[grid.element] add sub_entity_center to select codim at runtime

dune/xt/grid/element.hh

+// This file is part of the dune-xt project:
+//   https://github.com/dune-community/dune-xt
+// Copyright 2009-2020 dune-xt developers and contributors. All rights reserved.
+// License: Dual licensed as BSD 2-Clause License (http://opensource.org/licenses/BSD-2-Clause)
+//      or  GPL-2.0+ (http://opensource.org/licenses/gpl-license)
+//          with "runtime exception" (http://www.dune-project.org/license.html)
+// Authors:
+//   Felix Schindler (2020)
+
+#ifndef DUNE_XT_GRID_ELEMENT_HH
+#define DUNE_XT_GRID_ELEMENT_HH
+
+#include <dune/grid/common/entity.hh>
+
+#include <dune/xt/common/numeric_cast.hh>
+#include <dune/xt/grid/type_traits.hh>
+
+namespace Dune {
+namespace XT {
+namespace Grid {
+
+
+template <class E>
+class SubEntityCenter
+{
+  static_assert(is_entity<E>::value, "");
+  using G = extract_grid_t<E>;
+  using D = typename G::ctype;
+  static const constexpr size_t d = G::dimension;
+
+  template <int cd = d, bool anything = true>
+  struct subEntity
+  {
+    static FieldVector<D, d> center(const E& element, const int codim, const size_t i)
+    {
+      if (codim == cd) {
+        DUNE_THROW_IF(i >= element.subEntities(codim),
+                      Common::Exceptions::index_out_of_range,
+                      "element.subEntities(" << codim << ") = " << element.subEntities(codim) << "\n   i = " << i);
+        return element.subEntity<cd>(i).geometry().center();
+      } else
+        return subEntity<cd - 1>::center(element, codim, i);
+    } // ... center(...)
+  }; // struct subEntity
+
+  template <bool anything>
+  struct subEntity<0, anything>
+  {
+    static FieldVector<D, d> center(const E& element, const int codim, const size_t i)
+    {
+      DUNE_THROW_IF(codim != 0, Common::Exceptions::internal_error, "This must not happen");
+      return element.subEntity<0>(i).geometry().center();
+    }
+  }; // struct subEntity
+
+public:
+  static FieldVector<D, d> get(const E& element, const int codim, const size_t i)
+  {
+    return subEntity<>::center(element, codim, i);
+  }
+}; // class SubEntityCenter
+
+
+template <int dim, class GridImp, template <int, int, class> class EntityImp>
+FieldVector<typename GridImp::ctype, GridImp::dimension>
+sub_entity_center(const Dune::Entity<0, dim, GridImp, EntityImp> element, const int codim, const size_t i)
+{
+  return SubEntityCenter<Dune::Entity<0, dim, GridImp, EntityImp>>::get(element, codim, i);
+}
+
+
+} // namespace Grid
+} // namespace XT
+} // namespace Dune
+
+#endif // DUNE_XT_GRID_ELEMENT_HH