libcollections

1 files changed, 2404 insertions, 0 deletions

diff --git a/libcollections/vec_deque.rs b/libcollections/vec_deque.rs
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+// Copyright 2012-2014 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+//! VecDeque is a double-ended queue, which is implemented with the help of a
+//! growing ring buffer.
+//!
+//! This queue has `O(1)` amortized inserts and removals from both ends of the
+//! container. It also has `O(1)` indexing like a vector. The contained elements
+//! are not required to be copyable, and the queue will be sendable if the
+//! contained type is sendable.
+
+#![stable(feature = "rust1", since = "1.0.0")]
+
+use core::cmp::Ordering;
+use core::fmt;
+use core::iter::{repeat, FromIterator};
+use core::mem;
+use core::ops::{Index, IndexMut};
+use core::ptr;
+use core::slice;
+
+use core::hash::{Hash, Hasher};
+use core::cmp;
+
+use alloc::raw_vec::RawVec;
+
+use super::range::RangeArgument;
+
+const INITIAL_CAPACITY: usize = 7; // 2^3 - 1
+const MINIMUM_CAPACITY: usize = 1; // 2 - 1
+#[cfg(target_pointer_width = "32")]
+const MAXIMUM_ZST_CAPACITY: usize = 1 << (32 - 1); // Largest possible power of two
+#[cfg(target_pointer_width = "64")]
+const MAXIMUM_ZST_CAPACITY: usize = 1 << (64 - 1); // Largest possible power of two
+
+/// `VecDeque` is a growable ring buffer, which can be used as a double-ended
+/// queue efficiently.
+///
+/// The "default" usage of this type as a queue is to use `push_back` to add to
+/// the queue, and `pop_front` to remove from the queue. `extend` and `append`
+/// push onto the back in this manner, and iterating over `VecDeque` goes front
+/// to back.
+#[stable(feature = "rust1", since = "1.0.0")]
+pub struct VecDeque<T> {
+    // tail and head are pointers into the buffer. Tail always points
+    // to the first element that could be read, Head always points
+    // to where data should be written.
+    // If tail == head the buffer is empty. The length of the ringbuffer
+    // is defined as the distance between the two.
+    tail: usize,
+    head: usize,
+    buf: RawVec<T>,
+}
+
+#[stable(feature = "rust1", since = "1.0.0")]
+impl<T: Clone> Clone for VecDeque<T> {
+    fn clone(&self) -> VecDeque<T> {
+        self.iter().cloned().collect()
+    }
+}
+
+#[stable(feature = "rust1", since = "1.0.0")]
+impl<T> Drop for VecDeque<T> {
+    #[unsafe_destructor_blind_to_params]
+    fn drop(&mut self) {
+        let (front, back) = self.as_mut_slices();
+        unsafe {
+            // use drop for [T]
+            ptr::drop_in_place(front);
+            ptr::drop_in_place(back);
+        }
+        // RawVec handles deallocation
+    }
+}
+
+#[stable(feature = "rust1", since = "1.0.0")]
+impl<T> Default for VecDeque<T> {
+    #[inline]
+    fn default() -> VecDeque<T> {
+        VecDeque::new()
+    }
+}
+
+impl<T> VecDeque<T> {
+    /// Marginally more convenient
+    #[inline]
+    fn ptr(&self) -> *mut T {
+        self.buf.ptr()
+    }
+
+    /// Marginally more convenient
+    #[inline]
+    fn cap(&self) -> usize {
+        if mem::size_of::<T>() == 0 {
+            // For zero sized types, we are always at maximum capacity
+            MAXIMUM_ZST_CAPACITY
+        } else {
+            self.buf.cap()
+        }
+    }
+
+    /// Turn ptr into a slice
+    #[inline]
+    unsafe fn buffer_as_slice(&self) -> &[T] {
+        slice::from_raw_parts(self.ptr(), self.cap())
+    }
+
+    /// Turn ptr into a mut slice
+    #[inline]
+    unsafe fn buffer_as_mut_slice(&mut self) -> &mut [T] {
+        slice::from_raw_parts_mut(self.ptr(), self.cap())
+    }
+
+    /// Moves an element out of the buffer
+    #[inline]
+    unsafe fn buffer_read(&mut self, off: usize) -> T {
+        ptr::read(self.ptr().offset(off as isize))
+    }
+
+    /// Writes an element into the buffer, moving it.
+    #[inline]
+    unsafe fn buffer_write(&mut self, off: usize, value: T) {
+        ptr::write(self.ptr().offset(off as isize), value);
+    }
+
+    /// Returns true if and only if the buffer is at capacity
+    #[inline]
+    fn is_full(&self) -> bool {
+        self.cap() - self.len() == 1
+    }
+
+    /// Returns the index in the underlying buffer for a given logical element
+    /// index.
+    #[inline]
+    fn wrap_index(&self, idx: usize) -> usize {
+        wrap_index(idx, self.cap())
+    }
+
+    /// Returns the index in the underlying buffer for a given logical element
+    /// index + addend.
+    #[inline]
+    fn wrap_add(&self, idx: usize, addend: usize) -> usize {
+        wrap_index(idx.wrapping_add(addend), self.cap())
+    }
+
+    /// Returns the index in the underlying buffer for a given logical element
+    /// index - subtrahend.
+    #[inline]
+    fn wrap_sub(&self, idx: usize, subtrahend: usize) -> usize {
+        wrap_index(idx.wrapping_sub(subtrahend), self.cap())
+    }
+
+    /// Copies a contiguous block of memory len long from src to dst
+    #[inline]
+    unsafe fn copy(&self, dst: usize, src: usize, len: usize) {
+        debug_assert!(dst + len <= self.cap(),
+                      "cpy dst={} src={} len={} cap={}",
+                      dst,
+                      src,
+                      len,
+                      self.cap());
+        debug_assert!(src + len <= self.cap(),
+                      "cpy dst={} src={} len={} cap={}",
+                      dst,
+                      src,
+                      len,
+                      self.cap());
+        ptr::copy(self.ptr().offset(src as isize),
+                  self.ptr().offset(dst as isize),
+                  len);
+    }
+
+    /// Copies a contiguous block of memory len long from src to dst
+    #[inline]
+    unsafe fn copy_nonoverlapping(&self, dst: usize, src: usize, len: usize) {
+        debug_assert!(dst + len <= self.cap(),
+                      "cno dst={} src={} len={} cap={}",
+                      dst,
+                      src,
+                      len,
+                      self.cap());
+        debug_assert!(src + len <= self.cap(),
+                      "cno dst={} src={} len={} cap={}",
+                      dst,
+                      src,
+                      len,
+                      self.cap());
+        ptr::copy_nonoverlapping(self.ptr().offset(src as isize),
+                                 self.ptr().offset(dst as isize),
+                                 len);
+    }
+
+    /// Copies a potentially wrapping block of memory len long from src to dest.
+    /// (abs(dst - src) + len) must be no larger than cap() (There must be at
+    /// most one continuous overlapping region between src and dest).
+    unsafe fn wrap_copy(&self, dst: usize, src: usize, len: usize) {
+        #[allow(dead_code)]
+        fn diff(a: usize, b: usize) -> usize {
+            if a <= b {
+                b - a
+            } else {
+                a - b
+            }
+        }
+        debug_assert!(cmp::min(diff(dst, src), self.cap() - diff(dst, src)) + len <= self.cap(),
+                      "wrc dst={} src={} len={} cap={}",
+                      dst,
+                      src,
+                      len,
+                      self.cap());
+
+        if src == dst || len == 0 {
+            return;
+        }
+
+        let dst_after_src = self.wrap_sub(dst, src) < len;
+
+        let src_pre_wrap_len = self.cap() - src;
+        let dst_pre_wrap_len = self.cap() - dst;
+        let src_wraps = src_pre_wrap_len < len;
+        let dst_wraps = dst_pre_wrap_len < len;
+
+        match (dst_after_src, src_wraps, dst_wraps) {
+            (_, false, false) => {
+                // src doesn't wrap, dst doesn't wrap
+                //
+                //        S . . .
+                // 1 [_ _ A A B B C C _]
+                // 2 [_ _ A A A A B B _]
+                //            D . . .
+                //
+                self.copy(dst, src, len);
+            }
+            (false, false, true) => {
+                // dst before src, src doesn't wrap, dst wraps
+                //
+                //    S . . .
+                // 1 [A A B B _ _ _ C C]
+                // 2 [A A B B _ _ _ A A]
+                // 3 [B B B B _ _ _ A A]
+                //    . .           D .
+                //
+                self.copy(dst, src, dst_pre_wrap_len);
+                self.copy(0, src + dst_pre_wrap_len, len - dst_pre_wrap_len);
+            }
+            (true, false, true) => {
+                // src before dst, src doesn't wrap, dst wraps
+                //
+                //              S . . .
+                // 1 [C C _ _ _ A A B B]
+                // 2 [B B _ _ _ A A B B]
+                // 3 [B B _ _ _ A A A A]
+                //    . .           D .
+                //
+                self.copy(0, src + dst_pre_wrap_len, len - dst_pre_wrap_len);
+                self.copy(dst, src, dst_pre_wrap_len);
+            }
+            (false, true, false) => {
+                // dst before src, src wraps, dst doesn't wrap
+                //
+                //    . .           S .
+                // 1 [C C _ _ _ A A B B]
+                // 2 [C C _ _ _ B B B B]
+                // 3 [C C _ _ _ B B C C]
+                //              D . . .
+                //
+                self.copy(dst, src, src_pre_wrap_len);
+                self.copy(dst + src_pre_wrap_len, 0, len - src_pre_wrap_len);
+            }
+            (true, true, false) => {
+                // src before dst, src wraps, dst doesn't wrap
+                //
+                //    . .           S .
