mg_fas_fmg Subroutine

  subroutine mg_fas_fmg(mg, have_guess, max_res)
    type(mg_t), intent(inout)       :: mg
    logical, intent(in)             :: have_guess
    !< If false, start from phi = 0
    real(dp), intent(out), optional :: max_res
    !< Store max(abs(residual))
    integer                         :: lvl, i, id

    call check_methods(mg)
    if (timer_smoother == -1) call mg_add_timers(mg)

    call mg_timer_start(mg%timers(timer_total_fmg))

    if (.not. have_guess) then
       do lvl = mg%highest_lvl, mg%lowest_lvl, -1
          do i = 1, size(mg%lvls(lvl)%my_ids)
             id = mg%lvls(lvl)%my_ids(i)
             mg%boxes(id)%cc(:, mg_iphi) = 0.0_dp
          end do
       end do
    end if

    ! Ensure ghost cells are filled correctly
    call mg_fill_ghost_cells_lvl(mg, mg%highest_lvl, mg_iphi)

    do lvl = mg%highest_lvl,  mg%lowest_lvl+1, -1
       ! Set rhs on coarse grid and restrict phi
       call mg_timer_start(mg%timers(timer_update_coarse))
       call update_coarse(mg, lvl)
       call mg_timer_end(mg%timers(timer_update_coarse))
    end do

    if (mg%subtract_mean) then
       ! For fully periodic solutions, the mean source term has to be zero
       call subtract_mean(mg, mg_irhs, .false.)
    end if

    do lvl = mg%lowest_lvl, mg%highest_lvl
       ! Store phi_old
       do i = 1, size(mg%lvls(lvl)%my_ids)
          id = mg%lvls(lvl)%my_ids(i)
          mg%boxes(id)%cc(:, mg_iold) = mg%boxes(id)%cc(:, mg_iphi)
       end do

       if (lvl > mg%lowest_lvl) then
          ! Correct solution at this lvl using lvl-1 data
          ! phi = phi + prolong(phi_coarse - phi_old_coarse)
          call mg_timer_start(mg%timers(timer_correct))
          call correct_children(mg, lvl-1)
          call mg_timer_end(mg%timers(timer_correct))

          ! Update ghost cells
          call mg_fill_ghost_cells_lvl(mg, lvl, mg_iphi)
       end if

       ! Perform V-cycle, possibly set residual on last iteration
       if (lvl == mg%highest_lvl) then
          call mg_fas_vcycle(mg, lvl, max_res, standalone=.false.)
       else
          call mg_fas_vcycle(mg, lvl, standalone=.false.)
       end if
    end do

    call mg_timer_end(mg%timers(timer_total_fmg))
  end subroutine mg_fas_fmg