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## moving_targets
### Instructions
You will have a linked list of `Target` named `Field`.
You will handle recursive types and ownership and implement the following **associated functions**:
- `new`: which will initialize the `Field` with `head` set to `None`.
- `push`: which receives a `Target` and add it as a `Node` at the head of the list.
- `pop`: which returns the last added `Target` wrapped in an `Option` and removes it from the list.
- `peek`: which returns the last added `Target` as a reference wrapped in an `Option` but do not removes it from the list.
- `peek_mut`: which returns the last added `Target` as a mutable reference wrapped in an `Option` and also do not removes it from the list.
You must also implement a type named `Link`. This will be the connection between the `Field` and `Target` structures. This will be a recursion type, and it must point to `None` if there is no `Target` to point to.
### Expected Functions and structures
```rust
pub struct Field {
head: Link,
}
type Link = // To be implemented
struct Node {
elem: Target,
next: Link,
}
#[derive(Debug, PartialEq, Eq)]
pub struct Target {
pub size: u32,
pub xp: u32,
}
impl Field {
pub fn new() -> Self {}
pub fn push(&mut self, target: Target) {}
pub fn pop(&mut self) -> Option<Target> {}
pub fn peek(&self) -> Option<&Target> {}
pub fn peek_mut(&mut self) -> Option<&mut Target> {}
}
```
### Usage
Here is a program to test your function:
```rust
use moving_targets::*;
fn main() {
let mut field = Field::new();
println!("{:?}", field.pop());
field.push(Target { size: 12, xp: 2 });
println!("{:?}", *field.peek().unwrap());
field.push(Target { size: 24, xp: 4 });
println!("{:?}", field.pop());
let last_target = field.peek_mut().unwrap();
*last_target = Target { size: 2, xp: 0 };
println!("{:?}", field.pop());
}
```
And its output:
```console
$ cargo run
None
Target { size: 12, xp: 2 }
Some(Target { size: 24, xp: 4 })
Some(Target { size: 2, xp: 0 })
$
```
### Notions
- [Box\<T\>](https://doc.rust-lang.org/book/ch15-01-box.html)
- [Linked lists in Rust](https://rust-unofficial.github.io/too-many-lists/index.html)