function_derivative.cc

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// ------------------------------------------------------------------------
//
// SPDX-License-Identifier: LGPL-2.1-or-later
// Copyright (C) 2000 - 2025 by the deal.II authors
//
// This file is part of the deal.II library.
//
// Part of the source code is dual licensed under Apache-2.0 WITH
// LLVM-exception OR LGPL-2.1-or-later. Detailed license information
// governing the source code and code contributions can be found in
// LICENSE.md and CONTRIBUTING.md at the top level directory of deal.II.
//
// ------------------------------------------------------------------------
 
#include <deal.II/base/function_derivative.h>
#include <deal.II/base/point.h>
 
#include <deal.II/lac/vector.h>
 
#include <cmath>
#include <vector>
 
 
DEAL_II_NAMESPACE_OPEN
 
template <int dim>
FunctionDerivative<dim>::FunctionDerivative(const Function<dim> &f,
                                            const Point<dim>    &dir,
                                            const double         h)
  : AutoDerivativeFunction<dim>(h, f.n_components, f.get_time())
  , f(f)
  , h(h)
  , incr(1, h * dir)
{
  set_formula();
}
 
 
 
template <int dim>
FunctionDerivative<dim>::FunctionDerivative(const Function<dim>           &f,
                                            const std::vector<Point<dim>> &dir,
                                            const double                   h)
  : AutoDerivativeFunction<dim>(h, f.n_components, f.get_time())
  , f(f)
  , h(h)
  , incr(dir.size())
{
  for (unsigned int i = 0; i < incr.size(); ++i)
    incr[i] = h * dir[i];
  set_formula();
}
 
 
 
template <int dim>
void
FunctionDerivative<dim>::set_formula(
  typename AutoDerivativeFunction<dim>::DifferenceFormula form)
{
  // go through all known formulas, reject ones we don't know about
  // and don't handle in the member functions of this class
  switch (form)
    {
      case AutoDerivativeFunction<dim>::Euler:
      case AutoDerivativeFunction<dim>::UpwindEuler:
      case AutoDerivativeFunction<dim>::FourthOrder:
        break;
      default:
        Assert(false,
               ExcMessage("The argument passed to this function does not "
                          "match any known difference formula."));
    }
 
  formula = form;
}
 
 
 
template <int dim>
void
FunctionDerivative<dim>::set_h(const double new_h)
{
  for (unsigned int i = 0; i < incr.size(); ++i)
    incr[i] *= new_h / h;
  h = new_h;
}
 
 
 
template <int dim>
double
FunctionDerivative<dim>::value(const Point<dim>  &p,
                               const unsigned int component) const
{
  Assert(incr.size() == 1,
         ExcMessage(
           "FunctionDerivative was not initialized for constant direction"));
 
  switch (formula)
    {
      case AutoDerivativeFunction<dim>::Euler:
        return (f.value(p + incr[0], component) -
                f.value(p - incr[0], component)) /
               (2 * h);
      case AutoDerivativeFunction<dim>::UpwindEuler:
        return (f.value(p, component) - f.value(p - incr[0], component)) / h;
      case AutoDerivativeFunction<dim>::FourthOrder:
        return (-f.value(p + 2 * incr[0], component) +
                8 * f.value(p + incr[0], component) -
                8 * f.value(p - incr[0], component) +
                f.value(p - 2 * incr[0], component)) /
               (12 * h);
      default:
        DEAL_II_NOT_IMPLEMENTED();
    }
  return 0.;
}
 
 
 
template <int dim>
void
FunctionDerivative<dim>::vector_value(const Point<dim> &p,
                                      Vector<double>   &result) const
{
  Assert(incr.size() == 1,
         ExcMessage(
           "FunctionDerivative was not initialized for constant direction"));
  Vector<double> aux(result.size());
 