+                // 1 [A A B B _ _ _ C C]
+                // 2 [A A A A _ _ _ C C]
+                // 3 [C C A A _ _ _ C C]
+                //    D . . .
+                //
+                self.copy(dst + src_pre_wrap_len, 0, len - src_pre_wrap_len);
+                self.copy(dst, src, src_pre_wrap_len);
+            }
+            (false, true, true) => {
+                // dst before src, src wraps, dst wraps
+                //
+                //    . . .         S .
+                // 1 [A B C D _ E F G H]
+                // 2 [A B C D _ E G H H]
+                // 3 [A B C D _ E G H A]
+                // 4 [B C C D _ E G H A]
+                //    . .         D . .
+                //
+                debug_assert!(dst_pre_wrap_len > src_pre_wrap_len);
+                let delta = dst_pre_wrap_len - src_pre_wrap_len;
+                self.copy(dst, src, src_pre_wrap_len);
+                self.copy(dst + src_pre_wrap_len, 0, delta);
+                self.copy(0, delta, len - dst_pre_wrap_len);
+            }
+            (true, true, true) => {
+                // src before dst, src wraps, dst wraps
+                //
+                //    . .         S . .
+                // 1 [A B C D _ E F G H]
+                // 2 [A A B D _ E F G H]
+                // 3 [H A B D _ E F G H]
+                // 4 [H A B D _ E F F G]
+                //    . . .         D .
+                //
+                debug_assert!(src_pre_wrap_len > dst_pre_wrap_len);
+                let delta = src_pre_wrap_len - dst_pre_wrap_len;
+                self.copy(delta, 0, len - src_pre_wrap_len);
+                self.copy(0, self.cap() - delta, delta);
+                self.copy(dst, src, dst_pre_wrap_len);
+            }
+        }
+    }
+
+    /// Frobs the head and tail sections around to handle the fact that we
+    /// just reallocated. Unsafe because it trusts old_cap.
+    #[inline]
+    unsafe fn handle_cap_increase(&mut self, old_cap: usize) {
+        let new_cap = self.cap();
+
+        // Move the shortest contiguous section of the ring buffer
+        //    T             H
+        //   [o o o o o o o . ]
+        //    T             H
+        // A [o o o o o o o . . . . . . . . . ]
+        //        H T
+        //   [o o . o o o o o ]
+        //          T             H
+        // B [. . . o o o o o o o . . . . . . ]
+        //              H T
+        //   [o o o o o . o o ]
+        //              H                 T
+        // C [o o o o o . . . . . . . . . o o ]
+
+        if self.tail <= self.head {
+            // A
+            // Nop
+        } else if self.head < old_cap - self.tail {
+            // B
+            self.copy_nonoverlapping(old_cap, 0, self.head);
+            self.head += old_cap;
+            debug_assert!(self.head > self.tail);
+        } else {
+            // C
+            let new_tail = new_cap - (old_cap - self.tail);
+            self.copy_nonoverlapping(new_tail, self.tail, old_cap - self.tail);
+            self.tail = new_tail;
+            debug_assert!(self.head < self.tail);
+        }
+        debug_assert!(self.head < self.cap());
+        debug_assert!(self.tail < self.cap());
+        debug_assert!(self.cap().count_ones() == 1);
+    }
+}
+
+impl<T> VecDeque<T> {
+    /// Creates an empty `VecDeque`.
+    #[stable(feature = "rust1", since = "1.0.0")]
+    pub fn new() -> VecDeque<T> {
+        VecDeque::with_capacity(INITIAL_CAPACITY)
+    }
+
+    /// Creates an empty `VecDeque` with space for at least `n` elements.
+    #[stable(feature = "rust1", since = "1.0.0")]
+    pub fn with_capacity(n: usize) -> VecDeque<T> {
+        // +1 since the ringbuffer always leaves one space empty
+        let cap = cmp::max(n + 1, MINIMUM_CAPACITY + 1).next_power_of_two();
+        assert!(cap > n, "capacity overflow");
+
+        VecDeque {
+            tail: 0,
+            head: 0,
+            buf: RawVec::with_capacity(cap),
+        }
+    }
+
+    /// Retrieves an element in the `VecDeque` by index.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use std::collections::VecDeque;
+    ///
+    /// let mut buf = VecDeque::new();
+    /// buf.push_back(3);
+    /// buf.push_back(4);
+    /// buf.push_back(5);
+    /// assert_eq!(buf.get(1), Some(&4));
+    /// ```
+    #[stable(feature = "rust1", since = "1.0.0")]
+    pub fn get(&self, index: usize) -> Option<&T> {
+        if index < self.len() {
+            let idx = self.wrap_add(self.tail, index);
+            unsafe { Some(&*self.ptr().offset(idx as isize)) }
+        } else {
+            None
+        }
+    }
+
+    /// Retrieves an element in the `VecDeque` mutably by index.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use std::collections::VecDeque;
+    ///
+    /// let mut buf = VecDeque::new();
+    /// buf.push_back(3);
+    /// buf.push_back(4);
+    /// buf.push_back(5);
+    /// if let Some(elem) = buf.get_mut(1) {
+    ///     *elem = 7;
+    /// }
+    ///
+    /// assert_eq!(buf[1], 7);
+    /// ```
+    #[stable(feature = "rust1", since = "1.0.0")]
+    pub fn get_mut(&mut self, index: usize) -> Option<&mut T> {
+        if index < self.len() {
+            let idx = self.wrap_add(self.tail, index);
+            unsafe { Some(&mut *self.ptr().offset(idx as isize)) }
+        } else {
+            None
+        }
+    }
+
+    /// Swaps elements at indices `i` and `j`.
+    ///
+    /// `i` and `j` may be equal.
+    ///
+    /// Fails if there is no element with either index.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use std::collections::VecDeque;
+    ///
+    /// let mut buf = VecDeque::new();
+    /// buf.push_back(3);
+    /// buf.push_back(4);
+    /// buf.push_back(5);
+    /// buf.swap(0, 2);
+    /// assert_eq!(buf[0], 5);
+    /// assert_eq!(buf[2], 3);
+    /// ```
+    #[stable(feature = "rust1", since = "1.0.0")]
+    pub fn swap(&mut self, i: usize, j: usize) {
+        assert!(i < self.len());
+        assert!(j < self.len());
+        let ri = self.wrap_add(self.tail, i);
+        let rj = self.wrap_add(self.tail, j);
+        unsafe {
+            ptr::swap(self.ptr().offset(ri as isize),
+                      self.ptr().offset(rj as isize))
+        }
+    }
+
+    /// Returns the number of elements the `VecDeque` can hold without
+    /// reallocating.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use std::collections::VecDeque;
+    ///
+    /// let buf: VecDeque<i32> = VecDeque::with_capacity(10);
+    /// assert!(buf.capacity() >= 10);
+    /// ```
+    #[inline]
+    #[stable(feature = "rust1", since = "1.0.0")]
+    pub fn capacity(&self) -> usize {
+        self.cap() - 1
+    }
+
+    /// Reserves the minimum capacity for exactly `additional` more elements to be inserted in the
+    /// given `VecDeque`. Does nothing if the capacity is already sufficient.
+    ///
+    /// Note that the allocator may give the collection more space than it requests. Therefore
+    /// capacity can not be relied upon to be precisely minimal. Prefer `reserve` if future
+    /// insertions are expected.
+    ///
+    /// # Panics
+    ///
+    /// Panics if the new capacity overflows `usize`.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use std::collections::VecDeque;
+    ///
+    /// let mut buf: VecDeque<i32> = vec![1].into_iter().collect();
+    /// buf.reserve_exact(10);
+    /// assert!(buf.capacity() >= 11);
+    /// ```
+    #[stable(feature = "rust1", since = "1.0.0")]
+    pub fn reserve_exact(&mut self, additional: usize) {
+        self.reserve(additional);
+    }
+
+    /// Reserves capacity for at least `additional` more elements to be inserted in the given
+    /// `VecDeque`. The collection may reserve more space to avoid frequent reallocations.
+    ///
+    /// # Panics
+    ///
+    /// Panics if the new capacity overflows `usize`.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use std::collections::VecDeque;
+    ///
+    /// let mut buf: VecDeque<i32> = vec![1].into_iter().collect();
+    /// buf.reserve(10);
+    /// assert!(buf.capacity() >= 11);
+    /// ```
+    #[stable(feature = "rust1", since = "1.0.0")]
+    pub fn reserve(&mut self, additional: usize) {
+        let old_cap = self.cap();
+        let used_cap = self.len() + 1;
+        let new_cap = used_cap.checked_add(additional)
+                              .and_then(|needed_cap| needed_cap.checked_next_power_of_two())
+                              .expect("capacity overflow");
+
+        if new_cap > self.capacity() {
+            self.buf.reserve_exact(used_cap, new_cap - used_cap);
+            unsafe {
+                self.handle_cap_increase(old_cap);
+            }
+        }
+    }
+
+    /// Shrinks the capacity of the `VecDeque` as much as possible.
+    ///
+    /// It will drop down as close as possible to the length but the allocator may still inform the
+    /// `VecDeque` that there is space for a few more elements.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use std::collections::VecDeque;
+    ///
+    /// let mut buf = VecDeque::with_capacity(15);
+    /// buf.extend(0..4);
+    /// assert_eq!(buf.capacity(), 15);
+    /// buf.shrink_to_fit();
+    /// assert!(buf.capacity() >= 4);
+    /// ```
+    #[stable(feature = "deque_extras_15", since = "1.5.0")]
+    pub fn shrink_to_fit(&mut self) {
+        // +1 since the ringbuffer always leaves one space empty
+        // len + 1 can't overflow for an existing, well-formed ringbuffer.
