cow1 solution

exercises/19_smart_pointers/cow1.rs

@@ -1,24 +1,18 @@
-// This exercise explores the Cow, or Clone-On-Write type. Cow is a
-// clone-on-write smart pointer. It can enclose and provide immutable access to
-// borrowed data, and clone the data lazily when mutation or ownership is
-// required. The type is designed to work with general borrowed data via the
-// Borrow trait.
-//
-// This exercise is meant to show you what to expect when passing data to Cow.
-// Fix the unit tests by checking for Cow::Owned(_) and Cow::Borrowed(_) at the
-// TODO markers.
+// This exercise explores the `Cow` (Clone-On-Write) smart pointer. It can
+// enclose and provide immutable access to borrowed data and clone the data
+// lazily when mutation or ownership is required. The type is designed to work
+// with general borrowed data via the `Borrow` trait.
 
 use std::borrow::Cow;
 
-fn abs_all<'a, 'b>(input: &'a mut Cow<'b, [i32]>) -> &'a mut Cow<'b, [i32]> {
-    for i in 0..input.len() {
-        let v = input[i];
-        if v < 0 {
+fn abs_all(input: &mut Cow<[i32]>) {
+    for ind in 0..input.len() {
+        let value = input[ind];
+        if value < 0 {
             // Clones into a vector if not already owned.
-            input.to_mut()[i] = -v;
+            input.to_mut()[ind] = -value;
         }
     }
-    input
 }
 
 fn main() {
@@ -30,47 +24,45 @@ mod tests {
     use super::*;
 
     #[test]
-    fn reference_mutation() -> Result<(), &'static str> {
+    fn reference_mutation() {
         // Clone occurs because `input` needs to be mutated.
-        let slice = [-1, 0, 1];
-        let mut input = Cow::from(&slice[..]);
-        match abs_all(&mut input) {
-            Cow::Owned(_) => Ok(()),
-            _ => Err("Expected owned value"),
-        }
+        let vec = vec![-1, 0, 1];
+        let mut input = Cow::from(&vec);
+        abs_all(&mut input);
+        assert!(matches!(input, Cow::Owned(_)));
     }
 
     #[test]
-    fn reference_no_mutation() -> Result<(), &'static str> {
+    fn reference_no_mutation() {
         // No clone occurs because `input` doesn't need to be mutated.
-        let slice = [0, 1, 2];
-        let mut input = Cow::from(&slice[..]);
-        match abs_all(&mut input) {
-            // TODO
-        }
+        let vec = vec![0, 1, 2];
+        let mut input = Cow::from(&vec);
+        abs_all(&mut input);
+        // TODO: Replace `todo!()` with `Cow::Owned(_)` or `Cow::Borrowed(_)`.
+        assert!(matches!(input, todo!()));
     }
 
     #[test]
-    fn owned_no_mutation() -> Result<(), &'static str> {
-        // We can also pass `slice` without `&` so Cow owns it directly. In this
-        // case no mutation occurs and thus also no clone, but the result is
+    fn owned_no_mutation() {
+        // We can also pass `vec` without `&` so `Cow` owns it directly. In this
+        // case, no mutation occurs and thus also no clone. But the result is
         // still owned because it was never borrowed or mutated.
-        let slice = vec![0, 1, 2];
-        let mut input = Cow::from(slice);
-        match abs_all(&mut input) {
-            // TODO
-        }
+        let vec = vec![0, 1, 2];
+        let mut input = Cow::from(vec);
+        abs_all(&mut input);
+        // TODO: Replace `todo!()` with `Cow::Owned(_)` or `Cow::Borrowed(_)`.
+        assert!(matches!(input, todo!()));
     }
 
     #[test]
-    fn owned_mutation() -> Result<(), &'static str> {
+    fn owned_mutation() {
         // Of course this is also the case if a mutation does occur. In this
-        // case the call to `to_mut()` in the abs_all() function returns a
+        // case, the call to `to_mut()` in the `abs_all` function returns a
         // reference to the same data as before.
-        let slice = vec![-1, 0, 1];
-        let mut input = Cow::from(slice);
-        match abs_all(&mut input) {
-            // TODO
-        }
+        let vec = vec![-1, 0, 1];
+        let mut input = Cow::from(vec);
+        abs_all(&mut input);
+        // TODO: Replace `todo!()` with `Cow::Owned(_)` or `Cow::Borrowed(_)`.
+        assert!(matches!(input, todo!()));
     }
 }

rustlings-macros/info.toml

@@ -1020,11 +1020,11 @@ https://doc.rust-lang.org/stable/book/ch16-00-concurrency.html"""
 name = "cow1"
 dir = "19_smart_pointers"
 hint = """
-If `Cow` already owns the data it doesn't need to clone it when `to_mut()` is
+If `Cow` already owns the data, it doesn't need to clone it when `to_mut()` is
 called.
 
-Check out https://doc.rust-lang.org/std/borrow/enum.Cow.html for documentation
-on the `Cow` type."""
+Check out the documentation of the `Cow` type:
+https://doc.rust-lang.org/std/borrow/enum.Cow.html"""
 
 # THREADS
 

solutions/19_smart_pointers/cow1.rs

@@ -1 +1,68 @@
-// Solutions will be available before the stable release. Thank you for testing the beta version 🥰
+// This exercise explores the `Cow` (Clone-On-Write) smart pointer. It can
+// enclose and provide immutable access to borrowed data and clone the data
+// lazily when mutation or ownership is required. The type is designed to work
+// with general borrowed data via the `Borrow` trait.
+
+use std::borrow::Cow;
+
+fn abs_all(input: &mut Cow<[i32]>) {
+    for ind in 0..input.len() {
+        let value = input[ind];
+        if value < 0 {
+            // Clones into a vector if not already owned.
+            input.to_mut()[ind] = -value;
+        }
+    }
+}
+
+fn main() {
+    // You can optionally experiment here.
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn reference_mutation() {
+        // Clone occurs because `input` needs to be mutated.
+        let vec = vec![-1, 0, 1];
+        let mut input = Cow::from(&vec);
+        abs_all(&mut input);
+        assert!(matches!(input, Cow::Owned(_)));
+    }
+
+    #[test]
+    fn reference_no_mutation() {
+        // No clone occurs because `input` doesn't need to be mutated.
+        let vec = vec![0, 1, 2];
+        let mut input = Cow::from(&vec);
+        abs_all(&mut input);
+        assert!(matches!(input, Cow::Borrowed(_)));
+        //                      ^^^^^^^^^^^^^^^^
+    }
+
+    #[test]
+    fn owned_no_mutation() {
+        // We can also pass `vec` without `&` so `Cow` owns it directly. In this
+        // case, no mutation occurs and thus also no clone. But the result is
+        // still owned because it was never borrowed or mutated.
+        let vec = vec![0, 1, 2];
+        let mut input = Cow::from(vec);
+        abs_all(&mut input);
+        assert!(matches!(input, Cow::Owned(_)));
+        //                      ^^^^^^^^^^^^^
+    }
+
+    #[test]
+    fn owned_mutation() {
+        // Of course this is also the case if a mutation does occur. In this
+        // case, the call to `to_mut()` in the `abs_all` function returns a
+        // reference to the same data as before.
+        let vec = vec![-1, 0, 1];
+        let mut input = Cow::from(vec);
+        abs_all(&mut input);
+        assert!(matches!(input, Cow::Owned(_)));
+        //                      ^^^^^^^^^^^^^
+    }
+}