Fix deterministic behavior in move_agent_to_one_of with `selection=…

…"closest"` (#2118)

This PR addresses a bug in `move_agent_to_one_of` where agents exhibit deterministic behavior when multiple closest positions are available and `selection="closest"` is used.

mesa/space.py

@@ -459,15 +459,21 @@ def move_agent_to_one_of(
             elif selection == "closest":
                 current_pos = agent.pos
                 # Find the closest position without sorting all positions
-                closest_pos = None
+                # TODO: See if this method can be optimized further
+                closest_pos = []
                 min_distance = float("inf")
                 agent.random.shuffle(pos)
                 for p in pos:
                     distance = self._distance_squared(p, current_pos)
                     if distance < min_distance:
                         min_distance = distance
-                        closest_pos = p
-                chosen_pos = closest_pos
+                        closest_pos.clear()
+                        closest_pos.append(p)
+                    elif distance == min_distance:
+                        closest_pos.append(p)
+
+                chosen_pos = agent.random.choice(closest_pos)
+
             else:
                 raise ValueError(
                     f"Invalid selection method {selection}. Choose 'random' or 'closest'."

tests/test_space.py

@@ -509,6 +509,23 @@ def test_move_agent_closest_selection(self):
         self.space.move_agent_to_one_of(agent, possible_positions, selection="closest")
         assert agent.pos == (6, 6)
 
+    def test_move_agent_closest_selection_multiple(self):
+        random_locations = []
+        agent = self.agents[0]
+        agent.pos = (5, 5)
+        repetititions = 10
+
+        for _ in range(repetititions):
+            possible_positions = [(4, 4), (6, 6), (10, 10), (20, 20)]
+            self.space.move_agent_to_one_of(
+                agent, possible_positions, selection="closest"
+            )
+            random_locations.append(agent.pos)
+            assert agent.pos in possible_positions
+            self.space.move_agent_to_one_of(agent, [(5, 5)], selection="closest")
+        non_random_locations = [random_locations[0]] * repetititions
+        assert random_locations != non_random_locations
+
     def test_move_agent_invalid_selection(self):
         agent = self.agents[0]
         possible_positions = [(10, 10), (20, 20), (30, 30)]