[fdfd.waveguide_3d] improve handling of out-of-bounds overlap_e windows

meanas/fdfd/waveguide_3d.py

@@ -5,6 +5,8 @@ This module relies heavily on `waveguide_2d` and mostly just transforms
 its parameters into 2D equivalents and expands the results back into 3D.
 """
 from typing import Any, cast
+import warnings
+from typing import Any
 from collections.abc import Sequence
 import numpy
 from numpy.typing import NDArray
@@ -200,17 +202,33 @@ def compute_overlap_e(
     Ee = expand_e(E=E, wavenumber=wavenumber, dxes=dxes,
                   axis=axis, polarity=polarity, slices=slices)
 
-    start, stop = sorted((slices[axis].start, slices[axis].start - 2 * polarity))
+    axis_size = E.shape[axis + 1]
+    if polarity > 0:
+        start = slices[axis].start - 2
+        stop = slices[axis].start
+    else:
+        start = slices[axis].stop
+        stop = slices[axis].stop + 2
+
+    clipped_start = max(0, start)
+    clipped_stop = min(axis_size, stop)
+    if clipped_start >= clipped_stop:
+        raise ValueError('Requested overlap window lies outside the domain')
+    if clipped_start != start or clipped_stop != stop:
+        warnings.warn('Requested overlap window was clipped to fit within the domain', RuntimeWarning)
 
     slices2_l = list(slices)
-    slices2_l[axis] = slice(start, stop)
+    slices2_l[axis] = slice(clipped_start, clipped_stop)
     slices2 = (slice(None), *slices2_l)
 
     Etgt = numpy.zeros_like(Ee)
     Etgt[slices2] = Ee[slices2]
 
     # Note: We normalize so that (Etgt @ E.conj()) == 1, so (Etgt @ Etgt.conj) != 1
-    Etgt /= (Etgt.conj() * Etgt).sum()
+    norm = (Etgt.conj() * Etgt).sum()
+    if norm == 0:
+        raise ValueError('Requested overlap window contains no overlap field support')
+    Etgt /= norm
     return cfdfield_t(Etgt)