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## drop_the_thread
### Instructions
> Interior mutability is a design pattern in Rust that allows you to mutate data even when there are immutable references to that data.
In this exercise, you will create a Drop Checker API.
Define the following structures:
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- `Workers`: containing:
- `drops`: that will save the number of dropped threads.
- `states`: that will save the state of multiple threads. If the thread is not dropped, the state will be `false`, and will be `true` otherwise.
- `Thread`: containing:
- `pid`: the id of the thread.
- `cmd`: the name of the thread.
- `parent`: a link to the structure `Workers`. (Tip: this should be a reference).
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You'll need to also add the following associated functions to the structures:
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- `Workers` :
- `new`: that creates a default worker.
- `new_worker`: that returns a tuple with the `pid` and a new `Thread`. This function must receive a `String` representing the `cmd`.
- `is_dropped`: that receives a `pid` and returns a `bool` that indicates the state of the thread.
- `track_worker`: which returns a `usize` representing the length of the `states` vector. (The index of the next new thread).
- `add_drop`: which is **called by the `Drop` trait**. It will receive a `pid` that will be used to change the state of the thread. If the state of that thread is `true` then it will panic with the message `"X is already dropped"`, where `X` represents the `pid`). Otherwise it should change the state to `true` and increment the `drops` field by 1.
- `Thread`:
- `new_thread`: that initializes a new thread.
- `skill`: that drops the thread.
- You must implement the `Drop` trait for the `Thread` structure. In this trait you must call the function `add_drop` so that the state of the thread changes.
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### Expected Functions
```rust
use std::cell::{RefCell, Cell};
#[derive(Debug, Default, Clone, Eq, PartialEq)]
pub struct Workers {
pub drops: Cell<usize>,
pub states: RefCell<Vec<bool>>
}
impl Workers {
pub fn new() -> Workers {}
pub fn new_worker(&self, c: String) -> (usize, Thread) {}
pub fn track_worker(&self) -> usize {}
pub fn is_dropped(&self, id: usize) -> bool {}
pub fn add_drop(&self, id: usize) {}
}
#[derive(Debug, Clone, Eq, PartialEq)]
pub struct Thread<'a> {
// expected public fields
}
impl<'a> Thread<'a> {
pub fn new_thread(p: usize, c: String, t: &'a Workers) -> Thread {}
pub fn skill(self) {}
}
```
### Usage
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Here is a program to test your function,
```rust
use std::rc::Rc;
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use drop_the_thread::*;
fn main() {
let worker = Workers::new();
let (id, thread) = worker.new_worker(String::from("command"));
let (id1, thread1) = worker.new_worker(String::from("command1"));
thread.skill();
println!("{:?}", (worker.is_dropped(id), id, &worker.drops));
thread1.skill();
println!("{:?}", (worker.is_dropped(id1), id1, &worker.drops));
let (id2, thread2) = worker.new_worker(String::from("command2"));
let thread2 = Rc::new(thread2);
let thread2_clone = thread2.clone();
drop(thread2_clone);
println!("{:?}", (worker.is_dropped(id2), id2, &worker.drops, Rc::strong_count(&thread2)));
}
```
And its output:
```console
$ cargo run
(true, 0, Cell { value: 1 })
(true, 1, Cell { value: 2 })
(false, 2, Cell { value: 2 }, 1)
$
```
### Notions
- [Trait std::ops::Drop](https://doc.bccnsoft.com/docs/rust-1.36.0-docs-html/std/ops/trait.Drop.html)
- [Struct std::cell::RefCell](https://doc.rust-lang.org/std/cell/struct.RefCell.html)
- [Interior Mutability](https://doc.rust-lang.org/book/ch15-05-interior-mutability.html)