vec_deque.rs

// Copyright 2012-2014 The Rust Project Developers. See the COPYRIGHT
// file at the top-level directory of this distribution and at
// http://rust-lang.org/COPYRIGHT.
//
// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
// option. This file may not be copied, modified, or distributed
// except according to those terms.

//! VecDeque is a double-ended queue, which is implemented with the help of a
//! growing ring buffer.
//!
//! This queue has `O(1)` amortized inserts and removals from both ends of the
//! container. It also has `O(1)` indexing like a vector. The contained elements
//! are not required to be copyable, and the queue will be sendable if the
//! contained type is sendable.

#![stable(feature = "rust1", since = "1.0.0")]

use core::prelude::*;

use core::cmp::Ordering;
use core::default::Default;
use core::fmt;
use core::iter::{self, repeat, FromIterator, IntoIterator, RandomAccessIterator};
use core::mem;
use core::num::{Int, UnsignedInt};
use core::num::wrapping::WrappingOps;
use core::ops::{Index, IndexMut};
use core::ptr::{self, Unique};
use core::raw::Slice as RawSlice;

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

use alloc::heap;

#[deprecated(since = "1.0.0", reason = "renamed to VecDeque")]
#[unstable(feature = "collections")]
pub use VecDeque as RingBuf;

const INITIAL_CAPACITY: usize = 7; // 2^3 - 1
const MINIMUM_CAPACITY: usize = 1; // 2 - 1

/// `VecDeque` is a growable ring buffer, which can be used as a
/// double-ended queue efficiently.
#[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 ringbuf
    // is defined as the distance between the two.

    tail: usize,
    head: usize,
    cap: usize,
    ptr: Unique<T>,
}

#[stable(feature = "rust1", since = "1.0.0")]
impl<T: Clone> Clone for VecDeque<T> {
    fn clone(&self) -> VecDeque<T> {
        self.iter().cloned().collect()
    }
}

#[unsafe_destructor]
#[stable(feature = "rust1", since = "1.0.0")]
impl<T> Drop for VecDeque<T> {
    fn drop(&mut self) {
        self.clear();
        unsafe {
            if mem::size_of::<T>() != 0 {
                heap::deallocate(*self.ptr as *mut u8,
                                 self.cap * mem::size_of::<T>(),
                                 mem::min_align_of::<T>())
            }
        }
    }
}

#[stable(feature = "rust1", since = "1.0.0")]
impl<T> Default for VecDeque<T> {
    #[inline]
    fn default() -> VecDeque<T> { VecDeque::new() }
}

impl<T> VecDeque<T> {
    /// Turn ptr into a slice
    #[inline]
    unsafe fn buffer_as_slice(&self) -> &[T] {
        mem::transmute(RawSlice { data: *self.ptr as *const T, len: self.cap })
    }

    /// Turn ptr into a mut slice
    #[inline]
    unsafe fn buffer_as_mut_slice(&mut self) -> &mut [T] {
        mem::transmute(RawSlice { data: *self.ptr as *const T, len: 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, t: T) {
        ptr::write(self.ptr.offset(off as isize), t);
    }

    /// Returns true iff the buffer is at capacity
    #[inline]
    fn is_full(&self) -> bool { self.cap - self.len() == 1 }

    /// Returns the index in the underlying buffer for a given logical element
    /// index.
    #[inline]
    fn wrap_index(&self, idx: usize) -> usize { wrap_index(idx, self.cap) }

    /// Returns the index in the underlying buffer for a given logical element
    /// index + addend.
    #[inline]
    fn wrap_add(&self, idx: usize, addend: usize) -> usize {
        wrap_index(idx.wrapping_add(addend), self.cap)
    }

    /// Returns the index in the underlying buffer for a given logical element
    /// index - subtrahend.
    #[inline]
    fn wrap_sub(&self, idx: usize, subtrahend: usize) -> usize {
        wrap_index(idx.wrapping_sub(subtrahend), self.cap)
    }

    /// Copies a contiguous block of memory len long from src to dst
    #[inline]
    unsafe fn copy(&self, dst: usize, src: usize, len: usize) {
        debug_assert!(dst + len <= self.cap, "dst={} src={} len={} cap={}", dst, src, len,
                      self.cap);
        debug_assert!(src + len <= self.cap, "dst={} src={} len={} cap={}", dst, src, len,
                      self.cap);
        ptr::copy(
            self.ptr.offset(dst as isize),
            self.ptr.offset(src 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, "dst={} src={} len={} cap={}", dst, src, len,
                      self.cap);
        debug_assert!(src + len <= self.cap, "dst={} src={} len={} cap={}", dst, src, len,
                      self.cap);
        ptr::copy_nonoverlapping(
            self.ptr.offset(dst as isize),
            self.ptr.offset(src as isize),
            len);
    }
}

impl<T> VecDeque<T> {
    /// Creates an empty `VecDeque`.
    #[stable(feature = "rust1", since = "1.0.0")]
    pub fn new() -> VecDeque<T> {
        VecDeque::with_capacity(INITIAL_CAPACITY)
    }

    /// Creates an empty `VecDeque` with space for at least `n` elements.
    #[stable(feature = "rust1", since = "1.0.0")]
    pub fn with_capacity(n: usize) -> VecDeque<T> {
        // +1 since the ringbuffer always leaves one space empty
        let cap = cmp::max(n + 1, MINIMUM_CAPACITY + 1).next_power_of_two();
        assert!(cap > n, "capacity overflow");
        let size = cap.checked_mul(mem::size_of::<T>())
                      .expect("capacity overflow");

        let ptr = unsafe {
            if mem::size_of::<T>() != 0 {
                let ptr = heap::allocate(size, mem::min_align_of::<T>())  as *mut T;;
                if ptr.is_null() { ::alloc::oom() }
                Unique::new(ptr)
            } else {
                Unique::new(heap::EMPTY as *mut T)
            }
        };

        VecDeque {
            tail: 0,
            head: 0,
            cap: cap,
            ptr: ptr,
        }
    }

    /// Retrieves an element in the `VecDeque` by index.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::collections::VecDeque;
    ///
    /// let mut buf = VecDeque::new();
    /// buf.push_back(3);
    /// buf.push_back(4);
    /// buf.push_back(5);
    /// assert_eq!(buf.get(1).unwrap(), &4);
    /// ```
    #[stable(feature = "rust1", since = "1.0.0")]
    pub fn get(&self, i: usize) -> Option<&T> {
        if i < self.len() {
            let idx = self.wrap_add(self.tail, i);
            unsafe { Some(&*self.ptr.offset(idx as isize)) }
        } else {
            None
        }
    }

    /// Retrieves an element in the `VecDeque` mutably by index.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::collections::VecDeque;
    ///
    /// let mut buf = VecDeque::new();
    /// buf.push_back(3);
    /// buf.push_back(4);
    /// buf.push_back(5);
    /// match buf.get_mut(1) {
    ///     None => {}
    ///     Some(elem) => {
    ///         *elem = 7;
    ///     }
    /// }
    ///
    /// assert_eq!(buf[1], 7);
    /// ```
    #[stable(feature = "rust1", since = "1.0.0")]
    pub fn get_mut(&mut self, i: usize) -> Option<&mut T> {
        if i < self.len() {
            let idx = self.wrap_add(self.tail, i);
            unsafe { Some(&mut *self.ptr.offset(idx as isize)) }
        } else {
            None
        }
    }

    /// Swaps elements at indices `i` and `j`.
    ///
    /// `i` and `j` may be equal.
    ///
    /// Fails if there is no element with either index.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::collections::VecDeque;
    ///
    /// let mut buf = VecDeque::new();
    /// buf.push_back(3);
    /// buf.push_back(4);
    /// buf.push_back(5);
    /// buf.swap(0, 2);
    /// assert_eq!(buf[0], 5);
    /// assert_eq!(buf[2], 3);
    /// ```
    #[stable(feature = "rust1", since = "1.0.0")]
    pub fn swap(&mut self, i: usize, j: usize) {
        assert!(i < self.len());
        assert!(j < self.len());
        let ri = self.wrap_add(self.tail, i);
        let rj = self.wrap_add(self.tail, j);
        unsafe {
            ptr::swap(self.ptr.offset(ri as isize), self.ptr.offset(rj as isize))
        }
    }

