fill_edge_region_1d Subroutine

  subroutine fill_edge_region_1d(input, output)
    !! Fill edge region for 1D padding
    implicit none

    ! Arguments
    type(array_type), intent(in) :: input
    type(array_type), intent(inout) :: output

    ! Local variables
    integer :: i, m, s, f
    integer :: step, idx_in, idx_out
    integer :: input_size, output_size, pad_size

    input_size = input%shape(1)
    output_size = output%shape(1)
    pad_size = output%indices(2)

    do f = 1, output%indices(3)
       do concurrent( s = 1:size(output%val, dim=2), m = 1:output%shape(2) )
          select case(output%indices(1))
          case(3, 4) ! circular or reflection
             step = merge(1, -1, output%indices(1) .eq. 3)
             do i = 1, pad_size
                idx_in = output%adj_ja(1,(f-1)*2 + 1) + step * (i - 1) + &
                     (m-1)*input_size
                idx_out = output%adj_ja(2,(f-1)*2 + 1) + i - 1 + &
                     (m-1)*output_size
                output%val(idx_out, s) = input%val(idx_in, s)
             end do
          case(5) ! replication
             idx_in = output%adj_ja(1,(f-1)*2 + 1) + (m-1)*input_size
             do i = 1, pad_size
                idx_out = output%adj_ja(2,(f-1)*2 + 1) + i - 1 + &
                     (m-1)*output_size
                output%val(idx_out, s) = input%val(idx_in, s)
             end do
          end select
       end do
    end do

  end subroutine fill_edge_region_1d