#ifndef TLineFit_hxx_seen
#define TLineFit_hxx_seen

namespace COMET {
    class ILineFit;
};

/// Do a line fit of Y as a function of X to a set of data points using a chi2
/// fit.  The point values are not saved so this can be used to do running
/// fits, but has the disadvantage that you have to calculate chi2 for
/// yourself.  Almost no internal information is saved, so you can use this as
/// in inline variable.
///
/// \code
/// ILineFit fit;
///
/// cout << fit.GetIntercept();   -- prints "0"
/// cout << fit.GetSlope();       -- prints "0"
///
/// fit.AddPoint(1,1,1);
/// cout << fit.GetIntercept();   -- prints "1"
/// cout << fit.GetSlope();       -- prints "0"
///
/// fit.AddPoint(2,2,1);
/// cout << fit.GetIntercept();   -- prints "0"
/// cout << fit.GetSlope();       -- prints "1"
///
/// fit.AddPoint(3,1,1);
/// cout << fit.GetIntercept();   -- prints "1.33333"
/// cout << fit.GetSlope();       -- prints "0"
///
/// fit.RemovePoint(1,1,1);
/// cout << fit.GetIntercept();   -- prints "4"
/// cout << fit.GetSlope();       -- prints "-1"
/// \endcode
/// The code is protected against points with zero or negative uncertainty,
/// and against removing too many points.  Note that if you remove a point
/// that was not actually added, your screwed.
///
/// One final comment.  This code would make a numerist or computer algorithm
/// expert shudder since it will accumulate error over the calculation.  Don't
/// let that scare you away, but do let it warn you.  This is intended as a
/// way to incrementally follow a line, it is not intended as a way to get the
/// best fit for a line.  If you want numeric accuracy, use a global fit.  If
/// you want robust ray shooting, use this.
class COMET::ILineFit {
public:
    ILineFit();
    /// Do a line fit of n points with independent variable X, and dependent
    /// variable Y.
    ILineFit(int n, double x[], double y[], double sigma[]);

    /// Add a point to the fit with X as the independent variable, and Y as
    /// the dependent variable.
    void AddPoint(double x, double y, double sigma=1);

    /// Remove a point from the fit with X as the independent variable, and Y
    /// as the dependent variable.
    void RemovePoint(double x, double y, double sigma=1);

    /// Get the intercept.
    double GetIntercept(void) const;

    /// Get the uncertainty in the intercept position (doesn't include any
    /// correlation to the slope).
    double GetInterceptUncertainty(void) const;

    /// Get the slope
    double GetSlope(void) const;

    /// Get the uncertainty in the slope.
    double GetSlopeUncertainty(void) const;

    /// Get the value of the dependent variable as a function of the
    /// independent variable.
    double GetValue(double x) const;

    /// Get the central value of the independent variable.
    double GetCentralIndependent(void) const;

    /// Get the number of points in the current fit.
    int GetNumberOfPoints(void) const {return fPoints;}

    /// Return true if the fit will have a valid slope and intercept.
    bool IsValid(void) const;

private:
    int fPoints;
    double fX, fA, fB, fC, fD, fE;
};    
#endif