mg_restrict_lvl Subroutine

  subroutine mg_restrict_lvl(mg, iv, lvl)

    type(mg_t), intent(inout) :: mg
    integer, intent(in)       :: iv
    integer, intent(in)       :: lvl
    integer                   :: i, id, dsize, nc

    if (lvl <= mg%lowest_lvl) error stop "cannot restrict lvl <= lowest_lvl"

    nc = mg%box_size_lvl(lvl)

    if (lvl >= mg%first_normal_lvl) then
       dsize = (nc/2)**2

       mg%buf(:)%i_send = 0
       mg%buf(:)%i_ix   = 0

       do i = 1, size(mg%lvls(lvl)%my_ids)
          id = mg%lvls(lvl)%my_ids(i)
          call restrict_set_buffer(mg, id, iv)
       end do

       mg%buf(:)%i_recv = mg%comm_restrict%n_recv(:, lvl) * dsize
       call sort_and_transfer_buffers(mg, dsize)
       mg%buf(:)%i_recv = 0
    end if

    do i = 1, size(mg%lvls(lvl-1)%my_parents)
       id = mg%lvls(lvl-1)%my_parents(i)
       call restrict_onto(mg, id, nc, iv)
    end do
  end subroutine mg_restrict_lvl