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    // Copyright 2013-2014 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 priority queue implemented with a binary heap.
    //!
    //! Insertion and popping the largest element have `O(log n)` time complexity.
    //! Checking the largest element is `O(1)`. Converting a vector to a binary heap
    //! can be done in-place, and has `O(n)` complexity. A binary heap can also be
    //! converted to a sorted vector in-place, allowing it to be used for an `O(n
    //! log n)` in-place heapsort.
    //!
    //! # Examples
    //!
    //! This is a larger example that implements [Dijkstra's algorithm][dijkstra]
    //! to solve the [shortest path problem][sssp] on a [directed graph][dir_graph].
    //! It shows how to use `BinaryHeap` with custom types.
    //!
    //! [dijkstra]: http://en.wikipedia.org/wiki/Dijkstra%27s_algorithm
    //! [sssp]: http://en.wikipedia.org/wiki/Shortest_path_problem
    //! [dir_graph]: http://en.wikipedia.org/wiki/Directed_graph
    //!
    //! ```
    //! use std::cmp::Ordering;
    //! use std::collections::BinaryHeap;
    //! use std::usize;
    //!
    //! #[derive(Copy, Clone, Eq, PartialEq)]
    //! struct State {
    //!     cost: usize,
    //!     position: usize,
    //! }
    //!
    //! // The priority queue depends on `Ord`.
    //! // Explicitly implement the trait so the queue becomes a min-heap
    //! // instead of a max-heap.
    //! impl Ord for State {
    //!     fn cmp(&self, other: &State) -> Ordering {
    //!         // Notice that the we flip the ordering here
    //!         other.cost.cmp(&self.cost)
    //!     }
    //! }
    //!
    //! // `PartialOrd` needs to be implemented as well.
    //! impl PartialOrd for State {
    //!     fn partial_cmp(&self, other: &State) -> Option<Ordering> {
    //!         Some(self.cmp(other))
    //!     }
    //! }
    //!
    //! // Each node is represented as an `usize`, for a shorter implementation.
    //! struct Edge {
    //!     node: usize,
    //!     cost: usize,
    //! }
    //!
    //! // Dijkstra's shortest path algorithm.
    //!
    //! // Start at `start` and use `dist` to track the current shortest distance
    //! // to each node. This implementation isn't memory-efficient as it may leave duplicate
    //! // nodes in the queue. It also uses `usize::MAX` as a sentinel value,
    //! // for a simpler implementation.
    //! fn shortest_path(adj_list: &Vec<Vec<Edge>>, start: usize, goal: usize) -> Option<usize> {
    //!     // dist[node] = current shortest distance from `start` to `node`
    //!     let mut dist: Vec<_> = (0..adj_list.len()).map(|_| usize::MAX).collect();
    //!
    //!     let mut heap = BinaryHeap::new();
    //!
    //!     // We're at `start`, with a zero cost
    //!     dist[start] = 0;
    //!     heap.push(State { cost: 0, position: start });
    //!
    //!     // Examine the frontier with lower cost nodes first (min-heap)
    //!     while let Some(State { cost, position }) = heap.pop() {
    //!         // Alternatively we could have continued to find all shortest paths
    //!         if position == goal { return Some(cost); }
    //!
    //!         // Important as we may have already found a better way
    //!         if cost > dist[position] { continue; }
    //!
    //!         // For each node we can reach, see if we can find a way with
    //!         // a lower cost going through this node
    //!         for edge in &adj_list[position] {
    //!             let next = State { cost: cost + edge.cost, position: edge.node };
    //!
    //!             // If so, add it to the frontier and continue
    //!             if next.cost < dist[next.position] {
    //!                 heap.push(next);
    //!                 // Relaxation, we have now found a better way
    //!                 dist[next.position] = next.cost;
    //!             }
    //!         }
    //!     }
    //!
    //!     // Goal not reachable
    //!     None
    //! }
    //!
    //! fn main() {
    //!     // This is the directed graph we're going to use.
    //!     // The node numbers correspond to the different states,
    //!     // and the edge weights symbolize the cost of moving
    //!     // from one node to another.
    //!     // Note that the edges are one-way.
    //!     //
    //!     //                  7
    //!     //          +-----------------+
    //!     //          |                 |
    //!     //          v   1        2    |  2
    //!     //          0 -----> 1 -----> 3 ---> 4
    //!     //          |        ^        ^      ^
    //!     //          |        | 1      |      |
    //!     //          |        |        | 3    | 1
    //!     //          +------> 2 -------+      |
    //!     //           10      |               |
    //!     //                   +---------------+
    //!     //
    //!     // The graph is represented as an adjacency list where each index,
    //!     // corresponding to a node value, has a list of outgoing edges.
    //!     // Chosen for its efficiency.
    //!     let graph = vec![
    //!         // Node 0
    //!         vec![Edge { node: 2, cost: 10 },
    //!              Edge { node: 1, cost: 1 }],
    //!         // Node 1
    //!         vec![Edge { node: 3, cost: 2 }],
    //!         // Node 2
    //!         vec![Edge { node: 1, cost: 1 },
    //!              Edge { node: 3, cost: 3 },
    //!              Edge { node: 4, cost: 1 }],
    //!         // Node 3
    //!         vec![Edge { node: 0, cost: 7 },
    //!              Edge { node: 4, cost: 2 }],
    //!         // Node 4
    //!         vec![]];
    //!
    //!     assert_eq!(shortest_path(&graph, 0, 1), Some(1));
    //!     assert_eq!(shortest_path(&graph, 0, 3), Some(3));
    //!     assert_eq!(shortest_path(&graph, 3, 0), Some(7));
    //!     assert_eq!(shortest_path(&graph, 0, 4), Some(5));
    //!     assert_eq!(shortest_path(&graph, 4, 0), None);
    //! }
    //! ```