+        let target_cap = cmp::max(self.len() + 1, MINIMUM_CAPACITY + 1).next_power_of_two();
+        if target_cap < self.cap() {
+            // There are three cases of interest:
+            //   All elements are out of desired bounds
+            //   Elements are contiguous, and head is out of desired bounds
+            //   Elements are discontiguous, and tail is out of desired bounds
+            //
+            // At all other times, element positions are unaffected.
+            //
+            // Indicates that elements at the head should be moved.
+            let head_outside = self.head == 0 || self.head >= target_cap;
+            // Move elements from out of desired bounds (positions after target_cap)
+            if self.tail >= target_cap && head_outside {
+                //                    T             H
+                //   [. . . . . . . . o o o o o o o . ]
+                //    T             H
+                //   [o o o o o o o . ]
+                unsafe {
+                    self.copy_nonoverlapping(0, self.tail, self.len());
+                }
+                self.head = self.len();
+                self.tail = 0;
+            } else if self.tail != 0 && self.tail < target_cap && head_outside {
+                //          T             H
+                //   [. . . o o o o o o o . . . . . . ]
+                //        H T
+                //   [o o . o o o o o ]
+                let len = self.wrap_sub(self.head, target_cap);
+                unsafe {
+                    self.copy_nonoverlapping(0, target_cap, len);
+                }
+                self.head = len;
+                debug_assert!(self.head < self.tail);
+            } else if self.tail >= target_cap {
+                //              H                 T
+                //   [o o o o o . . . . . . . . . o o ]
+                //              H T
+                //   [o o o o o . o o ]
+                debug_assert!(self.wrap_sub(self.head, 1) < target_cap);
+                let len = self.cap() - self.tail;
+                let new_tail = target_cap - len;
+                unsafe {
+                    self.copy_nonoverlapping(new_tail, self.tail, len);
+                }
+                self.tail = new_tail;
+                debug_assert!(self.head < self.tail);
+            }
+
+            self.buf.shrink_to_fit(target_cap);
+
+            debug_assert!(self.head < self.cap());
+            debug_assert!(self.tail < self.cap());
+            debug_assert!(self.cap().count_ones() == 1);
+        }
+    }
+
+    /// Shortens a `VecDeque`, dropping excess elements from the back.
+    ///
+    /// If `len` is greater than the `VecDeque`'s current length, this has no
+    /// effect.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// #![feature(deque_extras)]
+    ///
+    /// use std::collections::VecDeque;
+    ///
+    /// let mut buf = VecDeque::new();
+    /// buf.push_back(5);
+    /// buf.push_back(10);
+    /// buf.push_back(15);
+    /// buf.truncate(1);
+    /// assert_eq!(buf.len(), 1);
+    /// assert_eq!(Some(&5), buf.get(0));
+    /// ```
+    #[unstable(feature = "deque_extras",
+               reason = "matches collection reform specification; waiting on panic semantics",
+               issue = "27788")]
+    pub fn truncate(&mut self, len: usize) {
+        for _ in len..self.len() {
+            self.pop_back();
+        }
+    }
+
+    /// Returns a front-to-back iterator.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use std::collections::VecDeque;
+    ///
+    /// let mut buf = VecDeque::new();
+    /// buf.push_back(5);
+    /// buf.push_back(3);
+    /// buf.push_back(4);
+    /// let b: &[_] = &[&5, &3, &4];
+    /// let c: Vec<&i32> = buf.iter().collect();
+    /// assert_eq!(&c[..], b);
+    /// ```
+    #[stable(feature = "rust1", since = "1.0.0")]
+    pub fn iter(&self) -> Iter<T> {
+        Iter {
+            tail: self.tail,
+            head: self.head,
+            ring: unsafe { self.buffer_as_slice() },
+        }
+    }
+
+    /// Returns a front-to-back iterator that returns mutable references.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use std::collections::VecDeque;
+    ///
+    /// let mut buf = VecDeque::new();
+    /// buf.push_back(5);
+    /// buf.push_back(3);
+    /// buf.push_back(4);
+    /// for num in buf.iter_mut() {
+    ///     *num = *num - 2;
+    /// }
+    /// let b: &[_] = &[&mut 3, &mut 1, &mut 2];
+    /// assert_eq!(&buf.iter_mut().collect::<Vec<&mut i32>>()[..], b);
+    /// ```
+    #[stable(feature = "rust1", since = "1.0.0")]
+    pub fn iter_mut(&mut self) -> IterMut<T> {
+        IterMut {
+            tail: self.tail,
+            head: self.head,
+            ring: unsafe { self.buffer_as_mut_slice() },
+        }
+    }
+
+    /// Returns a pair of slices which contain, in order, the contents of the
+    /// `VecDeque`.
+    #[inline]
+    #[stable(feature = "deque_extras_15", since = "1.5.0")]
+    pub fn as_slices(&self) -> (&[T], &[T]) {
+        unsafe {
+            let contiguous = self.is_contiguous();
+            let buf = self.buffer_as_slice();
+            if contiguous {
+                let (empty, buf) = buf.split_at(0);
+                (&buf[self.tail..self.head], empty)
+            } else {
+                let (mid, right) = buf.split_at(self.tail);
+                let (left, _) = mid.split_at(self.head);
+                (right, left)
+            }
+        }
+    }
+
+    /// Returns a pair of slices which contain, in order, the contents of the
+    /// `VecDeque`.
+    #[inline]
+    #[stable(feature = "deque_extras_15", since = "1.5.0")]
+    pub fn as_mut_slices(&mut self) -> (&mut [T], &mut [T]) {
+        unsafe {
+            let contiguous = self.is_contiguous();
+            let head = self.head;
+            let tail = self.tail;
+            let buf = self.buffer_as_mut_slice();
+
+            if contiguous {
+                let (empty, buf) = buf.split_at_mut(0);
+                (&mut buf[tail..head], empty)
+            } else {
+                let (mid, right) = buf.split_at_mut(tail);
+                let (left, _) = mid.split_at_mut(head);
+
+                (right, left)
+            }
+        }
+    }
+
+    /// Returns the number of elements in the `VecDeque`.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use std::collections::VecDeque;
+    ///
+    /// let mut v = VecDeque::new();
+    /// assert_eq!(v.len(), 0);
+    /// v.push_back(1);
+    /// assert_eq!(v.len(), 1);
+    /// ```
+    #[stable(feature = "rust1", since = "1.0.0")]
+    pub fn len(&self) -> usize {
+        count(self.tail, self.head, self.cap())
+    }
+
+    /// Returns true if the buffer contains no elements
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use std::collections::VecDeque;
+    ///
+    /// let mut v = VecDeque::new();
+    /// assert!(v.is_empty());
+    /// v.push_front(1);
+    /// assert!(!v.is_empty());
+    /// ```
+    #[stable(feature = "rust1", since = "1.0.0")]
+    pub fn is_empty(&self) -> bool {
+        self.len() == 0
+    }
+
+    /// Create a draining iterator that removes the specified range in the
+    /// `VecDeque` and yields the removed items.
+    ///
+    /// Note 1: The element range is removed even if the iterator is not
+    /// consumed until the end.
+    ///
+    /// Note 2: It is unspecified how many elements are removed from the deque,
+    /// if the `Drain` value is not dropped, but the borrow it holds expires
+    /// (eg. due to mem::forget).
+    ///
+    /// # Panics
+    ///
+    /// Panics if the starting point is greater than the end point or if
+    /// the end point is greater than the length of the vector.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use std::collections::VecDeque;
+
+    /// let mut v: VecDeque<_> = vec![1, 2, 3].into_iter().collect();
+    /// assert_eq!(vec![3].into_iter().collect::<VecDeque<_>>(), v.drain(2..).collect());
+    /// assert_eq!(vec![1, 2].into_iter().collect::<VecDeque<_>>(), v);
+    ///
+    /// // A full range clears all contents
+    /// v.drain(..);
+    /// assert!(v.is_empty());
+    /// ```
+    #[inline]
+    #[stable(feature = "drain", since = "1.6.0")]
+    pub fn drain<R>(&mut self, range: R) -> Drain<T>
+        where R: RangeArgument<usize>
+    {
+        // Memory safety
+        //
+        // When the Drain is first created, the source deque is shortened to
+        // make sure no uninitialized or moved-from elements are accessible at
+        // all if the Drain's destructor never gets to run.
+        //
+        // Drain will ptr::read out the values to remove.
+        // When finished, the remaining data will be copied back to cover the hole,
+        // and the head/tail values will be restored correctly.
+        //
+        let len = self.len();
+        let start = *range.start().unwrap_or(&0);
+        let end = *range.end().unwrap_or(&len);
+        assert!(start <= end, "drain lower bound was too large");
+        assert!(end <= len, "drain upper bound was too large");
+
+        // The deque's elements are parted into three segments:
+        // * self.tail  -> drain_tail
+        // * drain_tail -> drain_head
+        // * drain_head -> self.head
+        //
+        // T = self.tail; H = self.head; t = drain_tail; h = drain_head
+        //
+        // We store drain_tail as self.head, and drain_head and self.head as
+        // after_tail and after_head respectively on the Drain. This also
+        // truncates the effective array such that if the Drain is leaked, we
+        // have forgotten about the potentially moved values after the start of
+        // the drain.
+        //
+        //        T   t   h   H
+        // [. . . o o x x o o . . .]
+        //
+        let drain_tail = self.wrap_add(self.tail, start);
+        let drain_head = self.wrap_add(self.tail, end);
+        let head = self.head;
+
+        // "forget" about the values after the start of the drain until after
+        // the drain is complete and the Drain destructor is run.
