Decrease the runtime of GridBasedGrid::init

GloMethod::compute_new_local_cells uses GloMethod::rank_of internally to
determine if a cell is owned by the calling process.
This is very compute intensive for GridBasedGrid because it checks all
Octagons (initially *all*, later all neighboring).
This commit adds an override GridBasedGrid::compute_new_local_cells
that only checks the Octagon of the calling process itself.

Author: hirschsn

repa/grids/gridbased.cpp

@@ -114,6 +114,18 @@ util::const_span<rank_type> GridBasedGrid::neighbor_ranks() const
     return util::make_const_span(const_neighborhood);
 }
 
+std::vector<global_cell_index_type>
+GridBasedGrid::compute_new_local_cells() const
+{
+    std::vector<global_cell_index_type> res;
+    for (const auto i : gbox.global_cells()) {
+        if (my_dom.contains(gbox.midpoint(i))) {
+            res.push_back(i);
+        }
+    }
+    return res;
+}
+
 void GridBasedGrid::pre_init(bool firstcall)
 {
     if (firstcall) {

repa/grids/gridbased.hpp

@@ -49,6 +49,14 @@ struct GridBasedGrid : public GloMethod {
     // neighborhood.
     util::const_span<rank_type> neighbor_ranks() const override;
 
+    // GloMethod::compute_new_local_cells uses rank_of() to determine if a cell
+    // belongs to this process. This is, however, very compute intensive for
+    // GridBasedGrid because it checks all neighboring (and initially *all*)
+    // subdomains. This implementation only checks the own Octagon if it
+    // includes a certain cell or not.
+    virtual std::vector<global_cell_index_type>
+    compute_new_local_cells() const override;
+
 private:
     // Indicator if the decomposition currently is a regular grid,
     // which is the case directly after instantiation.