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

    use core::iter::FromIterator;
    use core::mem::swap;
    use core::ptr;
    use core::fmt;

    use slice;
    use vec::{self, Vec};

    /// A priority queue implemented with a binary heap.
    ///
    /// This will be a max-heap.
    ///
    /// It is a logic error for an item to be modified in such a way that the
    /// item's ordering relative to any other item, as determined by the `Ord`
    /// trait, changes while it is in the heap. This is normally only possible
    /// through `Cell`, `RefCell`, global state, I/O, or unsafe code.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::collections::BinaryHeap;
    ///
    /// // type inference lets us omit an explicit type signature (which
    /// // would be `BinaryHeap<i32>` in this example).
    /// let mut heap = BinaryHeap::new();
    ///
    /// // We can use peek to look at the next item in the heap. In this case,
    /// // there's no items in there yet so we get None.
    /// assert_eq!(heap.peek(), None);
    ///
    /// // Let's add some scores...
    /// heap.push(1);
    /// heap.push(5);
    /// heap.push(2);
    ///
    /// // Now peek shows the most important item in the heap.
    /// assert_eq!(heap.peek(), Some(&5));
    ///
    /// // We can check the length of a heap.
    /// assert_eq!(heap.len(), 3);
    ///
    /// // We can iterate over the items in the heap, although they are returned in
    /// // a random order.
    /// for x in heap.iter() {
    ///     println!("{}", x);
    /// }
    ///
    /// // If we instead pop these scores, they should come back in order.
    /// assert_eq!(heap.pop(), Some(5));
    /// assert_eq!(heap.pop(), Some(2));
    /// assert_eq!(heap.pop(), Some(1));
    /// assert_eq!(heap.pop(), None);
    ///
    /// // We can clear the heap of any remaining items.
    /// heap.clear();
    ///
    /// // The heap should now be empty.
    /// assert!(heap.is_empty())
    /// ```
    #[stable(feature = "rust1", since = "1.0.0")]
    pub struct BinaryHeap<T> {
        data: Vec<T>,
    }

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

        fn clone_from(&mut self, source: &Self) {
            self.data.clone_from(&source.data);
        }
    }

