robot-name: Redesign for testability

exercises/practice/robot-name/.meta/example.rs

@@ -57,6 +57,8 @@ impl Robot {
     // name conflicts with other robots from the same factory.
     pub fn reset<R: Rng>(&mut self, rng: &mut R) {
         let mut used_names = self.used_names.lock().unwrap();
-        self.name = generate_name(rng, &mut used_names);
+        let new_name = generate_name(rng, &mut used_names);
+        used_names.remove(&self.name);
+        self.name = new_name
     }
 }

exercises/practice/robot-name/tests/robot_name.rs

@@ -105,3 +105,16 @@ fn factory_prevents_name_collision_despite_reset() {
 
     assert!((0..3529).all(|_| !set.contains(factory.new_robot(&mut rng).name())));
 }
+
+#[test]
+#[ignore]
+fn old_name_becomes_available_after_reset() {
+    let mut rng = deterministic_rng();
+    let mut factory = RobotFactory::new();
+    let mut robot = factory.new_robot(&mut rng);
+    let first_name = robot.name().to_string();
+    robot.reset(&mut rng); // cause first name to become available again
+    let mut rng = deterministic_rng(); // reset rng
+    let second_robot = factory.new_robot(&mut rng);
+    assert_eq!(second_robot.name(), first_name);
+}