Add draw_spirograph with tests

Draws int(360 / size_of_gap) circles, each in a random RGB color, turning the heading
by the gap after each. A gap that is not positive raises ValueError, and the integer
count avoids the TypeError that range raises for a float.

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

src/turtle_challenges/__init__.py

@@ -3,10 +3,11 @@
 @brief Solutions to the turtle graphics challenges of day 18 of Udemy's
 100 Days of Code.
 
-The challenges so far are @ref turtle_challenges.square.draw_square,
+The five challenges are @ref turtle_challenges.square.draw_square,
 @ref turtle_challenges.dashed_line.draw_dashed_line,
-@ref turtle_challenges.shapes.draw_shape and
-@ref turtle_challenges.random_walk.random_walk. Each takes the turtle as its
+@ref turtle_challenges.shapes.draw_shape,
+@ref turtle_challenges.random_walk.random_walk and
+@ref turtle_challenges.spirograph.draw_spirograph. Each takes the turtle as its
 first argument, so it can be tested without a window.
 """
 
@@ -14,12 +15,14 @@ from turtle_challenges.colors import random_color, random_palette_color
 from turtle_challenges.dashed_line import draw_dashed_line
 from turtle_challenges.random_walk import random_walk
 from turtle_challenges.shapes import draw_shape, draw_shapes
+from turtle_challenges.spirograph import draw_spirograph
 from turtle_challenges.square import draw_square
 
 __all__ = [
     "draw_dashed_line",
     "draw_shape",
     "draw_shapes",
+    "draw_spirograph",
     "draw_square",
     "random_color",
     "random_palette_color",

src/turtle_challenges/constants.py

@@ -77,6 +77,12 @@ WALK_PEN_SIZE = 10
 ## Challenge 4: headings of a step: east, north, west and south, in degrees.
 WALK_HEADINGS: tuple[int, ...] = (0, 90, 180, 270)
 
+## Challenge 5: radius of every circle of the spirograph, in turtle units.
+SPIROGRAPH_RADIUS = 100
+
+## Challenge 5: degrees the heading turns after each circle.
+SPIROGRAPH_GAP_DEGREES = 5
+
 ## Command line name of challenge 1.
 CHALLENGE_SQUARE = "square"
 

src/turtle_challenges/spirograph.py

@@ -0,0 +1,47 @@
+"""!
+@file spirograph.py
+@brief Turtle Challenge 5: draw a spirograph.
+"""
+
+import random
+
+from turtle_challenges.colors import random_color
+from turtle_challenges.constants import (
+    FASTEST_SPEED,
+    FULL_TURN_DEGREES,
+    SPIROGRAPH_GAP_DEGREES,
+    SPIROGRAPH_RADIUS,
+)
+from turtle_challenges.pen import Pen
+
+
+def draw_spirograph(
+    pen: Pen,
+    size_of_gap: float = SPIROGRAPH_GAP_DEGREES,
+    radius: float = SPIROGRAPH_RADIUS,
+    rng: random.Random | None = None,
+) -> None:
+    """!
+    @brief Draw overlapping circles, turning a little after each one.
+
+    After every circle the heading grows by @p size_of_gap, so the circles
+    overlap into a spirograph once the heading has made a full turn. The number
+    of circles is 360 divided by the gap, rounded down: `range` takes only an
+    integer, and a float such as 72.0 would raise a `TypeError`.
+
+    @param pen The turtle that draws.
+    @param size_of_gap Degrees between two circles; must be positive.
+    @param radius Radius of every circle, in turtle units.
+    @param rng Random generator for the colors; a shared one is used if
+        omitted.
+    @exception ValueError If @p size_of_gap or @p radius is not positive.
+    """
+    if size_of_gap <= 0:
+        raise ValueError(f"size_of_gap must be positive, got {size_of_gap}")
+    if radius <= 0:
+        raise ValueError(f"radius must be positive, got {radius}")
+    pen.speed(FASTEST_SPEED)
+    for _ in range(int(FULL_TURN_DEGREES / size_of_gap)):
+        pen.color(random_color(rng))
+        pen.circle(radius)
+        pen.setheading(pen.heading() + size_of_gap)