MnParabola.h

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// @(#)root/minuit2:$Id: MnParabola.h 23522 2008-04-24 15:09:19Z moneta $
// 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

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