Add fourth example

src/first.rs

@@ -1,3 +1,4 @@
+//! # A Bad Stack
 use std::mem;
 
 pub struct List {

src/fourth.rs

@@ -0,0 +1,238 @@
+//! # A Bad Safe Deque
+use std::rc::Rc;
+use std::cell::{Ref, RefCell, RefMut};
+
+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>,
+}
+
+pub struct IntoIter<T>(List<T>);
+
+impl<T> Node<T> {
+    fn new(elem: T) -> Rc<RefCell<Self>> {
+        Rc::new(RefCell::new(Node {
+            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 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 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 peek_front(&self) -> Option<Ref<T>> {
+        self.head.as_ref().map(|node| {
+            Ref::map(node.borrow(), |node| &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 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 into_iter(self) -> IntoIter<T> {
+        IntoIter(self)
+    }
+}
+
+impl<T> Drop for List<T> {
+    fn drop(&mut self) {
+        while self.pop_front().is_some() {}
+    }
+}
+
+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_front(), 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);
+    }
+}

src/lib.rs

@@ -1,3 +1,4 @@
 pub mod first;
 pub mod second;
 pub mod third;
+pub mod fourth;

src/second.rs

@@ -1,3 +1,4 @@
+//! # An Ok Stack
 pub struct List<T> {
     head: Link<T>,
 }

src/third.rs

@@ -1,3 +1,4 @@
+//! # A Persistent Stack
 use std::rc::Rc;
 
 pub struct List<T> {