feat(discretization/domain): updated domain system to use unified mfem backend

This means that the domain system never directly uses mfem code any more

src/include/serif/discretization/domain/mesh/topology.hpp

@@ -10,17 +10,6 @@
 #include <cstddef>
 #include <vector>
 
-namespace mfem {
-    /**
-     * @brief Forward declaration of the mfem::Mesh class to avoid including the entire MFEM header.
-     *
-     * @todo We should probably consider doing this more often and in some central location since ideally we would avoid including MFEM headers in our own headers. Due to how
-     * heavily templated our code is that wont always be possible, but there are many places where we could. This has the potential to dramatically reduce compile times
-     * since our compile time is primarily limited by parsing through the MFEM headers at the moment.
-     */
-    class Mesh;
-}
-
 // MeshTopology
 namespace serif::discretization::domain::mesh {
     /**
@@ -32,6 +21,20 @@ namespace serif::discretization::domain::mesh {
         int secondElementID{-1};
     };
 
+    /**
+     * @brief Backend-independent material labels and face connectivity;
+     *
+     * @todo Implemented this with std::vector when I moved all the mfem code into backend/mfem; however,
+     * std::vector has all sorts of limitations and problems. We may want to consider using the
+     * types.hpp vector alias here instead? Or some other vector implementation.
+     */
+    struct MeshTopologyData {
+        std::vector<int> elementDomainIDs;
+        std::vector<FaceElements> faceElements;
+        std::vector<int> boundaryElementBoundaryIDs;
+        std::vector<int> boundaryElementFaceIDs;
+    };
+
     /**
      * @class MeshTopology
      * @brief Provides a convenient interface for querying the topological relationships between elements, faces, and boundaries in a mesh.
@@ -40,10 +43,10 @@ namespace serif::discretization::domain::mesh {
     public:
 
         /**
-         * @brief Constructs a MeshTopology object from an MFEM mesh. Note that MeshTopology does not own the mesh;
-         * @param mesh The MFEM mesh from which to extract topological information.
+         * @brief Owns material labels and connectivity independently of the mesh backend.
+         * @param data The topology data from which to derive element and boundary adjacency.
          */
-        explicit MeshTopology(const mfem::Mesh &mesh);
+        explicit MeshTopology(MeshTopologyData data);
 
         /**
          * @brief Returns the number of elements in the mesh.
@@ -68,7 +71,7 @@ namespace serif::discretization::domain::mesh {
          * @param elementID The ID of the element for which to retrieve the domain ID.
          * @return The domain ID of the specified element.
          *
-         * @note The domain ID is the MFEM element attribute (often called the material attribute) of a volume element, i.e. which DomainID the mesh claims this element belongs to.
+         * @note The domain ID is the material label of a volume element, i.e. which DomainID the mesh claims this element belongs to.
          */
         [[nodiscard]] int element_domain_id(int elementID) const;
 
@@ -77,7 +80,7 @@ namespace serif::discretization::domain::mesh {
          * @param boundaryElementID The ID of the boundary element for which to retrieve the boundary ID.
          * @return The boundary ID of the specified boundary element.
          *
-         * @note The boundary ID is the MFEM boundary attribute of a boundary element, i.e. which BoundaryID the mesh claims this boundary element belongs to.
+         * @note The boundary ID is the boundary label of a boundary element, i.e. which BoundaryID the mesh claims this boundary element belongs to.
          */
         [[nodiscard]] int boundary_element_boundary_id(int boundaryElementID) const;
 
@@ -100,7 +103,7 @@ namespace serif::discretization::domain::mesh {
         /**
          * @brief Returns the face a boundary element sits on.
          * @param boundaryElementID The ID of the boundary element for which to retrieve the face.
-         * @return The ID of the face the boundary element sits on, or -1 if MFEM reports none.
+         * @return The ID of the face the boundary element sits on, or -1 if no face was supplied.
          */
         [[nodiscard]] int boundary_element_face(int boundaryElementID) const;