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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.

    //! A double-ended queue implemented with a growable 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, FusedIterator};
    use core::mem;
    use core::ops::{Index, IndexMut, Place, Placer, InPlace};
    use core::ptr;
    use core::ptr::Shared;
    use core::slice;

    use core::hash::{Hash, Hasher};
    use core::cmp;

    use raw_vec::RawVec;

    use super::range::RangeArgument;
    use Bound::{Excluded, Included, Unbounded};
    use super::vec::Vec;

    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

    /// A double-ended queue implemented with a growable ring buffer.
    ///
    /// 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.
    ///
    /// [`push_back`]: #method.push_back
    /// [`pop_front`]: #method.pop_front
    /// [`extend`]: #method.extend
    /// [`append`]: #method.append
    #[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")]
    unsafe impl<#[may_dangle] T> Drop for VecDeque<T> {
        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> {
        /// Creates an empty `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 full 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`.
        ///
        /// # Examples
        ///
        /// ```
        /// use std::collections::VecDeque;
        ///
        /// let vector: VecDeque<u32> = VecDeque::new();
        /// ```
        #[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.
        ///
        /// # Examples
        ///
        /// ```
        /// use std::collections::VecDeque;
        ///
        /// let vector: VecDeque<u32> = VecDeque::with_capacity(10);
        /// ```
        #[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.
        ///
        /// Element at index 0 is the front of the queue.
        ///
        /// # 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.
        ///
        /// Element at index 0 is the front of the queue.
        ///
        /// # 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.
        ///
        /// Element at index 0 is the front of the queue.
        ///
        /// # 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, [3, 4, 5]);
        /// buf.swap(0, 2);
        /// assert_eq!(buf, [5, 4, 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);
        /// ```
        ///
        /// [`reserve`]: #method.reserve
        #[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 the `VecDeque`, dropping excess elements from the back.
        ///
        /// If `len` is greater than the `VecDeque`'s current length, this has no
        /// effect.
        ///
        /// # Examples
        ///
        /// ```
        /// use std::collections::VecDeque;
        ///
        /// let mut buf = VecDeque::new();
        /// buf.push_back(5);
        /// buf.push_back(10);
        /// buf.push_back(15);
        /// assert_eq!(buf, [5, 10, 15]);
        /// buf.truncate(1);
        /// assert_eq!(buf, [5]);
        /// ```
        #[stable(feature = "deque_extras", since = "1.16.0")]
        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`.
        ///
        /// # Examples
        ///
        /// ```
        /// use std::collections::VecDeque;
        ///
        /// let mut vector = VecDeque::new();
        ///
        /// vector.push_back(0);
        /// vector.push_back(1);
        /// vector.push_back(2);
        ///
        /// assert_eq!(vector.as_slices(), (&[0, 1, 2][..], &[][..]));
        ///
        /// vector.push_front(10);
        /// vector.push_front(9);
        ///
        /// assert_eq!(vector.as_slices(), (&[9, 10][..], &[0, 1, 2][..]));
        /// ```
        #[inline]
        #[stable(feature = "deque_extras_15", since = "1.5.0")]
        pub fn as_slices(&self) -> (&[T], &[T]) {
            unsafe {
                let buf = self.buffer_as_slice();
                RingSlices::ring_slices(buf, self.head, self.tail)
            }
        }

        /// Returns a pair of slices which contain, in order, the contents of the
        /// `VecDeque`.
        ///
        /// # Examples
        ///
        /// ```
        /// use std::collections::VecDeque;
        ///
        /// let mut vector = VecDeque::new();
        ///
        /// vector.push_back(0);
        /// vector.push_back(1);
        ///
        /// vector.push_front(10);
        /// vector.push_front(9);
        ///
        /// vector.as_mut_slices().0[0] = 42;
        /// vector.as_mut_slices().1[0] = 24;
        /// assert_eq!(vector.as_slices(), (&[42, 10][..], &[24, 1][..]));
        /// ```
        #[inline]
        #[stable(feature = "deque_extras_15", since = "1.5.0")]
        pub fn as_mut_slices(&mut self) -> (&mut [T], &mut [T]) {
            unsafe {
                let head = self.head;
                let tail = self.tail;
                let buf = self.buffer_as_mut_slice();
                RingSlices::ring_slices(buf, head, tail)
            }
        }