    /// Returns the number of elements the `VecDeque` can hold without
    /// reallocating.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::collections::VecDeque;
    ///
    /// let buf: VecDeque<i32> = VecDeque::with_capacity(10);
    /// assert!(buf.capacity() >= 10);
    /// ```
    #[inline]
    #[stable(feature = "rust1", since = "1.0.0")]
    pub fn capacity(&self) -> usize { self.cap - 1 }

    /// Reserves the minimum capacity for exactly `additional` more elements to be inserted in the
    /// given `VecDeque`. Does nothing if the capacity is already sufficient.
    ///
    /// Note that the allocator may give the collection more space than it requests. Therefore
    /// capacity can not be relied upon to be precisely minimal. Prefer `reserve` if future
    /// insertions are expected.
    ///
    /// # Panics
    ///
    /// Panics if the new capacity overflows `usize`.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::collections::VecDeque;
    ///
    /// let mut buf: VecDeque<i32> = vec![1].into_iter().collect();
    /// buf.reserve_exact(10);
    /// assert!(buf.capacity() >= 11);
    /// ```
    #[stable(feature = "rust1", since = "1.0.0")]
    pub fn reserve_exact(&mut self, additional: usize) {
        self.reserve(additional);
    }

    /// Reserves capacity for at least `additional` more elements to be inserted in the given
    /// `Ringbuf`. 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 new_len = self.len() + additional;
        assert!(new_len + 1 > self.len(), "capacity overflow");
        if new_len > self.capacity() {
            let count = (new_len + 1).next_power_of_two();
            assert!(count >= new_len + 1);

            if mem::size_of::<T>() != 0 {
                let old = self.cap * mem::size_of::<T>();
                let new = count.checked_mul(mem::size_of::<T>())
                               .expect("capacity overflow");
                unsafe {
                    let ptr = heap::reallocate(*self.ptr as *mut u8,
                                               old,
                                               new,
                                               mem::min_align_of::<T>()) as *mut T;
                    if ptr.is_null() { ::alloc::oom() }
                    self.ptr = Unique::new(ptr);
                }
            }

            // 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 ]

            let oldcap = self.cap;
            self.cap = count;

            if self.tail <= self.head { // A
                // Nop
            } else if self.head < oldcap - self.tail { // B
                unsafe {
                    self.copy_nonoverlapping(oldcap, 0, self.head);
                }
                self.head += oldcap;
                debug_assert!(self.head > self.tail);
            } else { // C
                let new_tail = count - (oldcap - self.tail);
                unsafe {
                    self.copy_nonoverlapping(new_tail, self.tail, oldcap - 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);
        }
    }

    /// Shrinks the capacity of the ringbuf as much as possible.
    ///
    /// It will drop down as close as possible to the length but the allocator may still inform the
    /// ringbuf 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);
    /// ```
    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 ringbuf.
        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);
            }

            if mem::size_of::<T>() != 0 {
                let old = self.cap * mem::size_of::<T>();
                let new_size = target_cap * mem::size_of::<T>();
                unsafe {
                    let ptr = heap::reallocate(*self.ptr as *mut u8,
                                               old,
                                               new_size,
                                               mem::min_align_of::<T>()) as *mut T;
                    if ptr.is_null() { ::alloc::oom() }
                    self.ptr = Unique::new(ptr);
                }
            }
            self.cap = target_cap;
            debug_assert!(self.head < self.cap);
            debug_assert!(self.tail < self.cap);
            debug_assert!(self.cap.count_ones() == 1);
        }
    }

    /// Shorten a ringbuf, dropping excess elements from the back.
    ///
    /// If `len` is greater than the ringbuf'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);
    /// buf.truncate(1);
    /// assert_eq!(buf.len(), 1);
    /// assert_eq!(Some(&5), buf.get(0));
    /// ```
    #[unstable(feature = "collections",
               reason = "matches collection reform specification; waiting on panic semantics")]
    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];
    /// assert_eq!(buf.iter().collect::<Vec<&i32>>().as_slice(), 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() },
        }
    }

    /// Consumes the list into an iterator yielding elements by value.
    #[stable(feature = "rust1", since = "1.0.0")]
    pub fn into_iter(self) -> IntoIter<T> {
        IntoIter {
            inner: self,
        }
    }

    /// Returns a pair of slices which contain, in order, the contents of the
    /// `VecDeque`.
    #[inline]
    #[unstable(feature = "collections",
               reason = "matches collection reform specification, waiting for dust to settle")]
    pub fn as_slices(&self) -> (&[T], &[T]) {
        unsafe {
            let contiguous = self.is_contiguous();
            let buf = self.buffer_as_slice();
            if contiguous {
                let (empty, buf) = buf.split_at(0);
                (&buf[self.tail..self.head], empty)
            } else {
                let (mid, right) = buf.split_at(self.tail);
                let (left, _) = mid.split_at(self.head);
                (right, left)
            }
        }
    }

    /// Returns a pair of slices which contain, in order, the contents of the
    /// `VecDeque`.
    #[inline]
    #[unstable(feature = "collections",
               reason = "matches collection reform specification, waiting for dust to settle")]
    pub fn as_mut_slices(&mut self) -> (&mut [T], &mut [T]) {
        unsafe {
            let contiguous = self.is_contiguous();
            let head = self.head;
            let tail = self.tail;
            let buf = self.buffer_as_mut_slice();

            if contiguous {
                let (empty, buf) = buf.split_at_mut(0);
                (&mut buf[tail .. head], empty)
            } else {
                let (mid, right) = buf.split_at_mut(tail);
                let (left, _) = mid.split_at_mut(head);

                (right, left)
            }
        }
    }

    /// Returns the number of elements in the `VecDeque`.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::collections::VecDeque;
    ///
    /// let mut v = VecDeque::new();
    /// assert_eq!(v.len(), 0);
    /// v.push_back(1);
    /// assert_eq!(v.len(), 1);
    /// ```
    #[stable(feature = "rust1", since = "1.0.0")]
    pub fn len(&self) -> usize { count(self.tail, self.head, self.cap) }

    /// Returns true if the buffer contains no elements
    ///
    /// # Examples
    ///
    /// ```
    /// use std::collections::VecDeque;
    ///
    /// let mut v = VecDeque::new();
    /// assert!(v.is_empty());
    /// v.push_front(1);
    /// assert!(!v.is_empty());
    /// ```
    #[stable(feature = "rust1", since = "1.0.0")]
    pub fn is_empty(&self) -> bool { self.len() == 0 }

    /// Creates a draining iterator that clears the `VecDeque` and iterates over
    /// the removed items from start to end.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::collections::VecDeque;
    ///
    /// let mut v = VecDeque::new();
    /// v.push_back(1);
    /// assert_eq!(v.drain().next(), Some(1));
    /// assert!(v.is_empty());
    /// ```
    #[inline]
    #[unstable(feature = "collections",
               reason = "matches collection reform specification, waiting for dust to settle")]
    pub fn drain(&mut self) -> Drain<T> {
        Drain {
            inner: self,
        }
    }

    /// Clears the buffer, removing all values.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::collections::VecDeque;
    ///
    /// let mut v = VecDeque::new();
    /// v.push_back(1);
    /// v.clear();
    /// assert!(v.is_empty());
    /// ```
    #[stable(feature = "rust1", since = "1.0.0")]
    #[inline]
    pub fn clear(&mut self) {
        self.drain();
    }

    /// Provides a reference to the front element, or `None` if the sequence is
    /// empty.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::collections::VecDeque;
    ///
    /// let mut d = VecDeque::new();
    /// assert_eq!(d.front(), None);
    ///
    /// d.push_back(1);
    /// d.push_back(2);
    /// assert_eq!(d.front(), Some(&1));
    /// ```
    #[stable(feature = "rust1", since = "1.0.0")]
    pub fn front(&self) -> Option<&T> {
        if !self.is_empty() { Some(&self[0]) } else { None }
    }

    /// Provides a mutable reference to the front element, or `None` if the
    /// sequence is empty.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::collections::VecDeque;
    ///
    /// let mut d = VecDeque::new();
    /// assert_eq!(d.front_mut(), None);
    ///
    /// d.push_back(1);
    /// d.push_back(2);
    /// match d.front_mut() {
    ///     Some(x) => *x = 9,
    ///     None => (),
    /// }
    /// assert_eq!(d.front(), Some(&9));
    /// ```
    #[stable(feature = "rust1", since = "1.0.0")]
    pub fn front_mut(&mut self) -> Option<&mut T> {
        if !self.is_empty() { Some(&mut self[0]) } else { None }
    }

