resolved a merge conflict

cmake/modules/CMakeLists.txt

-set(modules "DuneMultimeshMacros.cmake")
-
-install(FILES ${modules} DESTINATION ${DUNE_INSTALL_MODULEDIR})
+install(FILES
+    DuneMultimeshMacros.cmake
+    CXXFeatures.cmake
+  DESTINATION ${DUNE_INSTALL_MODULEDIR})

cmake/modules/CXXFeatures.cmake

+include(CheckCXXSourceCompiles)
+
+# support for C++17's variant implementation
+check_cxx_source_compiles("
+  #include <variant>
+  int main()
+  {
+    std::variant<int,double> a;
+    a = 1;
+    a = 2.0;
+    bool positive = std::visit([](auto&& x) { return x > 0; }, a);
+  }
+" DUNE_HAVE_CXX_VARIANT )

cmake/modules/DuneMultimeshMacros.cmake

 # File for module specific CMake tests.
+
+include(CXXFeatures)

config.h.cmake

@@ -40,6 +40,9 @@
 /* Define to the revision of dune-multimesh */
 #define DUNE_MULTIMESH_VERSION_REVISION @DUNE_MULTIMESH_VERSION_REVISION@
 
+/* some detected compiler features may be used in this module */
+#cmakedefine DUNE_HAVE_CXX_VARIANT 1
+
 /* end dune-multimesh
    Everything below here will be overwritten
 */

dune/multimesh/mmentity.hh

@@ -12,23 +12,23 @@
 
 namespace Dune
 {
-  template <class GridImp>
+  template <class HostGrid>
   class MultiEntity
-      : public std::vector<typename GridImp::HostGridType::Traits::template Codim<0>::Entity>
+      : public std::vector<typename HostGrid::Traits::template Codim<0>::Entity>
   {
   private:
-    using ctype = typename GridImp::ctype;
+    using ctype = typename HostGrid::ctype;
 
   public:
-    enum { dimension = GridImp::dimension };
+    enum { dimension = HostGrid::dimension };
 
     /// The type of a local geometry
-    using LocalGeometry = Dune::Geometry<dimension, dimension, const GridImp, MultiMeshLocalGeometry>;
+    using LocalGeometry = MultiMeshLocalGeometry<dimension, dimension, HostGrid>;
 
-    using EntitySeed = typename GridImp::Traits::template Codim<0>::EntitySeed;
+    using EntitySeed = MultiMeshEntitySeed<0, HostGrid>;
 
     /// Entity in the host grid
-    using HostGridEntity = typename GridImp::HostGridType::Traits::template Codim<0>::Entity;
+    using HostGridEntity = typename HostGrid::Traits::template Codim<0>::Entity;
 
     /// Containertype
     using Super = std::vector<HostGridEntity>;
@@ -39,15 +39,13 @@ namespace Dune
     /// Return a local geometry of source in target
     static LocalGeometry localGeometry (HostGridEntity const& source, HostGridEntity const& target)
     {
-      using LocalGeometryImp = MultiMeshLocalGeometry<dimension, dimension, const GridImp>;
-      return LocalGeometry{LocalGeometryImp{source, target}};
+      return {source, target};
     }
 
     /// Return a local geometry of source_i'th entity in target_t'th entity
     LocalGeometry localGeometry (std::size_t source_i, std::size_t target_i) const
     {
-      using LocalGeometryImp = MultiMeshLocalGeometry<dimension, dimension, const GridImp>;
-      return LocalGeometry{LocalGeometryImp{(*this)[source_i], (*this)[target_i]}};
+      return {(*this)[source_i], (*this)[target_i]};
     }
 
     /// Return coordinate `sourceLocal` in coordinates of target entity
@@ -73,8 +71,7 @@ namespace Dune
      **/
     EntitySeed seed (std::size_t entity_i) const
     {
-      using EntitySeedImp = MultiMeshEntitySeed<0, const GridImp>;
-      return EntitySeedImp{(*this)[entity_i], entity_i};
+      return {(*this)[entity_i], entity_i};
     }
 
