From 5eea2e170f2c83f00d2220f31d52647793963e0b Mon Sep 17 00:00:00 2001
From: Marc Fehling <mafehling.git@gmail.com>
Date: Wed, 28 Feb 2024 19:07:55 -0700
Subject: [PATCH] Fix limit_p_level_difference for p-coarsening.

---
 doc/news/changes/minor/20240229Fehling      |  4 +
 source/hp/refinement.cc                     |  3 +-
 tests/hp/limit_p_level_difference_06.cc     | 89 +++++++++++++++++++++
 tests/hp/limit_p_level_difference_06.output |  7 ++
 4 files changed, 102 insertions(+), 1 deletion(-)
 create mode 100644 doc/news/changes/minor/20240229Fehling
 create mode 100644 tests/hp/limit_p_level_difference_06.cc
 create mode 100644 tests/hp/limit_p_level_difference_06.output

diff --git a/doc/news/changes/minor/20240229Fehling b/doc/news/changes/minor/20240229Fehling
new file mode 100644
index 0000000000..eba14354b6
--- /dev/null
+++ b/doc/news/changes/minor/20240229Fehling
@@ -0,0 +1,4 @@
+Fixed: The function hp::Refinement::limit_p_level_difference() now
+correctly sets future FE indices in case of p-coarsening.
+<br>
+(Marc Fehling, 2024/02/29)
diff --git a/source/hp/refinement.cc b/source/hp/refinement.cc
index f0eff39172..c9a4b218a4 100644
--- a/source/hp/refinement.cc
+++ b/source/hp/refinement.cc
@@ -1063,9 +1063,10 @@ namespace hp
               const unsigned int fe_index =
                 fe_index_for_hierarchy_level[cell_level];
 
-              // only update if necessary
               if (fe_index != cell->active_fe_index())
                 cell->set_future_fe_index(fe_index);
+              else
+                cell->clear_future_fe_index();
             }
         }
 
diff --git a/tests/hp/limit_p_level_difference_06.cc b/tests/hp/limit_p_level_difference_06.cc
new file mode 100644
index 0000000000..583005709c
--- /dev/null
+++ b/tests/hp/limit_p_level_difference_06.cc
@@ -0,0 +1,89 @@
+// ------------------------------------------------------------------------
+//
+// SPDX-License-Identifier: LGPL-2.1-or-later
+// Copyright (C) 2021 - 2024 by the deal.II authors
+//
+// This file is part of the deal.II library.
+//
+// Part of the source code is dual licensed under Apache-2.0 WITH
+// LLVM-exception OR LGPL-2.1-or-later. Detailed license information
+// governing the source code and code contributions can be found in
+// LICENSE.md and CONTRIBUTING.md at the top level directory of deal.II.
+//
+// ------------------------------------------------------------------------
+
+
+// hp::Refinement::limit_p_level_difference() used to ignore updating future
+// FE indices when they coincide with the active FE index of a cell.
+// This posed a problem with p-coarsening.
+
+
+#include <deal.II/dofs/dof_handler.h>
+
+#include <deal.II/fe/fe_q.h>
+
+#include <deal.II/grid/grid_generator.h>
+#include <deal.II/grid/tria.h>
+
+#include <deal.II/hp/fe_collection.h>
+#include <deal.II/hp/refinement.h>
+
+#include "../tests.h"
+
+#include "../test_grids.h"
+
+
+template <int dim>
+void
+test()
+{
+  // setup FE collection
+  hp::FECollection<dim> fes;
+  while (fes.size() < 3)
+    fes.push_back(FE_Q<dim>(1));
+
+  // setup two cell triangulation
+  Triangulation<dim> tria;
+  TestGrids::hyper_line(tria, 2);
+
+  DoFHandler<dim> dofh(tria);
+  dofh.distribute_dofs(fes);
+
+  // set both cells to the same active FE index,
+  // and assign a future FE index one higher and one lower
+  for (const auto &cell : dofh.active_cell_iterators())
+    {
+      cell->set_active_fe_index(1);
+
+      if (cell->id().to_string() == "0_0:")
+        cell->set_future_fe_index(2);
+      else if (cell->id().to_string() == "1_0:")
+        cell->set_future_fe_index(0);
+      else
+        Assert(false, ExcInternalError());
+    }
+
+  const bool future_fe_indices_changed =
+    hp::Refinement::limit_p_level_difference(dofh);
+
+  (void)future_fe_indices_changed;
+  Assert(future_fe_indices_changed, ExcInternalError());
+
+  // display FE indices for all cells
+  for (const auto &cell : dofh.active_cell_iterators())
+    deallog << cell->id().to_string() << ": active:" << cell->active_fe_index()
+            << " future:" << cell->future_fe_index()
+            << " flag:" << cell->future_fe_index_set() << std::endl;
+
+  deallog << "OK" << std::endl;
+}
+
+
+int
+main()
+{
+  initlog();
+
+  test<2>();
+  test<3>();
+}
diff --git a/tests/hp/limit_p_level_difference_06.output b/tests/hp/limit_p_level_difference_06.output
new file mode 100644
index 0000000000..997950631b
--- /dev/null
+++ b/tests/hp/limit_p_level_difference_06.output
@@ -0,0 +1,7 @@
+
+DEAL::0_0:: active:1 future:2 flag:1
+DEAL::1_0:: active:1 future:1 flag:0
+DEAL::OK
+DEAL::0_0:: active:1 future:2 flag:1
+DEAL::1_0:: active:1 future:1 flag:0
+DEAL::OK
-- 
2.39.5