    /// Provides a reference to the back element, or `None` if the sequence is
    /// empty.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::collections::VecDeque;
    ///
    /// let mut d = VecDeque::new();
    /// assert_eq!(d.back(), None);
    ///
    /// d.push_back(1);
    /// d.push_back(2);
    /// assert_eq!(d.back(), Some(&2));
    /// ```
    #[stable(feature = "rust1", since = "1.0.0")]
    pub fn back(&self) -> Option<&T> {
        if !self.is_empty() { Some(&self[self.len() - 1]) } else { None }
    }

    /// Provides a mutable reference to the back element, or `None` if the
    /// sequence is empty.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::collections::VecDeque;
    ///
    /// let mut d = VecDeque::new();
    /// assert_eq!(d.back(), None);
    ///
    /// d.push_back(1);
    /// d.push_back(2);
    /// match d.back_mut() {
    ///     Some(x) => *x = 9,
    ///     None => (),
    /// }
    /// assert_eq!(d.back(), Some(&9));
    /// ```
    #[stable(feature = "rust1", since = "1.0.0")]
    pub fn back_mut(&mut self) -> Option<&mut T> {
        let len = self.len();
        if !self.is_empty() { Some(&mut self[len - 1]) } else { None }
    }

    /// Removes the first element and returns it, or `None` if the sequence is
    /// empty.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::collections::VecDeque;
    ///
    /// let mut d = VecDeque::new();
    /// d.push_back(1);
    /// d.push_back(2);
    ///
    /// assert_eq!(d.pop_front(), Some(1));
    /// assert_eq!(d.pop_front(), Some(2));
    /// assert_eq!(d.pop_front(), None);
    /// ```
    #[stable(feature = "rust1", since = "1.0.0")]
    pub fn pop_front(&mut self) -> Option<T> {
        if self.is_empty() {
            None
        } else {
            let tail = self.tail;
            self.tail = self.wrap_add(self.tail, 1);
            unsafe { Some(self.buffer_read(tail)) }
        }
    }

    /// Inserts an element first in the sequence.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::collections::VecDeque;
    ///
    /// let mut d = VecDeque::new();
    /// d.push_front(1);
    /// d.push_front(2);
    /// assert_eq!(d.front(), Some(&2));
    /// ```
    #[stable(feature = "rust1", since = "1.0.0")]
    pub fn push_front(&mut self, t: T) {
        if self.is_full() {
            self.reserve(1);
            debug_assert!(!self.is_full());
        }

        self.tail = self.wrap_sub(self.tail, 1);
        let tail = self.tail;
        unsafe { self.buffer_write(tail, t); }
    }

    /// Appends an element to the back of a buffer
    ///
    /// # Examples
    ///
    /// ```
    /// use std::collections::VecDeque;
    ///
    /// let mut buf = VecDeque::new();
    /// buf.push_back(1);
    /// buf.push_back(3);
    /// assert_eq!(3, *buf.back().unwrap());
    /// ```
    #[stable(feature = "rust1", since = "1.0.0")]
    pub fn push_back(&mut self, t: T) {
        if self.is_full() {
            self.reserve(1);
            debug_assert!(!self.is_full());
        }

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

    /// Removes the last element from a buffer and returns it, or `None` if
    /// it is empty.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::collections::VecDeque;
    ///
    /// let mut buf = VecDeque::new();
    /// assert_eq!(buf.pop_back(), None);
    /// buf.push_back(1);
    /// buf.push_back(3);
    /// assert_eq!(buf.pop_back(), Some(3));
    /// ```
    #[stable(feature = "rust1", since = "1.0.0")]
    pub fn pop_back(&mut self) -> Option<T> {
        if self.is_empty() {
            None
        } else {
            self.head = self.wrap_sub(self.head, 1);
            let head = self.head;
            unsafe { Some(self.buffer_read(head)) }
        }
    }

    #[inline]
    fn is_contiguous(&self) -> bool {
        self.tail <= self.head
    }

    /// Removes an element from anywhere in the ringbuf and returns it, replacing it with the last
    /// element.
    ///
    /// This does not preserve ordering, but is O(1).
    ///
    /// Returns `None` if `index` is out of bounds.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::collections::VecDeque;
    ///
    /// let mut buf = VecDeque::new();
    /// assert_eq!(buf.swap_back_remove(0), None);
    /// buf.push_back(5);
    /// buf.push_back(99);
    /// buf.push_back(15);
    /// buf.push_back(20);
    /// buf.push_back(10);
    /// assert_eq!(buf.swap_back_remove(1), Some(99));
    /// ```
    #[unstable(feature = "collections",
               reason = "the naming of this function may be altered")]
    pub fn swap_back_remove(&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 ringbuf and returns it, replacing it with the first
    /// element.
    ///
    /// This does not preserve ordering, but is O(1).
    ///
    /// Returns `None` if `index` is out of bounds.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::collections::VecDeque;
    ///
    /// let mut buf = VecDeque::new();
    /// assert_eq!(buf.swap_front_remove(0), None);
    /// buf.push_back(15);
    /// buf.push_back(5);
    /// buf.push_back(10);
    /// buf.push_back(99);
    /// buf.push_back(20);
    /// assert_eq!(buf.swap_front_remove(3), Some(99));
    /// ```
    #[unstable(feature = "collections",
               reason = "the naming of this function may be altered")]
    pub fn swap_front_remove(&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 position `i` within the ringbuf. Whichever
    /// end is closer to the insertion point will be moved to make room,
    /// and all the affected elements will be moved to new positions.
    ///
    /// # Panics
    ///
    /// Panics if `i` is greater than ringbuf's length
    ///
    /// # Examples
    /// ```
    /// use std::collections::VecDeque;
    ///
    /// let mut buf = VecDeque::new();
    /// buf.push_back(10);
    /// buf.push_back(12);
    /// buf.insert(1,11);
    /// assert_eq!(Some(&11), buf.get(1));
    /// ```
    pub fn insert(&mut self, i: usize, t: T) {
        assert!(i <= self.len(), "index out of bounds");
        if self.is_full() {
            self.reserve(1);
            debug_assert!(!self.is_full());
        }

        // Move the least number of elements in the ring buffer and insert
        // the given object
        //
        // At most len/2 - 1 elements will be moved. O(min(n, n-i))
        //
        // There are three main cases:
        //  Elements are contiguous
        //      - special case when tail is 0
        //  Elements are discontiguous and the insert is in the tail section
        //  Elements are discontiguous and the insert is in the head section
        //
        // For each of those there are two more cases:
        //  Insert is closer to tail
        //  Insert is closer to head
        //
        // Key: H - self.head
        //      T - self.tail
        //      o - Valid element
        //      I - Insertion element
        //      A - The element that should be after the insertion point
        //      M - Indicates element was moved

        let idx = self.wrap_add(self.tail, i);

        let distance_to_tail = i;
        let distance_to_head = self.len() - i;

        let contiguous = self.is_contiguous();

        match (contiguous, distance_to_tail <= distance_to_head, idx >= self.tail) {
            (true, true, _) if i == 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 `i - 1` elements.
                self.copy(self.tail, self.tail + 1, i - 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, i);
                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, i);
        unsafe {
            self.buffer_write(new_idx, t);
        }
    }

    /// Removes and returns the element at position `i` from the ringbuf.
    /// 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 `i` is out of bounds.
    ///
    /// # Examples
    /// ```
    /// use std::collections::VecDeque;
    ///
    /// let mut buf = VecDeque::new();
    /// buf.push_back(5);
    /// buf.push_back(10);
    /// buf.push_back(12);
    /// buf.push_back(15);
    /// buf.remove(2);
    /// assert_eq!(Some(&15), buf.get(2));
    /// ```
    #[stable(feature = "rust1", since = "1.0.0")]
    pub fn remove(&mut self, i: usize) -> Option<T> {
        if self.is_empty() || self.len() <= i {
            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, i);

        let elem = unsafe {
            Some(self.buffer_read(idx))
        };

        let distance_to_tail = i;
        let distance_to_head = self.len() - i;