+        self.head = drain_tail;
+
+        Drain {
+            deque: self as *mut _,
+            after_tail: drain_head,
+            after_head: head,
+            iter: Iter {
+                tail: drain_tail,
+                head: drain_head,
+                ring: unsafe { self.buffer_as_mut_slice() },
+            },
+        }
+    }
+
+    /// Clears the buffer, removing all values.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use std::collections::VecDeque;
+    ///
+    /// let mut v = VecDeque::new();
+    /// v.push_back(1);
+    /// v.clear();
+    /// assert!(v.is_empty());
+    /// ```
+    #[stable(feature = "rust1", since = "1.0.0")]
+    #[inline]
+    pub fn clear(&mut self) {
+        self.drain(..);
+    }
+
+    /// Provides a reference to the front element, or `None` if the sequence is
+    /// empty.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use std::collections::VecDeque;
+    ///
+    /// let mut d = VecDeque::new();
+    /// assert_eq!(d.front(), None);
+    ///
+    /// d.push_back(1);
+    /// d.push_back(2);
+    /// assert_eq!(d.front(), Some(&1));
+    /// ```
+    #[stable(feature = "rust1", since = "1.0.0")]
+    pub fn front(&self) -> Option<&T> {
+        if !self.is_empty() {
+            Some(&self[0])
+        } else {
+            None
+        }
+    }
+
+    /// Provides a mutable reference to the front element, or `None` if the
+    /// sequence is empty.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use std::collections::VecDeque;
+    ///
+    /// let mut d = VecDeque::new();
+    /// assert_eq!(d.front_mut(), None);
+    ///
+    /// d.push_back(1);
+    /// d.push_back(2);
+    /// match d.front_mut() {
+    ///     Some(x) => *x = 9,
+    ///     None => (),
+    /// }
+    /// assert_eq!(d.front(), Some(&9));
+    /// ```
+    #[stable(feature = "rust1", since = "1.0.0")]
+    pub fn front_mut(&mut self) -> Option<&mut T> {
+        if !self.is_empty() {
+            Some(&mut self[0])
+        } else {
+            None
+        }
+    }
+
+    /// Provides a reference to the back element, or `None` if the sequence is
+    /// empty.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use std::collections::VecDeque;
+    ///
+    /// let mut d = VecDeque::new();
+    /// assert_eq!(d.back(), None);
+    ///
+    /// d.push_back(1);
+    /// d.push_back(2);
+    /// assert_eq!(d.back(), Some(&2));
+    /// ```
+    #[stable(feature = "rust1", since = "1.0.0")]
+    pub fn back(&self) -> Option<&T> {
+        if !self.is_empty() {
+            Some(&self[self.len() - 1])
+        } else {
+            None
+        }
+    }
+
+    /// Provides a mutable reference to the back element, or `None` if the
+    /// sequence is empty.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use std::collections::VecDeque;
+    ///
+    /// let mut d = VecDeque::new();
+    /// assert_eq!(d.back(), None);
+    ///
+    /// d.push_back(1);
+    /// d.push_back(2);
+    /// match d.back_mut() {
+    ///     Some(x) => *x = 9,
+    ///     None => (),
+    /// }
+    /// assert_eq!(d.back(), Some(&9));
+    /// ```
+    #[stable(feature = "rust1", since = "1.0.0")]
+    pub fn back_mut(&mut self) -> Option<&mut T> {
+        let len = self.len();
+        if !self.is_empty() {
+            Some(&mut self[len - 1])
+        } else {
+            None
+        }
+    }
+
+    /// Removes the first element and returns it, or `None` if the sequence is
+    /// empty.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use std::collections::VecDeque;
+    ///
+    /// let mut d = VecDeque::new();
+    /// d.push_back(1);
+    /// d.push_back(2);
+    ///
+    /// assert_eq!(d.pop_front(), Some(1));
+    /// assert_eq!(d.pop_front(), Some(2));
+    /// assert_eq!(d.pop_front(), None);
+    /// ```
+    #[stable(feature = "rust1", since = "1.0.0")]
+    pub fn pop_front(&mut self) -> Option<T> {
+        if self.is_empty() {
+            None
+        } else {
+            let tail = self.tail;
+            self.tail = self.wrap_add(self.tail, 1);
+            unsafe { Some(self.buffer_read(tail)) }
+        }
+    }
+
+    /// Inserts an element first in the sequence.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use std::collections::VecDeque;
+    ///
+    /// let mut d = VecDeque::new();
+    /// d.push_front(1);
+    /// d.push_front(2);
+    /// assert_eq!(d.front(), Some(&2));
+    /// ```
+    #[stable(feature = "rust1", since = "1.0.0")]
+    pub fn push_front(&mut self, value: T) {
+        if self.is_full() {
+            let old_cap = self.cap();
+            self.buf.double();
+            unsafe {
+                self.handle_cap_increase(old_cap);
+            }
+            debug_assert!(!self.is_full());
+        }
+
+        self.tail = self.wrap_sub(self.tail, 1);
+        let tail = self.tail;
+        unsafe {
+            self.buffer_write(tail, value);
+        }
+    }
+
+    /// Appends an element to the back of a buffer
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use std::collections::VecDeque;
+    ///
+    /// let mut buf = VecDeque::new();
+    /// buf.push_back(1);
+    /// buf.push_back(3);
+    /// assert_eq!(3, *buf.back().unwrap());
+    /// ```
+    #[stable(feature = "rust1", since = "1.0.0")]
+    pub fn push_back(&mut self, value: T) {
+        if self.is_full() {
+            let old_cap = self.cap();
+            self.buf.double();
+            unsafe {
+                self.handle_cap_increase(old_cap);
+            }
+            debug_assert!(!self.is_full());
+        }
+
+        let head = self.head;
+        self.head = self.wrap_add(self.head, 1);
+        unsafe { self.buffer_write(head, value) }
+    }
+
+    /// Removes the last element from a buffer and returns it, or `None` if
+    /// it is empty.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use std::collections::VecDeque;
+    ///
+    /// let mut buf = VecDeque::new();
+    /// assert_eq!(buf.pop_back(), None);
+    /// buf.push_back(1);
+    /// buf.push_back(3);
+    /// assert_eq!(buf.pop_back(), Some(3));
+    /// ```
+    #[stable(feature = "rust1", since = "1.0.0")]
+    pub fn pop_back(&mut self) -> Option<T> {
+        if self.is_empty() {
+            None
+        } else {
+            self.head = self.wrap_sub(self.head, 1);
+            let head = self.head;
+            unsafe { Some(self.buffer_read(head)) }
+        }
+    }
+
+    #[inline]
+    fn is_contiguous(&self) -> bool {
+        self.tail <= self.head
+    }
+
+    /// Removes an element from anywhere in the `VecDeque` and returns it, replacing it with the
+    /// last element.
+    ///
+    /// This does not preserve ordering, but is O(1).
+    ///
+    /// Returns `None` if `index` is out of bounds.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use std::collections::VecDeque;
+    ///
+    /// let mut buf = VecDeque::new();
+    /// assert_eq!(buf.swap_remove_back(0), None);
+    /// buf.push_back(1);
+    /// buf.push_back(2);
+    /// buf.push_back(3);
+    ///
+    /// assert_eq!(buf.swap_remove_back(0), Some(1));
+    /// assert_eq!(buf.len(), 2);
+    /// assert_eq!(buf[0], 3);
+    /// assert_eq!(buf[1], 2);
+    /// ```
+    #[stable(feature = "deque_extras_15", since = "1.5.0")]
+    pub fn swap_remove_back(&mut self, index: usize) -> Option<T> {
+        let length = self.len();
+        if length > 0 && index < length - 1 {
+            self.swap(index, length - 1);
+        } else if index >= length {
+            return None;
+        }
+        self.pop_back()
+    }
+
+    /// Removes an element from anywhere in the `VecDeque` and returns it,
+    /// replacing it with the first element.
+    ///
+    /// This does not preserve ordering, but is O(1).
+    ///
+    /// Returns `None` if `index` is out of bounds.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use std::collections::VecDeque;
+    ///
+    /// let mut buf = VecDeque::new();
+    /// assert_eq!(buf.swap_remove_front(0), None);
+    /// buf.push_back(1);
+    /// buf.push_back(2);
+    /// buf.push_back(3);
+    ///
+    /// assert_eq!(buf.swap_remove_front(2), Some(3));
+    /// assert_eq!(buf.len(), 2);
+    /// assert_eq!(buf[0], 2);
+    /// assert_eq!(buf[1], 1);
+    /// ```
+    #[stable(feature = "deque_extras_15", since = "1.5.0")]
+    pub fn swap_remove_front(&mut self, index: usize) -> Option<T> {
+        let length = self.len();
+        if length > 0 && index < length && index != 0 {
+            self.swap(index, 0);
+        } else if index >= length {
+            return None;
+        }
+        self.pop_front()
+    }
+
+    /// Inserts an element at `index` within the `VecDeque`. Whichever
+    /// end is closer to the insertion point will be moved to make room,
+    /// and all the affected elements will be moved to new positions.
