Fix mesh remeshing performance: implement missing involved_faces index usage

src/YgorMath.cc

@@ -6325,6 +6325,47 @@ fv_surface_mesh<T,I>::recreate_involved_face_index(void){
 #endif
 
 
+// Apply a surgical update to this->involved_faces.
+template <class T, class I>
+void
+fv_surface_mesh<T,I>::apply_involved_face_index_diff(const involved_face_index_diff<I> &diff){
+    // Extend involved_faces for new vertices.
+    for(I i = 0; i < diff.new_vertex_count; ++i){
+        this->involved_faces.emplace_back();
+    }
+
+    // Apply removals.
+    for(const auto &entry : diff.entries_to_remove){
+        const auto v_idx = entry.first;
+        const auto f_idx = entry.second;
+        if(static_cast<size_t>(v_idx) >= this->involved_faces.size()) continue;
+        auto &face_list = this->involved_faces[v_idx];
+        auto it = std::find(face_list.begin(), face_list.end(), f_idx);
+        if(it != face_list.end()){
+            // Swap-and-pop for O(1) erasure (order of entries is not significant).
+            std::swap(*it, face_list.back());
+            face_list.pop_back();
+        }
+    }
+
+    // Apply additions.
+    for(const auto &entry : diff.entries_to_add){
+        const auto v_idx = entry.first;
+        const auto f_idx = entry.second;
+        if(static_cast<size_t>(v_idx) >= this->involved_faces.size()) continue;
+        this->involved_faces[v_idx].push_back(f_idx);
+    }
+    return;
+}
+#ifndef YGORMATH_DISABLE_ALL_SPECIALIZATIONS
+    template void fv_surface_mesh<float , uint32_t >::apply_involved_face_index_diff(const involved_face_index_diff<uint32_t> &);
+    template void fv_surface_mesh<float , uint64_t >::apply_involved_face_index_diff(const involved_face_index_diff<uint64_t> &);
+
+    template void fv_surface_mesh<double, uint32_t >::apply_involved_face_index_diff(const involved_face_index_diff<uint32_t> &);
+    template void fv_surface_mesh<double, uint64_t >::apply_involved_face_index_diff(const involved_face_index_diff<uint64_t> &);
+#endif
+
+
 // Re-compute this->vertex_normals from current face orientations.
 template <class T, class I>
 void

src/YgorMath.h

@@ -537,6 +537,34 @@ Estimate_Contour_Separation(const std::list<std::reference_wrapper<contour_colle
 //---------------------------------------------------------------------------------------------------------------------------
 //--------------- fv_surface_mesh: a 2D surface mesh embedded in 3D with a straightforward data structure -------------------
 //---------------------------------------------------------------------------------------------------------------------------
+//---------------------------------------------------------------------------------------------------------------------------
+// A lightweight description of surgical changes to an involved_faces index.
+//
+// Instead of rebuilding the entire involved_faces index after every mesh modification,
+// callers can construct an involved_face_index_diff describing only the changes and
+// pass it to fv_surface_mesh::apply_involved_face_index_diff().
+//
+// Application order: new vertices are appended first, then all removals are applied,
+// and finally all additions are applied. The result is identical to what
+// recreate_involved_face_index() would produce.
+//
+template <class I>
+struct involved_face_index_diff {
+    // Number of new vertices added to the mesh.
+    // The involved_faces vector will be extended with this many empty entries.
+    I new_vertex_count = 0;
+
+    // Specific (vertex_index, face_index) associations to remove.
+    // Each pair means: remove face_index from involved_faces[vertex_index].
+    std::vector<std::pair<I, I>> entries_to_remove;
+
+    // Specific (vertex_index, face_index) associations to add.
+    // Each pair means: add face_index to involved_faces[vertex_index].
+    std::vector<std::pair<I, I>> entries_to_add;
+};
+
+//---------------------------------------------------------------------------------------------------------------------------
+
 //Simple, direct face-vertex list data structure representing a 3D surface mesh. Few constraints are imposed by this
 // data structure, and the meshes are not guaranteed to have many specific qualities. For example, multiple disconnected
 // meshes can be combined together without specifying whether they represent internal/external surfaces or are nested
@@ -597,6 +625,16 @@ template <class T, class I>   class fv_surface_mesh {
         // Regenerates this->involved_faces using this->vertices and this->faces.
         void recreate_involved_face_index(void);
 
+        // Apply a surgical update to this->involved_faces.
+        //
+        // This is an efficient alternative to recreate_involved_face_index() when only
+        // a small number of faces or vertices have changed. The diff describes which
+        // (vertex, face) associations to remove and which to add. New vertex entries are
+        // appended first, then all removals are applied, then all additions.
+        // The resulting index is identical to what recreate_involved_face_index() would
+        // produce given the same mesh state.
+        void apply_involved_face_index_diff(const involved_face_index_diff<I> &diff);
+
         // Re-compute this->vertex_normals using the current face orientations.
         //
         // Each vertex normal is the area-weighted average of the face normals of all