Should fix #5.

voronoi/events/circle_event.py

@@ -90,7 +90,12 @@ def create_circle_event(left_node: LeafNode, middle_node: LeafNode, right_node:
     @staticmethod
     def create_circle(a, b, c):
 
-        a, b, c = sorted((a, b, c), key=lambda item: item.x)
+        # Due to small rounding errors, two circles that should have the same coordinates (e.g. in case of a 2x2 grid),
+        # can get slightly different coordinates from each other. This line will make sure this problem is avoided,
+        # by sorting the points, such that the calculations will be executed in the same order, and thus the rounding
+        # errors will be the same for both circles. This in turn, ensures that the priority of the two circle events
+        # is the same, so that first inserted circle event will be handled first.
+        a, b, c = sorted((a, b, c), key=lambda item: (item.y, item.x))
 
         # Algorithm from O'Rourke 2ed p. 189
         A = Decimal(b.x - a.x)

voronoi/tests/unit.py

@@ -382,3 +382,16 @@ def test_alexdiab():
     ])
     sizes = [14.04, 5.63, 6.11, 16.66, 11.57]
     _execute(polygon, points, sizes)
+
+# Test case added thanks to mars0001 (https://github.com/Yatoom/voronoi/issues/5)
+def test_mars0001():
+    points = [(20.1273, 18.7303), (26.5107, 18.7303), (20.1273, 23.8437), (26.5107, 23.8437)]
+
+    polygon = Polygon([
+        (15., 15.),
+        (15., 30.),
+        (30., 30.),
+        (30., 15.),
+    ])
+    sizes = [52.3, 42.0, 72.48, 58.21]
+    _execute(polygon, points, sizes)