Make creation of spatial trees based on usage for unstructured mesh. (#…

…2815)

include/openmc/mesh.h

@@ -75,6 +75,8 @@ class Mesh {
   virtual ~Mesh() = default;
 
   // Methods
+  //! Perform any preparation needed to support use in mesh filters
+  virtual void prepare_for_tallies() {};
 
   //! Update a position to the local coordinates of the mesh
   virtual void local_coords(Position& r) const {};
@@ -664,6 +666,9 @@ class MOABMesh : public UnstructuredMesh {
 
   // Overridden Methods
 
+  //! Perform any preparation needed to support use in mesh filters
+  void prepare_for_tallies() override;
+
   Position sample_element(int32_t bin, uint64_t* seed) const override;
 
   void bins_crossed(Position r0, Position r1, const Direction& u,

src/mesh.cpp

@@ -2057,6 +2057,13 @@ void MOABMesh::initialize()
       }
     }
   }
+}
+
+void MOABMesh::prepare_for_tallies()
+{
+  // if the KDTree has already been constructed, do nothing
+  if (kdtree_)
+    return;
 
   // build acceleration data structures
   compute_barycentric_data(ehs_);

src/tallies/filter_mesh.cpp

@@ -77,8 +77,10 @@ std::string MeshFilter::text_label(int bin) const
 
 void MeshFilter::set_mesh(int32_t mesh)
 {
+  // perform any additional perparation for mesh tallies here
   mesh_ = mesh;
   n_bins_ = model::meshes[mesh_]->n_bins();
+  model::meshes[mesh_]->prepare_for_tallies();
 }
 
 void MeshFilter::set_translation(const Position& translation)