Chapter 4 done

lists/src/fourth.rs

 use std::rc::Rc;
-use std::cell::RefCell;
+use std::cell::{Ref, RefMut, RefCell};
 
 pub struct List<T> {
     head: Link<T>,
             Rc::try_unwrap(old_head).ok().unwrap().into_inner().elem
         })
     }
-}
 
-impl<T> Drop for List<T> {
-    fn drop(&mut self) {
-        while self.pop_front().is_some() {}
+    pub fn peek_front(&self) -> Option<Ref<T>> {
+        self.head.as_ref().map(|node| {
+            Ref::map(node.borrow(), |node| &node.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);
+            }
+        }
     }
-}
 
-#[cfg(test)]
-mod test {
-    use super::List;
+    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
+        })
+    }
 
-    #[test]
-    fn basics() {
-        let mut list = List::new();
+    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)
+        })
+    }
 
-        assert_eq!(list.pop_front(), None);
+    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)
+        })
+    }
+}
 
-        list.push_front(1);
-        list.push_front(2);
-        list.push_front(3);
+pub struct IntoIter<T>(List<T>);
 
-        assert_eq!(list.pop_front(), Some(3));
-        assert_eq!(list.pop_front(), Some(2));
+impl<T> List<T> {
+    pub fn into_iter(self) -> IntoIter<T> {
+        IntoIter(self)
+    }
+}
 
-        list.push_front(4);
-        list.push_front(5);
+impl<T> Iterator for IntoIter<T> {
+    type Item = T;
+    fn next(&mut self) -> Option<T> {
+        self.0.pop_front()
+    }
+}
 
-        assert_eq!(list.pop_front(), Some(5));
-        assert_eq!(list.pop_front(), Some(4));
+impl<T> DoubleEndedIterator for IntoIter<T> {
+    fn next_back(&mut self) -> Option<T> {
+        self.0.pop_back()
+    }
+}
 
-        assert_eq!(list.pop_front(), Some(1));
-        assert_eq!(list.pop_front(), None);
+impl<T> Drop for List<T> {
+    fn drop(&mut self) {
+        while self.pop_front().is_some() {}
     }
 }