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

    #[stable(feature = "binaryheap_debug", since = "1.4.0")]
    impl<T: fmt::Debug + Ord> fmt::Debug for BinaryHeap<T> {
        fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
            f.debug_list().entries(self.iter()).finish()
        }
    }

    impl<T: Ord> BinaryHeap<T> {
        /// Creates an empty `BinaryHeap` as a max-heap.
        ///
        /// # Examples
        ///
        /// Basic usage:
        ///
        /// ```
        /// use std::collections::BinaryHeap;
        /// let mut heap = BinaryHeap::new();
        /// heap.push(4);
        /// ```
        #[stable(feature = "rust1", since = "1.0.0")]
        pub fn new() -> BinaryHeap<T> {
            BinaryHeap { data: vec![] }
        }

        /// Creates an empty `BinaryHeap` with a specific capacity.
        /// This preallocates enough memory for `capacity` elements,
        /// so that the `BinaryHeap` does not have to be reallocated
        /// until it contains at least that many values.
        ///
        /// # Examples
        ///
        /// Basic usage:
        ///
        /// ```
        /// use std::collections::BinaryHeap;
        /// let mut heap = BinaryHeap::with_capacity(10);
        /// heap.push(4);
        /// ```
        #[stable(feature = "rust1", since = "1.0.0")]
        pub fn with_capacity(capacity: usize) -> BinaryHeap<T> {
            BinaryHeap { data: Vec::with_capacity(capacity) }
        }

        /// Returns an iterator visiting all values in the underlying vector, in
        /// arbitrary order.
        ///
        /// # Examples
        ///
        /// Basic usage:
        ///
        /// ```
        /// use std::collections::BinaryHeap;
        /// let heap = BinaryHeap::from(vec![1, 2, 3, 4]);
        ///
        /// // Print 1, 2, 3, 4 in arbitrary order
        /// for x in heap.iter() {
        ///     println!("{}", x);
        /// }
        /// ```
        #[stable(feature = "rust1", since = "1.0.0")]
        pub fn iter(&self) -> Iter<T> {
            Iter { iter: self.data.iter() }
        }

        /// Returns the greatest item in the binary heap, or `None` if it is empty.
        ///
        /// # Examples
        ///
        /// Basic usage:
        ///
        /// ```
        /// use std::collections::BinaryHeap;
        /// let mut heap = BinaryHeap::new();
        /// assert_eq!(heap.peek(), None);
        ///
        /// heap.push(1);
        /// heap.push(5);
        /// heap.push(2);
        /// assert_eq!(heap.peek(), Some(&5));
        ///
        /// ```
        #[stable(feature = "rust1", since = "1.0.0")]
        pub fn peek(&self) -> Option<&T> {
            self.data.get(0)
        }

        /// Returns the number of elements the binary heap can hold without reallocating.
        ///
        /// # Examples
        ///
        /// Basic usage:
        ///
        /// ```
        /// use std::collections::BinaryHeap;
        /// let mut heap = BinaryHeap::with_capacity(100);
        /// assert!(heap.capacity() >= 100);
        /// heap.push(4);
        /// ```
        #[stable(feature = "rust1", since = "1.0.0")]
        pub fn capacity(&self) -> usize {
            self.data.capacity()
        }

        /// Reserves the minimum capacity for exactly `additional` more elements to be inserted in the
        /// given `BinaryHeap`. 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
        ///
        /// Basic usage:
        ///
        /// ```
        /// use std::collections::BinaryHeap;
        /// let mut heap = BinaryHeap::new();
        /// heap.reserve_exact(100);
        /// assert!(heap.capacity() >= 100);
        /// heap.push(4);
        /// ```
        #[stable(feature = "rust1", since = "1.0.0")]
        pub fn reserve_exact(&mut self, additional: usize) {
            self.data.reserve_exact(additional);
        }

