/** University College London Dept of P&A Course in OO 3C59 
| all rights reserved 1999 
|
| class: RanCom
|
| 
| This class provides a method to generates an angle in the range 0-180 degrees
| which has the correct probability distribution for Compton scattering.
|
| The method is:
|     float scatter( )
|
|
|
| NOTE: In this class the angles are in degrees !
|
| Author: P.Clarke 
*/

import java.lang.Math ;
import java.util.Random ;

public class RanCom {

    // Member variable to store electron mass in KeV
    private double electronMass ;

     // Member variable to store the incident photon energy in KeV
    private double incidentEnergy ;

    // Random number generator
	private Random rnums ;


  //--------------------------
  //Constructor which takes two parameters
  //- the electron mass is in KeV
  //- the incident photon energy in KeV
  public RanCom( double emass, double incident ) 
	{
      // Set the electron mass
      electronMass = emass ;

      // Set the incident photon energy
      incidentEnergy = incident ;

	  // get a randome generator
      rnums = new Random() ;

    }


    //------------------------
    //This is the method the user needs. 
    //It returns an angle picked randomly using the Compton scattering 
    //probability distribution.
    public double scatter( ) 
	{
 
      boolean notdone = true ;
      double phi = 0  ;
      double Pmax, Pphi, Ptest ;

      // The maximum of the compton distribution is at phi = 0
      // We need this value
      Pmax = this.probability( 0.) ;

      while( notdone ) {

        // Generate a random x value in range in [0,180]
        phi = this.uniform() * 180.0   ;

        //Calc the probability function at at phi 
        Pphi = this.probability(phi) ;

        //Generate a random number in the range [0,Pmax]
        Ptest = this.uniform() * Pmax ;

        //Test if Ptest < Pphi
        //If it is then we keep this value of phi and so can stop, 
        //otherwise continue in the loop
        if( Ptest < Pphi ) notdone = false ; 
    
        //System.out.println( " phi Ptest Pmax/ "+phi+" / "+Ptest+" / "+Pmax+" \n " );

      }

      return phi ;

    }


    //--------------
    // Method to calculate the Compton probability functin for a givven 
    // scattering angle
    private double probability( double phiDeg ) 
	  {

		double alpha = incidentEnergy/electronMass ;  

		double prob, exp1, exp2, exp3, exp4 ;

		double phi = phiDeg / 360.0 * 2.0 * Math.PI ;


		/* Parts of expression */

		exp1 = alpha * ( 1 - Math.cos(phi) ) ;

		exp2 = Math.pow( 1 / ( 1 + exp1 ),  2 ) ;

		exp3 = Math.pow(exp1, 2) / ( 1 + exp1 ) ;

		exp4 = 1 + Math.pow(Math.cos(phi),2.) + exp3 ;

		prob = exp2 * exp4 ;

		return prob ;

	}

    //---------------
    // To get a uniform random number in range 0-1
    private double uniform( )
	{
	   double x = rnums.nextDouble() ;
	   //x += 1. ;
	   //x /= 2. ;

	   //System.out.println( " uniform x= "+x+" \n " );

	   return x ;
     }
}