// ---------------------------------------------------------------------
//
// Copyright (C) 1998 - 2021 by the deal.II authors
//
// This file is part of the deal.II library.
//
// The deal.II library is free software; you can use it, redistribute
// it, and/or modify it under the terms of the GNU Lesser General
// Public License as published by the Free Software Foundation; either
// version 2.1 of the License, or (at your option) any later version.
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#ifndef dealii_error_estimator_h
#define dealii_error_estimator_h


#include <deal.II/base/config.h>

#include <deal.II/base/exceptions.h>
#include <deal.II/base/function.h>

#include <deal.II/fe/component_mask.h>

#include <map>

DEAL_II_NAMESPACE_OPEN

// Forward declarations
#ifndef DOXYGEN
template <int, int>
class DoFHandler;
template <int, int>
class Mapping;
template <int>
class Quadrature;

namespace hp
{
  template <int>
  class QCollection;
}
#endif


/**
 * Implementation of the error indicator by Kelly, De S. R. Gago, Zienkiewicz
 * and Babuska (see @cite KGZB83) and its modification for the hp-FEM. This
 * error indicator tries to approximate the error per cell by integration of the
 * jump of the gradient of the solution along the faces of each cell.  It can be
 * understood as a gradient recovery estimator; see the survey of Ainsworth
 * and Oden, "A Posteriori Error Estimation in Finite Element Analysis"
 * (Wiley, 2000) for a complete discussion.
 *
 * In the original Kelly error estimator, the contribution of each face to the
 * cell error is scaled with the cell diagonal. In the modified version,
 * however, we employ a scaling factor which depends on the face diagonal and
 * polynomial degrees of the adjacent elements. The choice between the two is
 * done by means of the enumerator, defined within the class.
 *
 * @note In spite of the name, Kelly estimator is not truly an a posteriori
 * error estimator, even if applied to the Poisson problem only. It gives good
 * hints for mesh refinement, but the estimate is not to be trusted. For
 * higher order trial spaces the integrals computed here tend to zero faster
 * than the error itself, thus ruling out the values as error estimators.
 * However, the modified version discussed below can be utilised to obtain the
 * reliable error estimator by adding the residual (volume) part.
 *
 * The error estimator really only estimates the error for the generalized
 * Poisson equation $-\nabla\cdot a(x) \nabla u = f$ with either Dirichlet
 * boundary conditions or generalized Neumann boundary conditions involving
 * the conormal derivative $a\frac{du}{dn} = g$.
 *
 * The error estimator returns a vector of estimated errors per cell which can
 * be used to feed the GridRefinement::refine_and_coarsen_fixed_fraction(),
 * GridRefinement::refine_and_coarsen_fixed_number(), and similar functions.
 * This vector contains elements of data type @p float, rather than @p double,
 * since accuracy is not important in the current context.
 *
 *
 * <h3>Implementation</h3>
 *
 * In principle, the implementation of the error estimation is simple: let
 * @f[
 *   \eta_K^2
 *   =
 *   \sum_{F\in\partial K}
 *     c_F \int_{\partial K_F} \jump{a \frac{\partial u_h}{\partial n}}^2
 * @f]
 * be the error estimator for cell $K$. $\jump{\cdot}$ denotes the jump of the
 * function in square brackets at the face, and $c_F$ is a factor discussed
 * below. This is the general form of the interface terms of the error
 * estimator derived by Kelly et al. in the paper referenced above. The overall
 * error estimate is then computed as
 * @f[
 *   \eta^2 = \sum_K \eta_K^2
 * @f]
 * so that $\eta \approx \|\nabla (u-u_h)\|$ for the Laplace equation. The
 * functions of this class compute a vector of values that corresponds to
 * $\eta_K$ (i.e., the square root of the quantity above).
 *
 * In the paper of Ainsworth $ c_F=\frac {h_K}{24} $, but this factor is a bit
 * esoteric, stemming from interpolation estimates and stability constants which
 * may hold for the Poisson problem, but may not hold for more general
 * situations. Alternatively, we consider the case when $c_F=\frac {h_F}{2p_F}$,
 * where $h_F$ is the diameter of the face and $p_F=max(p^+,p^-)$ is the maximum
 * polynomial degree of adjacent elements; or $c_F=h_K$. The choice between
 * these factors is done by means of the enumerator, provided as the last
 * argument in all functions.
 *
 * To perform the integration, use is made of the FEFaceValues and
 * FESubfaceValues classes. The integration is performed by looping over all
 * cells and integrating over faces that are not yet treated. This way we
 * avoid integration on faces twice, once for each time we visit one of the
 * adjacent cells. In a second loop over all cells, we sum up the
 * contributions of the faces (which are the integrated square of the jumps
 * times some factor) of each cell and take the square root.
 *
 * The integration is done using a quadrature formula on the face
 * provided by the caller of the estimate() functions declared by this
 * class. For linear trial functions (FE_Q(1)), QGauss with two points
 * or even the QMidpoint rule might actually suffice. For higher order
 * elements, it is necessary to utilize higher order quadrature
 * formulae with `fe.degree+1` Gauss points.
 *
 * We store the contribution of each face in a @p map, as provided by the C++
 * standard library, with the iterator pointing to that face being the key
 * into the map. When looping the second time over all cells, we have to sum
 * up the contributions of the faces and take the square root. For the Kelly
 * estimator, the multiplication with $\frac {h_K}{24}$ is done in the second
 * loop. By doing so we avoid problems to decide with which $h_K$ to multiply,
 * that of the cell on the one or that of the cell on the other side of the
 * face. Whereas for the hp-estimator the @p map stores integrals multiplied
 * by $\frac {h_F}{2p_F}$, which are then summed in the second loop.
 *
 * $h_K$ ($h_F$) is taken to be the greatest length of the diagonals of the cell
 * (face). For more or less uniform cells (faces) without deformed angles,
 * this coincides with the diameter of the cell (face).
 *
 *
 * <h3>Vector-valued functions</h3>
 *
 * If the finite element field for which the error is to be estimated is
 * vector-valued, i.e. the finite element has more than one component, then
 * all the above can be applied to all or only some components at the same
 * time. The main function of this class takes a list of flags denoting the
 * components for which components the error estimator is to be applied; by
 * default, it is a list of only @p trues, meaning that all variables shall be