+    ///
+    /// # Panics
+    ///
+    /// Panics if `index` is greater than `VecDeque`'s length
+    ///
+    /// # Examples
+    /// ```
+    /// use std::collections::VecDeque;
+    ///
+    /// let mut buf = VecDeque::new();
+    /// buf.push_back(10);
+    /// buf.push_back(12);
+    /// buf.insert(1, 11);
+    /// assert_eq!(Some(&11), buf.get(1));
+    /// ```
+    #[stable(feature = "deque_extras_15", since = "1.5.0")]
+    pub fn insert(&mut self, index: usize, value: T) {
+        assert!(index <= self.len(), "index out of bounds");
+        if self.is_full() {
+            let old_cap = self.cap();
+            self.buf.double();
+            unsafe {
+                self.handle_cap_increase(old_cap);
+            }
+            debug_assert!(!self.is_full());
+        }
+
+        // Move the least number of elements in the ring buffer and insert
+        // the given object
+        //
+        // At most len/2 - 1 elements will be moved. O(min(n, n-i))
+        //
+        // There are three main cases:
+        //  Elements are contiguous
+        //      - special case when tail is 0
+        //  Elements are discontiguous and the insert is in the tail section
+        //  Elements are discontiguous and the insert is in the head section
+        //
+        // For each of those there are two more cases:
+        //  Insert is closer to tail
+        //  Insert is closer to head
+        //
+        // Key: H - self.head
+        //      T - self.tail
+        //      o - Valid element
+        //      I - Insertion element
+        //      A - The element that should be after the insertion point
+        //      M - Indicates element was moved
+
+        let idx = self.wrap_add(self.tail, index);
+
+        let distance_to_tail = index;
+        let distance_to_head = self.len() - index;
+
+        let contiguous = self.is_contiguous();
+
+        match (contiguous,
+               distance_to_tail <= distance_to_head,
+               idx >= self.tail) {
+            (true, true, _) if index == 0 => {
+                // push_front
+                //
+                //       T
+                //       I             H
+                //      [A o o o o o o . . . . . . . . .]
+                //
+                //                       H         T
+                //      [A o o o o o o o . . . . . I]
+                //
+
+                self.tail = self.wrap_sub(self.tail, 1);
+            }
+            (true, true, _) => {
+                unsafe {
+                    // contiguous, insert closer to tail:
+                    //
+                    //             T   I         H
+                    //      [. . . o o A o o o o . . . . . .]
+                    //
+                    //           T               H
+                    //      [. . o o I A o o o o . . . . . .]
+                    //           M M
+                    //
+                    // contiguous, insert closer to tail and tail is 0:
+                    //
+                    //
+                    //       T   I         H
+                    //      [o o A o o o o . . . . . . . . .]
+                    //
+                    //                       H             T
+                    //      [o I A o o o o o . . . . . . . o]
+                    //       M                             M
+
+                    let new_tail = self.wrap_sub(self.tail, 1);
+
+                    self.copy(new_tail, self.tail, 1);
+                    // Already moved the tail, so we only copy `index - 1` elements.
+                    self.copy(self.tail, self.tail + 1, index - 1);
+
+                    self.tail = new_tail;
+                }
+            }
+            (true, false, _) => {
+                unsafe {
+                    //  contiguous, insert closer to head:
+                    //
+                    //             T       I     H
+                    //      [. . . o o o o A o o . . . . . .]
+                    //
+                    //             T               H
+                    //      [. . . o o o o I A o o . . . . .]
+                    //                       M M M
+
+                    self.copy(idx + 1, idx, self.head - idx);
+                    self.head = self.wrap_add(self.head, 1);
+                }
+            }
+            (false, true, true) => {
+                unsafe {
+                    // discontiguous, insert closer to tail, tail section:
+                    //
+                    //                   H         T   I
+                    //      [o o o o o o . . . . . o o A o o]
+                    //
+                    //                   H       T
+                    //      [o o o o o o . . . . o o I A o o]
+                    //                           M M
+
+                    self.copy(self.tail - 1, self.tail, index);
+                    self.tail -= 1;
+                }
+            }
+            (false, false, true) => {
+                unsafe {
+                    // discontiguous, insert closer to head, tail section:
+                    //
+                    //           H             T         I
+                    //      [o o . . . . . . . o o o o o A o]
+                    //
+                    //             H           T
+                    //      [o o o . . . . . . o o o o o I A]
+                    //       M M M                         M
+
+                    // copy elements up to new head
+                    self.copy(1, 0, self.head);
+
+                    // copy last element into empty spot at bottom of buffer
+                    self.copy(0, self.cap() - 1, 1);
+
+                    // move elements from idx to end forward not including ^ element
+                    self.copy(idx + 1, idx, self.cap() - 1 - idx);
+
+                    self.head += 1;
+                }
+            }
+            (false, true, false) if idx == 0 => {
+                unsafe {
+                    // discontiguous, insert is closer to tail, head section,
+                    // and is at index zero in the internal buffer:
+                    //
+                    //       I                   H     T
+                    //      [A o o o o o o o o o . . . o o o]
+                    //
+                    //                           H   T
+                    //      [A o o o o o o o o o . . o o o I]
+                    //                               M M M
+
+                    // copy elements up to new tail
+                    self.copy(self.tail - 1, self.tail, self.cap() - self.tail);
+
+                    // copy last element into empty spot at bottom of buffer
+                    self.copy(self.cap() - 1, 0, 1);
+
+                    self.tail -= 1;
+                }
+            }
+            (false, true, false) => {
+                unsafe {
+                    // discontiguous, insert closer to tail, head section:
+                    //
+                    //             I             H     T
+                    //      [o o o A o o o o o o . . . o o o]
+                    //
+                    //                           H   T
+                    //      [o o I A o o o o o o . . o o o o]
+                    //       M M                     M M M M
+
+                    // copy elements up to new tail
+                    self.copy(self.tail - 1, self.tail, self.cap() - self.tail);
+
+                    // copy last element into empty spot at bottom of buffer
+                    self.copy(self.cap() - 1, 0, 1);
+
+                    // move elements from idx-1 to end forward not including ^ element
+                    self.copy(0, 1, idx - 1);
+
+                    self.tail -= 1;
+                }
+            }
+            (false, false, false) => {
+                unsafe {
+                    // discontiguous, insert closer to head, head section:
+                    //
+                    //               I     H           T
+                    //      [o o o o A o o . . . . . . o o o]
+                    //
+                    //                     H           T
+                    //      [o o o o I A o o . . . . . o o o]
+                    //                 M M M
+
+                    self.copy(idx + 1, idx, self.head - idx);
+                    self.head += 1;
+                }
+            }
+        }
+
+        // tail might've been changed so we need to recalculate
+        let new_idx = self.wrap_add(self.tail, index);
+        unsafe {
+            self.buffer_write(new_idx, value);
+        }
+    }
+
+    /// Removes and returns the element at `index` from the `VecDeque`.
+    /// Whichever end is closer to the removal point will be moved to make
+    /// room, and all the affected elements will be moved to new positions.
+    /// Returns `None` if `index` is out of bounds.
+    ///
+    /// # Examples
+    /// ```
+    /// use std::collections::VecDeque;
+    ///
+    /// let mut buf = VecDeque::new();
+    /// buf.push_back(1);
+    /// buf.push_back(2);
+    /// buf.push_back(3);
+    ///
+    /// assert_eq!(buf.remove(1), Some(2));
+    /// assert_eq!(buf.get(1), Some(&3));
+    /// ```
+    #[stable(feature = "rust1", since = "1.0.0")]
+    pub fn remove(&mut self, index: usize) -> Option<T> {
+        if self.is_empty() || self.len() <= index {
+            return None;
+        }
+
+        // There are three main cases:
+        //  Elements are contiguous
+        //  Elements are discontiguous and the removal is in the tail section
+        //  Elements are discontiguous and the removal is in the head section
+        //      - special case when elements are technically contiguous,
+        //        but self.head = 0
+        //
+        // For each of those there are two more cases:
+        //  Insert is closer to tail
+        //  Insert is closer to head
+        //
+        // Key: H - self.head
+        //      T - self.tail
+        //      o - Valid element
+        //      x - Element marked for removal
+        //      R - Indicates element that is being removed
+        //      M - Indicates element was moved
+
+        let idx = self.wrap_add(self.tail, index);
+
+        let elem = unsafe { Some(self.buffer_read(idx)) };
+
+        let distance_to_tail = index;
+        let distance_to_head = self.len() - index;
+
+        let contiguous = self.is_contiguous();
+
+        match (contiguous,
+               distance_to_tail <= distance_to_head,
+               idx >= self.tail) {
+            (true, true, _) => {
+                unsafe {
+                    // contiguous, remove closer to tail:
+                    //
+                    //             T   R         H
+                    //      [. . . o o x o o o o . . . . . .]
+                    //
+                    //               T           H
+                    //      [. . . . o o o o o o . . . . . .]
+                    //               M M
+
+                    self.copy(self.tail + 1, self.tail, index);
+                    self.tail += 1;
+                }
+            }
+            (true, false, _) => {
+                unsafe {
+                    // contiguous, remove closer to head:
+                    //
+                    //             T       R     H
+                    //      [. . . o o o o x o o . . . . . .]
+                    //
+                    //             T           H
+                    //      [. . . o o o o o o . . . . . . .]
+                    //                     M M
+
+                    self.copy(idx, idx + 1, self.head - idx - 1);
+                    self.head -= 1;
+                }
+            }
+            (false, true, true) => {
+                unsafe {
+                    // discontiguous, remove closer to tail, tail section:
+                    //
+                    //                   H         T   R
+                    //      [o o o o o o . . . . . o o x o o]
+                    //
+                    //                   H           T
+                    //      [o o o o o o . . . . . . o o o o]
+                    //                               M M
+
+                    self.copy(self.tail + 1, self.tail, index);
+                    self.tail = self.wrap_add(self.tail, 1);
+                }
+            }
+            (false, false, false) => {
+                unsafe {
+                    // discontiguous, remove closer to head, head section:
+                    //
+                    //               R     H           T
+                    //      [o o o o x o o . . . . . . o o o]
+                    //
+                    //                   H             T
+                    //      [o o o o o o . . . . . . . o o o]
+                    //               M M
+
+                    self.copy(idx, idx + 1, self.head - idx - 1);
+                    self.head -= 1;
+                }
+            }
+            (false, false, true) => {
+                unsafe {
+                    // discontiguous, remove closer to head, tail section:
+                    //
+                    //             H           T         R
+                    //      [o o o . . . . . . o o o o o x o]
+                    //
+                    //           H             T
+                    //      [o o . . . . . . . o o o o o o o]
+                    //       M M                         M M
+                    //
+                    // or quasi-discontiguous, remove next to head, tail section:
+                    //
+                    //       H                 T         R
+                    //      [. . . . . . . . . o o o o o x o]
+                    //
+                    //                         T           H
+                    //      [. . . . . . . . . o o o o o o .]