        /// Reserves capacity for at least `additional` more elements to be inserted in the
        /// `BinaryHeap`. The collection may reserve more space to avoid frequent reallocations.
        ///
        /// # Panics
        ///
        /// Panics if the new capacity overflows `usize`.
        ///
        /// # Examples
        ///
        /// Basic usage:
        ///
        /// ```
        /// use std::collections::BinaryHeap;
        /// let mut heap = BinaryHeap::new();
        /// heap.reserve(100);
        /// assert!(heap.capacity() >= 100);
        /// heap.push(4);
        /// ```
        #[stable(feature = "rust1", since = "1.0.0")]
        pub fn reserve(&mut self, additional: usize) {
            self.data.reserve(additional);
        }

        /// Discards as much additional capacity as possible.
        ///
        /// # Examples
        ///
        /// Basic usage:
        ///
        /// ```
        /// use std::collections::BinaryHeap;
        /// let mut heap: BinaryHeap<i32> = BinaryHeap::with_capacity(100);
        ///
        /// assert!(heap.capacity() >= 100);
        /// heap.shrink_to_fit();
        /// assert!(heap.capacity() == 0);
        /// ```
        #[stable(feature = "rust1", since = "1.0.0")]
        pub fn shrink_to_fit(&mut self) {
            self.data.shrink_to_fit();
        }

        /// Removes the greatest item from the binary heap and returns it, or `None` if it
        /// is empty.
        ///
        /// # Examples
        ///
        /// Basic usage:
        ///
        /// ```
        /// use std::collections::BinaryHeap;
        /// let mut heap = BinaryHeap::from(vec![1, 3]);
        ///
        /// assert_eq!(heap.pop(), Some(3));
        /// assert_eq!(heap.pop(), Some(1));
        /// assert_eq!(heap.pop(), None);
        /// ```
        #[stable(feature = "rust1", since = "1.0.0")]
        pub fn pop(&mut self) -> Option<T> {
            self.data.pop().map(|mut item| {
                if !self.is_empty() {
                    swap(&mut item, &mut self.data[0]);
                    self.sift_down_to_bottom(0);
                }
                item
            })
        }

        /// Pushes an item onto the binary heap.
        ///
        /// # Examples
        ///
        /// Basic usage:
        ///
        /// ```
        /// use std::collections::BinaryHeap;
        /// let mut heap = BinaryHeap::new();
        /// heap.push(3);
        /// heap.push(5);
        /// heap.push(1);
        ///
        /// assert_eq!(heap.len(), 3);
        /// assert_eq!(heap.peek(), Some(&5));
        /// ```
        #[stable(feature = "rust1", since = "1.0.0")]
        pub fn push(&mut self, item: T) {
            let old_len = self.len();
            self.data.push(item);
            self.sift_up(0, old_len);
        }

        /// Pushes an item onto the binary heap, then pops the greatest item off the queue in
        /// an optimized fashion.
        ///
        /// # Examples
        ///
        /// Basic usage:
        ///
        /// ```
        /// #![feature(binary_heap_extras)]
        ///
        /// use std::collections::BinaryHeap;
        /// let mut heap = BinaryHeap::new();
        /// heap.push(1);
        /// heap.push(5);
        ///
        /// assert_eq!(heap.push_pop(3), 5);
        /// assert_eq!(heap.push_pop(9), 9);
        /// assert_eq!(heap.len(), 2);
        /// assert_eq!(heap.peek(), Some(&3));
        /// ```
        #[unstable(feature = "binary_heap_extras",
                   reason = "needs to be audited",
                   issue = "28147")]
        pub fn push_pop(&mut self, mut item: T) -> T {
            match self.data.get_mut(0) {
                None => return item,
                Some(top) => {
                    if *top > item {
                        swap(&mut item, top);
                    } else {
                        return item;
                    }
                }
            }