 * treated.
 *
 * In case the different components of a field have different physical meaning
 * (for example velocity and pressure in the Stokes equations), it would be
 * senseless to use the same coefficient for all components. In that case, you
 * can pass a function with as many components as there are components in the
 * finite element field and each component of the error estimator will then be
 * weighted by the respective component in this coefficient function. In the
 * other case, when all components have the same meaning (for example the
 * displacements in Lam&eacute;'s equations of elasticity), you can specify a
 * scalar coefficient which will then be used for all components.
 *
 *
 * <h3>Boundary values</h3>
 *
 * If the face is at the boundary, i.e. there is no neighboring cell to which
 * the jump in the gradient could be computed, there are two possibilities:
 * <ul>
 * <li> The face belongs to a Dirichlet boundary. Then the face is not
 * considered, which can be justified looking at a dual problem technique and
 * should hold exactly if the boundary can be approximated exactly by the
 * finite element used (i.e. it is a linear boundary for linear finite
 * elements, quadratic for isoparametric quadratic elements, etc). For
 * boundaries which can not be exactly approximated, one should consider the
 * difference $z-z_h$ on the face, $z$ being a dual problem's solution which
 * is zero at the true boundary and $z_h$ being an approximation, which in
 * most cases will be zero on the numerical boundary. Since on the numerical
 * boundary $z$ will not be zero in general, we would get another term here,
 * but this one is neglected for practical reasons, in the hope that the error
 * made here will tend to zero faster than the energy error we wish to
 * estimate.
 *
 * Though no integration is necessary, in the list of face contributions we
 * store a zero for this face, which makes summing up the contributions of the
 * different faces to the cells easier.
 *
 * <li> The face belongs to a Neumann boundary.  In this case, the
 * contribution of the face $F\in\partial K$ looks like \f[ n_F\int_F
 * \left|g-a\frac{\partial u_h}{\partial n}\right|^2 ds \f] where $g$ is the
 * Neumann boundary function, $n_F=\frac {h_K}{24}$ and $n_F=\frac {h_F}{p}$ for
 * the Kelly and hp-estimator, respectively. If the finite element is vector-
 * valued, then obviously the function denoting the Neumann boundary
 * conditions needs to be vector-valued as well.
 *
 * <li> No other boundary conditions are considered.
 * </ul>
 *
 * In practice, if you have Robin boundary conditions or are too lazy to
 * accurately describe Neumann values, then this is rarely an issue: if you
 * don't say anything in the map about a particular part of the boundary then
 * the Kelly indicator will simply assume that the solution is correct on that
 * part of the boundary and not touch it. Of course, if you have a have a
 * Neumann or Robin boundary, that isn't quite true, there is going to be a
 * difference between the normal derivative of the numerical solution and the
 * Neumann values these normal derivatives should equal. So if we simply
 * ignore those parts of the boundary, we'll underestimate the error. In
 * practice, this rarely appears to be a problem -- you may not refine the
 * cell this time around but you'll probably refine it in the next refinement
 * step and everything is good again. After all, for all problems but the
 * Laplace equation, the Kelly indicator is only an indicator, not an
 * estimator, and so the values it computes are not exact error
 * representations anyway.
 *
 *
 * <h3>Handling of hanging nodes</h3>
 *
 * The integration along faces with hanging nodes is quite tricky, since one
 * of the elements has to be shifted one level up or down. See the
 * documentation for the FESubFaceValues class for more information about
 * technical issues regarding this topic.
 *
 * In praxi, since we integrate over each face only once, we do this when we
 * are on the coarser one of the two cells adjacent to a subface (a subface is
 * defined to be the child of a face; seen from the coarse cell, it is a
 * subface, while seen from the refined cell it is one of its faces). The
 * reason is that finding neighborship information is a bit easier then, but
 * that's all practical reasoning, nothing fundamental.
 *
 * Since we integrate from the coarse side of the face, we have the mother
 * face readily at hand and store the result of the integration over that
 * mother face (being the sum of the integrals along the subfaces) in the
 * abovementioned map of integrals as well. This consumes some memory more
 * than needed, but makes the summing up of the face contributions to the
 * cells easier, since then we have the information from all faces of all
 * cells at hand and need not think about explicitly determining whether a
 * face was refined or not. The same applies for boundary faces, see above.
 *
 *
 * <h3>Multiple solution vectors</h3>
 *
 * In some cases, for example in time-dependent problems, one would like to
 * compute the error estimates for several solution vectors on the same grid
 * at once, with the same coefficients, boundary condition object, etc, e.g.
 * for the solutions on several successive time steps. One could then call the
 * functions of this class several times for each solution. However, the
 * largest factor in the computation of the error estimates (in terms of
 * computing time) is initialization of FEFaceValues and FESubFaceValues
 * objects, and iterating through all faces and subfaces. If the solution
 * vectors live on the same grid, this effort can be reduced significantly by
 * treating all solution vectors at the same time, initializing the
 * FEFaceValues objects only once per cell and for all solution vectors at
 * once, and also only looping through the triangulation only once. For this
 * reason, besides the @p estimate function in this class that takes a single
 * input vector and returns a single output vector, there is also a function
 * that accepts several in- and output vectors at the same time.
 *
 * @ingroup numerics
 */
template <int dim, int spacedim = dim>
class KellyErrorEstimator
{
public:
  /**
   * The enum type given to the class functions to decide on the scaling
   * factors of the face integrals.
   */
  enum Strategy
  {
    //! Kelly error estimator with the factor $\frac {h_K}{24}$.
    cell_diameter_over_24 = 0,
    //! the boundary residual estimator with the factor $\frac {h_F}{2
    //! max(p^+,p^-)}$.
    face_diameter_over_twice_max_degree,
    //! Kelly error estimator with the factor $h_K$.
    cell_diameter
  };

