Refactor path segment direction logic to ensure consistent offsets for reversed station pairs

Author: yanfengliu

PROGRESS.md

@@ -39,3 +39,4 @@
 - Added station-pool generation logic so unique shapes can only appear after the 10th station slot and at most once each per run.
 - Updated `ARCHITECTURE.md` to reflect the latest project file structure.
 - Fixed passenger wait blink logic so passengers at or past the max wait threshold also blink instead of only pre-timeout passengers.
+- Fixed path segment offset direction to use a stable station-pair orientation so reversed A/B segments stay parallel and no longer cross each other.

src/entity/path_segment.py

@@ -4,12 +4,28 @@
 from entity.segment import Segment
 from entity.station import Station
 from geometry.line import Line
+from geometry.point import Point
 from geometry.utils import direction
 from shortuuid import uuid  # type: ignore
 from type import Color
 
 
 class PathSegment(Segment):
+    @staticmethod
+    def _canonical_pair_direction(
+        start_station: Station, end_station: Station
+    ) -> Point:
+        start_position = start_station.position
+        end_position = end_station.position
+        # Use one stable ordering per station pair so offsets are consistent
+        # for both A->B and B->A segments.
+        if (
+            (start_position.left, start_position.top)
+            <= (end_position.left, end_position.top)
+        ):
+            return direction(start_position, end_position)
+        return direction(end_position, start_position)
+
     def __init__(
         self,
         color: Color,
@@ -23,9 +39,7 @@ def __init__(
         self.end_station = end_station
         self.path_order = path_order
 
-        start_point = start_station.position
-        end_point = end_station.position
-        direct = direction(start_point, end_point)
+        direct = self._canonical_pair_direction(start_station, end_station)
         buffer_vector = direct * path_order_shift
         buffer_vector = buffer_vector.rotate(90)
 

test/test_path.py

@@ -11,6 +11,7 @@
 from entity.get_entity import get_random_station, get_random_stations
 from entity.metro import Metro
 from entity.path import Path
+from entity.path_segment import PathSegment
 from entity.station import Station
 from geometry.circle import Circle
 from geometry.point import Point
@@ -178,6 +179,16 @@ def test_move_metro_looped_first_segment_backward_wraps(self):
         path.move_metro(metro, 100000)
         self.assertEqual(metro.current_segment_idx, len(path.segments) - 1)
 
+    def test_path_segment_keeps_parallel_offsets_for_reversed_station_pair(self):
+        station_a = Station(Circle(station_color, station_size), Point(0, 0))
+        station_b = Station(Circle(station_color, station_size), Point(100, 40))
+        segment_ab = PathSegment((10, 10, 10), station_a, station_b, path_order=2)
+        segment_ba = PathSegment((10, 10, 10), station_b, station_a, path_order=2)
+
+        offset_from_a = segment_ab.segment_start - station_a.position
+        offset_from_b = segment_ba.segment_start - station_b.position
+        self.assertEqual(offset_from_a, offset_from_b)
+
 
 if __name__ == "__main__":
     unittest.main()