     /// Return the entity with maximal level

dune/multimesh/mmgeometry.hh

@@ -9,23 +9,23 @@
 
 namespace Dune
 {
-  template <int mydim, int coorddim, class GridImp>
+  template <int mydim, int coorddim, class HostGrid>
   class MultiMeshLocalGeometry
-      : public GeometryDefaultImplementation<mydim, coorddim, GridImp, MultiMeshLocalGeometry>
+      : public GeometryDefaultImplementation<mydim, coorddim, HostGrid, MultiMeshLocalGeometry>
   {
   private:
-    using ctype = typename GridImp::ctype;
+    using ctype = typename HostGrid::ctype;
 
   public:
     // The codimension of this entitypointer wrt the host grid
-    enum { codimension = GridImp::HostGridType::dimension - mydim };
-    enum { dimensionworld = GridImp::HostGridType::dimensionworld };
+    enum { codimension = HostGrid::dimension - mydim };
+    enum { dimensionworld = HostGrid::dimensionworld };
 
     // select appropriate hostgrid geometry via typeswitch
-    using HostLocalGeometry = typename GridImp::HostGridType::Traits::template Codim<codimension>::LocalGeometry;
+    using HostLocalGeometry = typename HostGrid::Traits::template Codim<codimension>::LocalGeometry;
 
     /// entity in the host grid
-    using HostGridEntity = typename GridImp::HostGridType::Traits::template Codim<codimension>::Entity;
+    using HostGridEntity = typename HostGrid::Traits::template Codim<codimension>::Entity;
 
     //! type of jacobian transposed
     using JacobianInverseTransposed = typename HostLocalGeometry::JacobianInverseTransposed;

dune/multimesh/mmiterator.hh

@@ -5,8 +5,10 @@
 
 #include <numeric>
 #include <stack>
+#include <variant>
 
 #include <dune/common/iteratorfacades.hh>
+#include <dune/common/std/type_traits.hh>
 #include <dune/grid/common/exceptions.hh>
 #include <dune/grid/common/gridenums.hh>
 
@@ -14,10 +16,15 @@
 
 namespace Dune
 {
+  namespace tag
+  {
+    struct with_gridview {};
+  }
+
   /** \brief Iterator over all entities of a given codimension and level of a grid.
    *  \ingroup MultiMesh
    */
-  template <int codim, PartitionIteratorType pitype, class GridImp>
+  template <int codim, PartitionIteratorType pitype, class HostGrid>
   class MultiMeshIterator
   {
   public:
@@ -35,18 +42,20 @@ namespace Dune
 
 
   // Implemented for codim 0 entities only
-  template <PartitionIteratorType pitype, class GridImp>
-  class MultiMeshIterator<0, pitype, GridImp>
-      : public ForwardIteratorFacade<MultiMeshIterator<0,pitype,GridImp>,
-            MultiEntity<GridImp>,
-            MultiEntity<GridImp> const&>
+  template <PartitionIteratorType pitype, class HostGrid>
+  class MultiMeshIterator<0, pitype, HostGrid>
+      : public ForwardIteratorFacade<MultiMeshIterator<0,pitype,HostGrid>,
+            MultiEntity<HostGrid>,
+            MultiEntity<HostGrid> const&>
   {
   private:
     // LevelIterator to the equivalent entity in the host grid
     using HostGridLevelIterator =
-      typename GridImp::HostGridType::template Codim<0>::template Partition<pitype>::LevelIterator;
+      typename HostGrid::template Codim<0>::template Partition<pitype>::LevelIterator;
 
-    using HostEntity = typename GridImp::HostGridType::template Codim<0>::Entity;
+    using HostEntity = typename HostGrid::template Codim<0>::Entity;
+
+    using EntityTest = std::function<bool(HostEntity)>;
 
     struct EntityStackEntry
     {
@@ -88,21 +97,23 @@ namespace Dune
 
   public:
     /// Constructor. Stores a pointer to the grid
-    explicit MultiMeshIterator (const GridImp* multiMesh, int level)
-      : multiMesh_(multiMesh)
-      , incrementAllowed_(multiMesh->size(), true)
-      , level_(level)
+    template <class GridImp>
+    MultiMeshIterator (const GridImp* multiMesh, int level)
+      : incrementAllowed_(multiMesh->size(), true)
+      , contains_(multiMesh->size(), EntityTest{[level](const HostEntity& entity) { return entity.level() == level; }})
+      , maxLevel_(multiMesh->size(), level)
       , multiEntity_(multiMesh->size())
     {
-      for (auto const& grid : multiMesh_->grids_) {
+      for (auto const& grid : multiMesh->grids_) {
         macroIterators_.push_back(grid->levelGridView(0).template begin<0,pitype>());
         macroEndIterators_.push_back(grid->levelGridView(0).template end<0,pitype>());
       }
 
