Use caches when returning or counting subdomains

Especially n_subdomains() should be an O(1) call, not O(N).

Author: roystgnr

include/mesh/mesh_base.h

@@ -1492,6 +1492,8 @@ class MeshBase : public ParallelObject
    * materials in a solid mechanics application, or regions where different
    * physical processes are important.  The subdomain mapping is independent
    * from the parallel decomposition.
+   *
+   * This relies on the mesh cached element data being prepared.
    */
   subdomain_id_type n_subdomains () const;
 
@@ -1501,6 +1503,8 @@ class MeshBase : public ParallelObject
    * materials in a solid mechanics application, or regions where different
    * physical processes are important.  The subdomain mapping is independent
    * from the parallel decomposition.
+   *
+   * This relies on the mesh cached element data being prepared.
    */
   subdomain_id_type n_local_subdomains () const;
 

src/mesh/mesh_base.C

@@ -1080,26 +1080,12 @@ void MeshBase::remove_ghosting_functor(GhostingFunctor & ghosting_functor)
 
 void MeshBase::subdomain_ids (std::set<subdomain_id_type> & ids, const bool global /* = true */) const
 {
-  // This requires an inspection on every processor
-  if (global)
-    parallel_object_only();
-
-  ids.clear();
-
-  for (const auto & elem : this->active_local_element_ptr_range())
-    ids.insert(elem->subdomain_id());
+  libmesh_assert(this->has_cached_elem_data());
 
   if (global)
-    {
-      // Only include the unpartitioned elements if the user requests the global IDs.
-      // In the case of the local subdomain IDs, it doesn't make sense to include the
-      // unpartitioned elements because said elements do not have a sense of locality.
-      for (const auto & elem : this->active_unpartitioned_element_ptr_range())
-        ids.insert(elem->subdomain_id());
-
-      // Some subdomains may only live on other processors
-      this->comm().set_union(ids);
-    }
+    ids = this->get_mesh_subdomains();
+  else
+    ids = this->get_mesh_local_subdomains();
 }
 
 
@@ -1128,14 +1114,7 @@ void MeshBase::update_post_partitioning()
 
 subdomain_id_type MeshBase::n_subdomains() const
 {
-  // This requires an inspection on every processor
-  parallel_object_only();
-
-  std::set<subdomain_id_type> ids;
-
-  this->subdomain_ids (ids);
-
-  return cast_int<subdomain_id_type>(ids.size());
+  return cast_int<subdomain_id_type>(this->get_mesh_subdomains().size());
 }