        /// 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 `VecDeque` is empty.
        ///
        /// # 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.tail == self.head
        }

        /// 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();
        /// let drained = v.drain(2..).collect::<VecDeque<_>>();
        /// assert_eq!(drained, [3]);
        /// assert_eq!(v, [1, 2]);
        ///
        /// // 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 = match range.start() {
                Included(&n) => n,
                Excluded(&n) => n + 1,
                Unbounded    => 0,
            };
            let end = match range.end() {
                Included(&n) => n + 1,
                Excluded(&n) => n,
                Unbounded    => 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: unsafe { Shared::new(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(..);
        }

        /// Returns `true` if the `VecDeque` contains an element equal to the
        /// given value.
        ///
        /// # Examples
        ///
        /// ```
        /// use std::collections::VecDeque;
        ///
        /// let mut vector: VecDeque<u32> = VecDeque::new();
        ///
        /// vector.push_back(0);
        /// vector.push_back(1);
        ///
        /// assert_eq!(vector.contains(&1), true);
        /// assert_eq!(vector.contains(&10), false);
        /// ```
        #[stable(feature = "vec_deque_contains", since = "1.12.0")]
        pub fn contains(&self, x: &T) -> bool
            where T: PartialEq<T>
        {
            let (a, b) = self.as_slices();
            a.contains(x) || b.contains(x)
        }

        /// Provides a reference to the front element, or `None` if the `VecDeque` 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
        /// `VecDeque` 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 `VecDeque` 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
        /// `VecDeque` 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 `VecDeque` 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)) }
            }
        }

        /// Prepends an element to the `VecDeque`.
        ///
        /// # 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) {
            self.grow_if_necessary();

            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 the `VecDeque`.
        ///
        /// # 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) {
            self.grow_if_necessary();

            let head = self.head;
            self.head = self.wrap_add(self.head, 1);
            unsafe { self.buffer_write(head, value) }
        }

        /// Removes the last element from the `VecDeque` 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.
        ///
        /// Element at index 0 is the front of the queue.
        ///
        /// # 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, [1, 2, 3]);
        ///
        /// assert_eq!(buf.swap_remove_back(0), Some(1));
        /// assert_eq!(buf, [3, 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.
        ///
        /// Element at index 0 is the front of the queue.
        ///
        /// # 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, [1, 2, 3]);
        ///
        /// assert_eq!(buf.swap_remove_front(2), Some(3));
        /// assert_eq!(buf, [2, 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`, shifting all elements with indices
        /// greater than or equal to `index` towards the back.
        ///
        /// Element at index 0 is the front of the queue.
        ///
        /// # Panics
        ///
        /// Panics if `index` is greater than `VecDeque`'s length
        ///
        /// # Examples
        ///
        /// ```
        /// use std::collections::VecDeque;
        ///
        /// let mut vec_deque = VecDeque::new();
        /// vec_deque.push_back('a');
        /// vec_deque.push_back('b');
        /// vec_deque.push_back('c');
        /// assert_eq!(vec_deque, &['a', 'b', 'c']);
        ///
        /// vec_deque.insert(1, 'd');
        /// assert_eq!(vec_deque, &['a', 'd', 'b', 'c']);
        /// ```
        #[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");
            self.grow_if_necessary();

            // 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.
        ///
        /// Element at index 0 is the front of the queue.
        ///
        /// # 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, [1, 2, 3]);
        ///
        /// assert_eq!(buf.remove(1), Some(2));
        /// assert_eq!(buf, [1, 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.
        ///
        /// Element at index 0 is the front of the queue.
        ///
        /// # 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);
        /// assert_eq!(buf, [1]);
        /// assert_eq!(buf2, [2, 3]);
        /// ```
        #[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].into_iter().collect();
        /// let mut buf2: VecDeque<_> = vec![3, 4].into_iter().collect();
        /// buf.append(&mut buf2);
        /// assert_eq!(buf, [1, 2, 3, 4]);
        /// assert_eq!(buf2, []);
        /// ```
        #[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);
        /// assert_eq!(buf, [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);
            }
        }

