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

/**
 * This class implements random variate generators for the 
 * binomial distribution. It has parameters n and p with
 *  mass function
 * 
 * 

 * 

 * p(x) = nCr(n, x)px(1 - p)n-x = n!/(x!(n - x)!)  px(1 - p)n-x        for x = 0, 1, 2,..., n
 * 

 * where nCr(n, x) is the number of combinations of x objects
 * among n,
 * n is a positive integer, and 
 * 0 <= p <= 1.
 * 
 * 

 * The (non-static) nextInt method simply calls inverseF on the
 * distribution.
 * 
 */
public class BinomialGen extends RandomVariateGenInt  {
   protected int    n = -1;
   protected double p = -1.0;    
    


   /**
    * Creates a binomial random variate generator with parameters n and p,
    *   using stream s.
    * 
    */
   public BinomialGen (RandomStream s, int n, double p) {
      super (s, new BinomialDist (n, p));
      setParams (n, p);
   }


   /**
    * Creates a random variate generator for the binomial 
    *     distribution dist and the random stream s.
    * 
    */
   public BinomialGen (RandomStream s, BinomialDist dist)  {
      super (s, dist);
      if (dist != null)
         setParams (dist.getN(), dist.getP());
   }


   /**
    * Generates a new integer from the binomial distribution with
    *   parameters
    *    n = n and p = p, using the given stream s.
    * 
    */
   public static int nextInt (RandomStream s, int n, double p) {
      return BinomialDist.inverseF (n, p, s.nextDouble());
   }


   /**
    * Returns the parameter n of this object.
    * 
    */
   public int getN() {
      return n;
   }
   


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


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

}