014 透過連結串列學習Rust之持久化連結串列的Drop

linghuyichong發表於2021-06-20

影片地址:www.bilibili.com/video/av78062009/
相關原始碼:github.com/anonymousGiga/Rust-link...

本節我們實現可持久化連結串列的Drop。我們在之前實現可變連結串列的Drop的時候,我們是使用迴圈來進行回收的。這裡我們如果採用統一的方法,是行不通的,因為我們此處透過Rc進行共享,必須等待引用計數為0 才能將資料從Rc中移除。

Drop實現如下:

//實現Drop
impl<T> Drop for List<T> {
    fn drop(&mut self) {
        let mut head = self.head.take();    
        while let Some(node) = head {
            if let Ok(mut node) = Rc::try_unwrap(node) {
                head = node.next.take();
            } else {
                break;
            }
        }
    }
}
use std::rc::Rc;
pub struct List<T> {
    head: Link<T>,
}

type Link<T> = Option<Rc<Node<T>>>;

struct Node<T> {
    elem: T,
    next: Link<T>,
}

impl<T> List<T> {
    pub fn new() -> Self {
        List { head: None }
    }

    pub fn append(&mut self, elem: T) -> List<T> {
        List { head: Some(Rc::new(Node {
            elem: elem,
            next: self.head.clone(),
        }))}    
    }

    pub fn tail(&self) -> List<T> {
        List { head: self.head.as_ref().and_then(|node| {
            node.next.clone()
        })}
    }

    pub fn head(&self) -> Option<&T> {
        self.head.as_ref().map(|node| &node.elem)
    }
}

//實現Iter
pub struct Iter<'a, T> {
    next: Option<&'a Node<T>>,
}

impl<T> List<T> {
    pub fn iter(&self) -> Iter<T> {
        Iter { next: self.head.as_deref() }
    }
}

impl<'a, T> Iterator for Iter<'a, T> {
    type Item = &'a T;

    fn next(&mut self) -> Option<Self::Item> {
        self.next.map(|node| {
            self.next = node.next.as_deref();
            &node.elem
        })
    }
}

//實現Drop
impl<T> Drop for List<T> {
    fn drop(&mut self) {
        let mut head = self.head.take();    
        while let Some(node) = head {
            if let Ok(mut node) = Rc::try_unwrap(node) {
                head = node.next.take();
            } else {
                break;
            }
        }
    }
}

#[cfg(test)]
mod test {
    use super::List;
    #[test]
    fn basics() {
        let mut list = List::new();
        assert_eq!(list.head(), None);

        let list = list.append(1).append(2).append(3);
        assert_eq!(list.head(), Some(&3));

        let list = list.tail();
        assert_eq!(list.head(), Some(&2));

        let list = list.tail();
        assert_eq!(list.head(), Some(&1));

        let list = list.tail();
        assert_eq!(list.head(), None);
    }

    #[test]
    fn iter() {
        let list = List::new().append(1).append(2).append(3);
        let mut iter = list.iter();

        assert_eq!(iter.next(), Some(&3));
        assert_eq!(iter.next(), Some(&2));
        assert_eq!(iter.next(), Some(&1));
    }
}
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