        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, i);
                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, i);
                self.tail = self.wrap_add(self.tail, 1);
            },
            (false, false, false) => unsafe {
                // discontiguous, remove closer to head, head section:
                //
                //               R     H           T
                //      [o o o o x o o . . . . . . o o o]
                //
                //                   H             T
                //      [o o o o o o . . . . . . . o o o]
                //               M M

                self.copy(idx, idx + 1, self.head - idx - 1);
                self.head -= 1;
            },
            (false, false, true) => unsafe {
                // discontiguous, remove closer to head, tail section:
                //
                //             H           T         R
                //      [o o o . . . . . . o o o o o x o]
                //
                //           H             T
                //      [o o . . . . . . . o o o o o o o]
                //       M M                         M M
                //
                // or quasi-discontiguous, remove next to head, tail section:
                //
                //       H                 T         R
                //      [. . . . . . . . . o o o o o x o]
                //
                //                         T           H
                //      [. . . . . . . . . o o o o o o .]
                //                                   M

                // draw in elements in the tail section
                self.copy(idx, idx + 1, self.cap - idx - 1);

                // Prevents underflow.
                if self.head != 0 {
                    // copy first element into empty spot
                    self.copy(self.cap - 1, 0, 1);

                    // move elements in the head section backwards
                    self.copy(0, 1, self.head - 1);
                }

                self.head = self.wrap_sub(self.head, 1);
            },
            (false, true, false) => unsafe {
                // discontiguous, remove closer to tail, head section:
                //
                //           R               H     T
                //      [o o x o o o o o o o . . . o o o]
                //
                //                           H       T
                //      [o o o o o o o o o o . . . . o o]
                //       M M M                       M M

                // draw in elements up to idx
                self.copy(1, 0, idx);

                // copy last element into empty spot
                self.copy(0, self.cap - 1, 1);

                // move elements from tail to end forward, excluding the last one
                self.copy(self.tail + 1, self.tail, self.cap - self.tail - 1);

                self.tail = self.wrap_add(self.tail, 1);
            }
        }

        return elem;
    }

    /// Splits the collection into two at the given index.
    ///
    /// Returns a newly allocated `Self`. `self` contains elements `[0, at)`,
    /// and the returned `Self` contains elements `[at, len)`.
    ///
    /// Note that the capacity of `self` does not change.
    ///
    /// # Panics
    ///
    /// Panics if `at > len`
    ///
    /// # Examples
    ///
    /// ```
    /// use std::collections::VecDeque;
    ///
    /// let mut buf: VecDeque<_> = vec![1,2,3].into_iter().collect();
    /// let buf2 = buf.split_off(1);
    /// // buf = [1], buf2 = [2, 3]
    /// assert_eq!(buf.len(), 1);
    /// assert_eq!(buf2.len(), 2);
    /// ```
    #[inline]
    #[unstable(feature = "collections",
               reason = "new API, waiting for dust to settle")]
    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(*other.ptr,
                                         first_half.as_ptr().offset(at as isize),
                                         amount_in_first);

                // just take all of the second half.
                ptr::copy_nonoverlapping(other.ptr.offset(amount_in_first as isize),
                                         second_half.as_ptr(),
                                         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(*other.ptr,
                                         second_half.as_ptr().offset(offset as isize),
                                         amount_in_second);
            }
        }

        // Cleanup where the ends of the buffers are
        self.head = self.wrap_sub(self.head, other_len);
        other.head = other.wrap_index(other_len);

        other
    }

    /// Moves all the elements of `other` into `Self`, leaving `other` empty.
    ///
    /// # Panics
    ///
    /// Panics if the new number of elements in self overflows a `usize`.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::collections::VecDeque;
    ///
    /// let mut buf: VecDeque<_> = vec![1, 2, 3].into_iter().collect();
    /// let mut buf2: VecDeque<_> = vec![4, 5, 6].into_iter().collect();
    /// buf.append(&mut buf2);
    /// assert_eq!(buf.len(), 6);
    /// assert_eq!(buf2.len(), 0);
    /// ```
    #[inline]
    #[unstable(feature = "collections",
               reason = "new API, waiting for dust to settle")]
    pub fn append(&mut self, other: &mut Self) {
        // naive impl
        self.extend(other.drain());
    }
}

impl<T: Clone> VecDeque<T> {
    /// Modifies the ringbuf in-place so that `len()` is equal to new_len,
    /// either by removing excess elements or by appending copies of a value to the back.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::collections::VecDeque;
    ///
    /// let mut buf = VecDeque::new();
    /// buf.push_back(5);
    /// buf.push_back(10);
    /// buf.push_back(15);
    /// buf.resize(2, 0);
    /// buf.resize(6, 20);
    /// for (a, b) in [5, 10, 20, 20, 20, 20].iter().zip(buf.iter()) {
    ///     assert_eq!(a, b);
    /// }
    /// ```
    #[unstable(feature = "collections",
               reason = "matches collection reform specification; waiting on panic semantics")]
    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
    index & (size - 1)
}

/// Calculate the number of elements left to be read in the buffer
#[inline]
fn count(tail: usize, head: usize, size: usize) -> usize {
    // size is always a power of 2
    (head.wrapping_sub(tail)) & (size - 1)
}

/// `VecDeque` iterator.
#[stable(feature = "rust1", since = "1.0.0")]
pub struct Iter<'a, T:'a> {
    ring: &'a [T],
    tail: usize,
    head: usize
}

// FIXME(#19839) Remove in favor of `#[derive(Clone)]`
impl<'a, T> Clone for Iter<'a, T> {
    fn clone(&self) -> Iter<'a, T> {
        Iter {
            ring: self.ring,
            tail: self.tail,
            head: self.head
        }
    }
}

#[stable(feature = "rust1", since = "1.0.0")]
impl<'a, T> Iterator for Iter<'a, T> {
    type Item = &'a T;

    #[inline]
    fn next(&mut self) -> Option<&'a T> {
        if self.tail == self.head {
            return None;
        }
        let tail = self.tail;
        self.tail = wrap_index(self.tail.wrapping_add(1), self.ring.len());
        unsafe { Some(self.ring.get_unchecked(tail)) }
    }

    #[inline]
    fn size_hint(&self) -> (usize, Option<usize>) {
        let len = count(self.tail, self.head, self.ring.len());
        (len, Some(len))
    }
}

#[stable(feature = "rust1", since = "1.0.0")]
impl<'a, T> DoubleEndedIterator for Iter<'a, T> {
    #[inline]
    fn next_back(&mut self) -> Option<&'a T> {
        if self.tail == self.head {
            return None;
        }
        self.head = wrap_index(self.head.wrapping_sub(1), self.ring.len());
        unsafe { Some(self.ring.get_unchecked(self.head)) }
    }
}

#[stable(feature = "rust1", since = "1.0.0")]
impl<'a, T> ExactSizeIterator for Iter<'a, T> {}

#[stable(feature = "rust1", since = "1.0.0")]
impl<'a, T> RandomAccessIterator for Iter<'a, T> {
    #[inline]
    fn indexable(&self) -> usize {
        let (len, _) = self.size_hint();
        len
    }

    #[inline]
    fn idx(&mut self, j: usize) -> Option<&'a T> {
        if j >= self.indexable() {
            None
        } else {
            let idx = wrap_index(self.tail.wrapping_add(j), self.ring.len());
            unsafe { Some(self.ring.get_unchecked(idx)) }
        }
    }
}

/// `VecDeque` mutable iterator.
#[stable(feature = "rust1", since = "1.0.0")]
pub struct IterMut<'a, T:'a> {
    ring: &'a mut [T],
    tail: usize,
    head: usize,
}

#[stable(feature = "rust1", since = "1.0.0")]
impl<'a, T> Iterator for IterMut<'a, T> {
    type Item = &'a mut T;

    #[inline]
    fn next(&mut self) -> Option<&'a mut T> {
        if self.tail == self.head {
            return None;
        }
        let tail = self.tail;
        self.tail = wrap_index(self.tail.wrapping_add(1), self.ring.len());

        unsafe {
            let elem = self.ring.get_unchecked_mut(tail);
            Some(&mut *(elem as *mut _))
        }
    }

    #[inline]
    fn size_hint(&self) -> (usize, Option<usize>) {
        let len = count(self.tail, self.head, self.ring.len());
        (len, Some(len))
    }
}

#[stable(feature = "rust1", since = "1.0.0")]
impl<'a, T> DoubleEndedIterator for IterMut<'a, T> {
    #[inline]
    fn next_back(&mut self) -> Option<&'a mut T> {
        if self.tail == self.head {
            return None;
        }
        self.head = wrap_index(self.head.wrapping_sub(1), self.ring.len());

        unsafe {
            let elem = self.ring.get_unchecked_mut(self.head);
            Some(&mut *(elem as *mut _))
        }
    }
}

