robot-name: Redesign for testability

exercises/practice/robot-name/.meta/config.json

@@ -22,6 +22,7 @@
     "pminten",
     "razielgn",
     "rofrol",
+    "senekor",
     "spazm",
     "stringparser",
     "workingjubilee",

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

@@ -1,52 +1,62 @@
 use std::{
     collections::HashSet,
-    sync::{LazyLock, Mutex},
+    sync::{Arc, Mutex},
 };
 
-use rand::{Rng, thread_rng};
+use rand::Rng;
 
-static NAMES: LazyLock<Mutex<HashSet<String>>> = LazyLock::new(|| Mutex::new(HashSet::new()));
-
-pub struct Robot {
-    name: String,
-}
-
-fn generate_name() -> String {
+fn generate_name<R: Rng>(rng: &mut R, used_names: &mut HashSet<String>) -> String {
     loop {
         let mut s = String::with_capacity(5);
-        static LETTERS: &[u8] = b"ABCDEFGHIJKLMNOPQRSTUVWXYZ";
-        static NUMBERS: &[u8] = b"0123456789";
         for _ in 0..2 {
-            s.push(*thread_rng().choose(LETTERS).unwrap() as char);
+            s.push(rng.random_range('A'..='Z'));
         }
         for _ in 0..3 {
-            s.push(*thread_rng().choose(NUMBERS).unwrap() as char);
+            s.push(rng.random_range('0'..='9'));
         }
-
-        if NAMES.lock().unwrap().insert(s.clone()) {
+        if used_names.insert(s.clone()) {
             return s;
         }
     }
 }
 
-impl Robot {
-    pub fn new() -> Robot {
-        Robot {
-            name: generate_name(),
+/// A `RobotFactory` is responsible for ensuring that all robots produced by
+/// it have a unique name. Robots from different factories can have the same
+/// name.
+pub struct RobotFactory {
+    used_names: Arc<Mutex<HashSet<String>>>,
+}
+
+pub struct Robot {
+    used_names: Arc<Mutex<HashSet<String>>>,
+    name: String,
+}
+
+impl RobotFactory {
+    pub fn new() -> Self {
+        Self {
+            used_names: Arc::new(Mutex::new(HashSet::new())),
         }
     }
 
-    pub fn name(&self) -> &str {
-        &self.name[..]
+    pub fn new_robot<R: Rng>(&mut self, rng: &mut R) -> Robot {
+        let mut guard = self.used_names.lock().unwrap();
+        Robot {
+            used_names: Arc::clone(&self.used_names),
+            name: generate_name(rng, &mut guard),
+        }
     }
+}
 
-    pub fn reset_name(&mut self) {
-        self.name = generate_name();
+impl Robot {
+    pub fn name(&self) -> &str {
+        &self.name
     }
-}
 
-impl Default for Robot {
-    fn default() -> Self {
-        Self::new()
+    // When a robot is reset, its factory must still ensure that there are no
+    // 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);
     }
 }

exercises/practice/robot-name/Cargo.toml

@@ -7,6 +7,9 @@ edition = "2024"
 # The full list of available libraries is here:
 # https://github.com/exercism/rust-test-runner/blob/main/local-registry/Cargo.toml
 [dependencies]
+# We use a wild-card specifier to minimize the risk of breaking existing
+# solutions in case the version in the test runner is updated.
+rand = "*"
 
 [lints.clippy]
 new_without_default = "allow"

exercises/practice/robot-name/src/lib.rs

@@ -1,15 +1,28 @@
+use rand::Rng;
+
+/// A `RobotFactory` is responsible for ensuring that all robots produced by
+/// it have a unique name. Robots from different factories can have the same
+/// name.
+pub struct RobotFactory;
+
 pub struct Robot;
 
-impl Robot {
+impl RobotFactory {
     pub fn new() -> Self {
-        todo!("Construct a new Robot struct.");
+        todo!("Create a new robot factory")
     }
 
+    pub fn new_robot<R: Rng>(&mut self, _rng: &mut R) -> Robot {
+        todo!("Create a new robot with a unique name")
+    }
+}
+
+impl Robot {
     pub fn name(&self) -> &str {
-        todo!("Return the reference to the robot's name.");
+        todo!("Return a reference to the robot's name");
     }
 
-    pub fn reset_name(&mut self) {
-        todo!("Assign a new unique name to the robot.");
+    pub fn reset<R: Rng>(&mut self, _rng: &mut R) {
+        todo!("Assign a new unique name to the robot");
     }
 }

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

@@ -1,4 +1,12 @@
-use robot_name as robot;
+use std::collections::HashSet;
+
+use rand::SeedableRng as _;
+use rand::rngs::SmallRng;
+use robot_name::*;
+
+fn deterministic_rng() -> SmallRng {
+    SmallRng::seed_from_u64(0)
+}
 
 fn assert_name_matches_pattern(n: &str) {
     assert!(n.len() == 5, "name is exactly 5 characters long");
@@ -14,46 +22,86 @@ fn assert_name_matches_pattern(n: &str) {
 