        // This may panic or abort
        #[inline]
        fn grow_if_necessary(&mut self) {
            if self.is_full() {
                let old_cap = self.cap();
                self.buf.double();
                unsafe {
                    self.handle_cap_increase(old_cap);
                }
                debug_assert!(!self.is_full());
            }
        }

        /// Returns a place for insertion at the back of the `VecDeque`.
        ///
        /// Using this method with placement syntax is equivalent to [`push_back`](#method.push_back),
        /// but may be more efficient.
        ///
        /// # Examples
        ///
        /// ```
        /// #![feature(collection_placement)]
        /// #![feature(placement_in_syntax)]
        ///
        /// use std::collections::VecDeque;
        ///
        /// let mut buf = VecDeque::new();
        /// buf.place_back() <- 3;
        /// buf.place_back() <- 4;
        /// assert_eq!(&buf, &[3, 4]);
        /// ```
        #[unstable(feature = "collection_placement",
                   reason = "placement protocol is subject to change",
                   issue = "30172")]
        pub fn place_back(&mut self) -> PlaceBack<T> {
            PlaceBack { vec_deque: self }
        }

        /// Returns a place for insertion at the front of the `VecDeque`.
        ///
        /// Using this method with placement syntax is equivalent to [`push_front`](#method.push_front),
        /// but may be more efficient.
        ///
        /// # Examples
        ///
        /// ```
        /// #![feature(collection_placement)]
        /// #![feature(placement_in_syntax)]
        ///
        /// use std::collections::VecDeque;
        ///
        /// let mut buf = VecDeque::new();
        /// buf.place_front() <- 3;
        /// buf.place_front() <- 4;
        /// assert_eq!(&buf, &[4, 3]);
        /// ```
        #[unstable(feature = "collection_placement",
                   reason = "placement protocol is subject to change",
                   issue = "30172")]
        pub fn place_front(&mut self) -> PlaceFront<T> {
            PlaceFront { vec_deque: self }
        }
    }

    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 clones of `value` to the back.
        ///
        /// # Examples
        ///
        /// ```
        /// use std::collections::VecDeque;
        ///
        /// let mut buf = VecDeque::new();
        /// buf.push_back(5);
        /// buf.push_back(10);
        /// buf.push_back(15);
        /// assert_eq!(buf, [5, 10, 15]);
        ///
        /// buf.resize(2, 0);
        /// assert_eq!(buf, [5, 10]);
        ///
        /// buf.resize(5, 20);
        /// assert_eq!(buf, [5, 10, 20, 20, 20]);
        /// ```
        #[stable(feature = "deque_extras", since = "1.16.0")]
        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)
    }

    /// Returns the two slices that cover the `VecDeque`'s valid range
    trait RingSlices: Sized {
        fn slice(self, from: usize, to: usize) -> Self;
        fn split_at(self, i: usize) -> (Self, Self);

        fn ring_slices(buf: Self, head: usize, tail: usize) -> (Self, Self) {
            let contiguous = tail <= head;
            if contiguous {
                let (empty, buf) = buf.split_at(0);
                (buf.slice(tail, head), empty)
            } else {
                let (mid, right) = buf.split_at(tail);
                let (left, _) = mid.split_at(head);
                (right, left)
            }
        }
    }

    impl<'a, T> RingSlices for &'a [T] {
        fn slice(self, from: usize, to: usize) -> Self {
            &self[from..to]
        }
        fn split_at(self, i: usize) -> (Self, Self) {
            (*self).split_at(i)
        }
    }

    impl<'a, T> RingSlices for &'a mut [T] {
        fn slice(self, from: usize, to: usize) -> Self {
            &mut self[from..to]
        }
        fn split_at(self, i: usize) -> (Self, Self) {
            (*self).split_at_mut(i)
        }
    }

    /// 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)
    }

    /// An iterator over the elements of a `VecDeque`.
    ///
    /// This `struct` is created by the [`iter`] method on [`VecDeque`]. See its
    /// documentation for more.
    ///
    /// [`iter`]: struct.VecDeque.html#method.iter
    /// [`VecDeque`]: struct.VecDeque.html
    #[stable(feature = "rust1", since = "1.0.0")]
    pub struct Iter<'a, T: 'a> {
        ring: &'a [T],
        tail: usize,
        head: usize,
    }

