simplify assignment of neighboring tasks

[Pedro Costa]

The determination of neighboring tasks can be simplified by using the following functions I drafted:

   function get_id(coords,dims,periods) result(id)
     use mpi, only: MPI_PROC_NULL
     implicit none
     integer :: id
     integer, intent(in), dimension(3) :: coords,dims
     logical, intent(in), dimension(3), optional :: periods
     integer, dimension(3) :: coords_aux,shift
     coords_aux(:) = coords(:)
     if(present(periods)) then
       shift(:) = 0
       where(periods(:))
         where(coords_aux(:)>dims(:)-1) shift(:) =   (0        -coords_aux(:))/dims(:)
         where(coords_aux(:)<0        ) shift(:) =   (dims(:)-1-coords_aux(:))/dims(:)
         coords_aux(:) = coords_aux(:) + shift(:)*dims(:)
       end where
     endif
     if(all(coords_aux(:)<=dims(:)-1).and.all(coords_aux(:)>=0)) then
       id = coords_aux(1)+coords_aux(2)*dims(1)+coords_aux(3)*dims(2)
     else
       id = MPI_PROC_NULL
     endif
   end function get_id

   function get_coords(id,dims) result(coords)
     integer :: coords(3)
     integer, intent(in) :: id, dims(3)
     coords(:) = [mod(id,dims(1)),mod(id/dims(1),dims(2)),mod(id/(dims(1)*dims(2)),dims(3))]
   end function get_coords