From ac61b8bab7a0d435b55b3a696f83274ef8c85060 Mon Sep 17 00:00:00 2001
From: Jan Petykiewicz <jan@mpxd.net>
Date: Mon, 14 Sep 2026 21:52:18 -0700
Subject: [PATCH] [Arc] fix arc mirroring

---
 masque/shapes/arc.py                 |  9 ++++-----
 masque/test/test_arc.py              | 15 +++++++++++++++
 masque/test/test_shape_transforms.py |  3 ++-
 3 files changed, 21 insertions(+), 6 deletions(-)

diff --git a/masque/shapes/arc.py b/masque/shapes/arc.py
index 53fadb8..35362a9 100644
--- a/masque/shapes/arc.py
+++ b/masque/shapes/arc.py
@@ -423,11 +423,10 @@ class Arc(PositionableImpl, Shape):
         return self
 
     def mirror(self, axis: int = 0) -> 'Arc':
-        if self.angle_ref != ArcAngleRef.Center:
-            x_major = self.radius_x > self.radius_y
-            y_major = self.radius_y > self.radius_x
-            if (axis == 0 and y_major) or (axis == 1 and x_major):
-                self._swap_focus_ref()
+        # Both external reflections use a local Y reflection; the extra pi
+        # rotation below accounts for the external axis.
+        if self.angle_ref != ArcAngleRef.Center and self.radius_y > self.radius_x:
+            self._swap_focus_ref()
         self.rotation *= -1
         self.rotation += axis * pi
         self.angles *= -1
diff --git a/masque/test/test_arc.py b/masque/test/test_arc.py
index dc23144..ae2789a 100644
--- a/masque/test/test_arc.py
+++ b/masque/test/test_arc.py
@@ -14,6 +14,21 @@ def test_arc_init() -> None:
     assert_equal(a.angles, [0, pi / 2])
     assert a.width == 2
 
+
+@pytest.mark.parametrize('axis', [0, 1])
+@pytest.mark.parametrize('radii', [(10, 6), (6, 10), (10, 10)])
+@pytest.mark.parametrize('angle_ref', list(Arc.AngleRef))
+@pytest.mark.parametrize('rotation', [0, pi / 5])
+def test_arc_reflection_preserves_caps_and_bounds(axis: int, radii: tuple, angle_ref: Arc.AngleRef, rotation: float) -> None:
+    arc = Arc(radii=radii, angles=(-0.3, 1.1), width=1, angle_ref=angle_ref, rotation=rotation)
+    reflected = arc.deepcopy().mirror(axis)
+    signs = numpy.ones(2)
+    signs[1 - axis] = -1
+    assert_allclose(reflected.get_cap_edges(), arc.get_cap_edges() * signs, atol=1e-12)
+    expected = arc.get_bounds_single() * signs
+    assert_allclose(reflected.get_bounds_single(), numpy.sort(expected, axis=0), atol=1e-12)
+    assert_allclose(reflected.mirror(axis).get_cap_edges(), arc.get_cap_edges(), atol=1e-12)
+
 def test_arc_to_polygons() -> None:
     a = Arc(radii=(10, 10), angles=(0, pi / 2), width=2)
     polys = a.to_polygons(num_vertices=32)
diff --git a/masque/test/test_shape_transforms.py b/masque/test/test_shape_transforms.py
index 2a9092a..13bb77d 100644
--- a/masque/test/test_shape_transforms.py
+++ b/masque/test/test_shape_transforms.py
@@ -17,7 +17,8 @@ def test_shape_mirror() -> None:
 
     a = Arc(radii=(10, 5), angles=(0, pi / 4), width=2, angle_ref=Arc.AngleRef.FocusPos)
     a.mirror(1)
-    assert a.angle_ref == Arc.AngleRef.FocusNeg
+    # The pi rotation already reflects the X-major focus across the Y axis.
+    assert a.angle_ref == Arc.AngleRef.FocusPos
 
     a = Arc(radii=(5, 10), angles=(0, pi / 4), width=2, angle_ref=Arc.AngleRef.FocusPos)
     a.mirror(0)