Build boundary node multimap (libMesh#3385)

Author: dschwen

src/mesh/mesh_tools.C

@@ -537,7 +537,7 @@ MeshTools::find_block_boundary_nodes(const MeshBase & mesh)
   // mark them as true in on_boundary.
   for (const auto & elem : mesh.active_element_ptr_range())
     for (auto s : elem->side_index_range())
-      if (elem->neighbor_ptr(s) && (elem->neighbor_ptr(s)->subdomain_id() != elem->subdomain_id()))
+      if (elem->neighbor_ptr(s) && elem->neighbor_ptr(s)->subdomain_id() > elem->subdomain_id())
         {
           auto nodes_on_side = elem->nodes_on_side(s);
 
@@ -549,6 +549,31 @@ MeshTools::find_block_boundary_nodes(const MeshBase & mesh)
 }
 
 
+std::multimap<dof_id_type, std::pair<subdomain_id_type, subdomain_id_type>>
+MeshTools::build_block_boundary_node_map(const MeshBase & mesh)
+{
+  std::multimap<dof_id_type, std::pair<subdomain_id_type, subdomain_id_type>> block_boundary_node_map;
+
+  // Loop over elements, find those on boundary, and
+  // mark them as true in on_boundary.
+  for (const auto & elem : mesh.active_element_ptr_range())
+  {
+    const auto id1 = elem->subdomain_id();
+    for (auto s : elem->side_index_range())
+    {
+      const auto id2 = elem->neighbor_ptr(s)->subdomain_id();
+      if (elem->neighbor_ptr(s) && id2 > id1)
+        {
+          auto nodes_on_side = elem->nodes_on_side(s);
+
+          for (auto & local_id : nodes_on_side)
+            block_boundary_node_map.emplace(elem->node_ptr(local_id)->id(), id1, id2);
+        }
+    }
+
+  return block_boundary_node_map;
+}
+
 
 libMesh::BoundingBox
 MeshTools::create_bounding_box (const MeshBase & mesh)