  /**
   * Implementation of the error estimator described above. You may give a
   * coefficient, but there is a default value which denotes the constant
   * coefficient with value one. The coefficient function may either be a
   * scalar one, in which case it is used for all components of the finite
   * element, or a vector-valued one with as many components as there are in
   * the finite element; in the latter case, each component is weighted by the
   * respective component in the coefficient.
   *
   * You might give a list of components you want to evaluate, in case the
   * finite element used by the DoFHandler object is vector-valued. You then
   * have to set those entries to true in the bit-vector @p component_mask
   * (see
   * @ref GlossComponentMask
   * ) for which the respective component is to be used in the error
   * estimator. The default is to use all components, which is done by either
   * providing a bit-vector with all-set entries, or an empty bit-vector.
   *
   * The @p subdomain_id parameter indicates whether we shall compute
   * indicators for all cells (in case its value is the default,
   * <tt>numbers::invalid_unsigned_int</tt>), or only for the cells belonging
   * to a certain subdomain with the given indicator. The latter case is used
   * for parallel computations where all processor nodes have the global grid
   * stored, and could well compute all the indicators for all cells
   * themselves, but where it is more efficient to have each process compute
   * only indicators for the cells it owns, and have them exchange the
   * resulting information afterwards. This is in particular true for the case
   * where meshes are very large and computing indicators for @em every cell
   * is too expensive, while computing indicators for only local cells is
   * acceptable. Note that if you only ask for the indicators of a certain
   * subdomain to be computed, you must nevertheless make sure that this
   * function has access to the correct node values of @em all degrees of
   * freedom. This is since the function needs to access DoF values on
   * neighboring cells as well, even if they belong to a different subdomain.
   *
   * The @p material_id parameter has a similar meaning: if not set to its
   * default value (which is numbers::invalid_material_id), it means that
   * indicators will only be computed for cells with this particular material
   * id.
   *
   * The @p n_threads parameter used to indicate the number of threads to be
   * used to compute the error estimator. This parameter is now ignored, with
   * the number of threads determined automatically. The parameter is retained
   * for compatibility with old versions of the library.
   *
   * The @p strategy parameter is used to choose the scaling factor for the
   * integral over cell's faces.
   *
   * @note If the DoFHandler object given as an argument to this function
   * builds on a parallel::distributed::Triangulation, this function skips
   * computations on all cells that are not locally owned. In that case, the
   * only valid value for the subdomain_id argument (besides the invalid
   * value) is the subdomain id that is associated with the currently
   * processor, as reported by
   * parallel::distributed::Triangulation::locally_owned_subdomain(). Even
   * though nothing is computed on cells that we don't locally own, the error
   * indicator vector must still have a length equal to the number of active
   * cell in the mesh as reported by
   * parallel::distributed::Triangulation::n_locally_owned_active_cells().
   */
  template <typename InputVector>
  static void
  estimate(
    const Mapping<dim, spacedim> &   mapping,
    const DoFHandler<dim, spacedim> &dof,
    const Quadrature<dim - 1> &      quadrature,
    const std::map<types::boundary_id,
                   const Function<spacedim, typename InputVector::value_type> *>
      &                       neumann_bc,
    const InputVector &       solution,
    Vector<float> &           error,
    const ComponentMask &     component_mask = ComponentMask(),
    const Function<spacedim> *coefficients   = nullptr,
    const unsigned int        n_threads      = numbers::invalid_unsigned_int,
    const types::subdomain_id subdomain_id   = numbers::invalid_subdomain_id,
    const types::material_id  material_id    = numbers::invalid_material_id,
    const Strategy            strategy       = cell_diameter_over_24);

