Feature: boundary condition handler

example/remove/t8_example_gauss_blob.cxx

@@ -36,7 +36,7 @@ struct t8_adapt_data
 };
 
 static double
-t8_gausss_blob (const t8_3D_vec &center_elem, const t8_3D_vec &center_cube, const double radius)
+t8_gauss_blob (const t8_3D_vec &center_elem, const t8_3D_vec &center_cube, const double radius)
 {
   double expo = 0;
   for (int i = 0; i < 3; i++) {
@@ -68,7 +68,7 @@ t8_create_element_data (t8_forest_t forest, const t8_3D_vec &sphere_center, cons
       element = t8_forest_get_leaf_element_in_tree (forest, itree, ielement);
       t8_3D_vec center;
       t8_forest_element_centroid (forest, itree, element, center.data ());
-      element_data[current_index] = t8_gausss_blob (center, sphere_center, sphere_radius);
+      element_data[current_index] = t8_gauss_blob (center, sphere_center, sphere_radius);
     }
   }
   return element_data;

src/CMakeLists.txt

@@ -149,11 +149,14 @@ target_sources( T8 PRIVATE
     t8_cmesh/t8_cmesh_internal/t8_cmesh_copy.cxx
     t8_cmesh/t8_cmesh_internal/t8_cmesh_offset.c
     t8_cmesh/t8_cmesh_internal/t8_cmesh_partition.cxx
-    t8_cmesh/t8_cmesh_internal/t8_cmesh_stash.c
+    t8_cmesh/t8_cmesh_internal/t8_cmesh_stash.cxx
     t8_cmesh/t8_cmesh_internal/t8_cmesh_trees.cxx
     t8_cmesh/t8_cmesh_vertex_connectivity/t8_cmesh_vertex_conn_tree_to_vertex.cxx
     t8_cmesh/t8_cmesh_vertex_connectivity/t8_cmesh_vertex_conn_vertex_to_tree.cxx
     t8_cmesh/t8_cmesh_vertex_connectivity/t8_cmesh_vertex_connectivity.cxx
+    t8_cmesh/t8_cmesh_boundary_conditions/t8_cmesh_boundary_conditions.cxx
+    t8_cmesh/t8_cmesh_boundary_conditions/internal/t8_cmesh_boundary_condition_handler.cxx
+    t8_cmesh/t8_cmesh_boundary_conditions/internal/t8_cmesh_boundary_condition_handler_types.cxx
     t8_cmesh/t8_cmesh_io/t8_cmesh_readmshfile.cxx
     t8_cmesh/t8_cmesh_io/deprecated/deprecated_t8_cmesh_save.cxx
     t8_cmesh/t8_cmesh_io/deprecated/deprecated_t8_cmesh_triangle.cxx

src/t8_cmesh/t8_cmesh_boundary_conditions/internal/t8_cmesh_boundary_condition_handler.cxx

@@ -0,0 +1,93 @@
+/*
+  This file is part of t8code.
+  t8code is a C library to manage a collection (a forest) of multiple
+  connected adaptive space-trees of general element classes in parallel.
+
+  Copyright (C) 2026 the developers
+
+  t8code is free software; you can redistribute it and/or modify
+  it under the terms of the GNU General Public License as published by
+  the Free Software Foundation; either version 2 of the License, or
+  (at your option) any later version.
+
+  t8code is distributed in the hope that it will be useful,
+  but WITHOUT ANY WARRANTY; without even the implied warranty of
+  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+  GNU General Public License for more details.
+
+  You should have received a copy of the GNU General Public License
+  along with t8code; if not, write to the Free Software Foundation, Inc.,
+  51 Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA.
+*/
+
+/**
+ * \file t8_cmesh_boundary_condition_handler.cxx
+ * Implementation context of \ref t8_cmesh_boundary_condition_handler.hxx
+ */
+
+#include <t8_cmesh/t8_cmesh_boundary_conditions/internal/t8_cmesh_boundary_condition_handler.hxx>
+#include <t8_cmesh/t8_cmesh_boundary_conditions/internal/t8_cmesh_boundary_condition_handler_types.h>
+#include <t8_cmesh/t8_cmesh_internal/t8_cmesh_types.h>
+#include <t8_cmesh/t8_cmesh_internal/t8_cmesh_stash.h>
+
+using namespace detail;
+
+#if T8_ENABLE_DEBUG
+int
+t8_cmesh_boundary_condition_handler::verify () const
+{
+  T8_ASSERT (!t8_cmesh_is_committed (m_cmesh, 0));
+
+  /* Retrieve eclasses and boundary conditions from stash */
+  std::vector<std::pair<t8_gloidx_t, std::span<boundary_condition_hash>>> boundary_conditions
+    = t8_stash_extract_attribute_list<boundary_condition_hash> (m_cmesh->stash, t8_get_package_id (),
+                                                                T8_CMESH_BOUNDARY_CONDITION_ATTRIBUTE_KEY);
+  std::vector<std::pair<t8_gloidx_t, t8_eclass>> eclasses = t8_stash_extract_eclasses (m_cmesh->stash);
+
+  /* Check that every tree has a bc */
+  if (eclasses.size () != boundary_conditions.size ()) {
+    t8_errorf ("ERROR: Number of cmesh eclasses does not match the number of cmesh boundary conditions.\n"
+               "If boundary conditions are applied, all trees have to get them assigned. They can be left empty.\n");
+    return false;
+  }
+
+  /* Sort the bcs and eclasses by tree id. */
+  std::sort (boundary_conditions.begin (), boundary_conditions.end (),
+             [] (const auto &a, const auto &b) { return a.first < b.first; });
+  std::sort (eclasses.begin (), eclasses.end (), [] (const auto &a, const auto &b) { return a.first < b.first; });
+
+  auto eclasses_it = eclasses.cbegin ();
+  auto bc_it = boundary_conditions.cbegin ();
+  for (; eclasses_it != eclasses.end (); ++eclasses_it, ++bc_it) {
+    /* Make sure, that the eclasses and bcs tree id are the same.  */
+    if (eclasses_it->first != bc_it->first) {
+      t8_errorf ("ERROR: eclass tree id and boundary condition tree id do not match.\nProbably the boundary condition "
+                 "got a wrong tree id. Boundary condition tree id: %li \n",
+                 bc_it->first);
+      return false;
+    }
+    /* Make sure that the amount of registered bcs matches th number of tree faces. */
+    if (static_cast<size_t> (t8_eclass_num_faces[eclasses_it->second]) != bc_it->second.size ()) {
+      t8_errorf ("ERROR: Tree %li has a mismatch in its face count and boundary condition count.\n"
+                 "Face count: %i, boundary condition count: %li\n",
+                 eclasses_it->first, t8_eclass_num_faces[eclasses_it->second], bc_it->second.size ());
+      return false;
+    }
+    /* Check, that the hash of the bc attribute is present in the bc map. This failure cannot be caused by the user. */
+    for (const auto bc : bc_it->second) {
+      if (!get_boundary_condition_name_safe (bc).has_value ()) {
+        t8_errorf ("ERROR: Could not find name of applied boundary condition. You probably found a bug.\n");
+        return false;
+      }
+    }
+  }
+  return true;
+}
+
+#endif /* T8_ENABLE_DEBUG */
+
+int
+t8_cmesh_boundary_condition_handler::get_boundary_condition_attribute_key () const
+{
+  return T8_CMESH_BOUNDARY_CONDITION_ATTRIBUTE_KEY;
+}