use anyhow::Result;
use std::{cell::RefCell, rc::Rc};

#[derive(Debug)]
enum List {
    Cons(i32, RefCell<Rc<List>>),
    Nil,
}

impl List {
    fn tail(&self) -> Option<&RefCell<Rc<List>>> {
        match self {
            List::Cons(_, item) => Some(item),
            List::Nil => None,
        }
    }
}

fn main() -> Result<()> {
    use List::*;

    // Create a List call a, his tail is Nil
    let a = Rc::new(Cons(1, RefCell::new(Rc::new(Nil))));
    println!("a's strong count {}", Rc::strong_count(&a));

    // Create a List call b, his tail is a
    let b = Rc::new(Cons(2, RefCell::new(a.clone())));
    println!(
        "After b created, a's strong count is {}",
        Rc::strong_count(&a)
    );

    // Because the RefCell, we can get a's tail, and point it to b
    if let Some(link) = a.tail() {
        *link.borrow_mut() = b.clone();
    }
    println!(
        "After point a's tail to b, b's strong count is {}, And a's {}",
        Rc::strong_count(&b),
        Rc::strong_count(&a)
    );

    // And overflow
    // println!("a's tail {:?}", a.tail());

    Ok(())
}

use anyhow::Result;
use std::{
    cell::RefCell,
    rc::{Rc, Weak},
};

#[derive(Debug)]
enum List {
    Cons(i32, RefCell<Weak<List>>),
    Nil,
}

impl List {
    fn tail(&self) -> Option<&RefCell<Weak<List>>> {
        match self {
            List::Cons(_, item) => Some(item),
            List::Nil => None,
        }
    }
}

fn main() -> Result<()> {
    use List::*;

    let nil = Rc::new(Nil);
    // Create a List call a, his tail is Nil
    let a = Rc::new(Cons(1, RefCell::new(Rc::downgrade(&nil))));
    println!("a's strong count {}", Rc::strong_count(&a));

    // Create a List call b, his tail is a
    let b = Rc::new(Cons(2, RefCell::new(Rc::downgrade(&a))));
    println!(
        "After b created, a's strong count is {}",
        Rc::strong_count(&a)
    );

    // Because the RefCell, we can get a's tail, and point it to b
    if let Some(link) = a.tail() {
        *link.borrow_mut() = Rc::downgrade(&b);
    }
    println!(
        "After point a's tail to b, b's strong count is {}, And a's {}",
        Rc::strong_count(&b),
        Rc::strong_count(&a)
    );

    // And overflow
    println!("a's tail {:?}", a.tail());

    Ok(())
}