  /**
   * Call the @p estimate function, see above, with
   * <tt>mapping=MappingQ@<dim@>(1)</tt>.
   */
  template <typename InputVector>
  static void
  estimate(
    const DoFHandler<dim, spacedim> &dof,
    const Quadrature<dim - 1> &      quadrature,
    const std::map<types::boundary_id,
                   const Function<spacedim, typename InputVector::value_type> *>
      &                       neumann_bc,
    const InputVector &       solution,
    Vector<float> &           error,
    const ComponentMask &     component_mask = ComponentMask(),
    const Function<spacedim> *coefficients   = nullptr,
    const unsigned int        n_threads      = numbers::invalid_unsigned_int,
    const types::subdomain_id subdomain_id   = numbers::invalid_subdomain_id,
    const types::material_id  material_id    = numbers::invalid_material_id,
    const Strategy            strategy       = cell_diameter_over_24);

  /**
   * Same function as above, but accepts more than one solution vector and
   * returns one error vector for each solution vector. For the reason of
   * existence of this function, see the general documentation of this class.
   *
   * Since we do not want to force the user of this function to copy around
   * their solution vectors, the vector of solution vectors takes pointers to
   * the solutions, rather than being a vector of vectors. This makes it
   * simpler to have the solution vectors somewhere in memory, rather than to
   * have them collected somewhere special. (Note that it is not possible to
   * construct of vector of references, so we had to use a vector of
   * pointers.)
   */
  template <typename InputVector>
  static void
  estimate(
    const Mapping<dim, spacedim> &   mapping,
    const DoFHandler<dim, spacedim> &dof,
    const Quadrature<dim - 1> &      quadrature,
    const std::map<types::boundary_id,
                   const Function<spacedim, typename InputVector::value_type> *>
      &                                     neumann_bc,
    const std::vector<const InputVector *> &solutions,
    std::vector<Vector<float> *> &          errors,
    const ComponentMask &                   component_mask = ComponentMask(),
    const Function<spacedim> *              coefficients   = nullptr,
    const unsigned int        n_threads    = numbers::invalid_unsigned_int,
    const types::subdomain_id subdomain_id = numbers::invalid_subdomain_id,
    const types::material_id  material_id  = numbers::invalid_material_id,
    const Strategy            strategy     = cell_diameter_over_24);