    #[stable(feature = "collection_debug", since = "1.17.0")]
    impl<'a, T: 'a + fmt::Debug> fmt::Debug for Iter<'a, T> {
        fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
            f.debug_tuple("Iter")
             .field(&self.ring)
             .field(&self.tail)
             .field(&self.head)
             .finish()
        }
    }

    // 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))
        }

        fn fold<Acc, F>(self, mut accum: Acc, mut f: F) -> Acc
            where F: FnMut(Acc, Self::Item) -> Acc
        {
            let (front, back) = RingSlices::ring_slices(self.ring, self.head, self.tail);
            accum = front.iter().fold(accum, &mut f);
            back.iter().fold(accum, &mut f)
        }
    }

    #[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> {
        fn is_empty(&self) -> bool {
            self.head == self.tail
        }
    }

    #[unstable(feature = "fused", issue = "35602")]
    impl<'a, T> FusedIterator for Iter<'a, T> {}

    /// A mutable iterator over the elements of a `VecDeque`.
    ///
    /// This `struct` is created by the [`iter_mut`] method on [`VecDeque`]. See its
    /// documentation for more.
    ///
    /// [`iter_mut`]: struct.VecDeque.html#method.iter_mut
    /// [`VecDeque`]: struct.VecDeque.html
    #[stable(feature = "rust1", since = "1.0.0")]
    pub struct IterMut<'a, T: 'a> {
        ring: &'a mut [T],
        tail: usize,
        head: usize,
    }

    #[stable(feature = "collection_debug", since = "1.17.0")]
    impl<'a, T: 'a + fmt::Debug> fmt::Debug for IterMut<'a, T> {
        fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
            f.debug_tuple("IterMut")
             .field(&self.ring)
             .field(&self.tail)
             .field(&self.head)
             .finish()
        }
    }

    #[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))
        }

        fn fold<Acc, F>(self, mut accum: Acc, mut f: F) -> Acc
            where F: FnMut(Acc, Self::Item) -> Acc
        {
            let (front, back) = RingSlices::ring_slices(self.ring, self.head, self.tail);
            accum = front.iter_mut().fold(accum, &mut f);
            back.iter_mut().fold(accum, &mut f)
        }
    }

    #[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> {
        fn is_empty(&self) -> bool {
            self.head == self.tail
        }
    }

    #[unstable(feature = "fused", issue = "35602")]
    impl<'a, T> FusedIterator for IterMut<'a, T> {}

    /// An owning iterator over the elements of a `VecDeque`.
    ///
    /// This `struct` is created by the [`into_iter`] method on [`VecDeque`][`VecDeque`]
    /// (provided by the `IntoIterator` trait). See its documentation for more.
    ///
    /// [`into_iter`]: struct.VecDeque.html#method.into_iter
    /// [`VecDeque`]: struct.VecDeque.html
    #[derive(Clone)]
    #[stable(feature = "rust1", since = "1.0.0")]
    pub struct IntoIter<T> {
        inner: VecDeque<T>,
    }

    #[stable(feature = "collection_debug", since = "1.17.0")]
    impl<T: fmt::Debug> fmt::Debug for IntoIter<T> {
        fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
            f.debug_tuple("IntoIter")
             .field(&self.inner)
             .finish()
        }
    }

    #[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> {
        fn is_empty(&self) -> bool {
            self.inner.is_empty()
        }
    }

    #[unstable(feature = "fused", issue = "35602")]
    impl<T> FusedIterator for IntoIter<T> {}

    /// A draining iterator over the elements of a `VecDeque`.
    ///
    /// This `struct` is created by the [`drain`] method on [`VecDeque`]. See its
    /// documentation for more.
    ///
    /// [`drain`]: struct.VecDeque.html#method.drain
    /// [`VecDeque`]: struct.VecDeque.html
    #[stable(feature = "drain", since = "1.6.0")]
    pub struct Drain<'a, T: 'a> {
        after_tail: usize,
        after_head: usize,
        iter: Iter<'a, T>,
        deque: Shared<VecDeque<T>>,
    }