#[stable(feature = "rust1", since = "1.0.0")]
impl<'a, T> ExactSizeIterator for IterMut<'a, T> {}

/// A by-value VecDeque iterator
#[stable(feature = "rust1", since = "1.0.0")]
pub struct IntoIter<T> {
    inner: VecDeque<T>,
}

#[stable(feature = "rust1", since = "1.0.0")]
impl<T> Iterator for IntoIter<T> {
    type Item = T;

    #[inline]
    fn next(&mut self) -> Option<T> {
        self.inner.pop_front()
    }

    #[inline]
    fn size_hint(&self) -> (usize, Option<usize>) {
        let len = self.inner.len();
        (len, Some(len))
    }
}

#[stable(feature = "rust1", since = "1.0.0")]
impl<T> DoubleEndedIterator for IntoIter<T> {
    #[inline]
    fn next_back(&mut self) -> Option<T> {
        self.inner.pop_back()
    }
}

#[stable(feature = "rust1", since = "1.0.0")]
impl<T> ExactSizeIterator for IntoIter<T> {}

/// A draining VecDeque iterator
#[unstable(feature = "collections",
           reason = "matches collection reform specification, waiting for dust to settle")]
pub struct Drain<'a, T: 'a> {
    inner: &'a mut VecDeque<T>,
}

#[unsafe_destructor]
#[stable(feature = "rust1", since = "1.0.0")]
impl<'a, T: 'a> Drop for Drain<'a, T> {
    fn drop(&mut self) {
        for _ in self.by_ref() {}
        self.inner.head = 0;
        self.inner.tail = 0;
    }
}

#[stable(feature = "rust1", since = "1.0.0")]
impl<'a, T: 'a> Iterator for Drain<'a, T> {
    type Item = T;

    #[inline]
    fn next(&mut self) -> Option<T> {
        self.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<'a, T: 'a> DoubleEndedIterator for Drain<'a, T> {
    #[inline]
    fn next_back(&mut self) -> Option<T> {
        self.inner.pop_back()
    }
}

#[stable(feature = "rust1", since = "1.0.0")]
impl<'a, T: 'a> ExactSizeIterator for Drain<'a, T> {}

#[stable(feature = "rust1", since = "1.0.0")]
impl<A: PartialEq> PartialEq for VecDeque<A> {
    fn eq(&self, other: &VecDeque<A>) -> bool {
        self.len() == other.len() &&
            self.iter().zip(other.iter()).all(|(a, b)| a.eq(b))
    }
}

#[stable(feature = "rust1", since = "1.0.0")]
impl<A: Eq> Eq for VecDeque<A> {}

#[stable(feature = "rust1", since = "1.0.0")]
impl<A: PartialOrd> PartialOrd for VecDeque<A> {
    fn partial_cmp(&self, other: &VecDeque<A>) -> Option<Ordering> {
        iter::order::partial_cmp(self.iter(), other.iter())
    }
}

#[stable(feature = "rust1", since = "1.0.0")]
impl<A: Ord> Ord for VecDeque<A> {
    #[inline]
    fn cmp(&self, other: &VecDeque<A>) -> Ordering {
        iter::order::cmp(self.iter(), 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);
        for elt in self {
            elt.hash(state);
        }
    }
}

#[stable(feature = "rust1", since = "1.0.0")]
impl<A> Index<usize> for VecDeque<A> {
    type Output = A;

    #[inline]
    fn index(&self, i: &usize) -> &A {
        self.get(*i).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, i: &usize) -> &mut A {
        self.get_mut(*i).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>>(iterable: T) -> VecDeque<A> {
        let iterator = iterable.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>;

    fn into_iter(self) -> IntoIter<T> {
        self.into_iter()
    }
}

#[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 = "rust1", since = "1.0.0")]
impl<T: fmt::Debug> fmt::Debug for VecDeque<T> {
    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
        try!(write!(f, "["));

        for (i, e) in self.iter().enumerate() {
            if i != 0 { try!(write!(f, ", ")); }
            try!(write!(f, "{:?}", *e));
        }

        write!(f, "]")
    }
}

#[cfg(test)]
mod tests {
    use self::Taggy::*;
    use self::Taggypar::*;
    use prelude::*;
    use core::iter;
    use std::fmt::Debug;
    use std::hash::{self, SipHasher};
    use test::Bencher;
    use test;

    use super::VecDeque;

    #[test]
    #[allow(deprecated)]
    fn test_simple() {
        let mut d = VecDeque::new();
        assert_eq!(d.len(), 0);
        d.push_front(17);
        d.push_front(42);
        d.push_back(137);
        assert_eq!(d.len(), 3);
        d.push_back(137);
        assert_eq!(d.len(), 4);
        assert_eq!(*d.front().unwrap(), 42);
        assert_eq!(*d.back().unwrap(), 137);
        let mut i = d.pop_front();
        assert_eq!(i, Some(42));
        i = d.pop_back();
        assert_eq!(i, Some(137));
        i = d.pop_back();
        assert_eq!(i, Some(137));
        i = d.pop_back();
        assert_eq!(i, Some(17));
        assert_eq!(d.len(), 0);
        d.push_back(3);
        assert_eq!(d.len(), 1);
        d.push_front(2);
        assert_eq!(d.len(), 2);
        d.push_back(4);
        assert_eq!(d.len(), 3);
        d.push_front(1);
        assert_eq!(d.len(), 4);
        debug!("{}", d[0]);
        debug!("{}", d[1]);
        debug!("{}", d[2]);
        debug!("{}", d[3]);
        assert_eq!(d[0], 1);
        assert_eq!(d[1], 2);
        assert_eq!(d[2], 3);
        assert_eq!(d[3], 4);
    }

    #[cfg(test)]
    fn test_parameterized<T:Clone + PartialEq + Debug>(a: T, b: T, c: T, d: T) {
        let mut deq = VecDeque::new();
        assert_eq!(deq.len(), 0);
        deq.push_front(a.clone());
        deq.push_front(b.clone());
        deq.push_back(c.clone());
        assert_eq!(deq.len(), 3);
        deq.push_back(d.clone());
        assert_eq!(deq.len(), 4);
        assert_eq!((*deq.front().unwrap()).clone(), b.clone());
        assert_eq!((*deq.back().unwrap()).clone(), d.clone());
        assert_eq!(deq.pop_front().unwrap(), b.clone());
        assert_eq!(deq.pop_back().unwrap(), d.clone());
        assert_eq!(deq.pop_back().unwrap(), c.clone());
        assert_eq!(deq.pop_back().unwrap(), a.clone());
        assert_eq!(deq.len(), 0);
        deq.push_back(c.clone());
        assert_eq!(deq.len(), 1);
        deq.push_front(b.clone());
        assert_eq!(deq.len(), 2);
        deq.push_back(d.clone());
        assert_eq!(deq.len(), 3);
        deq.push_front(a.clone());
        assert_eq!(deq.len(), 4);
        assert_eq!(deq[0].clone(), a.clone());
        assert_eq!(deq[1].clone(), b.clone());
        assert_eq!(deq[2].clone(), c.clone());
        assert_eq!(deq[3].clone(), d.clone());
    }

    #[test]
    fn test_push_front_grow() {
        let mut deq = VecDeque::new();
        for i in 0..66 {
            deq.push_front(i);
        }
        assert_eq!(deq.len(), 66);

        for i in 0..66 {
            assert_eq!(deq[i], 65 - i);
        }

        let mut deq = VecDeque::new();
        for i in 0..66 {
            deq.push_back(i);
        }

        for i in 0..66 {
            assert_eq!(deq[i], i);
        }
    }

    #[test]
    fn test_index() {
        let mut deq = VecDeque::new();
        for i in 1..4 {
            deq.push_front(i);
        }
        assert_eq!(deq[1], 2);
    }

    #[test]
    #[should_panic]
    fn test_index_out_of_bounds() {
        let mut deq = VecDeque::new();
        for i in 1..4 {
            deq.push_front(i);
        }
        deq[3];
    }

    #[bench]
    fn bench_new(b: &mut test::Bencher) {
        b.iter(|| {
            let ring: VecDeque<i32> = VecDeque::new();
            test::black_box(ring);
        })
    }

    #[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());
            }
        })
    }

