Optimize counting.py: fast-reject, skip empty tracks, throttle purge

- Return early when tracks list is empty
- Fast-reject centroid by bounding rect before pointPolygonTest
- Purge stale tracks every 30 frames instead of every frame
- Skip deque append when trail_length == 0

src/chicken_counter/counting.py

@@ -51,12 +51,30 @@ class CountingZone:
         counting_paused: bool = False,
     ) -> list[CountEvent]:
         events: list[CountEvent] = []
+
+        if not tracks:
+            self.inside_box_count = 0
+            self.current_inside_ids.clear()
+            self.prev_inside_ids.clear()
+            self._purge_stale(frame_index, set())
+            return events
+
         active_ids = set()
         inside_ids = set()
+
+        rx1, ry1, rx2, ry2 = self._counting_rect
+
         for track in tracks:
             active_ids.add(track.track_id)
             self.last_seen_frame[track.track_id] = frame_index
-            self.histories[track.track_id].append(track.centroid)
+
+            if self.trail_length > 0:
+                self.histories[track.track_id].append(track.centroid)
+
+            # fast-reject: bounding rect check before pointPolygonTest
+            cx, cy = track.centroid
+            if not (rx1 <= cx <= rx2 and ry1 <= cy <= ry2):
+                continue
 
             if self._inside_roi(track.centroid):
                 inside_ids.add(track.track_id)
@@ -64,10 +82,9 @@ class CountingZone:
             if counting_paused:
                 continue
 
-            if track.track_id not in inside_ids or track.track_id in self.counted_ids:
+            if track.track_id in self.counted_ids:
                 continue
 
-            should_validate = False
             if self.validate_while_inside:
                 should_validate = self._meets_validation_thresholds(track)
             else:
@@ -102,7 +119,8 @@ class CountingZone:
         self.inside_box_count = len(inside_ids)
         self.current_inside_ids = inside_ids
         self.prev_inside_ids = inside_ids
-        self._purge_stale(frame_index, active_ids)
+        if frame_index % 30 == 0:
+            self._purge_stale(frame_index, active_ids)
         return events
 
     def trail_for(self, track_id: int) -> list[tuple[int, int]]: