AsymmetricPearsonVII.h

// Mantid Repository : https://github.com/mantidproject/mantid
//
// Copyright © 2022 ISIS Rutherford Appleton Laboratory UKRI,
//   NScD Oak Ridge National Laboratory, European Spallation Source,
//   Institut Laue - Langevin & CSNS, Institute of High Energy Physics, CAS
// SPDX - License - Identifier: GPL - 3.0 +
#pragma once

#include "MantidAPI/IPeakFunction.h"
#include "MantidCurveFitting/DllConfig.h"

namespace Mantid {
namespace CurveFitting {
namespace Functions {

/**
  Provides an implementation of the asymmetric PearsonVII function (sometimes it
  is also referred to as the split-PearsonVII function). This function combines
  two PearsonVII functions so to fit sharp peak curves. It is useful for analysis
  of X-ray diffractions peaks which consist of two separable components. The
  function takes 5 parameters: peak height (ph), peak center (pc), FWHM (w), left
  (ml) and right (mr) shape parameters.

  The function is defined as:

  AsymmetricPearsonVII(x, ph, pc, w, ml, mr) =
    PearsonVII(x, ph, pc, w, ml)*θ(pc-x) + PearsonVII(x, ph, pc, w, mr)*θ(x-pc),

  which is a superposition of left and right K-alpha peaks. θ denotes the Heavyside
  step function. The parameters ml and mr determine (for the left and right side of
  the function, respectively) whether the function is more close to a Lorentzian or
  a Gaussian. When ml/mr=1 it's a Lorentzian, whereas when ml/mr->infinity it's a
  Gaussian.

  @author Oleksandr Koshchii, Forshungsztentrum Jülich at MLZ, Garching
  @date 18/02/2022
*/

class MANTID_CURVEFITTING_DLL AsymmetricPearsonVII : public API::IPeakFunction {
public:
  /// Constructor
  AsymmetricPearsonVII();

  /// Override IPeakFunction base class methods
  double height() const override;
  double centre() const override;
  double fwhm() const override;
  double leftShape() const;
  double rightShape() const;

  void setCentre(const double newCentre) override;
  void setHeight(const double newHight) override;
  void setFwhm(const double newFwhm) override;
  void setLeftShape(const double newLeftShape);
  void setRightShape(const double newRightShape);

  /// Override IFunction base class methods
  std::string name() const override { return "AsymmetricPearsonVII"; }
  const std::string category() const override { return "XrayDiffraction"; }

  double activeParameter(size_t i) const override;
  void setActiveParameter(size_t i, double value) override;

  double getPearsonVII(double peak_height, double offset, double weight, double m) const;
  double getPearsonVIIDerivWRTh(double offset, double weight, double m) const;
  double getPearsonVIIDerivWRTc(double peak_height, double offset, double weight, double m) const;
  double getPearsonVIIDerivWRTw(double peak_height, double offset, double weight, double m) const;
  double getPearsonVIIDerivWRTm(double peak_height, double offset, double weight, double m) const;

  double getPearsonVIILimitmEq0(double peak_height) const;
  double getPearsonVIIDerivWRThLimitmEq0() const;
  double getPearsonVIIDerivWRTcLimitmEq0() const;
  double getPearsonVIIDerivWRTwLimitmEq0() const;
  double getPearsonVIIDerivWRTmLimitmEq0(double peak_height, double offset, double weight) const;

protected:
  void functionLocal(double *out, const double *xValues, const size_t nData) const override;
  void functionDerivLocal(API::Jacobian *out, const double *xValues, const size_t nData) override;
  /// Override IFunction base class method, which declares function parameters
  void init() override;
};

double denominator_function(double offset_sq, double weight_sq, double m);
double derivative_function(double peak_height, double offset, double weight, double m);
double m_derivative_function(double peak_height, double offset_sq, double weight_sq, double m);

} // namespace Functions
} // namespace CurveFitting
} // namespace Mantid