 #[test]
 fn name_should_match_expected_pattern() {
-    let r = robot::Robot::new();
-    assert_name_matches_pattern(r.name());
+    let mut rng = deterministic_rng();
+    let mut factory = RobotFactory::new();
+    let robot = factory.new_robot(&mut rng);
+    assert_name_matches_pattern(robot.name());
 }
 
 #[test]
 #[ignore]
-fn different_robots_have_different_names() {
-    let r1 = robot::Robot::new();
-    let r2 = robot::Robot::new();
-    assert_ne!(r1.name(), r2.name(), "Robot names should be different");
+fn factory_prevents_name_collisions() {
+    let mut factory = RobotFactory::new();
+    let robot_1 = factory.new_robot(&mut deterministic_rng());
+    let robot_2 = factory.new_robot(&mut deterministic_rng());
+    assert_ne!(robot_1.name(), robot_2.name());
 }
 
 #[test]
 #[ignore]
-fn many_different_robots_have_different_names() {
-    use std::collections::HashSet;
+fn robot_name_depends_on_rng() {
+    let mut rng = deterministic_rng();
+    let robot_1 = RobotFactory::new().new_robot(&mut rng);
+    let robot_2 = RobotFactory::new().new_robot(&mut rng);
+    assert_ne!(robot_1.name(), robot_2.name());
+}
 
-    // In 3,529 random robot names, there is ~99.99% chance of a name collision
-    let vec: Vec<_> = (0..3529).map(|_| robot::Robot::new()).collect();
-    let set: HashSet<_> = vec.iter().map(|robot| robot.name()).collect();
+#[test]
+#[ignore]
+fn robot_name_only_depends_on_rng() {
+    let robot_1 = RobotFactory::new().new_robot(&mut deterministic_rng());
+    let robot_2 = RobotFactory::new().new_robot(&mut deterministic_rng());
+    assert_eq!(robot_1.name(), robot_2.name());
+}
 
-    let number_of_collisions = vec.len() - set.len();
-    assert_eq!(number_of_collisions, 0);
+#[test]
+#[ignore]
+fn many_different_robots_have_different_names() {
+    // In 3,529 random robot names, there is ~99.99% chance of a name collision
+    let mut rng = deterministic_rng();
+    let mut factory = RobotFactory::new();
+    let robots: Vec<_> = (0..3529).map(|_| factory.new_robot(&mut rng)).collect();
+    let mut set = HashSet::new();
+    assert!(robots.iter().all(|robot| set.insert(robot.name())));
 }
 
 #[test]
 #[ignore]
 fn new_name_should_match_expected_pattern() {
-    let mut r = robot::Robot::new();
-    assert_name_matches_pattern(r.name());
-    r.reset_name();
-    assert_name_matches_pattern(r.name());
+    let mut rng = deterministic_rng();
+    let mut factory = RobotFactory::new();
+    let mut robot = factory.new_robot(&mut rng);
+    assert_name_matches_pattern(robot.name());
+    robot.reset(&mut rng);
+    assert_name_matches_pattern(robot.name());
 }
 
 #[test]
 #[ignore]
 fn new_name_is_different_from_old_name() {
-    let mut r = robot::Robot::new();
-    let n1 = r.name().to_string();
-    r.reset_name();
-    let n2 = r.name().to_string();
-    assert_ne!(n1, n2, "Robot name should change when reset");
+    let mut rng = deterministic_rng();
+    let mut factory = RobotFactory::new();
+    let mut robot = factory.new_robot(&mut rng);
+    let name_1 = robot.name().to_string();
+    robot.reset(&mut rng);
+    let name_2 = robot.name().to_string();
+    assert_ne!(name_1, name_2, "Robot name should change when reset");
+}
+
+#[test]
+#[ignore]
+fn factory_prevents_name_collision_despite_reset() {
+    // To keep the same probablity as the first test with many robots, we
+    // generate 3,529 robots and reset their names, then generate another 3,529
+    // robots and check if there are collisions across these two groups.
+    let mut rng = deterministic_rng();
+    let mut factory = RobotFactory::new();
+    let mut reset_robots: Vec<_> = (0..3529).map(|_| factory.new_robot(&mut rng)).collect();
+    for robot in &mut reset_robots {
+        robot.reset(&mut rng);
+    }
+    let mut set = HashSet::new();
+    assert!(reset_robots.iter().all(|robot| set.insert(robot.name())));
+
+    assert!((0..3529).all(|_| !set.contains(factory.new_robot(&mut rng).name())));
 }

rust-tooling/ci-tests/tests/no_underscore_prefixed_idents.rs

@@ -56,6 +56,7 @@ static EXCEPTIONS: &[&str] = &[
     "roman-numerals",     // std::fmt::Formatter is not Debug
     "simple-linked-list", // has generics
     "sublist",            // has generics
+    "robot-name",         // has generics
 ];
 
 fn line_is_not_a_comment(line: &&str) -> bool {