  /**
   * Call the @p estimate function, see above, with
   * <tt>mapping=MappingQ@<dim@>(1)</tt>.
   */
  template <typename InputVector>
  static void
  estimate(
    const DoFHandler<dim, spacedim> &dof,
    const Quadrature<dim - 1> &      quadrature,
    const std::map<types::boundary_id,
                   const Function<spacedim, typename InputVector::value_type> *>
      &                                     neumann_bc,
    const std::vector<const InputVector *> &solutions,
    std::vector<Vector<float> *> &          errors,
    const ComponentMask &                   component_mask = ComponentMask(),
    const Function<spacedim> *              coefficients   = nullptr,
    const unsigned int        n_threads    = numbers::invalid_unsigned_int,
    const types::subdomain_id subdomain_id = numbers::invalid_subdomain_id,
    const types::material_id  material_id  = numbers::invalid_material_id,
    const Strategy            strategy     = cell_diameter_over_24);


  /**
   * Equivalent to the set of functions above, except that this one takes a
   * quadrature collection for hp-finite element dof handlers.
   */
  template <typename InputVector>
  static void
  estimate(
    const Mapping<dim, spacedim> &   mapping,
    const DoFHandler<dim, spacedim> &dof,
    const hp::QCollection<dim - 1> & quadrature,
    const std::map<types::boundary_id,
                   const Function<spacedim, typename InputVector::value_type> *>
      &                       neumann_bc,
    const InputVector &       solution,
    Vector<float> &           error,
    const ComponentMask &     component_mask = ComponentMask(),
    const Function<spacedim> *coefficients   = nullptr,
    const unsigned int        n_threads      = numbers::invalid_unsigned_int,
    const types::subdomain_id subdomain_id   = numbers::invalid_subdomain_id,
    const types::material_id  material_id    = numbers::invalid_material_id,
    const Strategy            strategy       = cell_diameter_over_24);


  /**
   * Equivalent to the set of functions above, except that this one takes a
   * quadrature collection for hp-finite element dof handlers.
   */
  template <typename InputVector>
  static void
  estimate(
    const DoFHandler<dim, spacedim> &dof,
    const hp::QCollection<dim - 1> & quadrature,
    const std::map<types::boundary_id,
                   const Function<spacedim, typename InputVector::value_type> *>
      &                       neumann_bc,
    const InputVector &       solution,
    Vector<float> &           error,
    const ComponentMask &     component_mask = ComponentMask(),
    const Function<spacedim> *coefficients   = nullptr,
    const unsigned int        n_threads      = numbers::invalid_unsigned_int,
    const types::subdomain_id subdomain_id   = numbers::invalid_subdomain_id,
    const types::material_id  material_id    = numbers::invalid_material_id,
    const Strategy            strategy       = cell_diameter_over_24);


  /**
   * Equivalent to the set of functions above, except that this one takes a
   * quadrature collection for hp-finite element dof handlers.
   */
  template <typename InputVector>
  static void
  estimate(
    const Mapping<dim, spacedim> &   mapping,
    const DoFHandler<dim, spacedim> &dof,
    const hp::QCollection<dim - 1> & quadrature,
    const std::map<types::boundary_id,
                   const Function<spacedim, typename InputVector::value_type> *>
      &                                     neumann_bc,
    const std::vector<const InputVector *> &solutions,
    std::vector<Vector<float> *> &          errors,
    const ComponentMask &                   component_mask = ComponentMask(),
    const Function<spacedim> *              coefficients   = nullptr,
    const unsigned int        n_threads    = numbers::invalid_unsigned_int,
    const types::subdomain_id subdomain_id = numbers::invalid_subdomain_id,
    const types::material_id  material_id  = numbers::invalid_material_id,
    const Strategy            strategy     = cell_diameter_over_24);


