Adopt constants.py and its test

Copied from the base with identity-only edits: the docstrings name days 20 and 21, and the package main page describes the whole game. The values are unchanged; MIL-003 checks them against the day-21 lectures.

Task: MIL-001#3
Closes #3

tests/test_constants.py

@@ -0,0 +1,127 @@
+"""Tests of the constants that the day-20 and day-21 lectures fix."""
+
+from snake_game import constants
+
+HALF_WIDTH = constants.SCREEN_WIDTH / 2
+HALF_HEIGHT = constants.SCREEN_HEIGHT / 2
+
+
+def test_screen_size_matches_lecture() -> None:
+    assert (constants.SCREEN_WIDTH, constants.SCREEN_HEIGHT) == (600, 600)
+
+
+def test_screen_is_black_and_titled_as_in_the_lecture() -> None:
+    assert constants.SCREEN_BACKGROUND_COLOR == "black"
+    assert constants.SCREEN_TITLE == "My Snake Game"
+
+
+def test_segments_are_white_squares() -> None:
+    assert constants.SEGMENT_SHAPE == "square"
+    assert constants.SEGMENT_COLOR == "white"
+
+
+def test_snake_starts_with_three_segments_at_the_lecture_positions() -> None:
+    assert constants.STARTING_POSITIONS == ((0, 0), (-20, 0), (-40, 0))
+
+
+def test_starting_positions_are_distinct_on_one_row() -> None:
+    positions = constants.STARTING_POSITIONS
+
+    assert len(set(positions)) == len(positions)
+    assert len({y for _, y in positions}) == 1
+
+
+def test_starting_positions_run_from_head_to_tail_leftwards() -> None:
+    x_values = [x for x, _ in constants.STARTING_POSITIONS]
+
+    assert x_values == sorted(x_values, reverse=True)
+
+
+def test_starting_positions_are_one_move_distance_apart() -> None:
+    x_values = [x for x, _ in constants.STARTING_POSITIONS]
+    gaps = {left - right for left, right in zip(x_values, x_values[1:], strict=False)}
+
+    assert gaps == {constants.MOVE_DISTANCE}
+
+
+def test_starting_positions_are_inside_the_screen() -> None:
+    assert all(
+        abs(x) < HALF_WIDTH and abs(y) < HALF_HEIGHT
+        for x, y in constants.STARTING_POSITIONS
+    )
+
+
+def test_move_distance_is_twenty_pixels() -> None:
+    assert constants.MOVE_DISTANCE == 20
+
+
+def test_directions_match_the_lecture_headings() -> None:
+    assert constants.UP == 90
+    assert constants.DOWN == 270
+    assert constants.LEFT == 180
+    assert constants.RIGHT == 0
+
+
+def test_four_directions_are_distinct() -> None:
+    directions = (constants.UP, constants.DOWN, constants.LEFT, constants.RIGHT)
+
+    assert len(set(directions)) == len(directions)
+
+
+def test_opposite_directions_differ_by_half_a_turn() -> None:
+    assert (constants.UP - constants.DOWN) % 360 == 180
+    assert (constants.LEFT - constants.RIGHT) % 360 == 180
+
+
+def test_refresh_delay_is_a_tenth_of_a_second() -> None:
+    assert constants.REFRESH_DELAY_SECONDS == 0.1
+
+
+def test_food_is_a_small_blue_circle_without_animation() -> None:
+    assert constants.FOOD_SHAPE == "circle"
+    assert constants.FOOD_SIZE == 0.5
+    assert constants.FOOD_COLOR == "blue"
+    assert constants.FOOD_SPEED == "fastest"
+
+
+def test_wall_is_inside_the_screen() -> None:
+    assert 0 < constants.WALL_LIMIT < HALF_WIDTH
+    assert constants.WALL_LIMIT < HALF_HEIGHT
+
+
+def test_wall_is_280_pixels_from_the_centre() -> None:
+    assert constants.WALL_LIMIT == 280
+
+
+def test_food_is_eaten_closer_than_15_pixels_which_is_less_than_a_move() -> None:
+    assert constants.FOOD_COLLISION_DISTANCE == 15
+    assert constants.FOOD_COLLISION_DISTANCE < constants.MOVE_DISTANCE
+
+
+def test_scoreboard_is_white_text_at_the_top_centre_inside_the_screen() -> None:
+    x, y = constants.SCOREBOARD_POSITION
+
+    assert constants.SCOREBOARD_COLOR == "white"
+    assert constants.SCOREBOARD_ALIGNMENT == "center"
+    assert (x, y) == (0, 270)
+    assert abs(x) < HALF_WIDTH
+    assert 0 < y < HALF_HEIGHT
+
+
+def test_scoreboard_text_is_score_label_and_arial_24() -> None:
+    assert constants.SCORE_LABEL == "Score: "
+    assert constants.SCOREBOARD_FONT == ("Arial", 24, "normal")
+
+
+def test_tail_is_touched_closer_than_10_pixels_which_is_less_than_a_move() -> None:
+    assert constants.TAIL_COLLISION_DISTANCE == 10
+    assert 0 < constants.TAIL_COLLISION_DISTANCE < constants.MOVE_DISTANCE
+
+
+def test_game_over_text_is_written_at_the_centre_inside_the_wall() -> None:
+    x, y = constants.GAME_OVER_POSITION
+
+    assert constants.GAME_OVER_TEXT == "GAME OVER"
+    assert (x, y) == (0, 0)
+    assert abs(x) < constants.WALL_LIMIT
+    assert abs(y) < constants.WALL_LIMIT