    #[bench]
    fn bench_grow_1025(b: &mut test::Bencher) {
        b.iter(|| {
            let mut deq = VecDeque::new();
            for i in 0..1025 {
                deq.push_front(i);
            }
            test::black_box(deq);
        })
    }

    #[bench]
    fn bench_iter_1000(b: &mut test::Bencher) {
        let ring: VecDeque<_> = (0..1000).collect();

        b.iter(|| {
            let mut sum = 0;
            for &i in &ring {
                sum += i;
            }
            test::black_box(sum);
        })
    }

    #[bench]
    fn bench_mut_iter_1000(b: &mut test::Bencher) {
        let mut ring: VecDeque<_> = (0..1000).collect();

        b.iter(|| {
            let mut sum = 0;
            for i in &mut ring {
                sum += *i;
            }
            test::black_box(sum);
        })
    }

    #[derive(Clone, PartialEq, Debug)]
    enum Taggy {
        One(i32),
        Two(i32, i32),
        Three(i32, i32, i32),
    }

    #[derive(Clone, PartialEq, Debug)]
    enum Taggypar<T> {
        Onepar(T),
        Twopar(T, T),
        Threepar(T, T, T),
    }

    #[derive(Clone, PartialEq, Debug)]
    struct RecCy {
        x: i32,
        y: i32,
        t: Taggy
    }

    #[test]
    fn test_param_int() {
        test_parameterized::<i32>(5, 72, 64, 175);
    }

    #[test]
    fn test_param_taggy() {
        test_parameterized::<Taggy>(One(1), Two(1, 2), Three(1, 2, 3), Two(17, 42));
    }

    #[test]
    fn test_param_taggypar() {
        test_parameterized::<Taggypar<i32>>(Onepar::<i32>(1),
                                            Twopar::<i32>(1, 2),
                                            Threepar::<i32>(1, 2, 3),
                                            Twopar::<i32>(17, 42));
    }

    #[test]
    fn test_param_reccy() {
        let reccy1 = RecCy { x: 1, y: 2, t: One(1) };
        let reccy2 = RecCy { x: 345, y: 2, t: Two(1, 2) };
        let reccy3 = RecCy { x: 1, y: 777, t: Three(1, 2, 3) };
        let reccy4 = RecCy { x: 19, y: 252, t: Two(17, 42) };
        test_parameterized::<RecCy>(reccy1, reccy2, reccy3, reccy4);
    }

    #[test]
    fn test_with_capacity() {
        let mut d = VecDeque::with_capacity(0);
        d.push_back(1);
        assert_eq!(d.len(), 1);
        let mut d = VecDeque::with_capacity(50);
        d.push_back(1);
        assert_eq!(d.len(), 1);
    }

    #[test]
    fn test_with_capacity_non_power_two() {
        let mut d3 = VecDeque::with_capacity(3);
        d3.push_back(1);

        // X = None, | = lo
        // [|1, X, X]
        assert_eq!(d3.pop_front(), Some(1));
        // [X, |X, X]
        assert_eq!(d3.front(), None);

        // [X, |3, X]
        d3.push_back(3);
        // [X, |3, 6]
        d3.push_back(6);
        // [X, X, |6]
        assert_eq!(d3.pop_front(), Some(3));

        // Pushing the lo past half way point to trigger
        // the 'B' scenario for growth
        // [9, X, |6]
        d3.push_back(9);
        // [9, 12, |6]
        d3.push_back(12);

        d3.push_back(15);
        // There used to be a bug here about how the
        // VecDeque made growth assumptions about the
        // underlying Vec which didn't hold and lead
        // to corruption.
        // (Vec grows to next power of two)
        //good- [9, 12, 15, X, X, X, X, |6]
        //bug-  [15, 12, X, X, X, |6, X, X]
        assert_eq!(d3.pop_front(), Some(6));

        // Which leads us to the following state which
        // would be a failure case.
        //bug-  [15, 12, X, X, X, X, |X, X]
        assert_eq!(d3.front(), Some(&9));
    }

    #[test]
    fn test_reserve_exact() {
        let mut d = VecDeque::new();
        d.push_back(0);
        d.reserve_exact(50);
        assert!(d.capacity() >= 51);
    }

    #[test]
    fn test_reserve() {
        let mut d = VecDeque::new();
        d.push_back(0);
        d.reserve(50);
        assert!(d.capacity() >= 51);
    }

    #[test]
    fn test_swap() {
        let mut d: VecDeque<_> = (0..5).collect();
        d.pop_front();
        d.swap(0, 3);
        assert_eq!(d.iter().cloned().collect::<Vec<_>>(), [4, 2, 3, 1]);
    }

    #[test]
    fn test_iter() {
        let mut d = VecDeque::new();
        assert_eq!(d.iter().next(), None);
        assert_eq!(d.iter().size_hint(), (0, Some(0)));

        for i in 0..5 {
            d.push_back(i);
        }
        {
            let b: &[_] = &[&0,&1,&2,&3,&4];
            assert_eq!(d.iter().collect::<Vec<_>>(), b);
        }

        for i in 6..9 {
            d.push_front(i);
        }
        {
            let b: &[_] = &[&8,&7,&6,&0,&1,&2,&3,&4];
            assert_eq!(d.iter().collect::<Vec<_>>(), b);
        }

        let mut it = d.iter();
        let mut len = d.len();
        loop {
            match it.next() {
                None => break,
                _ => { len -= 1; assert_eq!(it.size_hint(), (len, Some(len))) }
            }
        }
    }

    #[test]
    fn test_rev_iter() {
        let mut d = VecDeque::new();
        assert_eq!(d.iter().rev().next(), None);

        for i in 0..5 {
            d.push_back(i);
        }
        {
            let b: &[_] = &[&4,&3,&2,&1,&0];
            assert_eq!(d.iter().rev().collect::<Vec<_>>(), b);
        }

        for i in 6..9 {
            d.push_front(i);
        }
        let b: &[_] = &[&4,&3,&2,&1,&0,&6,&7,&8];
        assert_eq!(d.iter().rev().collect::<Vec<_>>(), b);
    }

    #[test]
    fn test_mut_rev_iter_wrap() {
        let mut d = VecDeque::with_capacity(3);
        assert!(d.iter_mut().rev().next().is_none());

        d.push_back(1);
        d.push_back(2);
        d.push_back(3);
        assert_eq!(d.pop_front(), Some(1));
        d.push_back(4);

        assert_eq!(d.iter_mut().rev().cloned().collect::<Vec<_>>(),
                   vec![4, 3, 2]);
    }

    #[test]
    fn test_mut_iter() {
        let mut d = VecDeque::new();
        assert!(d.iter_mut().next().is_none());

        for i in 0..3 {
            d.push_front(i);
        }

        for (i, elt) in d.iter_mut().enumerate() {
            assert_eq!(*elt, 2 - i);
            *elt = i;
        }

        {
            let mut it = d.iter_mut();
            assert_eq!(*it.next().unwrap(), 0);
            assert_eq!(*it.next().unwrap(), 1);
            assert_eq!(*it.next().unwrap(), 2);
            assert!(it.next().is_none());
        }
    }

    #[test]
    fn test_mut_rev_iter() {
        let mut d = VecDeque::new();
        assert!(d.iter_mut().rev().next().is_none());

        for i in 0..3 {
            d.push_front(i);
        }

        for (i, elt) in d.iter_mut().rev().enumerate() {
            assert_eq!(*elt, i);
            *elt = i;
        }

        {
            let mut it = d.iter_mut().rev();
            assert_eq!(*it.next().unwrap(), 0);
            assert_eq!(*it.next().unwrap(), 1);
            assert_eq!(*it.next().unwrap(), 2);
            assert!(it.next().is_none());
        }
    }

    #[test]
    fn test_into_iter() {

        // Empty iter
        {
            let d: VecDeque<i32> = VecDeque::new();
            let mut iter = d.into_iter();

            assert_eq!(iter.size_hint(), (0, Some(0)));
            assert_eq!(iter.next(), None);
            assert_eq!(iter.size_hint(), (0, Some(0)));
        }

        // simple iter
        {
            let mut d = VecDeque::new();
            for i in 0..5 {
                d.push_back(i);
            }

            let b = vec![0,1,2,3,4];
            assert_eq!(d.into_iter().collect::<Vec<_>>(), b);
        }

