Feature: ancestor face computation function for the scheme

src/t8_schemes/t8_scheme.hxx

@@ -628,6 +628,27 @@ class t8_scheme {
                        eclass_schemes[tree_class]);
   };
 
+  /** Given a face of an element and a level coarser than (or equal to)
+   * the element's level, return the face number
+   * of the ancestor of the element that matches the element's face. Or return -1 if
+   * no face of the ancestor matches the face.
+   * \param [in] tree_class    The eclass of the current tree.
+   * \param [in]  element    The element.
+   * \param [in]  ancestor_level A refinement level smaller than (or equal to) \a element's level.
+   * \param [in]  face    Then number of a face of \a element.
+   * \return              If \a face of \a element is a subface of a face of \a element's ancestor at level \a ancestor_level,
+   *                      the face number of this face. Otherwise -1.
+   * \note For the root element this function always returns \a face.
+   */
+  constexpr int
+  element_face_get_ancestor_face (const t8_eclass_t tree_class, const t8_element_t *element, const int ancestor_level,
+                                  const int face) const
+  {
+    return std::visit (
+      [&] (auto &&scheme) { return scheme.element_face_get_ancestor_face (element, ancestor_level, face); },
+      eclass_schemes[tree_class]);
+  }
+
   /** Given an element and a face of this element. If the face lies on the
    * tree boundary, return the face number of the tree face.
    * If not the return value is arbitrary.

src/t8_schemes/t8_scheme_helpers.hxx

@@ -63,6 +63,50 @@ class t8_scheme_helpers: public t8_crtp_basic<TUnderlyingEclassScheme> {
   {
     return TEclass;
   }
+
+  /** Given a face of an element and a level coarser than (or equal to)
+   * the element's level, return the face number
+   * of the ancestor of the element that matches the element's face. Or return -1 if
+   * no face of the ancestor matches the face.
+   * \param [in] tree_class    The eclass of the current tree.
+   * \param [in]  element    The element.
+   * \param [in]  ancestor_level A refinement level smaller than (or equal to) \a element's level.
+   * \param [in]  face    Then number of a face of \a element.
+   * \return              If \a face of \a element is a subface of a face of \a element's ancestor at level \a ancestor_level,
+   *                      the face number of this face. Otherwise -1.
+   * \note For the root element this function always returns \a face.
+   */
+  inline int
+  element_face_get_ancestor_face (const t8_element_t *element, const int ancestor_level, const int face) const
+  {
+    auto underlying_impl = this->underlying ();  // Reference to the underlying scheme implementation
+
+    const int element_level = underlying_impl.element_get_level (element);
+    T8_ASSERT (element_level >= ancestor_level);
+    if (element_level == ancestor_level) {
+      // On the same level, the return value is the face itself
+      return face;
+    }
+    // Allocate memory for a temporary element.
+    t8_element_t *parent;
+    underlying_impl.element_new (1, &parent);
+    // Pointer to a temoporary element, that will move up the refinement hierarchy
+    const t8_element_t *temp_element = element;
+    int temp_face = face;
+    for (int ilevel = element_level; ilevel > ancestor_level; --ilevel) {
+      // Go one level up in the refinement hierarchy with the face
+      temp_face = underlying_impl.element_face_get_parent_face (temp_element, temp_face);
+      if (temp_face == -1) {
+        // This face is not a subface of an ancestor face.
+        underlying_impl.element_destroy (1, &parent);
+        return -1;
+      }
+      underlying_impl.element_get_parent (temp_element, parent);
+      temp_element = parent;
+    }
+    underlying_impl.element_destroy (1, &parent);
+    return temp_face;
+  }
 };
 
 #endif /* T8_SCHEME_HELPERS_HXX */

test/t8_schemes/t8_gtest_child_parent_face.cxx

@@ -27,6 +27,10 @@
 #include "t8_gtest_dfs_base.hxx"
 #include <test/t8_gtest_macros.hxx>
 
+/* TODO: extend this test or new test. On each level,
+ *        go multiple levels below and check against ancestor face.
+ *       See https://github.com/DLR-AMR/t8code/issues/2011 */
+
 class class_child_parent_face: public TestDFS {
   void
   check_element () override
@@ -49,6 +53,17 @@ class class_child_parent_face: public TestDFS {
         const int parentface = scheme->element_face_get_parent_face (eclass, children[ifacechild], childface);
         /* Check, that this is equal to the face that we started with */
         EXPECT_EQ (iface, parentface);
+        const int element_level = scheme->element_get_level (eclass, element);
+        // Check the ancestor face function when input is the child and the level of the child.
+        // We expect the output face to be the input face
+        const int ancestor_face_same_level
+          = scheme->element_face_get_ancestor_face (eclass, children[ifacechild], element_level + 1, childface);
+        EXPECT_EQ (childface, ancestor_face_same_level);
+        // Check the ancestor face function when input is the element level/
+        // We expect the output face to be the original face
+        const int ancestor_face_one_level_higher
+          = scheme->element_face_get_ancestor_face (eclass, children[ifacechild], element_level, childface);
+        EXPECT_EQ (iface, ancestor_face_one_level_higher);
       }
       scheme->element_destroy (eclass, num_face_children, children);
       T8_TESTSUITE_FREE (children);