From ce1dc71d988c53b224a4bdadd97ee1fc0e9ea267 Mon Sep 17 00:00:00 2001
From: Wolfgang Bangerth <bangerth@colostate.edu>
Date: Mon, 18 Dec 2023 16:41:55 -0700
Subject: [PATCH] Some cleanups to create_description_from_triangulation().

Specifically, (i) group assertions at the top of the function, (ii) do
not insist on p::d::Triangulation as a parallel triangulation but
accept p::s::Triangulation as well by checking for
parallel::TriangulationBase as the common base class.
---
 source/grid/tria_description.cc | 65 ++++++++++++++++++---------------
 1 file changed, 35 insertions(+), 30 deletions(-)

diff --git a/source/grid/tria_description.cc b/source/grid/tria_description.cc
index cc8258aeaa..cc86a67111 100644
--- a/source/grid/tria_description.cc
+++ b/source/grid/tria_description.cc
@@ -741,42 +741,47 @@ namespace TriangulationDescription
       const TriangulationDescription::Settings    settings,
       const unsigned int                          my_rank_in)
     {
-      if (const auto tria_pdt = dynamic_cast<
-            const parallel::distributed::Triangulation<dim, spacedim> *>(&tria))
-        Assert(comm == tria_pdt->get_communicator(),
-               ExcMessage("MPI communicators do not match."));
-
-      // First, figure out for what rank we are supposed to build the
-      // TriangulationDescription::Description object
-      unsigned int my_rank = my_rank_in;
-      Assert(my_rank == numbers::invalid_unsigned_int ||
-               my_rank < dealii::Utilities::MPI::n_mpi_processes(comm),
-             ExcMessage("Rank has to be smaller than available processes."));
-
-      if (auto tria_pdt = dynamic_cast<
-            const parallel::distributed::Triangulation<dim, spacedim> *>(&tria))
+      if (const auto ptria =
+            dynamic_cast<const parallel::TriangulationBase<dim, spacedim> *>(
+              &tria))
         {
-          Assert(my_rank == numbers::invalid_unsigned_int ||
-                   my_rank == dealii::Utilities::MPI::this_mpi_process(comm),
+          Assert(comm == ptria->get_communicator(),
+                 ExcMessage("MPI communicators do not match."));
+          Assert(my_rank_in == numbers::invalid_unsigned_int ||
+                   my_rank_in == dealii::Utilities::MPI::this_mpi_process(comm),
                  ExcMessage(
-                   "For creation from a parallel::distributed::Triangulation, "
-                   "my_rank has to equal global rank."));
-
-          my_rank = dealii::Utilities::MPI::this_mpi_process(comm);
-        }
-      else if (auto tria_serial =
-                 dynamic_cast<const dealii::Triangulation<dim, spacedim> *>(
-                   &tria))
-        {
-          if (my_rank == numbers::invalid_unsigned_int)
-            my_rank = dealii::Utilities::MPI::this_mpi_process(comm);
+                   "For creation from a parallel::Triangulation, "
+                   "my_rank has to equal the rank of the current process "
+                   "in the given communicator."));
         }
-      else
+
+      // If we are dealing with a sequential triangulation, then someone
+      // will have needed to set the subdomain_ids by hand. Make sure that
+      // all ids we see are less than the number of processes we are
+      // supposed to split the triangulation into.
+      if (dynamic_cast<const parallel::TriangulationBase<dim, spacedim> *>(
+            &tria) == nullptr)
         {
-          Assert(false,
-                 ExcMessage("This type of triangulation is not supported!"));
+#if DEBUG
+          const unsigned int n_mpi_processes =
+            dealii::Utilities::MPI::n_mpi_processes(comm);
+          for (const auto &cell : tria.active_cell_iterators())
+            Assert(cell->subdomain_id() < n_mpi_processes,
+                   ExcMessage("You can't have a cell with subdomain_id of " +
+                              std::to_string(cell->subdomain_id()) +
+                              " when splitting the triangulation using an MPI "
+                              " communicator with only " +
+                              std::to_string(n_mpi_processes) + " processes."));
+#endif
         }
 
+      // First, figure out for what rank we are supposed to build the
+      // TriangulationDescription::Description object
+      const unsigned int my_rank =
+        (my_rank_in == numbers::invalid_unsigned_int ?
+           dealii::Utilities::MPI::this_mpi_process(comm) :
+           my_rank_in);
+
       const auto subdomain_id_function = [](const auto &cell) {
         return cell->subdomain_id();
       };
-- 
2.39.5