        // wrapped iter
        {
            let mut d = VecDeque::new();
            for i in 0..5 {
                d.push_back(i);
            }
            for i in 6..9 {
                d.push_front(i);
            }

            let b = vec![8,7,6,0,1,2,3,4];
            assert_eq!(d.into_iter().collect::<Vec<_>>(), b);
        }

        // partially used
        {
            let mut d = VecDeque::new();
            for i in 0..5 {
                d.push_back(i);
            }
            for i in 6..9 {
                d.push_front(i);
            }

            let mut it = d.into_iter();
            assert_eq!(it.size_hint(), (8, Some(8)));
            assert_eq!(it.next(), Some(8));
            assert_eq!(it.size_hint(), (7, Some(7)));
            assert_eq!(it.next_back(), Some(4));
            assert_eq!(it.size_hint(), (6, Some(6)));
            assert_eq!(it.next(), Some(7));
            assert_eq!(it.size_hint(), (5, Some(5)));
        }
    }

    #[test]
    fn test_drain() {

        // Empty iter
        {
            let mut d: VecDeque<i32> = VecDeque::new();

            {
                let mut iter = d.drain();

                assert_eq!(iter.size_hint(), (0, Some(0)));
                assert_eq!(iter.next(), None);
                assert_eq!(iter.size_hint(), (0, Some(0)));
            }

            assert!(d.is_empty());
        }

        // simple iter
        {
            let mut d = VecDeque::new();
            for i in 0..5 {
                d.push_back(i);
            }

            assert_eq!(d.drain().collect::<Vec<_>>(), [0, 1, 2, 3, 4]);
            assert!(d.is_empty());
        }

        // wrapped iter
        {
            let mut d = VecDeque::new();
            for i in 0..5 {
                d.push_back(i);
            }
            for i in 6..9 {
                d.push_front(i);
            }

            assert_eq!(d.drain().collect::<Vec<_>>(), [8,7,6,0,1,2,3,4]);
            assert!(d.is_empty());
        }

        // partially used
        {
            let mut d: VecDeque<_> = VecDeque::new();
            for i in 0..5 {
                d.push_back(i);
            }
            for i in 6..9 {
                d.push_front(i);
            }

            {
                let mut it = d.drain();
                assert_eq!(it.size_hint(), (8, Some(8)));
                assert_eq!(it.next(), Some(8));
                assert_eq!(it.size_hint(), (7, Some(7)));
                assert_eq!(it.next_back(), Some(4));
                assert_eq!(it.size_hint(), (6, Some(6)));
                assert_eq!(it.next(), Some(7));
                assert_eq!(it.size_hint(), (5, Some(5)));
            }
            assert!(d.is_empty());
        }
    }

    #[test]
    fn test_from_iter() {
        use core::iter;
        let v = vec!(1,2,3,4,5,6,7);
        let deq: VecDeque<_> = v.iter().cloned().collect();
        let u: Vec<_> = deq.iter().cloned().collect();
        assert_eq!(u, v);

        let seq = iter::count(0, 2).take(256);
        let deq: VecDeque<_> = seq.collect();
        for (i, &x) in deq.iter().enumerate() {
            assert_eq!(2*i, x);
        }
        assert_eq!(deq.len(), 256);
    }

    #[test]
    fn test_clone() {
        let mut d = VecDeque::new();
        d.push_front(17);
        d.push_front(42);
        d.push_back(137);
        d.push_back(137);
        assert_eq!(d.len(), 4);
        let mut e = d.clone();
        assert_eq!(e.len(), 4);
        while !d.is_empty() {
            assert_eq!(d.pop_back(), e.pop_back());
        }
        assert_eq!(d.len(), 0);
        assert_eq!(e.len(), 0);
    }

    #[test]
    fn test_eq() {
        let mut d = VecDeque::new();
        assert!(d == VecDeque::with_capacity(0));
        d.push_front(137);
        d.push_front(17);
        d.push_front(42);
        d.push_back(137);
        let mut e = VecDeque::with_capacity(0);
        e.push_back(42);
        e.push_back(17);
        e.push_back(137);
        e.push_back(137);
        assert!(&e == &d);
        e.pop_back();
        e.push_back(0);
        assert!(e != d);
        e.clear();
        assert!(e == VecDeque::new());
    }

    #[test]
    fn test_hash() {
      let mut x = VecDeque::new();
      let mut y = VecDeque::new();

      x.push_back(1);
      x.push_back(2);
      x.push_back(3);

      y.push_back(0);
      y.push_back(1);
      y.pop_front();
      y.push_back(2);
      y.push_back(3);

      assert!(hash::hash::<_, SipHasher>(&x) == hash::hash::<_, SipHasher>(&y));
    }

    #[test]
    fn test_ord() {
        let x = VecDeque::new();
        let mut y = VecDeque::new();
        y.push_back(1);
        y.push_back(2);
        y.push_back(3);
        assert!(x < y);
        assert!(y > x);
        assert!(x <= x);
        assert!(x >= x);
    }

    #[test]
    fn test_show() {
        let ringbuf: VecDeque<_> = (0..10).collect();
        assert_eq!(format!("{:?}", ringbuf), "[0, 1, 2, 3, 4, 5, 6, 7, 8, 9]");

        let ringbuf: VecDeque<_> = vec!["just", "one", "test", "more"].iter()
                                                                        .cloned()
                                                                        .collect();
        assert_eq!(format!("{:?}", ringbuf), "[\"just\", \"one\", \"test\", \"more\"]");
    }

    #[test]
    fn test_drop() {
        static mut drops: i32 = 0;
        struct Elem;
        impl Drop for Elem {
            fn drop(&mut self) {
                unsafe { drops += 1; }
            }
        }

        let mut ring = VecDeque::new();
        ring.push_back(Elem);
        ring.push_front(Elem);
        ring.push_back(Elem);
        ring.push_front(Elem);
        drop(ring);

        assert_eq!(unsafe {drops}, 4);
    }

    #[test]
    fn test_drop_with_pop() {
        static mut drops: i32 = 0;
        struct Elem;
        impl Drop for Elem {
            fn drop(&mut self) {
                unsafe { drops += 1; }
            }
        }

        let mut ring = VecDeque::new();
        ring.push_back(Elem);
        ring.push_front(Elem);
        ring.push_back(Elem);
        ring.push_front(Elem);

        drop(ring.pop_back());
        drop(ring.pop_front());
        assert_eq!(unsafe {drops}, 2);

        drop(ring);
        assert_eq!(unsafe {drops}, 4);
    }

    #[test]
    fn test_drop_clear() {
        static mut drops: i32 = 0;
        struct Elem;
        impl Drop for Elem {
            fn drop(&mut self) {
                unsafe { drops += 1; }
            }
        }

        let mut ring = VecDeque::new();
        ring.push_back(Elem);
        ring.push_front(Elem);
        ring.push_back(Elem);
        ring.push_front(Elem);
        ring.clear();
        assert_eq!(unsafe {drops}, 4);

        drop(ring);
        assert_eq!(unsafe {drops}, 4);
    }

    #[test]
    fn test_reserve_grow() {
        // test growth path A
        // [T o o H] -> [T o o H . . . . ]
        let mut ring = VecDeque::with_capacity(4);
        for i in 0..3 {
            ring.push_back(i);
        }
        ring.reserve(7);
        for i in 0..3 {
            assert_eq!(ring.pop_front(), Some(i));
        }

        // test growth path B
        // [H T o o] -> [. T o o H . . . ]
        let mut ring = VecDeque::with_capacity(4);
        for i in 0..1 {
            ring.push_back(i);
            assert_eq!(ring.pop_front(), Some(i));
        }
        for i in 0..3 {
            ring.push_back(i);
        }
        ring.reserve(7);
        for i in 0..3 {
            assert_eq!(ring.pop_front(), Some(i));
        }

        // test growth path C
        // [o o H T] -> [o o H . . . . T ]
        let mut ring = VecDeque::with_capacity(4);
        for i in 0..3 {
            ring.push_back(i);
            assert_eq!(ring.pop_front(), Some(i));
        }
        for i in 0..3 {
            ring.push_back(i);
        }
        ring.reserve(7);
        for i in 0..3 {
            assert_eq!(ring.pop_front(), Some(i));
        }
    }

