get_partial_avgpool1d_val Subroutine

  pure subroutine get_partial_avgpool1d_val(this, upstream_grad, output)
    !! Optimised backward pass for 1D average pooling
    implicit none

    ! Arguments
    class(array_type), intent(in) :: this
    real(real32), dimension(:,:), intent(in) :: upstream_grad
    real(real32), dimension(:,:), intent(out) :: output

    ! Local variables
    integer :: i, m, s, p
    integer :: base_idx, out_idx, input_h
    real(real32) :: pool_norm, grad_val
    integer, dimension(3) :: input_shape
    integer, dimension(1) :: pool_size, stride

    ! Unpack parameters
    input_shape = [ this%left_operand%shape, size(this%val, dim=2) ]
    pool_size(1) = this%adj_ja(1,1)
    stride(1) = this%adj_ja(1,2)
    input_h = input_shape(1)

    output = 0._real32

    pool_norm = 1.0_real32 / real(pool_size(1), real32)

    ! Parallelised over batch and spatial/channel dimensions
    do concurrent(s = 1:input_shape(3), m = 1:this%shape(2), &
         i = 1:this%shape(1))

       ! Compute indices once
       base_idx = (i - 1) * stride(1) + (m - 1) * input_h
       out_idx = i + (m - 1) * this%shape(1)
       grad_val = upstream_grad(out_idx, s) * pool_norm

       ! Distribute gradient over pooling window
       do p = 0, pool_size(1) - 1
          output(base_idx + p + 1, s) = output(base_idx + p + 1, s) + grad_val
       end do
    end do

  end subroutine get_partial_avgpool1d_val