    #[stable(feature = "collection_debug", since = "1.17.0")]
    impl<'a, T: 'a + fmt::Debug> fmt::Debug for Drain<'a, T> {
        fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
            f.debug_tuple("Drain")
             .field(&self.after_tail)
             .field(&self.after_head)
             .field(&self.iter)
             .finish()
        }
    }

    #[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 = "drain", since = "1.6.0")]
    impl<'a, T: 'a> Drop for Drain<'a, T> {
        fn drop(&mut self) {
            for _ in self.by_ref() {}

            let source_deque = unsafe { self.deque.as_mut() };

            // 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 = "drain", since = "1.6.0")]
    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 = "drain", since = "1.6.0")]
    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 = "drain", since = "1.6.0")]
    impl<'a, T: 'a> ExactSizeIterator for Drain<'a, T> {}

    #[unstable(feature = "fused", issue = "35602")]
    impl<'a, T: 'a> FusedIterator 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> {}

    macro_rules! __impl_slice_eq1 {
        ($Lhs: ty, $Rhs: ty) => {
            __impl_slice_eq1! { $Lhs, $Rhs, Sized }
        };
        ($Lhs: ty, $Rhs: ty, $Bound: ident) => {
            #[stable(feature = "vec-deque-partial-eq-slice", since = "1.17.0")]
            impl<'a, 'b, A: $Bound, B> PartialEq<$Rhs> for $Lhs where A: PartialEq<B> {
                fn eq(&self, other: &$Rhs) -> bool {
                    if self.len() != other.len() {
                        return false;
                    }
                    let (sa, sb) = self.as_slices();
                    let (oa, ob) = other[..].split_at(sa.len());
                    sa == oa && sb == ob
                }
            }
        }
    }

    __impl_slice_eq1! { VecDeque<A>, Vec<B> }
    __impl_slice_eq1! { VecDeque<A>, &'b [B] }
    __impl_slice_eq1! { VecDeque<A>, &'b mut [B] }

    macro_rules! array_impls {
        ($($N: expr)+) => {
            $(
                __impl_slice_eq1! { VecDeque<A>, [B; $N] }
                __impl_slice_eq1! { VecDeque<A>, &'b [B; $N] }
                __impl_slice_eq1! { VecDeque<A>, &'b mut [B; $N] }
            )+
        }
    }

    array_impls! {
         0  1  2  3  4  5  6  7  8  9
        10 11 12 13 14 15 16 17 18 19
        20 21 22 23 24 25 26 27 28 29
        30 31 32
    }

    #[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()
        }
    }

    #[stable(feature = "vecdeque_vec_conversions", since = "1.10.0")]
    impl<T> From<Vec<T>> for VecDeque<T> {
        fn from(mut other: Vec<T>) -> Self {
            unsafe {
                let other_buf = other.as_mut_ptr();
                let mut buf = RawVec::from_raw_parts(other_buf, other.capacity());
                let len = other.len();
                mem::forget(other);

                // We need to extend the buf if it's not a power of two, too small
                // or doesn't have at least one free space
                if !buf.cap().is_power_of_two() || (buf.cap() < (MINIMUM_CAPACITY + 1)) ||
                   (buf.cap() == len) {
                    let cap = cmp::max(buf.cap() + 1, MINIMUM_CAPACITY + 1).next_power_of_two();
                    buf.reserve_exact(len, cap - len);
                }

                VecDeque {
                    tail: 0,
                    head: len,
                    buf: buf,
                }
            }
        }
    }

    #[stable(feature = "vecdeque_vec_conversions", since = "1.10.0")]
    impl<T> From<VecDeque<T>> for Vec<T> {
        fn from(other: VecDeque<T>) -> Self {
            unsafe {
                let buf = other.buf.ptr();
                let len = other.len();
                let tail = other.tail;
                let head = other.head;
                let cap = other.cap();

