MnParabola.h

Go to the documentation of this file.

// @(#)root/minuit2:$Id$
// Authors: M. Winkler, F. James, L. Moneta, A. Zsenei   2003-2005
 
/**********************************************************************
 *                                                                    *
 * Copyright (c) 2005 LCG ROOT Math team,  CERN/PH-SFT                *
 *                                                                    *
 **********************************************************************/
 
#ifndef ROOT_Minuit2_MnParabola
#define ROOT_Minuit2_MnParabola
 
#include <math.h>
 
namespace ROOT {
 
   namespace Minuit2 {
 
 
/**
 
This class defines a parabola of the form a*x*x + b*x + c
 
@author Fred James and Matthias Winkler; comments added by Andras Zsenei
and Lorenzo Moneta
 
@ingroup Minuit
 
 */
 
class MnParabola {
 
public:
 
 
  /**
 
  Constructor that initializes the parabola with its three parameters.
 
  @param a the coefficient of the quadratic term
  @param b the coefficient of the linear term
  @param c the constant
 
  */
 
  MnParabola(double a, double b, double c) : fA(a), fB(b), fC(c) {}
 
 
  ~MnParabola() {}
 
 
  /**
 
  Evaluates the parabola a the point x.
 
  @param x the coordinate where the parabola needs to be evaluated.
 
  @return the y coordinate of the parabola corresponding to x.
 
  */
 
  double Y(double x) const {return (fA*x*x + fB*x +fC);}
 
 
  /**
 
  Calculates the bigger of the two x values corresponding to the
  given y Value.
 
  <p>
 
  ???????!!!!!!!!! And when there is none?? it looks like it will
  crash?? what is sqrt (-1.0) ?
 
  @param y the y Value for which the x Value is to be calculated.
 
  @return the bigger one of the two corresponding values.
 
  */
 
  // ok, at first glance it does not look like the formula for the quadratic
  // equation, but it is!  ;-)
  double X_pos(double y) const {return (sqrt(y/fA + Min()*Min() - fC/fA) + Min());}
  // maybe it is worth to check the performance improvement with the below formula??
  //   double X_pos(double y) const {return (sqrt(y/fA + fB*fB/(4.*fA*fA) - fC/fA)  - fB/(2.*fA));}
 
 
 
  /**
 
  Calculates the smaller of the two x values corresponding to the
  given y Value.
 
  <p>
 
  ???????!!!!!!!!! And when there is none?? it looks like it will
  crash?? what is sqrt (-1.0) ?
 
  @param y the y Value for which the x Value is to be calculated.
 
  @return the smaller one of the two corresponding values.
 
  */
 
  double X_neg(double y) const {return (-sqrt(y/fA + Min()*Min() - fC/fA) + Min());}
 
 
  /**
 
  Calculates the x coordinate of the Minimum of the parabola.
 
  @return x coordinate of the Minimum.
 
  */
 
  double Min() const {return -fB/(2.*fA);}
 
 
  /**
 
  Calculates the y coordinate of the Minimum of the parabola.
 
  @return y coordinate of the Minimum.
 
  */
 
  double YMin() const {return (-fB*fB/(4.*fA) + fC);}
 
 
  /**
 
  Accessor to the coefficient of the quadratic term.
 
  @return the coefficient of the quadratic term.
 
   */
 
  double A() const {return fA;}
 
 
  /**
 
  Accessor to the coefficient of the linear term.
 
  @return the coefficient of the linear term.
 
  */
 
  double B() const {return fB;}
 
 
  /**
 
  Accessor to the coefficient of the constant term.
 
  @return the coefficient of the constant term.
 
  */
 
  double C() const {return fC;}
 
private:
 
  double fA;
  double fB;
  double fC;
};
 
  }  // namespace Minuit2
 
}  // namespace ROOT
 
#endif  // ROOT_Minuit2_MnParabola