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exercises/19_smart_pointers/README.md

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+# Smart Pointers
+
+In Rust, smart pointers are variables that contain an address in memory and reference some other data, but they also have additional metadata and capabilities.
+Smart pointers in Rust often own the data they point to, while references only borrow data.
+
+## Further Information
+
+- [Smart Pointers](https://doc.rust-lang.org/book/ch15-00-smart-pointers.html)
+- [Using Box to Point to Data on the Heap](https://doc.rust-lang.org/book/ch15-01-box.html)
+- [Rc\<T\>, the Reference Counted Smart Pointer](https://doc.rust-lang.org/book/ch15-04-rc.html)
+- [Shared-State Concurrency](https://doc.rust-lang.org/book/ch16-03-shared-state.html)
+- [Cow Documentation](https://doc.rust-lang.org/std/borrow/enum.Cow.html)

exercises/19_smart_pointers/arc1.rs

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+// In this exercise, we are given a `Vec` of `u32` called `numbers` with values
+// ranging from 0 to 99. We would like to use this set of numbers within 8
+// different threads simultaneously. Each thread is going to get the sum of
+// every eighth value with an offset.
+//
+// The first thread (offset 0), will sum 0, 8, 16, …
+// The second thread (offset 1), will sum 1, 9, 17, …
+// The third thread (offset 2), will sum 2, 10, 18, …
+// …
+// The eighth thread (offset 7), will sum 7, 15, 23, …
+//
+// Each thread should own a reference-counting pointer to the vector of
+// numbers. But `Rc` isn't thread-safe. Therefore, we need to use `Arc`.
+//
+// Don't get distracted by how threads are spawned and joined. We will practice
+// that later in the exercises about threads.
+
+// Don't change the lines below.
+#![forbid(unused_imports)]
+use std::{sync::Arc, thread};
+
+fn main() {
+    let numbers: Vec<_> = (0..100u32).collect();
+
+    // TODO: Define `shared_numbers` by using `Arc`.
+    // let shared_numbers = ???;
+    let shared_numbers = Arc::new(numbers);
+
+    let mut join_handles = Vec::new();
+
+    for offset in 0..8 {
+        // TODO: Define `child_numbers` using `shared_numbers`.
+        // let child_numbers = ???;
+        let child_numbers = Arc::clone(&shared_numbers);
+
+        let handle = thread::spawn(move || {
+            let sum: u32 = child_numbers.iter().filter(|&&n| n % 8 == offset).sum();
+            println!("Sum of offset {offset} is {sum}");
+        });
+
+        join_handles.push(handle);
+    }
+
+    for handle in join_handles.into_iter() {
+        handle.join().unwrap();
+    }
+}

exercises/19_smart_pointers/box1.rs

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+// At compile time, Rust needs to know how much space a type takes up. This
+// becomes problematic for recursive types, where a value can have as part of
+// itself another value of the same type. To get around the issue, we can use a
+// `Box` - a smart pointer used to store data on the heap, which also allows us
+// to wrap a recursive type.
+//
+// The recursive type we're implementing in this exercise is the "cons list", a
+// data structure frequently found in functional programming languages. Each
+// item in a cons list contains two elements: The value of the current item and
+// the next item. The last item is a value called `Nil`.
+
+// TODO: Use a `Box` in the enum definition to make the code compile.
+#[derive(PartialEq, Debug)]
+enum List {
+    Cons(i32, Box<List>),
+    Nil,
+}
+
+// TODO: Create an empty cons list.
+fn create_empty_list() -> List {
+    List::Nil
+}
+
+// TODO: Create a non-empty cons list.
+fn create_non_empty_list() -> List {
+    List::Cons(42, Box::new(List::Nil))
+}
+
+fn main() {
+    println!("This is an empty cons list: {:?}", create_empty_list());
+    println!(
+        "This is a non-empty cons list: {:?}",
+        create_non_empty_list(),
+    );
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn test_create_empty_list() {
+        assert_eq!(create_empty_list(), List::Nil);
+    }
+
+    #[test]
+    fn test_create_non_empty_list() {
+        assert_ne!(create_empty_list(), create_non_empty_list());
+    }
+}