            self.sift_down(0);
            item
        }

        /// Pops the greatest item off the binary heap, then pushes an item onto the queue in
        /// an optimized fashion. The push is done regardless of whether the binary heap
        /// was empty.
        ///
        /// # Examples
        ///
        /// Basic usage:
        ///
        /// ```
        /// #![feature(binary_heap_extras)]
        ///
        /// use std::collections::BinaryHeap;
        /// let mut heap = BinaryHeap::new();
        ///
        /// assert_eq!(heap.replace(1), None);
        /// assert_eq!(heap.replace(3), Some(1));
        /// assert_eq!(heap.len(), 1);
        /// assert_eq!(heap.peek(), Some(&3));
        /// ```
        #[unstable(feature = "binary_heap_extras",
                   reason = "needs to be audited",
                   issue = "28147")]
        pub fn replace(&mut self, mut item: T) -> Option<T> {
            if !self.is_empty() {
                swap(&mut item, &mut self.data[0]);
                self.sift_down(0);
                Some(item)
            } else {
                self.push(item);
                None
            }
        }

        /// Consumes the `BinaryHeap` and returns the underlying vector
        /// in arbitrary order.
        ///
        /// # Examples
        ///
        /// Basic usage:
        ///
        /// ```
        /// use std::collections::BinaryHeap;
        /// let heap = BinaryHeap::from(vec![1, 2, 3, 4, 5, 6, 7]);
        /// let vec = heap.into_vec();
        ///
        /// // Will print in some order
        /// for x in vec {
        ///     println!("{}", x);
        /// }
        /// ```
        #[stable(feature = "binary_heap_extras_15", since = "1.5.0")]
        pub fn into_vec(self) -> Vec<T> {
            self.into()
        }

        /// Consumes the `BinaryHeap` and returns a vector in sorted
        /// (ascending) order.
        ///
        /// # Examples
        ///
        /// Basic usage:
        ///
        /// ```
        /// use std::collections::BinaryHeap;
        ///
        /// let mut heap = BinaryHeap::from(vec![1, 2, 4, 5, 7]);
        /// heap.push(6);
        /// heap.push(3);
        ///
        /// let vec = heap.into_sorted_vec();
        /// assert_eq!(vec, [1, 2, 3, 4, 5, 6, 7]);
        /// ```
        #[stable(feature = "binary_heap_extras_15", since = "1.5.0")]
        pub fn into_sorted_vec(mut self) -> Vec<T> {
            let mut end = self.len();
            while end > 1 {
                end -= 1;
                self.data.swap(0, end);
                self.sift_down_range(0, end);
            }
            self.into_vec()
        }

        // The implementations of sift_up and sift_down use unsafe blocks in
        // order to move an element out of the vector (leaving behind a
        // hole), shift along the others and move the removed element back into the
        // vector at the final location of the hole.
        // The `Hole` type is used to represent this, and make sure
        // the hole is filled back at the end of its scope, even on panic.
        // Using a hole reduces the constant factor compared to using swaps,
        // which involves twice as many moves.
        fn sift_up(&mut self, start: usize, pos: usize) {
            unsafe {
                // Take out the value at `pos` and create a hole.
                let mut hole = Hole::new(&mut self.data, pos);

                while hole.pos() > start {
                    let parent = (hole.pos() - 1) / 2;
                    if hole.element() <= hole.get(parent) {
                        break;
                    }
                    hole.move_to(parent);
                }
            }
        }

        /// Take an element at `pos` and move it down the heap,
        /// while its children are larger.
        fn sift_down_range(&mut self, pos: usize, end: usize) {
            unsafe {
                let mut hole = Hole::new(&mut self.data, pos);
                let mut child = 2 * pos + 1;
                while child < end {
                    let right = child + 1;
                    // compare with the greater of the two children
                    if right < end && !(hole.get(child) > hole.get(right)) {
                        child = right;
                    }
                    // if we are already in order, stop.
                    if hole.element() >= hole.get(child) {
                        break;
                    }
                    hole.move_to(child);
                    child = 2 * hole.pos() + 1;
                }
            }
        }

        fn sift_down(&mut self, pos: usize) {
            let len = self.len();
            self.sift_down_range(pos, len);
        }

