      super (s, dist);
      if (dist != null)
         setParams (dist.getGamma(), dist.getP());
   }

    
   /**
    * Generates a new variate from the negative binomial distribution,
    *  with parameters γ = gamma and p = p,
    *  using stream s.
    * 
    */
   public static int nextInt (RandomStream s, double gamma, double p)  {
      return NegativeBinomialDist.inverseF (gamma, p, s.nextDouble());
   }


   /**
    * Returns the parameter γ of this object.
    * 
    */
   public double getGamma() {
      return gamma;
   }


   /**
    * Returns the parameter p of this object.
    * 
    * 
    */
   public double getP() {
      return p;
   }


   /**
    * Sets the parameter γ and p of this object.
    * 
    */
   protected void setParams (double gamma, double p) {
      if (p < 0.0 || p > 1.0)
         throw new IllegalArgumentException ("p not in [0, 1]");
      if (gamma <= 0.0)
         throw new IllegalArgumentException ("gamma <= 0");
      this.p = p;
      this.gamma = gamma;
   }

}