public/subjects/drop_the_blog

drop_the_blog

Instructions

Define the following structures:

• Blog: containing:

  • drops that will save the number of dropped articles.
  • states that will save the state of multiple articles. If the article is not dropped, the state will be false, otherwise it will be true.

• Article: containing:

  • id as usize.
  • body as String.
  • parent a link to the structure Blog. (Tip: this should be a reference).

You'll need to also add the following associated functions to the structures:

• Blog:

  • new that creates an empty blog.
  • new_article that receives a String for the body and returns a tuple with the id and a new Article.
  • is_dropped that receives an id and returns a bool that indicates the state of the article.
  • new_id which returns a usize representing the length of the states vector. (Which is also the first available id).
  • add_drop which is called by the Drop trait. It will receive an id that will be used to change the state of the article. If the state of that article is true then it will panic with the message "X is already dropped", where X represents the id). Otherwise it should change the state to true and increment the drops field by 1.

• Article:

  • new that initializes a new article.
  • discard that drops the article.

You must implement the Drop trait for the Article structure. In this trait you must call the function add_drop so that the state of the article changes.

Expected Functions

use std::cell::{RefCell, Cell};

#[derive(Debug, Default, Clone, Eq, PartialEq)]
pub struct Blog {
    pub drops: Cell<usize>,
    pub states: RefCell<Vec<bool>>
}

impl Blog {
    pub fn new() -> Blog {}
    pub fn new_article(&self, body: String) -> (usize, Article) {}
    pub fn new_id(&self) -> usize {}
    pub fn is_dropped(&self, id: usize) -> bool {}
    pub fn add_drop(&self, id: usize) {}
}

#[derive(Debug, Clone, Eq, PartialEq)]
pub struct Article<'a> {
    // expected public fields
}

impl<'a> Article<'a> {
    pub fn new(id: usize, body: String, blog: &'a Blog) -> Article {}
    pub fn discard(self) {}
}

Usage

Here is a program to test your function,

use drop_the_blog::*;
use std::rc::Rc;

fn main() {
    let blog = Blog::new();
    let (id, article) = blog.new_article(String::from("Winter is coming"));
    let (id1, article1) = blog.new_article(String::from("The story of the universe"));

    article.discard();

    println!("{:?}", (blog.is_dropped(id), id, &blog.drops));

    article1.discard();
    println!("{:?}", (blog.is_dropped(id1), id1, &blog.drops));

    let (id2, article2) = blog.new_article(String::from("How to cook 101"));
    let article2 = Rc::new(article2);
    let article2_clone = article2.clone();

    drop(article2_clone);

    println!("{:?}",
      (blog.is_dropped(id2), id2, &blog.drops, Rc::strong_count(&article2)));
}

And its output:

$ cargo run
(true, 0, Cell { value: 1 })
(true, 1, Cell { value: 2 })
(false, 2, Cell { value: 2 }, 1)
$

Notions

• Trait std::ops::Drop
• Struct std::cell::RefCell
• Interior Mutability