Implement a second way to select a quadrature and mapping object.

diff --git a/include/deal.II/fe/fe_interface_values.h b/include/deal.II/fe/fe_interface_values.h
index 805d84f48d75b451ab62642f34e930bbeeed4747..835f9cb89549d49992258c2cf7cef922b36862d9 100644 (file)
--- a/include/deal.II/fe/fe_interface_values.h
+++ b/include/deal.II/fe/fe_interface_values.h
@@ -1483,8 +1483,13 @@ public:
    * -# If one of these arguments is left at its default value, then the
    *    function will need to choose a quadrature and/or mapping that is
    *    appropriate for the two finite element spaces used on the two cells
-   *    adjacent to the current interface. To this end, we try to find out
-   *    which of the two spaces is "larger" (say, if you had used $Q_2$
+   *    adjacent to the current interface. As the first choice, if the
+   *    quadrature or mapping collection we are considering has only one
+   *    element, then that is clearly the one that should be used.
+   * -# If the quadrature or mapping collection have multiple elements,
+   *    then we need to dig further. To this end, we try to find out
+   *    which of the two finite element spaces on the two adjacent cells
+   *    is "larger" (say, if you had used $Q_2$
    *    and $Q_4$ elements on the two adjacent cells, then the $Q_4$
    *    element is the larger one); the determination of which space
    *    is "larger" is made using the hp::FECollection::find_dominated_fe()
@@ -1509,7 +1514,7 @@ public:
    *      hp::MappingCollection to retrieve quadrature and mapping objects
    *      appropriate for the current face.
    *    - There are cases where neither element dominates the other. For
-   * example, if one uses $Q_2\times Q_1$ and $Q_1\times Q_2$ elements on
+   *      example, if one uses $Q_2\times Q_1$ and $Q_1\times Q_2$ elements on
    *      neighboring cells, neither of the two spaces dominates the other --
    *      or, in the context of the current function, neither space is "larger"
    *      than the other. In that case, there is no way for the current function
@@ -2543,34 +2548,53 @@ FEInterfaceValues<dim, spacedim>::reinit(
     }
   else if (internal_hp_fe_face_values)
     {
+      unsigned int used_q_index       = q_index;
+      unsigned int used_mapping_index = mapping_index;
+
+      // First check. If there is only one element in a collection, and if none
+      // had been specified explicitly, then that's clearly the one to take:
+      if (used_q_index == numbers::invalid_unsigned_int)
+        if (internal_hp_fe_face_values->get_quadrature_collection().size() == 1)
+          used_q_index = 0;
+
+      if (used_mapping_index == numbers::invalid_unsigned_int)
+        if (internal_hp_fe_face_values->get_mapping_collection().size() == 1)
+          used_mapping_index = 0;
+
+      // Second check, if the above did not already suffice. We see if we
+      // can get somewhere via the dominated's finite element index.
       const unsigned int dominated_fe_index =
-        internal_hp_fe_face_values->get_fe_collection().find_dominated_fe(
-          {cell->active_fe_index(), cell_neighbor->active_fe_index()});
-
-      if (q_index == numbers::invalid_unsigned_int)
-        Assert(dominated_fe_index != numbers::invalid_fe_index,
-               ExcMessage("You called this function with 'q_index' left at its "
-                          "default value, but this can only work if one of "
-                          "the two finite elements adjacent to this face "
-                          "dominates the other. See the documentation "
-                          "of this function for more information of how "
-                          "to deal with this situation."));
-      if (mapping_index == numbers::invalid_unsigned_int)
-        Assert(dominated_fe_index != numbers::invalid_fe_index,
-               ExcMessage("You called this function with 'mapping_index' left "
-                          "at its default value, but this can only work if one "
-                          "of the two finite elements adjacent to this face "
-                          "dominates the other. See the documentation "
-                          "of this function for more information of how "
-                          "to deal with this situation."));
-
-
-      const unsigned int used_q_index =
-        (q_index == numbers::invalid_unsigned_int ? dominated_fe_index :
-                                                    q_index);
-      const unsigned int used_mapping_index =
-        (mapping_index == numbers::invalid_unsigned_int ? dominated_fe_index :
-                                                          mapping_index);
+        ((used_q_index == numbers::invalid_unsigned_int) ||
+             (used_mapping_index == numbers::invalid_unsigned_int) ?
+           internal_hp_fe_face_values->get_fe_collection().find_dominated_fe(
+             {cell->active_fe_index(), cell_neighbor->active_fe_index()}) :
+           numbers::invalid_unsigned_int);
+
+      if (used_q_index == numbers::invalid_unsigned_int)
+        {
+          Assert(dominated_fe_index != numbers::invalid_fe_index,
+                 ExcMessage(
+                   "You called this function with 'q_index' left at its "
+                   "default value, but this can only work if one of "
+                   "the two finite elements adjacent to this face "
+                   "dominates the other. See the documentation "
+                   "of this function for more information of how "
+                   "to deal with this situation."));
+          used_q_index = dominated_fe_index;
+        }
+
+      if (used_mapping_index == numbers::invalid_unsigned_int)
+        {
+          Assert(dominated_fe_index != numbers::invalid_fe_index,
+                 ExcMessage(
+                   "You called this function with 'mapping_index' left "
+                   "at its default value, but this can only work if one "
+                   "of the two finite elements adjacent to this face "
+                   "dominates the other. See the documentation "
+                   "of this function for more information of how "
+                   "to deal with this situation."));
+          used_mapping_index = dominated_fe_index;
+        }
 
       // Same as if above, but when hp is enabled.
       if (sub_face_no == numbers::invalid_unsigned_int)