+                    //                                   M
+
+                    // draw in elements in the tail section
+                    self.copy(idx, idx + 1, self.cap() - idx - 1);
+
+                    // Prevents underflow.
+                    if self.head != 0 {
+                        // copy first element into empty spot
+                        self.copy(self.cap() - 1, 0, 1);
+
+                        // move elements in the head section backwards
+                        self.copy(0, 1, self.head - 1);
+                    }
+
+                    self.head = self.wrap_sub(self.head, 1);
+                }
+            }
+            (false, true, false) => {
+                unsafe {
+                    // discontiguous, remove closer to tail, head section:
+                    //
+                    //           R               H     T
+                    //      [o o x o o o o o o o . . . o o o]
+                    //
+                    //                           H       T
+                    //      [o o o o o o o o o o . . . . o o]
+                    //       M M M                       M M
+
+                    // draw in elements up to idx
+                    self.copy(1, 0, idx);
+
+                    // copy last element into empty spot
+                    self.copy(0, self.cap() - 1, 1);
+
+                    // move elements from tail to end forward, excluding the last one
+                    self.copy(self.tail + 1, self.tail, self.cap() - self.tail - 1);
+
+                    self.tail = self.wrap_add(self.tail, 1);
+                }
+            }
+        }
+
+        return elem;
+    }
+
+    /// Splits the collection into two at the given index.
+    ///
+    /// Returns a newly allocated `Self`. `self` contains elements `[0, at)`,
+    /// and the returned `Self` contains elements `[at, len)`.
+    ///
+    /// Note that the capacity of `self` does not change.
+    ///
+    /// # Panics
+    ///
+    /// Panics if `at > len`
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use std::collections::VecDeque;
+    ///
+    /// let mut buf: VecDeque<_> = vec![1,2,3].into_iter().collect();
+    /// let buf2 = buf.split_off(1);
+    /// // buf = [1], buf2 = [2, 3]
+    /// assert_eq!(buf.len(), 1);
+    /// assert_eq!(buf2.len(), 2);
+    /// ```
+    #[inline]
+    #[stable(feature = "split_off", since = "1.4.0")]
+    pub fn split_off(&mut self, at: usize) -> Self {
+        let len = self.len();
+        assert!(at <= len, "`at` out of bounds");
+
+        let other_len = len - at;
+        let mut other = VecDeque::with_capacity(other_len);
+
+        unsafe {
+            let (first_half, second_half) = self.as_slices();
+
+            let first_len = first_half.len();
+            let second_len = second_half.len();
+            if at < first_len {
+                // `at` lies in the first half.
+                let amount_in_first = first_len - at;
+
+                ptr::copy_nonoverlapping(first_half.as_ptr().offset(at as isize),
+                                         other.ptr(),
+                                         amount_in_first);
+
+                // just take all of the second half.
+                ptr::copy_nonoverlapping(second_half.as_ptr(),
+                                         other.ptr().offset(amount_in_first as isize),
+                                         second_len);
+            } else {
+                // `at` lies in the second half, need to factor in the elements we skipped
+                // in the first half.
+                let offset = at - first_len;
+                let amount_in_second = second_len - offset;
+                ptr::copy_nonoverlapping(second_half.as_ptr().offset(offset as isize),
+                                         other.ptr(),
+                                         amount_in_second);
+            }
+        }
+
+        // Cleanup where the ends of the buffers are
+        self.head = self.wrap_sub(self.head, other_len);
+        other.head = other.wrap_index(other_len);
+
+        other
+    }
+
+    /// Moves all the elements of `other` into `Self`, leaving `other` empty.
+    ///
+    /// # Panics
+    ///
+    /// Panics if the new number of elements in self overflows a `usize`.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use std::collections::VecDeque;
+    ///
+    /// let mut buf: VecDeque<_> = vec![1, 2, 3].into_iter().collect();
+    /// let mut buf2: VecDeque<_> = vec![4, 5, 6].into_iter().collect();
+    /// buf.append(&mut buf2);
+    /// assert_eq!(buf.len(), 6);
+    /// assert_eq!(buf2.len(), 0);
+    /// ```
+    #[inline]
+    #[stable(feature = "append", since = "1.4.0")]
+    pub fn append(&mut self, other: &mut Self) {
+        // 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 = "1.4.0")]
+    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> {
+    /// Modifies the `VecDeque` in-place so that `len()` is equal to new_len,
+    /// either by removing excess elements or by appending copies of a value to the back.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// #![feature(deque_extras)]
+    ///
+    /// use std::collections::VecDeque;
+    ///
+    /// let mut buf = VecDeque::new();
+    /// buf.push_back(5);
+    /// buf.push_back(10);
+    /// buf.push_back(15);
+    /// buf.resize(2, 0);
+    /// buf.resize(6, 20);
+    /// for (a, b) in [5, 10, 20, 20, 20, 20].iter().zip(&buf) {
+    ///     assert_eq!(a, b);
+    /// }
+    /// ```
+    #[unstable(feature = "deque_extras",
+               reason = "matches collection reform specification; waiting on panic semantics",
+               issue = "27788")]
+    pub fn resize(&mut self, new_len: usize, value: T) {
+        let len = self.len();
+
+        if new_len > len {
+            self.extend(repeat(value).take(new_len - len))
+        } else {
+            self.truncate(new_len);
+        }
+    }
+}
+
+/// Returns the index in the underlying buffer for a given logical element index.
+#[inline]
+fn wrap_index(index: usize, size: usize) -> usize {
+    // size is always a power of 2
+    debug_assert!(size.is_power_of_two());
+    index & (size - 1)
+}
+
+/// Calculate the number of elements left to be read in the buffer
+#[inline]
+fn count(tail: usize, head: usize, size: usize) -> usize {
+    // size is always a power of 2
+    (head.wrapping_sub(tail)) & (size - 1)
+}
+
+/// `VecDeque` iterator.
+#[stable(feature = "rust1", since = "1.0.0")]
+pub struct Iter<'a, T: 'a> {
+    ring: &'a [T],
+    tail: usize,
+    head: usize,
+}
+
+// FIXME(#19839) Remove in favor of `#[derive(Clone)]`
+#[stable(feature = "rust1", since = "1.0.0")]
+impl<'a, T> Clone for Iter<'a, T> {
+    fn clone(&self) -> Iter<'a, T> {
+        Iter {
+            ring: self.ring,
+            tail: self.tail,
+            head: self.head,
+        }
+    }
+}
+
+#[stable(feature = "rust1", since = "1.0.0")]
+impl<'a, T> Iterator for Iter<'a, T> {
+    type Item = &'a T;
+
+    #[inline]
+    fn next(&mut self) -> Option<&'a T> {
+        if self.tail == self.head {
+            return None;
+        }
+        let tail = self.tail;
+        self.tail = wrap_index(self.tail.wrapping_add(1), self.ring.len());
+        unsafe { Some(self.ring.get_unchecked(tail)) }
+    }
+
+    #[inline]
+    fn size_hint(&self) -> (usize, Option<usize>) {
+        let len = count(self.tail, self.head, self.ring.len());
+        (len, Some(len))
+    }
+}
+
+#[stable(feature = "rust1", since = "1.0.0")]
+impl<'a, T> DoubleEndedIterator for Iter<'a, T> {
+    #[inline]
+    fn next_back(&mut self) -> Option<&'a T> {
+        if self.tail == self.head {
+            return None;
+        }
+        self.head = wrap_index(self.head.wrapping_sub(1), self.ring.len());
+        unsafe { Some(self.ring.get_unchecked(self.head)) }
+    }
+}
+
+#[stable(feature = "rust1", since = "1.0.0")]
+impl<'a, T> ExactSizeIterator for Iter<'a, T> {}
+
+/// `VecDeque` mutable iterator.
