fix: reduce size before broadcasting

src/scippneutron/absorption/base.py

@@ -16,9 +16,6 @@ def compute_transmission_map(
     quadrature_kind: Any = 'medium',
 ) -> sc.DataArray:
     points, weights = sample_shape.quadrature(quadrature_kind)
-    scatter_direction = detector_position - points.to(unit=detector_position.unit)
-    scatter_direction /= sc.norm(scatter_direction)
-
     transmission = _integrate_transmission_fraction(
         partial(
             _single_scatter_distance_through_sample,
@@ -29,7 +26,7 @@ def compute_transmission_map(
         partial(_transmission_fraction, sample_material),
         points,
         weights,
-        scatter_direction,
+        detector_position,
         wavelength,
     )
     return sc.DataArray(
@@ -54,41 +51,38 @@ def _transmission_fraction(material, distance_through_sample, wavelength):
     )
 
 
-def _size_after_broadcast(a, b):
-    'Size of the result of broadcasting a and b'
-    size = 1
-    for s in {**a.sizes, **b.sizes}.values():
-        size *= s
-    return size
-
-
 def _integrate_transmission_fraction(
     distance_through_sample,
     transmission,
     points,
     weights,
-    scatter_direction,
+    detector_position,
     wavelengths,
 ):
     # If size after broadcast is too large
     # then don't vectorize the operation to avoid OOM
-    if _size_after_broadcast(points, scatter_direction) > 100_000_000:
-        dim = scatter_direction.dims[0]
-        return sc.concat(
-            [
+    if points.size * detector_position.size > 20_000_000:
+        out = []
+        dim = detector_position.dims[0]
+        for i in range(detector_position.sizes[dim]):
+            out.append(  # noqa: PERF401
                 _integrate_transmission_fraction(
                     distance_through_sample,
                     transmission,
                     points,
                     weights,
-                    scatter_direction[dim, i],
+                    detector_position[dim, i],
                     wavelengths,
                 )
-                for i in range(scatter_direction.shape[0])
-            ],
+            )
+
+        return sc.concat(
+            out,
             dim=dim,
         )
 
+    scatter_direction = detector_position - points.to(unit=detector_position.unit)
+    scatter_direction /= sc.norm(scatter_direction)
     Ltot = distance_through_sample(scatter_direction)
 
     # The Ltot array is already large, to avoid OOM, don't vectorize this operation