mg_set_next_level_ids Subroutine

  subroutine mg_set_next_level_ids(mg, lvl)
    type(mg_t), intent(inout)  :: mg
    integer, intent(in)        :: lvl
    integer                    :: n, i, id

    if (allocated(mg%lvls(lvl+1)%ids)) deallocate(mg%lvls(lvl+1)%ids)

    ! Set next level ids to children of this level
    if (mg%box_size_lvl(lvl+1) == mg%box_size_lvl(lvl)) then
       n = mg_num_children * size(mg%lvls(lvl)%parents)
       allocate(mg%lvls(lvl+1)%ids(n))

       n = mg_num_children
       do i = 1, size(mg%lvls(lvl)%parents)
          id = mg%lvls(lvl)%parents(i)
          mg%lvls(lvl+1)%ids(n*(i-1)+1:n*i) = mg%boxes(id)%children
       end do
    else
       n = size(mg%lvls(lvl)%parents)
       allocate(mg%lvls(lvl+1)%ids(n))

       n = 1
       do i = 1, size(mg%lvls(lvl)%parents)
          id = mg%lvls(lvl)%parents(i)
          mg%lvls(lvl+1)%ids(i) = mg%boxes(id)%children(1)
       end do
    end if

  end subroutine mg_set_next_level_ids