  /**
   * Equivalent to the set of functions above, except that this one takes a
   * quadrature collection for hp-finite element dof handlers.
   */
  template <typename InputVector>
  static void
  estimate(
    const DoFHandler<dim, spacedim> &dof,
    const hp::QCollection<dim - 1> & quadrature,
    const std::map<types::boundary_id,
                   const Function<spacedim, typename InputVector::value_type> *>
      &                                     neumann_bc,
    const std::vector<const InputVector *> &solutions,
    std::vector<Vector<float> *> &          errors,
    const ComponentMask &                   component_mask = ComponentMask(),
    const Function<spacedim> *              coefficients   = nullptr,
    const unsigned int        n_threads    = numbers::invalid_unsigned_int,
    const types::subdomain_id subdomain_id = numbers::invalid_subdomain_id,
    const types::material_id  material_id  = numbers::invalid_material_id,
    const Strategy            strategy     = cell_diameter_over_24);

  /**
   * Exception
   */
  DeclExceptionMsg(ExcInvalidComponentMask,
                   "You provided a ComponentMask argument that is invalid. "
                   "Component masks need to be either default constructed "
                   "(in which case they indicate that every component is "
                   "selected) or need to have a length equal to the number "
                   "of vector components of the finite element in use "
                   "by the DoFHandler object. In the latter case, at "
                   "least one component needs to be selected.");
  /**
   * Exception
   */
  DeclExceptionMsg(
    ExcInvalidCoefficient,
    "If you do specify the argument for a (possibly "
    "spatially variable) coefficient function for this function, "
    "then it needs to refer to a coefficient that is either "
    "scalar (has one vector component) or has as many vector "
    "components as there are in the finite element used by "
    "the DoFHandler argument.");
  /**
   * Exception
   */
  DeclException3(ExcInvalidBoundaryFunction,
                 types::boundary_id,
                 int,
                 int,
                 << "You provided a function map that for boundary indicator "
                 << arg1 << " specifies a function with " << arg2
                 << " vector components. However, the finite "
                    "element in use has "
                 << arg3
                 << " components, and these two numbers need to match.");
  /**
   * Exception
   */
  DeclException2(ExcIncompatibleNumberOfElements,
                 int,
                 int,
                 << "The number of input vectors, " << arg1
                 << " needs to be equal to the number of output vectors, "
                 << arg2
                 << ". This is not the case in your call of this function.");
  /**
   * Exception
   */
  DeclExceptionMsg(ExcNoSolutions,
                   "You need to specify at least one solution vector as "
                   "input.");
};



/**
 * This is a specialization of the general template for 1d. The implementation
 * is sufficiently different for 1d to justify this specialization. The basic
 * difference between 1d and all other space dimensions is that in 1d, there
 * are no faces of cells, just the vertices between line segments, so we have
 * to compute the jump terms differently. However, this class offers exactly
 * the same public functions as the general template, so that a user will not
 * see any difference.
 */
template <int spacedim>
class KellyErrorEstimator<1, spacedim>
{
public:
  /**
   * The enum type given to the class functions to decide on the scaling
   * factors of the face integrals.
   */
  enum Strategy
  {
    //! Kelly error estimator with the factor $\frac {h_K}{24}$.
    cell_diameter_over_24 = 0,
    //! the boundary residual estimator with the factor $\frac {h_F}{2
    //! max(p^+,p^-)}$.
    face_diameter_over_twice_max_degree,
    //! Kelly error estimator with the factor $h_K$.
    cell_diameter
  };



  /**
   * Implementation of the error estimator described above. You may give a
   * coefficient, but there is a default value which denotes the constant
   * coefficient with value one. The coefficient function may either be a
   * scalar one, in which case it is used for all components of the finite
   * element, or a vector-valued one with as many components as there are in
   * the finite element; in the latter case, each component is weighted by the
   * respective component in the coefficient.
   *
   * You might give a list of components you want to evaluate, in case the
   * finite element used by the DoFHandler object is vector-valued. You then
   * have to set those entries to true in the bit-vector @p component_mask for
   * which the respective component is to be used in the error estimator. The
   * default is to use all components, which is done by either providing a
   * bit-vector with all-set entries, or an empty bit-vector. All the other
   * parameters are as in the general case used for 2d and higher.
   *
   * The parameter n_threads is no longer used and will be ignored.
   * Multithreading is not presently implemented for 1d, but we retain the
   * respective parameter for compatibility with the function signature in the
   * general case.
   */
  template <typename InputVector>
  static void
  estimate(
    const Mapping<1, spacedim> &   mapping,
    const DoFHandler<1, spacedim> &dof,
    const Quadrature<0> &          quadrature,
    const std::map<types::boundary_id,
                   const Function<spacedim, typename InputVector::value_type> *>
      &                       neumann_bc,
    const InputVector &       solution,
    Vector<float> &           error,
    const ComponentMask &     component_mask = ComponentMask(),
    const Function<spacedim> *coefficient    = nullptr,
    const unsigned int        n_threads      = numbers::invalid_unsigned_int,
    const types::subdomain_id subdomain_id   = numbers::invalid_subdomain_id,
    const types::material_id  material_id    = numbers::invalid_material_id,
    const Strategy            strategy       = cell_diameter_over_24);



