最终代码

这一章真不好写( 也很难翻译... )，最终我们实现了一个 100% 安全但是功能残缺的双向链表。

同时在实现中，还有大量 Rc 和 RefCell 引起的运行时检查，最终会影响链表的性能。整个双向链表实现史就是一部别名和所有权的奋斗史。

总之，不管爱与不爱，它就这样了，特别是如果我们不在意内部的细节暴露给外面用户时。

而从下一章开始，我们将实现一个真正能够全盘掌控的链表，当然...通过 unsafe 代码实现！

#![allow(unused)]
fn main() {
use std::rc::Rc;
use std::cell::{Ref, RefMut, RefCell};

pub struct List<T> {
    head: Link<T>,
    tail: Link<T>,
}

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

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


impl<T> Node<T> {
    fn new(elem: T) -> Rc<RefCell<Self>> {
        Rc::new(RefCell::new(Node {
            elem: elem,
            prev: None,
            next: None,
        }))
    }
}

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

    pub fn push_front(&mut self, elem: T) {
        let new_head = Node::new(elem);
        match self.head.take() {
            Some(old_head) => {
                old_head.borrow_mut().prev = Some(new_head.clone());
                new_head.borrow_mut().next = Some(old_head);
                self.head = Some(new_head);
            }
            None => {
                self.tail = Some(new_head.clone());
                self.head = Some(new_head);
            }
        }
    }

    pub fn push_back(&mut self, elem: T) {
        let new_tail = Node::new(elem);
        match self.tail.take() {
            Some(old_tail) => {
                old_tail.borrow_mut().next = Some(new_tail.clone());
                new_tail.borrow_mut().prev = Some(old_tail);
                self.tail = Some(new_tail);
            }
            None => {
                self.head = Some(new_tail.clone());
                self.tail = Some(new_tail);
            }
        }
    }

    pub fn pop_back(&mut self) -> Option<T> {
        self.tail.take().map(|old_tail| {
            match old_tail.borrow_mut().prev.take() {
                Some(new_tail) => {
                    new_tail.borrow_mut().next.take();
                    self.tail = Some(new_tail);
                }
                None => {
                    self.head.take();
                }
            }
            Rc::try_unwrap(old_tail).ok().unwrap().into_inner().elem
        })
    }

    pub fn pop_front(&mut self) -> Option<T> {
        self.head.take().map(|old_head| {
            match old_head.borrow_mut().next.take() {
                Some(new_head) => {
                    new_head.borrow_mut().prev.take();
                    self.head = Some(new_head);
                }
                None => {
                    self.tail.take();
                }
            }
            Rc::try_unwrap(old_head).ok().unwrap().into_inner().elem
        })
    }

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

    pub fn peek_back(&self) -> Option<Ref<T>> {
        self.tail.as_ref().map(|node| {
            Ref::map(node.borrow(), |node| &node.elem)
        })
    }

    pub fn peek_back_mut(&mut self) -> Option<RefMut<T>> {
        self.tail.as_ref().map(|node| {
            RefMut::map(node.borrow_mut(), |node| &mut node.elem)
        })
    }

    pub fn peek_front_mut(&mut self) -> Option<RefMut<T>> {
        self.head.as_ref().map(|node| {
            RefMut::map(node.borrow_mut(), |node| &mut node.elem)
        })
    }

    pub fn into_iter(self) -> IntoIter<T> {
        IntoIter(self)
    }
}

impl<T> Drop for List<T> {
    fn drop(&mut self) {
        while self.pop_front().is_some() {}
    }
}

pub struct IntoIter<T>(List<T>);

impl<T> Iterator for IntoIter<T> {
    type Item = T;

    fn next(&mut self) -> Option<T> {
        self.0.pop_front()
    }
}

impl<T> DoubleEndedIterator for IntoIter<T> {
    fn next_back(&mut self) -> Option<T> {
        self.0.pop_back()
    }
}

#[cfg(test)]
mod test {
    use super::List;

    #[test]
    fn basics() {
        let mut list = List::new();

        // Check empty list behaves right
        assert_eq!(list.pop_front(), None);

        // Populate list
        list.push_front(1);
        list.push_front(2);
        list.push_front(3);

        // Check normal removal
        assert_eq!(list.pop_front(), Some(3));
        assert_eq!(list.pop_front(), Some(2));

        // Push some more just to make sure nothing's corrupted
        list.push_front(4);
        list.push_front(5);

        // Check normal removal
        assert_eq!(list.pop_front(), Some(5));
        assert_eq!(list.pop_front(), Some(4));

        // Check exhaustion
        assert_eq!(list.pop_front(), Some(1));
        assert_eq!(list.pop_front(), None);

        // ---- back -----

        // Check empty list behaves right
        assert_eq!(list.pop_back(), None);

        // Populate list
        list.push_back(1);
        list.push_back(2);
        list.push_back(3);

        // Check normal removal
        assert_eq!(list.pop_back(), Some(3));
        assert_eq!(list.pop_back(), Some(2));

        // Push some more just to make sure nothing's corrupted
        list.push_back(4);
        list.push_back(5);

        // Check normal removal
        assert_eq!(list.pop_back(), Some(5));
        assert_eq!(list.pop_back(), Some(4));

        // Check exhaustion
        assert_eq!(list.pop_back(), Some(1));
        assert_eq!(list.pop_back(), None);
    }

    #[test]
    fn peek() {
        let mut list = List::new();
        assert!(list.peek_front().is_none());
        assert!(list.peek_back().is_none());
        assert!(list.peek_front_mut().is_none());
        assert!(list.peek_back_mut().is_none());

        list.push_front(1); list.push_front(2); list.push_front(3);

        assert_eq!(&*list.peek_front().unwrap(), &3);
        assert_eq!(&mut *list.peek_front_mut().unwrap(), &mut 3);
        assert_eq!(&*list.peek_back().unwrap(), &1);
        assert_eq!(&mut *list.peek_back_mut().unwrap(), &mut 1);
    }

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

        let mut iter = list.into_iter();
        assert_eq!(iter.next(), Some(3));
        assert_eq!(iter.next_back(), Some(1));
        assert_eq!(iter.next(), Some(2));
        assert_eq!(iter.next_back(), None);
        assert_eq!(iter.next(), None);
    }
}
}