Add the CarManager and car-shaped cars

Task: MIL-004#1
Task: MIL-004#2
Task: MIL-004#3
Task: MIL-004#4

tests/test_car_manager.py

@@ -0,0 +1,105 @@
+"""Tests for the CarManager, using a fake car so no window is needed."""
+
+import random
+
+from car_manager import CarManager
+from constants import (
+    CAR_REMOVE_X,
+    CAR_START_X,
+    CAR_Y_LIMIT,
+    MOVE_INCREMENT,
+    SAFE_ZONE,
+    SCREEN_HEIGHT,
+    SPAWN_ODDS,
+    STARTING_MOVE_DISTANCE,
+)
+
+
+class FakeCar:
+    """Stands in for a turtle car."""
+
+    def __init__(self) -> None:
+        self.x = 0.0
+        self.y = 0.0
+        self.shape_name = ""
+        self.hidden = False
+
+    def penup(self) -> None:
+        """Nothing to lift: a fake draws nothing."""
+
+    def shape(self, name: str) -> None:
+        self.shape_name = name
+
+    def goto(self, x: float, y: float) -> None:
+        self.x, self.y = x, y
+
+    def xcor(self) -> float:
+        return self.x
+
+    def setx(self, x: float) -> None:
+        self.x = x
+
+    def hideturtle(self) -> None:
+        self.hidden = True
+
+
+class AlwaysOne(random.Random):
+    """Random source whose die always shows 1, so a car is always created."""
+
+    def randint(self, a: int, b: int) -> int:
+        return 1 if (a, b) == (1, SPAWN_ODDS) else super().randint(a, b)
+
+
+def make_manager(rng: random.Random | None = None) -> CarManager:
+    return CarManager(car_factory=FakeCar, rng=rng or AlwaysOne(1))
+
+
+def test_create_cars_places_car_at_right_edge() -> None:
+    manager = make_manager()
+    manager.create_cars()
+    assert [car.x for car in manager.all_cars] == [CAR_START_X]
+
+
+def test_cars_stay_out_of_the_safe_zone() -> None:
+    manager = make_manager()
+    for _ in range(500):
+        manager.create_cars()
+    ys = [car.y for car in manager.all_cars]
+    assert min(ys) >= -(SCREEN_HEIGHT / 2 - SAFE_ZONE)
+    assert max(ys) <= SCREEN_HEIGHT / 2 - SAFE_ZONE
+    assert max(abs(y) for y in ys) <= CAR_Y_LIMIT
+
+
+def test_a_car_is_created_about_one_call_in_six() -> None:
+    manager = make_manager(random.Random(7))
+    for _ in range(6000):
+        manager.create_cars()
+    assert 800 < len(manager.all_cars) < 1200
+
+
+def test_cars_get_a_registered_car_shape() -> None:
+    manager = make_manager()
+    manager.create_cars()
+    assert manager.all_cars[0].shape_name.startswith("car-")
+
+
+def test_move_cars_moves_left_by_move_distance() -> None:
+    manager = make_manager()
+    manager.create_cars()
+    manager.move_cars()
+    assert manager.all_cars[0].x == CAR_START_X - STARTING_MOVE_DISTANCE
+
+
+def test_cars_past_the_left_edge_are_removed() -> None:
+    manager = make_manager()
+    manager.create_cars()
+    car = manager.all_cars[0]
+    car.x = CAR_REMOVE_X
+    manager.move_cars()
+    assert manager.all_cars == [] and car.hidden
+
+
+def test_level_up_increases_speed() -> None:
+    manager = make_manager()
+    manager.level_up()
+    assert manager.move_distance == STARTING_MOVE_DISTANCE + MOVE_INCREMENT