  /**
   * Call the @p estimate function, see above, with
   * <tt>mapping=MappingQ1<1>()</tt>.
   */
  template <typename InputVector>
  static void
  estimate(
    const DoFHandler<1, spacedim> &dof,
    const Quadrature<0> &          quadrature,
    const std::map<types::boundary_id,
                   const Function<spacedim, typename InputVector::value_type> *>
      &                       neumann_bc,
    const InputVector &       solution,
    Vector<float> &           error,
    const ComponentMask &     component_mask = ComponentMask(),
    const Function<spacedim> *coefficients   = nullptr,
    const unsigned int        n_threads      = numbers::invalid_unsigned_int,
    const types::subdomain_id subdomain_id   = numbers::invalid_subdomain_id,
    const types::material_id  material_id    = numbers::invalid_material_id,
    const Strategy            strategy       = cell_diameter_over_24);



  /**
   * Same function as above, but accepts more than one solution vectors and
   * returns one error vector for each solution vector. For the reason of
   * existence of this function, see the general documentation of this class.
   *
   * Since we do not want to force the user of this function to copy around
   * their solution vectors, the vector of solution vectors takes pointers to
   * the solutions, rather than being a vector of vectors. This makes it
   * simpler to have the solution vectors somewhere in memory, rather than to
   * have them collected somewhere special. (Note that it is not possible to
   * construct of vector of references, so we had to use a vector of
   * pointers.)
   */
  template <typename InputVector>
  static void
  estimate(
    const Mapping<1, spacedim> &   mapping,
    const DoFHandler<1, spacedim> &dof,
    const Quadrature<0> &          quadrature,
    const std::map<types::boundary_id,
                   const Function<spacedim, typename InputVector::value_type> *>
      &                                     neumann_bc,
    const std::vector<const InputVector *> &solutions,
    std::vector<Vector<float> *> &          errors,
    const ComponentMask &                   component_mask = ComponentMask(),
    const Function<spacedim> *              coefficients   = nullptr,
    const unsigned int        n_threads    = numbers::invalid_unsigned_int,
    const types::subdomain_id subdomain_id = numbers::invalid_subdomain_id,
    const types::material_id  material_id  = numbers::invalid_material_id,
    const Strategy            strategy     = cell_diameter_over_24);



  /**
   * Call the @p estimate function, see above, with
   * <tt>mapping=MappingQ1<1>()</tt>.
   */
  template <typename InputVector>
  static void
  estimate(
    const DoFHandler<1, spacedim> &dof,
    const Quadrature<0> &          quadrature,
    const std::map<types::boundary_id,
                   const Function<spacedim, typename InputVector::value_type> *>
      &                                     neumann_bc,
    const std::vector<const InputVector *> &solutions,
    std::vector<Vector<float> *> &          errors,
    const ComponentMask &                   component_mask = ComponentMask(),
    const Function<spacedim> *              coefficients   = nullptr,
    const unsigned int        n_threads    = numbers::invalid_unsigned_int,
    const types::subdomain_id subdomain_id = numbers::invalid_subdomain_id,
    const types::material_id  material_id  = numbers::invalid_material_id,
    const Strategy            strategy     = cell_diameter_over_24);



  /**
   * Equivalent to the set of functions above, except that this one takes a
   * quadrature collection for hp-finite element dof handlers.
   */
  template <typename InputVector>
  static void
  estimate(
    const Mapping<1, spacedim> &   mapping,
    const DoFHandler<1, spacedim> &dof,
    const hp::QCollection<0> &     quadrature,
    const std::map<types::boundary_id,
                   const Function<spacedim, typename InputVector::value_type> *>
      &                       neumann_bc,
    const InputVector &       solution,
    Vector<float> &           error,
    const ComponentMask &     component_mask = ComponentMask(),
    const Function<spacedim> *coefficients   = nullptr,
    const unsigned int        n_threads      = numbers::invalid_unsigned_int,
    const types::subdomain_id subdomain_id   = numbers::invalid_subdomain_id,
    const types::material_id  material_id    = numbers::invalid_material_id,
    const Strategy            strategy       = cell_diameter_over_24);



