[constraints] Add utitily function for computing Dirichlet constraints

dune/fufem/constraints/CMakeLists.txt

 install(FILES
     affineconstraints.hh
+    boundaryconstraints.hh
     continuityconstraints.hh
     DESTINATION ${CMAKE_INSTALL_INCLUDEDIR}/dune/fufem/constraints)

dune/fufem/constraints/boundaryconstraints.hh

+// -*- tab-width: 4; indent-tabs-mode: nil; c-basic-offset: 2 -*-
+// vi: set et ts=4 sw=2 sts=2:
+#ifndef DUNE_FUFEM_CONSTRAINTS_BOUNDARYCONSTRAINTS_HH
+#define DUNE_FUFEM_CONSTRAINTS_BOUNDARYCONSTRAINTS_HH
+
+#include <variant>
+
+#include <dune/functions/functionspacebases/interpolate.hh>
+
+#include <dune/fufem/backends/traversal.hh>
+#include <dune/fufem/boundarypatch.hh>
+#include <dune/fufem/functiontools/boundarydofs.hh>
+#include <dune/fufem/constraints/affineconstraints.hh>
+
+
+
+namespace Dune::Fufem {
+
+/**
+ * \brief Compute constraints for essential boundary conditions
+ * \ingroup Constraints
+ *
+ * \tparam Basis Type of the global basis
+ * \tparam Function Type of the function to interpolate
+ *
+ * \param constraints An initialized AffineConstraints object
+ * \param basis A global basis
+ * \param f The Dirichlet values given as a function
+ * \param boundaryPatch The BoundaryPath where the boundary condition should be applied
+ *
+ * This will interpolate the function `f` on the `boundaryPatch`
+ * add constraints for all corresponding DOFs to `constraints`.
+ *
+ * This method assumes that the `constraints` has already been initialized.
+ * If `constraints` already contains constraint DOFs, then interpolation
+ * and the newly added constraints are reduced to the previously unconstrained
+ * DOFs.
+ *
+ * \warning This function is still experimental and the interface may change.
+ */
+template<class BV, class V, class MI, class C, class Basis, class Function>
+auto computeBoundaryConstraints(AffineConstraints<BV, V, MI, C>& constraints, const Basis& basis, Function&& f, const BoundaryPatch<typename Basis::GridView>& boundaryPatch)
+{
+  auto&& isConstrained = Dune::Functions::istlVectorBackend(constraints.isConstrained());
+  auto isNewConstrained = constraints.isConstrained();
+  Dune::Fufem::markBoundaryPatchDofs(boundaryPatch, basis, isNewConstrained);
+  Dune::Fufem::recursiveForEachVectorEntry(isNewConstrained, [&](auto&& isNewConstrained_i, const auto& i)
+  {
+    isNewConstrained_i = isNewConstrained_i and not isConstrained[i];
+    isConstrained[i] =  isNewConstrained_i or isConstrained[i];
+  });
+  Dune::Functions::interpolate(basis, constraints.constantTerm(), f, isNewConstrained);
+  constraints.resolveDependencies();
+}
+
+
+/**
+ * \brief Compute constraints for essential boundary conditions
+ * \ingroup Constraints
+ *
+ * \tparam BitVector A bit vector type suitable for the global indices of the basis
+ * \tparam Vector A coefficient vector type suitable for the global indices of the basis
+ * \tparam Basis Type of the global basis
+ * \tparam Function Type of the function to interpolate
+ *
+ * \param basis A global basis
+ * \param f The Dirichlet values given as a function
+ * \param boundaryPatch The BoundaryPath where the boundary condition should be applied
+ *
+ * This will interpolate the function `f` on the `boundaryPatch`
+ * return an `AffineConstraints` object constraining all corresponding DOFs.
+ *
+ * If the template default value `std::monostate` is used for the template
+ * parameters `BitVector` or `Vector` then the actual container types are deduced
+ * using `Dune::Functions::makeContainer(...)` and `Dune::Functions::makeISTLVector(...)`
+ * from the container descriptor provided by `basis` (only with dune-function>=2.11).
+ *
+ * \warning This function is still experimental and the interface may change.
+ */
+template<class BitVector=std::monostate, class Vector=std::monostate, class Basis, class Function>
+auto makeBoundaryConstraints(const Basis& basis, Function&& f, const BoundaryPatch<typename Basis::GridView>& boundaryPatch)
+{
+  auto constraints = makeAffineConstraints<BitVector, Vector>(basis);
+  computeBoundaryConstraints(constraints, basis, f, boundaryPatch);
+  return constraints;
+}
+
+} // namespace Dune::Fufem
+
+
+
+#endif // DUNE_FUFEM_CONSTRAINTS_BOUNDARYCONSTRAINTS_HH