                // Need to move the ring to the front of the buffer, as vec will expect this.
                if other.is_contiguous() {
                    ptr::copy(buf.offset(tail as isize), buf, len);
                } else {
                    if (tail - head) >= cmp::min((cap - tail), head) {
                        // There is enough free space in the centre for the shortest block so we can
                        // do this in at most three copy moves.
                        if (cap - tail) > head {
                            // right hand block is the long one; move that enough for the left
                            ptr::copy(buf.offset(tail as isize),
                                      buf.offset((tail - head) as isize),
                                      cap - tail);
                            // copy left in the end
                            ptr::copy(buf, buf.offset((cap - head) as isize), head);
                            // shift the new thing to the start
                            ptr::copy(buf.offset((tail - head) as isize), buf, len);
                        } else {
                            // left hand block is the long one, we can do it in two!
                            ptr::copy(buf, buf.offset((cap - tail) as isize), head);
                            ptr::copy(buf.offset(tail as isize), buf, cap - tail);
                        }
                    } else {
                        // Need to use N swaps to move the ring
                        // We can use the space at the end of the ring as a temp store

                        let mut left_edge: usize = 0;
                        let mut right_edge: usize = tail;

                        // The general problem looks like this
                        // GHIJKLM...ABCDEF - before any swaps
                        // ABCDEFM...GHIJKL - after 1 pass of swaps
                        // ABCDEFGHIJM...KL - swap until the left edge reaches the temp store
                        //                  - then restart the algorithm with a new (smaller) store
                        // Sometimes the temp store is reached when the right edge is at the end
                        // of the buffer - this means we've hit the right order with fewer swaps!
                        // E.g
                        // EF..ABCD
                        // ABCDEF.. - after four only swaps we've finished

                        while left_edge < len && right_edge != cap {
                            let mut right_offset = 0;
                            for i in left_edge..right_edge {
                                right_offset = (i - left_edge) % (cap - right_edge);
                                let src: isize = (right_edge + right_offset) as isize;
                                ptr::swap(buf.offset(i as isize), buf.offset(src));
                            }
                            let n_ops = right_edge - left_edge;
                            left_edge += n_ops;
                            right_edge += right_offset + 1;

                        }
                    }

                }
                let out = Vec::from_raw_parts(buf, len, cap);
                mem::forget(other);
                out
            }
        }
    }

    /// A place for insertion at the back of a `VecDeque`.
    ///
    /// See [`VecDeque::place_back`](struct.VecDeque.html#method.place_back) for details.
    #[must_use = "places do nothing unless written to with `<-` syntax"]
    #[unstable(feature = "collection_placement",
               reason = "struct name and placement protocol are subject to change",
               issue = "30172")]
    #[derive(Debug)]
    pub struct PlaceBack<'a, T: 'a> {
        vec_deque: &'a mut VecDeque<T>,
    }

    #[unstable(feature = "collection_placement",
               reason = "placement protocol is subject to change",
               issue = "30172")]
    impl<'a, T> Placer<T> for PlaceBack<'a, T> {
        type Place = PlaceBack<'a, T>;

        fn make_place(self) -> Self {
            self.vec_deque.grow_if_necessary();
            self
        }
    }

    #[unstable(feature = "collection_placement",
               reason = "placement protocol is subject to change",
               issue = "30172")]
    impl<'a, T> Place<T> for PlaceBack<'a, T> {
        fn pointer(&mut self) -> *mut T {
            unsafe { self.vec_deque.ptr().offset(self.vec_deque.head as isize) }
        }
    }

    #[unstable(feature = "collection_placement",
               reason = "placement protocol is subject to change",
               issue = "30172")]
    impl<'a, T> InPlace<T> for PlaceBack<'a, T> {
        type Owner = &'a mut T;

        unsafe fn finalize(mut self) -> &'a mut T {
            let head = self.vec_deque.head;
            self.vec_deque.head = self.vec_deque.wrap_add(head, 1);
            &mut *(self.vec_deque.ptr().offset(head as isize))
        }
    }

    /// A place for insertion at the front of a `VecDeque`.
    ///
    /// See [`VecDeque::place_front`](struct.VecDeque.html#method.place_front) for details.
    #[must_use = "places do nothing unless written to with `<-` syntax"]
    #[unstable(feature = "collection_placement",
               reason = "struct name and placement protocol are subject to change",
               issue = "30172")]
    #[derive(Debug)]
    pub struct PlaceFront<'a, T: 'a> {
        vec_deque: &'a mut VecDeque<T>,
    }