exercises/19_smart_pointers/cow1.rs

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+// 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);
+        // TODO: Replace `todo!()` with `Cow::Owned(_)` or `Cow::Borrowed(_)`.
+        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 (all numbers are already absolute) 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);
+        // TODO: Replace `todo!()` with `Cow::Owned(_)` or `Cow::Borrowed(_)`.
+        assert!(matches!(input, Cow::Owned(_)));
+    }
+
+    #[test]
+    fn owned_mutation() {
+        // Of course this is also the case if a mutation does occur (not all
+        // numbers are absolute). 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);
+        // TODO: Replace `todo!()` with `Cow::Owned(_)` or `Cow::Borrowed(_)`.
+        assert!(matches!(input, Cow::Owned(_)));
+    }
+}

exercises/19_smart_pointers/rc1.rs

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+// In this exercise, we want to express the concept of multiple owners via the
+// `Rc<T>` type. This is a model of our solar system - there is a `Sun` type and
+// multiple `Planet`s. The planets take ownership of the sun, indicating that
+// they revolve around the sun.
+
+use std::rc::Rc;
+
+#[derive(Debug)]
+struct Sun;
+
+#[derive(Debug)]
+enum Planet {
+    Mercury(Rc<Sun>),
+    Venus(Rc<Sun>),
+    Earth(Rc<Sun>),
+    Mars(Rc<Sun>),
+    Jupiter(Rc<Sun>),
+    Saturn(Rc<Sun>),
+    Uranus(Rc<Sun>),
+    Neptune(Rc<Sun>),
+}
+
+impl Planet {
+    fn details(&self) {
+        println!("Hi from {self:?}!");
+    }
+}
+
+fn main() {
+    // You can optionally experiment here.
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn rc1() {
+        let sun = Rc::new(Sun);
+        println!("reference count = {}", Rc::strong_count(&sun)); // 1 reference
+
+        let mercury = Planet::Mercury(Rc::clone(&sun));
+        println!("reference count = {}", Rc::strong_count(&sun)); // 2 references
+        mercury.details();
+
+        let venus = Planet::Venus(Rc::clone(&sun));
+        println!("reference count = {}", Rc::strong_count(&sun)); // 3 references
+        venus.details();
+
+        let earth = Planet::Earth(Rc::clone(&sun));
+        println!("reference count = {}", Rc::strong_count(&sun)); // 4 references
+        earth.details();
+
+        let mars = Planet::Mars(Rc::clone(&sun));
+        println!("reference count = {}", Rc::strong_count(&sun)); // 5 references
+        mars.details();
+
+        let jupiter = Planet::Jupiter(Rc::clone(&sun));
+        println!("reference count = {}", Rc::strong_count(&sun)); // 6 references
+        jupiter.details();
+
+        // TODO
+        let saturn = Planet::Saturn(Rc::clone(&sun));
+        println!("reference count = {}", Rc::strong_count(&sun)); // 7 references
+        saturn.details();
+
+        // TODO
+        let uranus = Planet::Uranus(Rc::clone(&sun));
+        println!("reference count = {}", Rc::strong_count(&sun)); // 8 references
+        uranus.details();
+
+        // TODO
+        let neptune = Planet::Neptune(Rc::clone(&sun));
+        println!("reference count = {}", Rc::strong_count(&sun)); // 9 references
+        neptune.details();
+
+        assert_eq!(Rc::strong_count(&sun), 9);
+
+        drop(neptune);
+        println!("reference count = {}", Rc::strong_count(&sun)); // 8 references
+
+        drop(uranus);
+        println!("reference count = {}", Rc::strong_count(&sun)); // 7 references
+
+        drop(saturn);
+        println!("reference count = {}", Rc::strong_count(&sun)); // 6 references
+
+        drop(jupiter);
+        println!("reference count = {}", Rc::strong_count(&sun)); // 5 references
+
+        drop(mars);
+        println!("reference count = {}", Rc::strong_count(&sun)); // 4 references
+
+        // TODO
+        drop(earth);
+        println!("reference count = {}", Rc::strong_count(&sun)); // 3 references
+
+        // TODO
+        drop(venus);
+        println!("reference count = {}", Rc::strong_count(&sun)); // 2 references
+
+        // TODO
+        drop(mercury);
+        println!("reference count = {}", Rc::strong_count(&sun)); // 1 reference
+
+        assert_eq!(Rc::strong_count(&sun), 1);
+    }
+}