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


/**
 * This class implements normal random variate generators using
 *  the polar method with rejection.
 * Since the method generates two variates at a time,
 * the second variate is returned upon the next call to {@link #nextDouble nextDouble}.
 * 
 */
public class NormalPolarGen extends NormalGen  {

   // used by polar method which calculate always two random values;
   private boolean available = false;
   private double[] variates = new double[2];
   private static double[] staticVariates = new double[2];


   /**
    * Creates a normal random variate generator with mean mu
    *   and standard deviation sigma, using stream s.
    * 
    */
   public NormalPolarGen (RandomStream s, double mu, double sigma)  {
      super (s, null);
      setParams (mu, sigma);
   }


   /**
    * Creates a standard normal random variate generator with μ = 0
    *  and σ = 1, using stream s.
    * 
    */
   public NormalPolarGen (RandomStream s)  {
      this (s, 0.0, 1.0);
   }


   /**
    * Creates a random variate generator for
    *   the normal distribution dist and stream s.
    * 
    */
   public NormalPolarGen (RandomStream s, NormalDist dist)  {
      super (s, dist);
      if (dist != null)
         setParams (dist.getMu(), dist.getSigma());
   }


   public double nextDouble() {
      if (available) {
         available = false;
         return mu + sigma*variates[1];
      }
      else {
         polar (stream, mu, sigma, variates);
         available = true;
         return mu + sigma*variates[0];
      }
   }

   public static double nextDouble (RandomStream s, double mu, double sigma) {
      polar (s, mu, sigma, staticVariates);
      return mu + sigma*staticVariates[0];
   }
   /**
    * Generates a variate from the normal distribution with
    *    parameters μ = mu and σ = sigma, using
    *    stream s.
    * 
    */


//>>>>>>>>>>>>>>>>>>>>  P R I V A T E     M E T H O D S   <<<<<<<<<<<<<<<<<<<<
   // Polar method with rejection
   private static void polar (RandomStream stream, double mu,
                              double sigma, double[] variates) {
      double x, y, s;
      do {
        x = 2*stream.nextDouble() - 1;
        y = 2*stream.nextDouble() - 1;
        s = x*x + y*y;
      } while (s > 1.0 || s == 0.0);

      double temp = Math.sqrt (-2.0*Math.log (s)/s);

      variates[0] = y*temp;
      variates[1] = x*temp;
   }

}