Fix bug creating MPI Cart comm ending at a grid boundary

gprMax/grid/mpi_grid.py

@@ -139,13 +139,16 @@ class MPIGrid(FDTDGrid):
         return self.rank == self.COORDINATOR_RANK
 
     def create_sub_communicator(
-        self, start: npt.NDArray[np.int32], stop: npt.NDArray[np.int32]
+        self, local_start: npt.NDArray[np.int32], local_stop: npt.NDArray[np.int32]
     ) -> Optional[MPI.Cartcomm]:
-        if self.local_bounds_overlap_grid(start, stop):
+        if self.local_bounds_overlap_grid(local_start, local_stop):
             comm = self.comm.Split()
             assert isinstance(comm, MPI.Intracomm)
-            start_grid_coord = self.get_grid_coord_from_local_coordinate(start)
-            stop_grid_coord = self.get_grid_coord_from_local_coordinate(stop) + 1
+            start_grid_coord = self.get_grid_coord_from_local_coordinate(local_start)
+            # Subtract 1 from local_stop as the upper extent is
+            # exclusive meaning the last coordinate included in the sub
+            # communicator is actually (local_stop - 1).
+            stop_grid_coord = self.get_grid_coord_from_local_coordinate(local_stop - 1) + 1
             comm = comm.Create_cart((stop_grid_coord - start_grid_coord).tolist())
             return comm
         else:

gprMax/output_controllers/grid_view.py

@@ -320,14 +320,20 @@ class MPIGridView(GridView[MPIGrid]):
 
         comm = grid.comm.Split()
         assert isinstance(comm, MPI.Intracomm)
-        start_grid_coord = grid.get_grid_coord_from_local_coordinate(self.start)
-        stop_grid_coord = grid.get_grid_coord_from_local_coordinate(self.stop) + 1
-        self.comm = comm.Create_cart((stop_grid_coord - start_grid_coord).tolist())
 
+        # Calculate start, stop and size for the global grid view
+        self.global_start = self.grid.local_to_global_coordinate(self.start)
+        self.global_stop = self.grid.local_to_global_coordinate(self.stop)
         self.global_size = self.size
 
-        # Calculate start for the local grid
-        self.global_start = self.grid.local_to_global_coordinate(self.start)
+        # Create new cartesean communicator by finding MPI grid coords
+        # for the start and end of the grid view.
+        # Subtract 1 from global_stop as the upper extent is exclusive
+        # meaning the last coordinate included in the grid view is
+        # actually (global_stop - 1).
+        start_grid_coord = grid.get_grid_coord_from_coordinate(self.global_start)
+        stop_grid_coord = grid.get_grid_coord_from_coordinate(self.global_stop - 1) + 1
+        self.comm = comm.Create_cart((stop_grid_coord - start_grid_coord).tolist())
 
         self.has_negative_neighbour = self.start < self.grid.negative_halo_offset
 
@@ -340,9 +346,6 @@ class MPIGridView(GridView[MPIGrid]):
             self.start,
         )
 
-        # Calculate stop for the local grid
-        self.global_stop = self.grid.local_to_global_coordinate(self.stop)
-
         self.has_positive_neighbour = self.stop > self.grid.size
 
         # Limit stop such that it is at most one step beyond the max