From 850e6a714a4fbff8a11b6992fac0a07d5d1aec1f Mon Sep 17 00:00:00 2001
From: Steven Allen <steven@stebalien.com>
Date: Mon, 27 Apr 2015 13:47:19 -0400
Subject: [PATCH] Implement retain for vec_deque

---
 src/libcollections/vec_deque.rs     | 34 +++++++++++++++++++++++++++++
 src/libcollectionstest/vec_deque.rs |  9 ++++++++
 2 files changed, 43 insertions(+)

diff --git a/src/libcollections/vec_deque.rs b/src/libcollections/vec_deque.rs
index 61369b30dea17..dcb9e398240a3 100644
--- a/src/libcollections/vec_deque.rs
+++ b/src/libcollections/vec_deque.rs
@@ -1396,6 +1396,40 @@ impl<T> VecDeque<T> {
         // naive impl
         self.extend(other.drain());
     }
+
+    /// Retains only the elements specified by the predicate.
+    ///
+    /// In other words, remove all elements `e` such that `f(&e)` returns false.
+    /// This method operates in place and preserves the order of the retained
+    /// elements.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use std::collections::VecDeque;
+    ///
+    /// let mut buf = VecDeque::new();
+    /// buf.extend(1..5);
+    /// buf.retain(|&x| x%2 == 0);
+    ///
+    /// let v: Vec<_> = buf.into_iter().collect();
+    /// assert_eq!(&v[..], &[2, 4]);
+    /// ```
+    #[stable(feature = "vec_deque_retain", since = "1.1.0")]
+    pub fn retain<F>(&mut self, mut f: F) where F: FnMut(&T) -> bool {
+        let len = self.len();
+        let mut del = 0;
+        for i in 0..len {
+            if !f(&self[i]) {
+                del += 1;
+            } else if del > 0 {
+                self.swap(i-del, i);
+            }
+        }
+        if del > 0 {
+            self.truncate(len - del);
+        }
+    }
 }
 
 impl<T: Clone> VecDeque<T> {
diff --git a/src/libcollectionstest/vec_deque.rs b/src/libcollectionstest/vec_deque.rs
index 12323286f6b4b..7870447281bc1 100644
--- a/src/libcollectionstest/vec_deque.rs
+++ b/src/libcollectionstest/vec_deque.rs
@@ -885,3 +885,12 @@ fn test_append() {
     assert_eq!(b.iter().cloned().collect::<Vec<_>>(), [1, 2, 3, 4, 5, 6]);
     assert_eq!(a.iter().cloned().collect::<Vec<_>>(), []);
 }
+
+#[test]
+fn test_retain() {
+    let mut buf = VecDeque::new();
+    buf.extend(1..5);
+    buf.retain(|&x| x % 2 == 0);
+    let v: Vec<_> = buf.into_iter().collect();
+    assert_eq!(&v[..], &[2, 4]);
+}