  /**
   * Equivalent to the set of functions above, except that this one takes a
   * quadrature collection for hp-finite element dof handlers.
   */
  template <typename InputVector>
  static void
  estimate(
    const DoFHandler<1, spacedim> &dof,
    const hp::QCollection<0> &     quadrature,
    const std::map<types::boundary_id,
                   const Function<spacedim, typename InputVector::value_type> *>
      &                       neumann_bc,
    const InputVector &       solution,
    Vector<float> &           error,
    const ComponentMask &     component_mask = ComponentMask(),
    const Function<spacedim> *coefficients   = nullptr,
    const unsigned int        n_threads      = numbers::invalid_unsigned_int,
    const types::subdomain_id subdomain_id   = numbers::invalid_subdomain_id,
    const types::material_id  material_id    = numbers::invalid_material_id,
    const Strategy            strategy       = cell_diameter_over_24);



  /**
   * Equivalent to the set of functions above, except that this one takes a
   * quadrature collection for hp-finite element dof handlers.
   */
  template <typename InputVector>
  static void
  estimate(
    const Mapping<1, spacedim> &   mapping,
    const DoFHandler<1, spacedim> &dof,
    const hp::QCollection<0> &     quadrature,
    const std::map<types::boundary_id,
                   const Function<spacedim, typename InputVector::value_type> *>
      &                                     neumann_bc,
    const std::vector<const InputVector *> &solutions,
    std::vector<Vector<float> *> &          errors,
    const ComponentMask &                   component_mask = ComponentMask(),
    const Function<spacedim> *              coefficients   = nullptr,
    const unsigned int        n_threads    = numbers::invalid_unsigned_int,
    const types::subdomain_id subdomain_id = numbers::invalid_subdomain_id,
    const types::material_id  material_id  = numbers::invalid_material_id,
    const Strategy            strategy     = cell_diameter_over_24);



  /**
   * Equivalent to the set of functions above, except that this one takes a
   * quadrature collection for hp-finite element dof handlers.
   */
  template <typename InputVector>
  static void
  estimate(
    const DoFHandler<1, spacedim> &dof,
    const hp::QCollection<0> &     quadrature,
    const std::map<types::boundary_id,
                   const Function<spacedim, typename InputVector::value_type> *>
      &                                     neumann_bc,
    const std::vector<const InputVector *> &solutions,
    std::vector<Vector<float> *> &          errors,
    const ComponentMask &                   component_mask = ComponentMask(),
    const Function<spacedim> *              coefficients   = nullptr,
    const unsigned int        n_threads    = numbers::invalid_unsigned_int,
    const types::subdomain_id subdomain_id = numbers::invalid_subdomain_id,
    const types::material_id  material_id  = numbers::invalid_material_id,
    const Strategy            strategy     = cell_diameter_over_24);



  /**
   * Exception
   */
  DeclExceptionMsg(ExcInvalidComponentMask,
                   "You provided a ComponentMask argument that is invalid. "
                   "Component masks need to be either default constructed "
                   "(in which case they indicate that every component is "
                   "selected) or need to have a length equal to the number "
                   "of vector components of the finite element in use "
                   "by the DoFHandler object. In the latter case, at "
                   "least one component needs to be selected.");

  /**
   * Exception
   */
  DeclExceptionMsg(
    ExcInvalidCoefficient,
    "If you do specify the argument for a (possibly "
    "spatially variable) coefficient function for this function, "
    "then it needs to refer to a coefficient that is either "
    "scalar (has one vector component) or has as many vector "
    "components as there are in the finite element used by "
    "the DoFHandler argument.");

  /**
   * Exception
   */
  DeclException3(ExcInvalidBoundaryFunction,
                 types::boundary_id,
                 int,
                 int,
                 << "You provided a function map that for boundary indicator "
                 << arg1 << " specifies a function with " << arg2
                 << " vector components. However, the finite "
                    "element in use has "
                 << arg3
                 << " components, and these two numbers need to match.");

  /**
   * Exception
   */
  DeclException2(ExcIncompatibleNumberOfElements,
                 int,
                 int,
                 << "The number of input vectors, " << arg1
                 << " needs to be equal to the number of output vectors, "
                 << arg2
                 << ". This is not the case in your call of this function.");

  /**
   * Exception
   */
  DeclExceptionMsg(ExcNoSolutions,
                   "You need to specify at least one solution vector as "
                   "input.");
};


DEAL_II_NAMESPACE_CLOSE

#endif