        /// Take an element at `pos` and move it all the way down the heap,
        /// then sift it up to its position.
        ///
        /// Note: This is faster when the element is known to be large / should
        /// be closer to the bottom.
        fn sift_down_to_bottom(&mut self, mut pos: usize) {
            let end = self.len();
            let start = pos;
            unsafe {
                let mut hole = Hole::new(&mut self.data, pos);
                let mut child = 2 * pos + 1;
                while child < end {
                    let right = child + 1;
                    // compare with the greater of the two children
                    if right < end && !(hole.get(child) > hole.get(right)) {
                        child = right;
                    }
                    hole.move_to(child);
                    child = 2 * hole.pos() + 1;
                }
                pos = hole.pos;
            }
            self.sift_up(start, pos);
        }

        /// Returns the length of the binary heap.
        ///
        /// # Examples
        ///
        /// Basic usage:
        ///
        /// ```
        /// use std::collections::BinaryHeap;
        /// let heap = BinaryHeap::from(vec![1, 3]);
        ///
        /// assert_eq!(heap.len(), 2);
        /// ```
        #[stable(feature = "rust1", since = "1.0.0")]
        pub fn len(&self) -> usize {
            self.data.len()
        }

        /// Checks if the binary heap is empty.
        ///
        /// # Examples
        ///
        /// Basic usage:
        ///
        /// ```
        /// use std::collections::BinaryHeap;
        /// let mut heap = BinaryHeap::new();
        ///
        /// assert!(heap.is_empty());
        ///
        /// heap.push(3);
        /// heap.push(5);
        /// heap.push(1);
        ///
        /// assert!(!heap.is_empty());
        /// ```
        #[stable(feature = "rust1", since = "1.0.0")]
        pub fn is_empty(&self) -> bool {
            self.len() == 0
        }

        /// Clears the binary heap, returning an iterator over the removed elements.
        ///
        /// The elements are removed in arbitrary order.
        ///
        /// # Examples
        ///
        /// Basic usage:
        ///
        /// ```
        /// use std::collections::BinaryHeap;
        /// let mut heap = BinaryHeap::from(vec![1, 3]);
        ///
        /// assert!(!heap.is_empty());
        ///
        /// for x in heap.drain() {
        ///     println!("{}", x);
        /// }
        ///
        /// assert!(heap.is_empty());
        /// ```
        #[inline]
        #[stable(feature = "drain", since = "1.6.0")]
        pub fn drain(&mut self) -> Drain<T> {
            Drain { iter: self.data.drain(..) }
        }

        /// Drops all items from the binary heap.
        ///
        /// # Examples
        ///
        /// Basic usage:
        ///
        /// ```
        /// use std::collections::BinaryHeap;
        /// let mut heap = BinaryHeap::from(vec![1, 3]);
        ///
        /// assert!(!heap.is_empty());
        ///
        /// heap.clear();
        ///
        /// assert!(heap.is_empty());
        /// ```
        #[stable(feature = "rust1", since = "1.0.0")]
        pub fn clear(&mut self) {
            self.drain();
        }
    }

    /// Hole represents a hole in a slice i.e. an index without valid value
    /// (because it was moved from or duplicated).
    /// In drop, `Hole` will restore the slice by filling the hole
    /// position with the value that was originally removed.
    struct Hole<'a, T: 'a> {
        data: &'a mut [T],
        /// `elt` is always `Some` from new until drop.
        elt: Option<T>,
        pos: usize,
    }

    impl<'a, T> Hole<'a, T> {
        /// Create a new Hole at index `pos`.
        fn new(data: &'a mut [T], pos: usize) -> Self {
            unsafe {
                let elt = ptr::read(&data[pos]);
                Hole {
                    data: data,
                    elt: Some(elt),
                    pos: pos,
                }
            }
        }

        #[inline(always)]
        fn pos(&self) -> usize {
            self.pos
        }

        /// Return a reference to the element removed
        #[inline(always)]
        fn element(&self) -> &T {
            self.elt.as_ref().unwrap()
        }

        /// Return a reference to the element at `index`.
        ///
        /// Panics if the index is out of bounds.
        ///
        /// Unsafe because index must not equal pos.
        #[inline(always)]
        unsafe fn get(&self, index: usize) -> &T {
            debug_assert!(index != self.pos);
            &self.data[index]
        }