+#[stable(feature = "rust1", since = "1.0.0")]
+pub struct IterMut<'a, T: 'a> {
+    ring: &'a mut [T],
+    tail: usize,
+    head: usize,
+}
+
+#[stable(feature = "rust1", since = "1.0.0")]
+impl<'a, T> Iterator for IterMut<'a, T> {
+    type Item = &'a mut T;
+
+    #[inline]
+    fn next(&mut self) -> Option<&'a mut T> {
+        if self.tail == self.head {
+            return None;
+        }
+        let tail = self.tail;
+        self.tail = wrap_index(self.tail.wrapping_add(1), self.ring.len());
+
+        unsafe {
+            let elem = self.ring.get_unchecked_mut(tail);
+            Some(&mut *(elem as *mut _))
+        }
+    }
+
+    #[inline]
+    fn size_hint(&self) -> (usize, Option<usize>) {
+        let len = count(self.tail, self.head, self.ring.len());
+        (len, Some(len))
+    }
+}
+
+#[stable(feature = "rust1", since = "1.0.0")]
+impl<'a, T> DoubleEndedIterator for IterMut<'a, T> {
+    #[inline]
+    fn next_back(&mut self) -> Option<&'a mut T> {
+        if self.tail == self.head {
+            return None;
+        }
+        self.head = wrap_index(self.head.wrapping_sub(1), self.ring.len());
+
+        unsafe {
+            let elem = self.ring.get_unchecked_mut(self.head);
+            Some(&mut *(elem as *mut _))
+        }
+    }
+}
+
+#[stable(feature = "rust1", since = "1.0.0")]
+impl<'a, T> ExactSizeIterator for IterMut<'a, T> {}
+
+/// A by-value VecDeque iterator
+#[derive(Clone)]
+#[stable(feature = "rust1", since = "1.0.0")]
+pub struct IntoIter<T> {
+    inner: VecDeque<T>,
+}
+
+#[stable(feature = "rust1", since = "1.0.0")]
+impl<T> Iterator for IntoIter<T> {
+    type Item = T;
+
+    #[inline]
+    fn next(&mut self) -> Option<T> {
+        self.inner.pop_front()
+    }
+
+    #[inline]
+    fn size_hint(&self) -> (usize, Option<usize>) {
+        let len = self.inner.len();
+        (len, Some(len))
+    }
+}
+
+#[stable(feature = "rust1", since = "1.0.0")]
+impl<T> DoubleEndedIterator for IntoIter<T> {
+    #[inline]
+    fn next_back(&mut self) -> Option<T> {
+        self.inner.pop_back()
+    }
+}
+
+#[stable(feature = "rust1", since = "1.0.0")]
+impl<T> ExactSizeIterator for IntoIter<T> {}
+
+/// A draining VecDeque iterator
+#[stable(feature = "drain", since = "1.6.0")]
+pub struct Drain<'a, T: 'a> {
+    after_tail: usize,
+    after_head: usize,
+    iter: Iter<'a, T>,
+    deque: *mut VecDeque<T>,
+}
+
+#[stable(feature = "drain", since = "1.6.0")]
+unsafe impl<'a, T: Sync> Sync for Drain<'a, T> {}
+#[stable(feature = "drain", since = "1.6.0")]
+unsafe impl<'a, T: Send> Send for Drain<'a, T> {}
+
+#[stable(feature = "rust1", since = "1.0.0")]
+impl<'a, T: 'a> Drop for Drain<'a, T> {
+    fn drop(&mut self) {
+        for _ in self.by_ref() {}
+
+        let source_deque = unsafe { &mut *self.deque };
+
+        // T = source_deque_tail; H = source_deque_head; t = drain_tail; h = drain_head
+        //
+        //        T   t   h   H
+        // [. . . o o x x o o . . .]
+        //
+        let orig_tail = source_deque.tail;
+        let drain_tail = source_deque.head;
+        let drain_head = self.after_tail;
+        let orig_head = self.after_head;
+
+        let tail_len = count(orig_tail, drain_tail, source_deque.cap());
+        let head_len = count(drain_head, orig_head, source_deque.cap());
+
+        // Restore the original head value
+        source_deque.head = orig_head;
+
+        match (tail_len, head_len) {
+            (0, 0) => {
+                source_deque.head = 0;
+                source_deque.tail = 0;
+            }
+            (0, _) => {
+                source_deque.tail = drain_head;
+            }
+            (_, 0) => {
+                source_deque.head = drain_tail;
+            }
+            _ => {
+                unsafe {
+                    if tail_len <= head_len {
+                        source_deque.tail = source_deque.wrap_sub(drain_head, tail_len);
+                        source_deque.wrap_copy(source_deque.tail, orig_tail, tail_len);
+                    } else {
+                        source_deque.head = source_deque.wrap_add(drain_tail, head_len);
+                        source_deque.wrap_copy(drain_tail, drain_head, head_len);
+                    }
+                }
+            }
+        }
+    }
+}
+
+#[stable(feature = "rust1", since = "1.0.0")]
+impl<'a, T: 'a> Iterator for Drain<'a, T> {
+    type Item = T;
+
+    #[inline]
+    fn next(&mut self) -> Option<T> {
+        self.iter.next().map(|elt| unsafe { ptr::read(elt) })
+    }
+
+    #[inline]
+    fn size_hint(&self) -> (usize, Option<usize>) {
+        self.iter.size_hint()
+    }
+}
+
+#[stable(feature = "rust1", since = "1.0.0")]
+impl<'a, T: 'a> DoubleEndedIterator for Drain<'a, T> {
+    #[inline]
+    fn next_back(&mut self) -> Option<T> {
+        self.iter.next_back().map(|elt| unsafe { ptr::read(elt) })
+    }
+}
+
+#[stable(feature = "rust1", since = "1.0.0")]
+impl<'a, T: 'a> ExactSizeIterator for Drain<'a, T> {}
+
+#[stable(feature = "rust1", since = "1.0.0")]
+impl<A: PartialEq> PartialEq for VecDeque<A> {
+    fn eq(&self, other: &VecDeque<A>) -> bool {
+        if self.len() != other.len() {
+            return false;
+        }
+        let (sa, sb) = self.as_slices();
+        let (oa, ob) = other.as_slices();
+        if sa.len() == oa.len() {
+            sa == oa && sb == ob
+        } else if sa.len() < oa.len() {
+            // Always divisible in three sections, for example:
+            // self:  [a b c|d e f]
+            // other: [0 1 2 3|4 5]
+            // front = 3, mid = 1,
+            // [a b c] == [0 1 2] && [d] == [3] && [e f] == [4 5]
+            let front = sa.len();
+            let mid = oa.len() - front;
+
+            let (oa_front, oa_mid) = oa.split_at(front);
+            let (sb_mid, sb_back) = sb.split_at(mid);
+            debug_assert_eq!(sa.len(), oa_front.len());
+            debug_assert_eq!(sb_mid.len(), oa_mid.len());
+            debug_assert_eq!(sb_back.len(), ob.len());
+            sa == oa_front && sb_mid == oa_mid && sb_back == ob
+        } else {
+            let front = oa.len();
+            let mid = sa.len() - front;
+
+            let (sa_front, sa_mid) = sa.split_at(front);
+            let (ob_mid, ob_back) = ob.split_at(mid);
+            debug_assert_eq!(sa_front.len(), oa.len());
+            debug_assert_eq!(sa_mid.len(), ob_mid.len());
+            debug_assert_eq!(sb.len(), ob_back.len());
+            sa_front == oa && sa_mid == ob_mid && sb == ob_back
+        }
+    }
+}
+
+#[stable(feature = "rust1", since = "1.0.0")]
+impl<A: Eq> Eq for VecDeque<A> {}
+
+#[stable(feature = "rust1", since = "1.0.0")]
+impl<A: PartialOrd> PartialOrd for VecDeque<A> {
+    fn partial_cmp(&self, other: &VecDeque<A>) -> Option<Ordering> {
+        self.iter().partial_cmp(other.iter())
+    }
+}
+
+#[stable(feature = "rust1", since = "1.0.0")]
+impl<A: Ord> Ord for VecDeque<A> {
+    #[inline]
+    fn cmp(&self, other: &VecDeque<A>) -> Ordering {
+        self.iter().cmp(other.iter())
+    }
+}
+
+#[stable(feature = "rust1", since = "1.0.0")]
+impl<A: Hash> Hash for VecDeque<A> {
+    fn hash<H: Hasher>(&self, state: &mut H) {
+        self.len().hash(state);
+        let (a, b) = self.as_slices();
+        Hash::hash_slice(a, state);
+        Hash::hash_slice(b, state);
+    }
+}
+
+#[stable(feature = "rust1", since = "1.0.0")]
+impl<A> Index<usize> for VecDeque<A> {
+    type Output = A;
+
+    #[inline]
+    fn index(&self, index: usize) -> &A {
+        self.get(index).expect("Out of bounds access")
+    }
+}
+
+#[stable(feature = "rust1", since = "1.0.0")]
+impl<A> IndexMut<usize> for VecDeque<A> {
+    #[inline]
+    fn index_mut(&mut self, index: usize) -> &mut A {
+        self.get_mut(index).expect("Out of bounds access")
+    }
+}
+
+#[stable(feature = "rust1", since = "1.0.0")]
+impl<A> FromIterator<A> for VecDeque<A> {
+    fn from_iter<T: IntoIterator<Item = A>>(iter: T) -> VecDeque<A> {
+        let iterator = iter.into_iter();
+        let (lower, _) = iterator.size_hint();
+        let mut deq = VecDeque::with_capacity(lower);
+        deq.extend(iterator);
+        deq
+    }
+}
+
+#[stable(feature = "rust1", since = "1.0.0")]
+impl<T> IntoIterator for VecDeque<T> {
+    type Item = T;
+    type IntoIter = IntoIter<T>;
+
+    /// Consumes the list into a front-to-back iterator yielding elements by
+    /// value.