    #[unstable(feature = "collection_placement",
               reason = "placement protocol is subject to change",
               issue = "30172")]
    impl<'a, T> Placer<T> for PlaceFront<'a, T> {
        type Place = PlaceFront<'a, T>;

        fn make_place(self) -> Self {
            self.vec_deque.grow_if_necessary();
            self
        }
    }

    #[unstable(feature = "collection_placement",
               reason = "placement protocol is subject to change",
               issue = "30172")]
    impl<'a, T> Place<T> for PlaceFront<'a, T> {
        fn pointer(&mut self) -> *mut T {
            let tail = self.vec_deque.wrap_sub(self.vec_deque.tail, 1);
            unsafe { self.vec_deque.ptr().offset(tail as isize) }
        }
    }

    #[unstable(feature = "collection_placement",
               reason = "placement protocol is subject to change",
               issue = "30172")]
    impl<'a, T> InPlace<T> for PlaceFront<'a, T> {
        type Owner = &'a mut T;

        unsafe fn finalize(mut self) -> &'a mut T {
            self.vec_deque.tail = self.vec_deque.wrap_sub(self.vec_deque.tail, 1);
            &mut *(self.vec_deque.ptr().offset(self.vec_deque.tail as isize))
        }
    }

    #[cfg(test)]
    mod tests {
        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: VecDeque<_> = 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::<VecDeque<_>>();
                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::<VecDeque<_>>();
                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::<VecDeque<_>>();
                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::<VecDeque<_>>();
                    // at, at + 1, .., len - 1 (may be empty)
                    let expected_other = (at..).take(len - at).collect::<VecDeque<_>>();

                    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);
                    }
                }
            }
        }

        #[test]
        fn test_from_vec() {
            use super::super::vec::Vec;
            for cap in 0..35 {
                for len in 0..cap + 1 {
                    let mut vec = Vec::with_capacity(cap);
                    vec.extend(0..len);

                    let vd = VecDeque::from(vec.clone());
                    assert!(vd.cap().is_power_of_two());
                    assert_eq!(vd.len(), vec.len());
                    assert!(vd.into_iter().eq(vec));
                }
            }
        }

        #[test]
        fn test_vec_from_vecdeque() {
            use super::super::vec::Vec;

            fn create_vec_and_test_convert(cap: usize, offset: usize, len: usize) {
                let mut vd = VecDeque::with_capacity(cap);
                for _ in 0..offset {
                    vd.push_back(0);
                    vd.pop_front();
                }
                vd.extend(0..len);

                let vec: Vec<_> = Vec::from(vd.clone());
                assert_eq!(vec.len(), vd.len());
                assert!(vec.into_iter().eq(vd));
            }

            for cap_pwr in 0..7 {
                // Make capacity as a (2^x)-1, so that the ring size is 2^x
                let cap = (2i32.pow(cap_pwr) - 1) as usize;

                // In these cases there is enough free space to solve it with copies
                for len in 0..((cap + 1) / 2) {
                    // Test contiguous cases
                    for offset in 0..(cap - len) {
                        create_vec_and_test_convert(cap, offset, len)
                    }

                    // Test cases where block at end of buffer is bigger than block at start
                    for offset in (cap - len)..(cap - (len / 2)) {
                        create_vec_and_test_convert(cap, offset, len)
                    }

                    // Test cases where block at start of buffer is bigger than block at end
                    for offset in (cap - (len / 2))..cap {
                        create_vec_and_test_convert(cap, offset, len)
                    }
                }

                // Now there's not (necessarily) space to straighten the ring with simple copies,
                // the ring will use swapping when:
                // (cap + 1 - offset) > (cap + 1 - len) && (len - (cap + 1 - offset)) > (cap + 1 - len))
                //  right block size  >   free space    &&      left block size       >    free space
                for len in ((cap + 1) / 2)..cap {
                    // Test contiguous cases
                    for offset in 0..(cap - len) {
                        create_vec_and_test_convert(cap, offset, len)
                    }

                    // Test cases where block at end of buffer is bigger than block at start
                    for offset in (cap - len)..(cap - (len / 2)) {
                        create_vec_and_test_convert(cap, offset, len)
                    }

                    // Test cases where block at start of buffer is bigger than block at end
                    for offset in (cap - (len / 2))..cap {
                        create_vec_and_test_convert(cap, offset, len)
                    }
                }
            }
        }

    }

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