       // go to first leaf entity on all grids
-      entityStacks_.reserve(multiMesh_->size());
-      for (std::size_t i = 0; i < multiMesh_->size(); ++i) {
-        entityStacks_.emplace_back((*multiMesh_)[i].maxLevel());
+      entityStacks_.reserve(multiMesh->size());
+      for (std::size_t i = 0; i < multiMesh->size(); ++i) {
+        maxLevel_[i] = (*multiMesh)[i].maxLevel();
+        entityStacks_.emplace_back((*multiMesh)[i].maxLevel());
         initialIncrement(i);
       }
     }
@@ -112,26 +123,84 @@ namespace Dune
      * \param  multiMesh  Pointer to grid instance
      * \param  endDummy   Here only to distinguish it from the other constructor
      */
+    template <class GridImp>
     MultiMeshIterator (const GridImp* multiMesh, int level, bool endDummy)
-      : multiMesh_(multiMesh)
-      , level_(level)
     {
-      for (auto const& grid : multiMesh_->grids_) {
+      for (auto const& grid : multiMesh->grids_)
         macroIterators_.push_back(grid->levelGridView(0).template end<0,pitype>());
-        macroEndIterators_.push_back(grid->levelGridView(0).template end<0,pitype>());
-      }
     }
 
     /// Constructor which create the leaf-iterator
+    template <class GridImp>
     MultiMeshIterator (const GridImp* multiMesh)
       : MultiMeshIterator{multiMesh, -1}
     {}
 
     /// Constructor which create the end leaf-iterator
+    template <class GridImp>
     MultiMeshIterator (const GridImp* multiMesh, bool endDummy)
       : MultiMeshIterator{multiMesh, -1, endDummy}
     {}
 
+    template <class... GridViews,
+      std::enable_if_t<not Std::disjunction<std::is_same<GridViews,bool>...>::value, int> = 0>
+    MultiMeshIterator (tag::with_gridview, GridViews const&... gridViews)
+      : incrementAllowed_(sizeof...(GridViews), true)
+      , maxLevel_{gridViews.grid().maxLevel()...}
+      , multiEntity_(sizeof...(GridViews))
+      , macroIterators_{gridViews.grid().levelGridView(0).template begin<0,pitype>()...}
+      , macroEndIterators_{gridViews.grid().levelGridView(0).template end<0,pitype>()...}
+      , entityStacks_{EntityStack{gridViews.grid().maxLevel()}...}
+    {
+      contains_.reserve(sizeof...(GridViews));
+      Hybrid::forEach(std::forward_as_tuple(gridViews...), [this](auto const& gv) {
+        contains_.emplace_back([gv](const HostEntity& entity) { return gv.contains(entity); });
+      });
+
+      for (std::size_t i = 0; i < sizeof...(GridViews); ++i)
+        initialIncrement(i);
+    }
+
+    template <class... GridViews>
+    MultiMeshIterator (tag::with_gridview, bool endDummy, GridViews const&... gridViews)
+      : macroIterators_{gridViews.grid().levelGridView(0).template end<0,pitype>()...}
+    {}
+
+#if DUNE_HAVE_CXX_VARIANT
+    template <class... GV>
+    MultiMeshIterator (const std::vector<std::variant<GV...>>& gridViews)
+      : incrementAllowed_(gridViews.size(), true)
+      , multiEntity_(gridViews.size())
+    {
+      contains_.reserve(gridViews.size());
+      entityStacks_.reserve(gridViews.size());
+      maxLevel_.reserve(gridViews.size());
+
+      for (auto const& gvs : gridViews) {
+        macroIterators_.push_back(std::visit([](auto const& gv) {
+          return gv.grid().levelGridView(0).template begin<0,pitype>(); }, gvs));
+        macroEndIterators_.push_back(std::visit([](auto const& gv) {
+          return gv.grid().levelGridView(0).template end<0,pitype>(); }, gvs));
+        contains_.emplace_back(std::visit([](auto const& gv) {
+          return EntityTest{[gv](const HostEntity& entity) { return gv.contains(entity); }}; }, gvs));
+        maxLevel_.push_back(std::visit([](auto const& gv) { return gv.grid().maxLevel(); }, gvs));
+        entityStacks_.emplace_back(std::visit([](auto const& gv) { return gv.grid().maxLevel(); }, gvs));
+      }
+
+      // go to first leaf entity on all grids
+      for (std::size_t i = 0; i < entityStacks_.size(); ++i)
+        initialIncrement(i);
+    }
+
+    template <class... GV>
+    MultiMeshIterator (const std::vector<std::variant<GV...>>& gridViews, bool endDummy)
+    {
+      for (auto const& gvs : gridViews) {
+        macroIterators_.push_back(std::visit([](auto const& gv) {
+          return gv.grid().levelGridView(0).template end<0,pitype>(); }, gvs));
+      }
+    }
+#endif
 
