StretchExp.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 +
//----------------------------------------------------------------------
// Includes
//----------------------------------------------------------------------
#include "MantidCurveFitting/Functions/StretchExp.h"
#include "MantidAPI/FunctionFactory.h"
#include <cmath>
 
namespace Mantid::CurveFitting::Functions {
 
using namespace CurveFitting;
 
using namespace Kernel;
 
using namespace API;
 
DECLARE_FUNCTION(StretchExp)
 
StretchExp::StretchExp() {
  declareParameter("Height", 1.0, "Height at time zero");
  declareParameter("Lifetime", 1.0, "Relaxation time of the standard exponential");
  declareParameter("Stretching", 1.0, "Stretching exponent");
}
 
void StretchExp::function1D(double *out, const double *xValues, const size_t nData) const {
  const double h = getParameter("Height");
  const double t = getParameter("Lifetime");
  const double b = getParameter("Stretching");
 
  for (size_t i = 0; i < nData; i++) {
    double x = xValues[i];
    if (x < 0.0) {
      // although it is defined for integer b's we don't allow negative x in
      // fitting
      throw std::runtime_error("StretchExp is undefined for negative argument.");
    }
    out[i] = h * exp(-pow(x / t, b));
  }
}
 
void StretchExp::functionDeriv1D(API::Jacobian *out, const double *xValues, const size_t nData) {
  const double h = getParameter("Height");
  const double t = getParameter("Lifetime");
  const double b = getParameter("Stretching");
 
  for (size_t i = 0; i < nData; i++) {
    double x = xValues[i];
    double a = pow(x / t, b);
    double e = exp(-a);
    out->set(i, 0, e);                 // derivative with respect to h
    out->set(i, 1, h * a * b * e / t); // derivative with respect to t
    if (x == 0.0) {
      // derivative with respect to b at x = 0 is undefined, set to 0
      out->set(i, 2, 0.0);
    } else {
      out->set(i, 2, -h * a * e * log(x / t)); // derivative with respect to b
    }
  }
}
 
} // namespace Mantid::CurveFitting::Functions