feat(cordyceps): add List::cursor_back_mut (#227)

This adds `List::cursor_back_mut` returning a mutable cursor at the back
of the list.

Closes #224

Signed-off-by: Eliza Weisman <[email protected]>

cordyceps/src/list.rs

@@ -617,6 +617,19 @@ impl<T: Linked<Links<T>> + ?Sized> List<T> {
         }
     }
 
+    /// Returns a [`Cursor`] s+tarting at the last element.
+    ///
+    /// The [`Cursor`] type can be used as a mutable [`Iterator`]. In addition,
+    /// however, it also permits modifying the *structure* of the list by
+    /// inserting or removing elements at the cursor's current position.
+    #[must_use]
+    pub fn cursor_back_mut(&mut self) -> Cursor<'_, T> {
+        Cursor {
+            curr: self.tail,
+            list: self,
+        }
+    }
+
     /// Returns an iterator over the items in this list, by reference.
     #[must_use]
     pub fn iter(&self) -> Iter<'_, T> {

cordyceps/src/list/tests/cursor.rs

@@ -43,23 +43,21 @@ fn move_peek() {
     assert_eq!(val(cursor.peek_prev()), Some(1));
     // assert_eq!(cursor.index(), Some(1));
 
-    // TODO(eliza): we don't have a `cursor_back` yet...
-
-    // let mut cursor = m.cursor_back();
-    // assert_eq!(val(cursor.current()), Some(&6));
-    // assert_eq!(val(cursor.peek_next()), None);
-    // assert_eq!(val(cursor.peek_prev()), Some(&5));
+    let mut cursor = list.cursor_back_mut();
+    assert_eq!(val(cursor.current()), Some(6));
+    assert_eq!(val(cursor.peek_next()), None);
+    assert_eq!(val(cursor.peek_prev()), Some(5));
     // assert_eq!(cursor.index(), Some(5));
-    // cursor.move_next();
-    // assert_eq!(val(cursor.current()), None);
-    // assert_eq!(val(cursor.peek_next()), Some(&1));
-    // assert_eq!(val(cursor.peek_prev()), Some(&6));
+    cursor.move_next();
+    assert_eq!(val(cursor.current()), None);
+    assert_eq!(val(cursor.peek_next()), Some(1));
+    assert_eq!(val(cursor.peek_prev()), Some(6));
     // assert_eq!(cursor.index(), None);
-    // cursor.move_prev();
-    // cursor.move_prev();
-    // assert_eq!(val(cursor.current()), Some(&5));
-    // assert_eq!(val(cursor.peek_next()), Some(&6));
-    // assert_eq!(val(cursor.peek_prev()), Some(&4));
+    cursor.move_prev();
+    cursor.move_prev();
+    assert_eq!(val(cursor.current()), Some(5));
+    assert_eq!(val(cursor.peek_next()), Some(6));
+    assert_eq!(val(cursor.peek_prev()), Some(4));
     // assert_eq!(cursor.index(), Some(4));
 
     // let mut m: LinkedList<u32> = LinkedList::new();