/* University College London Dept of P&A course in C++ 3C59 
| all rights reserved 2000
|
| Module:  Consolidation excercise: 
|
| Title: ComptonAnalysis 
|
| Class: Datum
|   
| Description:
|   This class is used to create objects which can hold the information pertaining to a
|   single datum as measured in the 3rd year lab compton scattering experiment.
|   A datum consists of: 
|      - The angle at which a photopeak energy was measured
|      - The measured photopeak energy
|      - The uncertainty on the energy measurement
|
| Author: P.Clarke 
*/

#ifndef  _datum_h
#define  _datum_h 1


#include <iostream>
#include <cmath>


class Datum {

  private:

	// These are the member variables which each object of this class will have
    float m_angle ;
    float m_energy ;
    float m_errEnergy ;


  public:

	// --------------------------
	// This method is a used to initialise an object after it has been created.
	// (this is provided because at this stage we have not covered constructors)

	  void initialise( float inputAngle, float inputEnergy, float inputError  )
	  {
		  // Students should be clear that here we set:
		  //   member variable = value input as argument to method 

		  m_angle     = inputAngle ;
		  m_energy    = inputEnergy ;
		  m_errEnergy = inputError ;

		  // Note the useful convention: prefix all member variables with m_....
		  // This means that in all your methods it is crystal clear which variables
		  // are member variables as opposed to temporary local variables

		  return ;

    }  // End of method



	// ---------------------------
  // This method asks the Datum object to calculate its contribution to the chi-squared
	// What this means is that it has to form the quantity 
	//    (Epredicted-Emeasured)^2 / error^2
	// corresponding to this Datum object
	//
	// In this calculation it already knows everything it needs to know except the mass
	// of the elctron which we are treating as an unknown.
	// Therefore when we ask a Datum object to do this calculation we have to pass it
	// the electron mass we want it to use.

    float chiSquared( float mass )
	  {

      // First lets calculate the predicted energy from the formula
		
        float eIncident  = 662.0 ;
        float invE       = 0.0 ;
        float ePredicted = 0.0;
		
        float angleInRadians = m_angle * 3.14159 / 180.0 ;

		    invE = (1.0/eIncident) + (1.0/mass) - ( cos( angleInRadians ) / mass ) ;

		    ePredicted = 1.0/invE ;

		  // Now calculate the chi-squared contribution

        float chisq = 0 ;

        chisq = (m_energy - ePredicted)*(m_energy - ePredicted)/(m_errEnergy*m_errEnergy);


      // Finally return the chi-squared contribution to the caller of this method

        return chisq ;

    }  // End of method


  // -------------------------
  // Method to dump contents for verification

    void dump()
    {
      std::cout << " Datum contents : " 
                << " Angle = " << m_angle
                << " / Energy = " << m_energy << " +- " << m_errEnergy 
                << std::endl;
    }

  // --------------------------
  // Please note how clear I make the boundary between each method. I use comment line
  // extensively to do this.
  // --------------------------

} ;

#endif