From c9074c7b38e19ee54b143661efbe0515bc4eb563 Mon Sep 17 00:00:00 2001
From: Wolfgang Bangerth <bangerth@colostate.edu>
Date: Sun, 31 Dec 2023 18:18:14 -0700
Subject: [PATCH] Deal with IndexSet operations in a more efficient way.

---
 source/lac/trilinos_sparse_matrix.cc | 29 +++++++++++++++-------------
 1 file changed, 16 insertions(+), 13 deletions(-)

diff --git a/source/lac/trilinos_sparse_matrix.cc b/source/lac/trilinos_sparse_matrix.cc
index 1d0fec67b2..0a9ce64272 100644
--- a/source/lac/trilinos_sparse_matrix.cc
+++ b/source/lac/trilinos_sparse_matrix.cc
@@ -664,19 +664,22 @@ namespace TrilinosWrappers
       std::vector<TrilinosWrappers::types::int_type> ghost_rows;
       std::vector<int> n_entries_per_row(row_space_map.NumMyElements());
       std::vector<int> n_entries_per_ghost_row;
-      for (unsigned int i = 0, own = 0; i < n_rows; ++i)
-        {
-          const TrilinosWrappers::types::int_type global_row =
-            relevant_rows.nth_index_in_set(i);
-          if (row_space_map.MyGID(global_row))
-            n_entries_per_row[own++] = sparsity_pattern.row_length(global_row);
-          else if (sparsity_pattern.row_length(global_row) > 0)
-            {
-              ghost_rows.push_back(global_row);
-              n_entries_per_ghost_row.push_back(
-                sparsity_pattern.row_length(global_row));
-            }
-        }
+      {
+        SparseMatrix::size_type own = 0;
+        for (const auto global_row : relevant_rows)
+          {
+            if (row_space_map.MyGID(
+                  static_cast<TrilinosWrappers::types::int_type>(global_row)))
+              n_entries_per_row[own++] =
+                sparsity_pattern.row_length(global_row);
+            else if (sparsity_pattern.row_length(global_row) > 0)
+              {
+                ghost_rows.push_back(global_row);
+                n_entries_per_ghost_row.push_back(
+                  sparsity_pattern.row_length(global_row));
+              }
+          }
+      }
 
       Epetra_Map off_processor_map(-1,
                                    ghost_rows.size(),
-- 
2.39.5