cavity_diff_impl Subroutine

   pure subroutine cavity_diff_impl(a, b, active, buf)
      !! `a - b` with the cavity missing-value convention.
      ! assumed-shape-ok: diag fill — fires once per output frame (cadence-bounded).
      real(wp), intent(in)    :: a(:, :), b(:, :), active(:, :)  ! assumed-shape-ok: diag fill — cadence-bounded
      real(wp), intent(inout) :: buf(:, :, :)  ! assumed-shape-ok: diag fill — cadence-bounded
      integer :: i, j, nx, ny
      real(wp) :: qnan
      nx = min(size(buf, 1), size(a, 1), size(b, 1), size(active, 1))
      ny = min(size(buf, 2), size(a, 2), size(b, 2), size(active, 2))
      qnan = ieee_value(0.0_wp, ieee_quiet_nan)
      do concurrent(j=1:ny, i=1:nx)
         if (active(i, j) > 0.5_wp) then
            buf(i, j, 1) = a(i, j) - b(i, j)
         else
            buf(i, j, 1) = qnan
         end if
      end do
   end subroutine cavity_diff_impl