    #[test]
    fn test_get() {
        let mut ring = VecDeque::new();
        ring.push_back(0);
        assert_eq!(ring.get(0), Some(&0));
        assert_eq!(ring.get(1), None);

        ring.push_back(1);
        assert_eq!(ring.get(0), Some(&0));
        assert_eq!(ring.get(1), Some(&1));
        assert_eq!(ring.get(2), None);

        ring.push_back(2);
        assert_eq!(ring.get(0), Some(&0));
        assert_eq!(ring.get(1), Some(&1));
        assert_eq!(ring.get(2), Some(&2));
        assert_eq!(ring.get(3), None);

        assert_eq!(ring.pop_front(), Some(0));
        assert_eq!(ring.get(0), Some(&1));
        assert_eq!(ring.get(1), Some(&2));
        assert_eq!(ring.get(2), None);

        assert_eq!(ring.pop_front(), Some(1));
        assert_eq!(ring.get(0), Some(&2));
        assert_eq!(ring.get(1), None);

        assert_eq!(ring.pop_front(), Some(2));
        assert_eq!(ring.get(0), None);
        assert_eq!(ring.get(1), None);
    }

    #[test]
    fn test_get_mut() {
        let mut ring = VecDeque::new();
        for i in 0..3 {
            ring.push_back(i);
        }

        match ring.get_mut(1) {
            Some(x) => *x = -1,
            None => ()
        };

        assert_eq!(ring.get_mut(0), Some(&mut 0));
        assert_eq!(ring.get_mut(1), Some(&mut -1));
        assert_eq!(ring.get_mut(2), Some(&mut 2));
        assert_eq!(ring.get_mut(3), None);

        assert_eq!(ring.pop_front(), Some(0));
        assert_eq!(ring.get_mut(0), Some(&mut -1));
        assert_eq!(ring.get_mut(1), Some(&mut 2));
        assert_eq!(ring.get_mut(2), None);
    }

    #[test]
    fn test_swap_front_back_remove() {
        fn test(back: bool) {
            // This test checks that every single combination of tail position and length is tested.
            // Capacity 15 should be large enough to cover every case.
            let mut tester = VecDeque::with_capacity(15);
            let usable_cap = tester.capacity();
            let final_len = usable_cap / 2;

            for len in 0..final_len {
                let expected = if back {
                    (0..len).collect()
                } else {
                    (0..len).rev().collect()
                };
                for tail_pos in 0..usable_cap {
                    tester.tail = tail_pos;
                    tester.head = tail_pos;
                    if back {
                        for i in 0..len * 2 {
                            tester.push_front(i);
                        }
                        for i in 0..len {
                            assert_eq!(tester.swap_back_remove(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_front_remove(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 = iter::count(0, 1).take(len).collect();
            for tail_pos in 0..cap {
                for to_insert in 0..len {
                    tester.tail = tail_pos;
                    tester.head = tail_pos;
                    for i in 0..len {
                        if i != to_insert {
                            tester.push_back(i);
                        }
                    }
                    tester.insert(to_insert, to_insert);
                    assert!(tester.tail < tester.cap);
                    assert!(tester.head < tester.cap);
                    assert_eq!(tester, expected);
                }
            }
        }
    }

    #[test]
    fn test_remove() {
        // This test checks that every single combination of tail position, length, and
        // removal position is tested. Capacity 15 should be large enough to cover every case.

        let mut tester = VecDeque::with_capacity(15);
        // can't guarantee we got 15, so have to get what we got.
        // 15 would be great, but we will definitely get 2^k - 1, for k >= 4, or else
        // this test isn't covering what it wants to
        let cap = tester.capacity();

        // len is the length *after* removal
        for len in 0..cap - 1 {
            // 0, 1, 2, .., len - 1
            let expected = iter::count(0, 1).take(len).collect();
            for tail_pos in 0..cap {
                for to_remove in 0..len + 1 {
                    tester.tail = tail_pos;
                    tester.head = tail_pos;
                    for i in 0..len {
                        if i == to_remove {
                            tester.push_back(1234);
                        }
                        tester.push_back(i);
                    }
                    if to_remove == len {
                        tester.push_back(1234);
                    }
                    tester.remove(to_remove);
                    assert!(tester.tail < tester.cap);
                    assert!(tester.head < tester.cap);
                    assert_eq!(tester, expected);
                }
            }
        }
    }

    #[test]
    fn test_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 = iter::count(0, 1).take(len).collect();
            for tail_pos in 0..max_cap + 1 {
                tester.tail = tail_pos;
                tester.head = tail_pos;
                tester.reserve(63);
                for i in 0..len {
                    tester.push_back(i);
                }
                tester.shrink_to_fit();
                assert!(tester.capacity() <= cap);
                assert!(tester.tail < tester.cap);
                assert!(tester.head < tester.cap);
                assert_eq!(tester, expected);
            }
        }
    }

    #[test]
    fn test_front() {
        let mut ring = VecDeque::new();
        ring.push_back(10);
        ring.push_back(20);
        assert_eq!(ring.front(), Some(&10));
        ring.pop_front();
        assert_eq!(ring.front(), Some(&20));
        ring.pop_front();
        assert_eq!(ring.front(), None);
    }

    #[test]
    fn test_as_slices() {
        let mut ring: VecDeque<i32> = VecDeque::with_capacity(127);
        let cap = ring.capacity() as i32;
        let first = cap/2;
        let last  = cap - first;
        for i in 0..first {
            ring.push_back(i);

            let (left, right) = ring.as_slices();
            let expected: Vec<_> = (0..i+1).collect();
            assert_eq!(left, expected);
            assert_eq!(right, []);
        }

        for j in -last..0 {
            ring.push_front(j);
            let (left, right) = ring.as_slices();
            let expected_left: Vec<_> = (-last..j+1).rev().collect();
            let expected_right: Vec<_> = (0..first).collect();
            assert_eq!(left, expected_left);
            assert_eq!(right, expected_right);
        }

        assert_eq!(ring.len() as i32, cap);
        assert_eq!(ring.capacity() as i32, cap);
    }

    #[test]
    fn test_as_mut_slices() {
        let mut ring: VecDeque<i32> = VecDeque::with_capacity(127);
        let cap = ring.capacity() as i32;
        let first = cap/2;
        let last  = cap - first;
        for i in 0..first {
            ring.push_back(i);

            let (left, right) = ring.as_mut_slices();
            let expected: Vec<_> = (0..i+1).collect();
            assert_eq!(left, expected);
            assert_eq!(right, []);
        }

        for j in -last..0 {
            ring.push_front(j);
            let (left, right) = ring.as_mut_slices();
            let expected_left: Vec<_> = (-last..j+1).rev().collect();
            let expected_right: Vec<_> = (0..first).collect();
            assert_eq!(left, expected_left);
            assert_eq!(right, expected_right);
        }

        assert_eq!(ring.len() as i32, cap);
        assert_eq!(ring.capacity() as i32, cap);
    }

    #[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 = iter::count(0, 1).take(at).collect();
                // at, at + 1, .., len - 1 (may be empty)
                let expected_other = iter::count(at, 1).take(len - at).collect();

                for tail_pos in 0..cap {
                    tester.tail = tail_pos;
                    tester.head = tail_pos;
                    for i in 0..len {
                        tester.push_back(i);
                    }
                    let result = tester.split_off(at);
                    assert!(tester.tail < tester.cap);
                    assert!(tester.head < tester.cap);
                    assert!(result.tail < result.cap);
                    assert!(result.head < result.cap);
                    assert_eq!(tester, expected_self);
                    assert_eq!(result, expected_other);
                }
            }
        }
    }

    #[test]
    fn test_append() {
        let mut a: VecDeque<_> = vec![1, 2, 3].into_iter().collect();
        let mut b: VecDeque<_> = vec![4, 5, 6].into_iter().collect();

        // normal append
        a.append(&mut b);
        assert_eq!(a.iter().cloned().collect::<Vec<_>>(), [1, 2, 3, 4, 5, 6]);
        assert_eq!(b.iter().cloned().collect::<Vec<_>>(), []);

        // append nothing to something
        a.append(&mut b);
        assert_eq!(a.iter().cloned().collect::<Vec<_>>(), [1, 2, 3, 4, 5, 6]);
        assert_eq!(b.iter().cloned().collect::<Vec<_>>(), []);

        // append something to nothing
        b.append(&mut a);
        assert_eq!(b.iter().cloned().collect::<Vec<_>>(), [1, 2, 3, 4, 5, 6]);
        assert_eq!(a.iter().cloned().collect::<Vec<_>>(), []);
    }
}