+    fn into_iter(self) -> IntoIter<T> {
+        IntoIter { inner: self }
+    }
+}
+
+#[stable(feature = "rust1", since = "1.0.0")]
+impl<'a, T> IntoIterator for &'a VecDeque<T> {
+    type Item = &'a T;
+    type IntoIter = Iter<'a, T>;
+
+    fn into_iter(self) -> Iter<'a, T> {
+        self.iter()
+    }
+}
+
+#[stable(feature = "rust1", since = "1.0.0")]
+impl<'a, T> IntoIterator for &'a mut VecDeque<T> {
+    type Item = &'a mut T;
+    type IntoIter = IterMut<'a, T>;
+
+    fn into_iter(mut self) -> IterMut<'a, T> {
+        self.iter_mut()
+    }
+}
+
+#[stable(feature = "rust1", since = "1.0.0")]
+impl<A> Extend<A> for VecDeque<A> {
+    fn extend<T: IntoIterator<Item = A>>(&mut self, iter: T) {
+        for elt in iter {
+            self.push_back(elt);
+        }
+    }
+}
+
+#[stable(feature = "extend_ref", since = "1.2.0")]
+impl<'a, T: 'a + Copy> Extend<&'a T> for VecDeque<T> {
+    fn extend<I: IntoIterator<Item = &'a T>>(&mut self, iter: I) {
+        self.extend(iter.into_iter().cloned());
+    }
+}
+
+#[stable(feature = "rust1", since = "1.0.0")]
+impl<T: fmt::Debug> fmt::Debug for VecDeque<T> {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        f.debug_list().entries(self).finish()
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use core::iter::Iterator;
+    use core::option::Option::Some;
+
+    use test;
+
+    use super::VecDeque;
+
+    #[bench]
+    fn bench_push_back_100(b: &mut test::Bencher) {
+        let mut deq = VecDeque::with_capacity(101);
+        b.iter(|| {
+            for i in 0..100 {
+                deq.push_back(i);
+            }
+            deq.head = 0;
+            deq.tail = 0;
+        })
+    }
+
+    #[bench]
+    fn bench_push_front_100(b: &mut test::Bencher) {
+        let mut deq = VecDeque::with_capacity(101);
+        b.iter(|| {
+            for i in 0..100 {
+                deq.push_front(i);
+            }
+            deq.head = 0;
+            deq.tail = 0;
+        })
+    }
+
+    #[bench]
+    fn bench_pop_back_100(b: &mut test::Bencher) {
+        let mut deq = VecDeque::<i32>::with_capacity(101);
+
+        b.iter(|| {
+            deq.head = 100;
+            deq.tail = 0;
+            while !deq.is_empty() {
+                test::black_box(deq.pop_back());
+            }
+        })
+    }
+
+    #[bench]
+    fn bench_pop_front_100(b: &mut test::Bencher) {
+        let mut deq = VecDeque::<i32>::with_capacity(101);
+
+        b.iter(|| {
+            deq.head = 100;
+            deq.tail = 0;
+            while !deq.is_empty() {
+                test::black_box(deq.pop_front());
+            }
+        })
+    }
+
+    #[test]
+    fn test_swap_front_back_remove() {
+        fn test(back: bool) {
+            // This test checks that every single combination of tail position and length is tested.
+            // Capacity 15 should be large enough to cover every case.
+            let mut tester = VecDeque::with_capacity(15);
+            let usable_cap = tester.capacity();
+            let final_len = usable_cap / 2;
+
+            for len in 0..final_len {
+                let expected = if back {
+                    (0..len).collect()
+                } else {
+                    (0..len).rev().collect()
+                };
+                for tail_pos in 0..usable_cap {
+                    tester.tail = tail_pos;
+                    tester.head = tail_pos;
+                    if back {
+                        for i in 0..len * 2 {
+                            tester.push_front(i);
+                        }
+                        for i in 0..len {
+                            assert_eq!(tester.swap_remove_back(i), Some(len * 2 - 1 - i));
+                        }
+                    } else {
+                        for i in 0..len * 2 {
+                            tester.push_back(i);
+                        }
+                        for i in 0..len {
+                            let idx = tester.len() - 1 - i;
+                            assert_eq!(tester.swap_remove_front(idx), Some(len * 2 - 1 - i));
+                        }
+                    }
+                    assert!(tester.tail < tester.cap());
+                    assert!(tester.head < tester.cap());
+                    assert_eq!(tester, expected);
+                }
+            }
+        }
+        test(true);
+        test(false);
+    }
+
+    #[test]
+    fn test_insert() {
+        // This test checks that every single combination of tail position, length, and
+        // insertion position is tested. Capacity 15 should be large enough to cover every case.
+
+        let mut tester = VecDeque::with_capacity(15);
+        // can't guarantee we got 15, so have to get what we got.
+        // 15 would be great, but we will definitely get 2^k - 1, for k >= 4, or else
+        // this test isn't covering what it wants to
+        let cap = tester.capacity();
+
+
+        // len is the length *after* insertion
+        for len in 1..cap {
+            // 0, 1, 2, .., len - 1
+            let expected = (0..).take(len).collect();
+            for tail_pos in 0..cap {
+                for to_insert in 0..len {
+                    tester.tail = tail_pos;
+                    tester.head = tail_pos;
+                    for i in 0..len {
+                        if i != to_insert {
+                            tester.push_back(i);
+                        }
+                    }
+                    tester.insert(to_insert, to_insert);
+                    assert!(tester.tail < tester.cap());
+                    assert!(tester.head < tester.cap());
+                    assert_eq!(tester, expected);
+                }
+            }
+        }
+    }
+
+    #[test]
+    fn test_remove() {
+        // This test checks that every single combination of tail position, length, and
+        // removal position is tested. Capacity 15 should be large enough to cover every case.
+
+        let mut tester = VecDeque::with_capacity(15);
+        // can't guarantee we got 15, so have to get what we got.
+        // 15 would be great, but we will definitely get 2^k - 1, for k >= 4, or else
+        // this test isn't covering what it wants to
+        let cap = tester.capacity();
+
+        // len is the length *after* removal
+        for len in 0..cap - 1 {
+            // 0, 1, 2, .., len - 1
+            let expected = (0..).take(len).collect();
+            for tail_pos in 0..cap {
+                for to_remove in 0..len + 1 {
+                    tester.tail = tail_pos;
+                    tester.head = tail_pos;
+                    for i in 0..len {
+                        if i == to_remove {
+                            tester.push_back(1234);
+                        }
+                        tester.push_back(i);
+                    }
+                    if to_remove == len {
+                        tester.push_back(1234);
+                    }
+                    tester.remove(to_remove);
+                    assert!(tester.tail < tester.cap());
+                    assert!(tester.head < tester.cap());
+                    assert_eq!(tester, expected);
+                }
+            }
+        }
+    }
+
+    #[test]
+    fn test_drain() {
+        let mut tester: VecDeque<usize> = VecDeque::with_capacity(7);
+
+        let cap = tester.capacity();
+        for len in 0..cap + 1 {
+            for tail in 0..cap + 1 {
+                for drain_start in 0..len + 1 {
+                    for drain_end in drain_start..len + 1 {
+                        tester.tail = tail;
+                        tester.head = tail;
+                        for i in 0..len {
+                            tester.push_back(i);
+                        }
+
+                        // Check that we drain the correct values
+                        let drained: VecDeque<_> = tester.drain(drain_start..drain_end).collect();
+                        let drained_expected: VecDeque<_> = (drain_start..drain_end).collect();
+                        assert_eq!(drained, drained_expected);
+
+                        // We shouldn't have changed the capacity or made the
+                        // head or tail out of bounds
+                        assert_eq!(tester.capacity(), cap);
+                        assert!(tester.tail < tester.cap());
+                        assert!(tester.head < tester.cap());
+
+                        // We should see the correct values in the VecDeque
+                        let expected: VecDeque<_> = (0..drain_start)
+                                                        .chain(drain_end..len)
+                                                        .collect();
+                        assert_eq!(expected, tester);
+                    }
+                }
+            }
+        }
+    }
+
+    #[test]
+    fn test_shrink_to_fit() {
+        // This test checks that every single combination of head and tail position,
+        // is tested. Capacity 15 should be large enough to cover every case.
+
+        let mut tester = VecDeque::with_capacity(15);
+        // can't guarantee we got 15, so have to get what we got.
+        // 15 would be great, but we will definitely get 2^k - 1, for k >= 4, or else
+        // this test isn't covering what it wants to
+        let cap = tester.capacity();
+        tester.reserve(63);
+        let max_cap = tester.capacity();
+
+        for len in 0..cap + 1 {
+            // 0, 1, 2, .., len - 1
+            let expected = (0..).take(len).collect();
+            for tail_pos in 0..max_cap + 1 {
+                tester.tail = tail_pos;
+                tester.head = tail_pos;
+                tester.reserve(63);
+                for i in 0..len {
+                    tester.push_back(i);
+                }
+                tester.shrink_to_fit();
+                assert!(tester.capacity() <= cap);
+                assert!(tester.tail < tester.cap());
+                assert!(tester.head < tester.cap());
+                assert_eq!(tester, expected);
+            }
+        }
+    }
+
+    #[test]
+    fn test_split_off() {
+        // This test checks that every single combination of tail position, length, and
+        // split position is tested. Capacity 15 should be large enough to cover every case.
+
+        let mut tester = VecDeque::with_capacity(15);
+        // can't guarantee we got 15, so have to get what we got.
+        // 15 would be great, but we will definitely get 2^k - 1, for k >= 4, or else
+        // this test isn't covering what it wants to
+        let cap = tester.capacity();
+
+        // len is the length *before* splitting
+        for len in 0..cap {
+            // index to split at
+            for at in 0..len + 1 {
+                // 0, 1, 2, .., at - 1 (may be empty)
+                let expected_self = (0..).take(at).collect();
+                // at, at + 1, .., len - 1 (may be empty)
+                let expected_other = (at..).take(len - at).collect();
+
+                for tail_pos in 0..cap {
+                    tester.tail = tail_pos;
+                    tester.head = tail_pos;
+                    for i in 0..len {
+                        tester.push_back(i);
+                    }
+                    let result = tester.split_off(at);
+                    assert!(tester.tail < tester.cap());
+                    assert!(tester.head < tester.cap());
+                    assert!(result.tail < result.cap());
+                    assert!(result.head < result.cap());
+                    assert_eq!(tester, expected_self);
+                    assert_eq!(result, expected_other);
+                }
+            }
+        }
+    }
+}