     /// prefix increment
     void increment ()
@@ -146,7 +215,7 @@ namespace Dune
     }
 
     /// dereferencing
-    MultiEntity<GridImp> const& dereference () const
+    MultiEntity<HostGrid> const& dereference () const
     {
       // update entries in multiEntity that have changed
       for (std::size_t i = 0; i < entityStacks_.size(); ++i) {
@@ -189,10 +258,10 @@ namespace Dune
       }
 
       // 3. go down in tree until leaf entity
-      for (auto child = dereference(i); !entityTest(child);
+      for (auto child = dereference(i); !entityTest(i,child);
                 child = dereference(i)) {
         entityStack.emplace(child);
-        assert( entityStack.size() <= (*multiMesh_)[i].maxLevel() );
+        assert( entityStack.size() <= maxLevel_[i] );
       }
     }
 
@@ -218,10 +287,10 @@ namespace Dune
         return;
 
       // 2. go down in tree until leaf entity
-      for (auto child = dereference(i); !entityTest(child);
+      for (auto child = dereference(i); !entityTest(i,child);
                 child = dereference(i)) {
         entityStack.emplace(child);
-        assert( entityStack.size() <= (*multiMesh_)[i].maxLevel() );
+        assert( entityStack.size() <= maxLevel_[i] );
       }
     }
 
@@ -236,24 +305,35 @@ namespace Dune
       }
     }
 
-    bool entityTest (HostEntity const& entity)
+    bool entityTest (std::size_t i, HostEntity const& entity) const
     {
-      return entity.level() == level_ || entity.isLeaf() || !entity.isRegular();
+      return contains_[i](entity) || entity.isLeaf() || !entity.isRegular();
     }
 
   private:
-    const GridImp* multiMesh_;
+    std::vector<bool> incrementAllowed_;
+    std::vector<EntityTest> contains_;
+    std::vector<int> maxLevel_;
+
+    mutable MultiEntity<HostGrid> multiEntity_;
 
     std::vector<HostGridLevelIterator> macroIterators_;
     std::vector<HostGridLevelIterator> macroEndIterators_;
-
     std::vector<EntityStack> entityStacks_;
-    std::vector<bool> incrementAllowed_;
-    int level_ = -1;
-
-    mutable MultiEntity<GridImp> multiEntity_;
   };
 
+  template <class> class MultiMesh;
+
+  template <class... GridViews>
+  inline auto multi_elements(GridViews const&... gridViews)
+  {
+    using GridView0 = std::tuple_element_t<0,std::tuple<GridViews...>>;
+    using Iterator = MultiMeshIterator<0,All_Partition,typename GridView0::Grid>;
+    using Range = IteratorRange<Iterator>;
+    return Range{ Iterator{tag::with_gridview{}, gridViews...},
+                  Iterator{tag::with_gridview{}, true, gridViews...} };
+  }
+
 }  // end namespace Dune
 
