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


/**
 * This class implements methods for generating random variates from the
 * Student distribution with n > 0 degrees of freedom.
 * Its density function is
 * 

 * 
 * 

 * f (x) = [Γ((n + 1)/2)/(Γ(n/2)(πn)1/2)][1 + x2/n]-(n+1)/2                for  - ∞ < x < ∞,
 * 

 * where 
 * Γ(x) is the gamma function defined in
 * {@link GammaGen}.
 * 
 * 

 * The nextDouble method simply calls inverseF on the
 * distribution.
 * 
 * 

 * The following table gives the CPU time needed to generate 107 Student
 *  random variates using the different implementations available in SSJ.
 *  The second test (Q) was made with the inverse in
 *   {@link StudentDistQuick},
 *  while the first test was made with the inverse in {@link StudentDist}.
 * These tests were made on a machine with processor AMD Athlon 4000, running
 * Red Hat Linux, with clock speed at 2400 MHz.
 * 
 * 

 * 

 * 
 * 
 * 
 * 
 * 
 * 
 * 
 * 
 * 
 * 
 * 
 * 
 * 
 * 
Generator
time in seconds
StudentGen
22.4
StudentGen(Q)
6.5
StudentPolarGen
1.4
 * 
 * 
 */
public class StudentGen extends RandomVariateGen  {
   protected int n = -1;


   /**
    * Creates a Student random variate generator with
    *  n degrees of freedom, using stream s.
    * 
    */
   public StudentGen (RandomStream s, int n)  {
      super (s, new StudentDist(n));
      setN (n);
   }


   /**
    * Creates a new generator for the Student distribution dist
    *      and stream s.
    * 
    */
   public StudentGen (RandomStream s, StudentDist dist)  {
      super (s, dist);
      if (dist != null)
         setN (dist.getN ());
   }


   /**
    * Generates a new variate from the Student distribution
    *    with n = n degrees of freedom, using stream s.
    * 
    */
   public static double nextDouble (RandomStream s, int n)  {
      return StudentDist.inverseF (n, s.nextDouble());
    }


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



   protected void setN (int nu) {
      if (nu <= 0)
         throw new IllegalArgumentException ("n <= 0");
      this.n = nu;
   }
}