ElasticDiffRotDiscreteCircle.cpp

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// Mantid Repository : https://github.com/mantidproject/mantid
//
// Copyright © 2018 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 +
#include "MantidCurveFitting/Functions/ElasticDiffRotDiscreteCircle.h"
#include "MantidAPI/FunctionFactory.h"
#include "MantidAPI/IFunction.h"
#include "MantidCurveFitting/Constraints/BoundaryConstraint.h"
 
#include <cmath>
#include <limits>
 
using BConstraint = Mantid::CurveFitting::Constraints::BoundaryConstraint;
 
namespace {
Mantid::Kernel::Logger g_log("ElasticDiffRotDiscreteCircle");
}
 
namespace Mantid::CurveFitting::Functions {
 
DECLARE_FUNCTION(ElasticDiffRotDiscreteCircle)
 
 
ElasticDiffRotDiscreteCircle::ElasticDiffRotDiscreteCircle() {
  // declareParameter("Height", 1.0); //parameter "Height" already declared in
  // constructor of base class DeltaFunction
  this->declareParameter("Radius", 1.0, "Circle radius [Angstroms] ");
  this->declareAttribute("N", API::IFunction::Attribute(3));
  FunctionQDepends::declareAttributes();
}
 
void ElasticDiffRotDiscreteCircle::init() {
  // Ensure positive values for Height and Radius
  auto HeightConstraint = std::make_unique<BConstraint>(this, "Height", std::numeric_limits<double>::epsilon(), true);
  this->addConstraint(std::move(HeightConstraint));
 
  auto RadiusConstraint = std::make_unique<BConstraint>(this, "Radius", std::numeric_limits<double>::epsilon(), true);
  this->addConstraint(std::move(RadiusConstraint));
}
 
double ElasticDiffRotDiscreteCircle::HeightPrefactor() const {
  auto R = this->getParameter("Radius");
  auto Q = this->getAttribute("Q").asDouble();
  auto N = this->getAttribute("N").asInt();
  double aN = 0;
  for (int k = 1; k < N; k++) {
    double x = 2 * Q * R * sin(M_PI * k / N);
    aN += sin(x) / x; // spherical Besell function of order zero j0==sin(x)/x
  }
  aN += 1; // limit for j0 when k==N, or x==0
  return aN / N;
}
 
} // namespace Mantid::CurveFitting::Functions