 #endif // DUNE_MULTIMESH_ITERATOR_HH

dune/multimesh/multigridview.hh

+// -*- tab-width: 4; indent-tabs-mode: nil; c-basic-offset: 2 -*-
+// vi: set et ts=4 sw=2 sts=2:
+#ifndef DUNE_MULTI_GRIDVIEW_HH
+#define DUNE_MULTI_GRIDVIEW_HH
+
+#if ! DUNE_HAVE_CXX_VARIANT
+#error "Require C++17 variant!"
+#endif
+
+#include <variant>
+
+#include <dune/common/typetraits.hh>
+#include <dune/common/exceptions.hh>
+#include <dune/common/std/type_traits.hh>
+
+#include <dune/grid/common/capabilities.hh>
+#include <dune/grid/common/gridview.hh>
+
+#include "mmiterator.hh"
+#include "multiindexset.hh"
+
+namespace Dune
+{
+  // forward declaration
+  template <class HostGrid>
+  class MultiGridView;
+
+  template <class HostGrid>
+  struct MultiGridViewTraits
+  {
+    using Grid = typename std::remove_const<HostGrid>::type;
+
+    using IndexSet = MultiIndexSet<HostGrid>;
+
+    using Intersection = std::variant<
+      typename HostGrid::Traits::LeafIntersection, typename HostGrid::Traits::LevelIntersection>;
+
+    using IntersectionIterator = std::variant<
+      typename HostGrid::Traits::LeafIntersectionIterator, typename HostGrid::Traits::LevelIntersectionIterator>;
+
+    using CollectiveCommunication = typename HostGrid::Traits::CollectiveCommunication;
+
+    template <int cd>
+    struct Codim
+    {
+      using Entity = typename Grid::Traits::template Codim<cd>::Entity;
+      using Geometry = typename Grid::template Codim<cd>::Geometry;
+      using LocalGeometry = typename Grid::template Codim<cd>::LocalGeometry;
+
+      template <PartitionIteratorType pit>
+      struct Partition
+      {
+        using Iterator = MultiMeshIterator<cd,pit,HostGrid>;
+      };
+    };
+
+    enum { conforming = Capabilities::isLevelwiseConforming<HostGrid>::v };
+  };
+
+
+  template <class HostGrid>
+  class MultiGridView
+  {
+  public:
+    using Traits = MultiGridViewTraits<HostGrid>;
+
+    /// The MultiMesh GridType
+    using Grid = typename Traits::Grid;
+
+    using GridViewTypes = std::variant<typename HostGrid::LeafGridView, typename HostGrid::LevelGridView>;
+
+    using IndexSet = typename Traits::IndexSet;
+    using IndexSetTypes = typename IndexSet::IndexSetTypes;
+    using IntersectionIterator = typename Traits::IntersectionIterator;
+
+    using CollectiveCommunication = typename Traits::CollectiveCommunication;
+
+    template <class GV>
+    using IsGridView = Std::disjunction<
+      std::is_same<std::decay_t<GV>,typename HostGrid::LeafGridView>,
+      std::is_same<std::decay_t<GV>,typename HostGrid::LevelGridView> >;
+
+    template <int cd>
+    struct Codim : public Traits::template Codim<cd> {};
+
+    enum { dimension = HostGrid::dimension };
+    enum { dimensionworld = HostGrid::dimensionworld };
+    enum { conforming = Traits::conforming };
+
+  public:
+    /// Constructor.
+    template <class... GridViews,
+      std::enable_if_t<Std::conjunction<IsGridView<GridViews>...>::value, int> = 0>
+    MultiGridView (GridViews&&... gridViews)
+      : gridViews_{std::forward<GridViews>(gridViews)...}
+    {}
+
+    template <class GridView,
+      std::enable_if_t<IsGridView<GridView>::value, int> = 0>
+    MultiGridView (const std::vector<GridView>& gridViews)
+      : gridViews_(gridViews.begin(), gridViews.end())
+    {}
+
+    template <class Iter,
+      std::enable_if_t<IsGridView<typename std::iterator_traits<Iter>::value_type>::