  // Formulas are the same as in
  // value, but here we have to use
  // Vector arithmetic
  switch (formula)
    {
      case AutoDerivativeFunction<dim>::Euler:
        f.vector_value(p + incr[0], result);
        f.vector_value(p - incr[0], aux);
        result.sadd(1. / (2 * h), -1. / (2 * h), aux);
        return;
      case AutoDerivativeFunction<dim>::UpwindEuler:
        f.vector_value(p, result);
        f.vector_value(p - incr[0], aux);
        result.sadd(1. / h, -1. / h, aux);
        return;
      case AutoDerivativeFunction<dim>::FourthOrder:
        f.vector_value(p - 2 * incr[0], result);
        f.vector_value(p + 2 * incr[0], aux);
        result.add(-1., aux);
        f.vector_value(p - incr[0], aux);
        result.add(-8., aux);
        f.vector_value(p + incr[0], aux);
        result.add(8., aux);
        result /= (12. * h);
        return;
      default:
        DEAL_II_NOT_IMPLEMENTED();
    }
}
 
 
 
template <int dim>
void
FunctionDerivative<dim>::value_list(const std::vector<Point<dim>> &points,
                                    std::vector<double>           &values,
                                    const unsigned int component) const
{
  const unsigned int n                  = points.size();
  const bool         variable_direction = (incr.size() == 1) ? false : true;
  if (variable_direction)
    Assert(incr.size() == points.size(),
           ExcDimensionMismatch(incr.size(), points.size()));
 
  // Vector of auxiliary values
  std::vector<double> aux(n);
  // Vector of auxiliary points
  std::vector<Point<dim>> paux(n);
  // Use the same formulas as in
  // value, but with vector
  // arithmetic
  switch (formula)
    {
      case AutoDerivativeFunction<dim>::Euler:
        for (unsigned int i = 0; i < n; ++i)
          paux[i] = points[i] + incr[(variable_direction) ? i : 0U];
        f.value_list(paux, values, component);
        for (unsigned int i = 0; i < n; ++i)
          paux[i] = points[i] - incr[(variable_direction) ? i : 0U];
        f.value_list(paux, aux, component);
        for (unsigned int i = 0; i < n; ++i)
          values[i] = (values[i] - aux[i]) / (2 * h);
        return;
      case AutoDerivativeFunction<dim>::UpwindEuler:
        f.value_list(points, values, component);
        for (unsigned int i = 0; i < n; ++i)
          paux[i] = points[i] - incr[(variable_direction) ? i : 0U];
        f.value_list(paux, aux, component);
        for (unsigned int i = 0; i < n; ++i)
          values[i] = (values[i] - aux[i]) / h;
        return;
      case AutoDerivativeFunction<dim>::FourthOrder:
        for (unsigned int i = 0; i < n; ++i)
          paux[i] = points[i] - 2 * incr[(variable_direction) ? i : 0U];
        f.value_list(paux, values, component);
        for (unsigned int i = 0; i < n; ++i)
          paux[i] = points[i] + 2 * incr[(variable_direction) ? i : 0U];
        f.value_list(paux, aux, component);
        for (unsigned int i = 0; i < n; ++i)
          values[i] -= aux[i];
        for (unsigned int i = 0; i < n; ++i)
          paux[i] = points[i] + incr[(variable_direction) ? i : 0U];
        f.value_list(paux, aux, component);
        for (unsigned int i = 0; i < n; ++i)
          values[i] += 8. * aux[i];
        for (unsigned int i = 0; i < n; ++i)
          paux[i] = points[i] - incr[(variable_direction) ? i : 0U];
        f.value_list(paux, aux, component);
        for (unsigned int i = 0; i < n; ++i)
          values[i] = (values[i] - 8. * aux[i]) / (12 * h);
        return;
      default:
        DEAL_II_NOT_IMPLEMENTED();
    }
}
 
 
 
template <int dim>
std::size_t
FunctionDerivative<dim>::memory_consumption() const
{
  // only simple data elements, so
  // use sizeof operator
  return sizeof(*this);
}
 
 
 
// explicit instantiations
template class FunctionDerivative<1>;
template class FunctionDerivative<2>;
template class FunctionDerivative<3>;
 
DEAL_II_NAMESPACE_CLOSE