        /// Move hole to new location
        ///
        /// Unsafe because index must not equal pos.
        #[inline(always)]
        unsafe fn move_to(&mut self, index: usize) {
            debug_assert!(index != self.pos);
            let index_ptr: *const _ = &self.data[index];
            let hole_ptr = &mut self.data[self.pos];
            ptr::copy_nonoverlapping(index_ptr, hole_ptr, 1);
            self.pos = index;
        }
    }

    impl<'a, T> Drop for Hole<'a, T> {
        fn drop(&mut self) {
            // fill the hole again
            unsafe {
                let pos = self.pos;
                ptr::write(&mut self.data[pos], self.elt.take().unwrap());
            }
        }
    }

    /// `BinaryHeap` iterator.
    #[stable(feature = "rust1", since = "1.0.0")]
    pub struct Iter<'a, T: 'a> {
        iter: slice::Iter<'a, T>,
    }

    // 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 { iter: self.iter.clone() }
        }
    }

    #[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> {
            self.iter.next()
        }

        #[inline]
        fn size_hint(&self) -> (usize, Option<usize>) {
            self.iter.size_hint()
        }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<'a, T> DoubleEndedIterator for Iter<'a, T> {
        #[inline]
        fn next_back(&mut self) -> Option<&'a T> {
            self.iter.next_back()
        }
    }

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

    /// An iterator that moves out of a `BinaryHeap`.
    #[stable(feature = "rust1", since = "1.0.0")]
    pub struct IntoIter<T> {
        iter: vec::IntoIter<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.iter.next()
        }

        #[inline]
        fn size_hint(&self) -> (usize, Option<usize>) {
            self.iter.size_hint()
        }
    }

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

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

    /// An iterator that drains a `BinaryHeap`.
    #[stable(feature = "drain", since = "1.6.0")]
    pub struct Drain<'a, T: 'a> {
        iter: vec::Drain<'a, T>,
    }

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

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

        #[inline]
        fn size_hint(&self) -> (usize, Option<usize>) {
            self.iter.size_hint()
        }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<'a, T: 'a> DoubleEndedIterator for Drain<'a, T> {
        #[inline]
        fn next_back(&mut self) -> Option<T> {
            self.iter.next_back()
        }
    }

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

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<T: Ord> From<Vec<T>> for BinaryHeap<T> {
        fn from(vec: Vec<T>) -> BinaryHeap<T> {
            let mut heap = BinaryHeap { data: vec };
            let mut n = heap.len() / 2;
            while n > 0 {
                n -= 1;
                heap.sift_down(n);
            }
            heap
        }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<T> From<BinaryHeap<T>> for Vec<T> {
        fn from(heap: BinaryHeap<T>) -> Vec<T> {
            heap.data
        }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<T: Ord> FromIterator<T> for BinaryHeap<T> {
        fn from_iter<I: IntoIterator<Item = T>>(iter: I) -> BinaryHeap<T> {
            BinaryHeap::from(iter.into_iter().collect::<Vec<_>>())
        }
    }

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

        /// Creates a consuming iterator, that is, one that moves each value out of
        /// the binary heap in arbitrary order. The binary heap cannot be used
        /// after calling this.
        ///
        /// # Examples
        ///
        /// Basic usage:
        ///
        /// ```
        /// use std::collections::BinaryHeap;
        /// let heap = BinaryHeap::from(vec![1, 2, 3, 4]);
        ///
        /// // Print 1, 2, 3, 4 in arbitrary order
        /// for x in heap.into_iter() {
        ///     // x has type i32, not &i32
        ///     println!("{}", x);
        /// }
        /// ```
        fn into_iter(self) -> IntoIter<T> {
            IntoIter { iter: self.data.into_iter() }
        }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<'a, T> IntoIterator for &'a BinaryHeap<T> where T: Ord {
        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<T: Ord> Extend<T> for BinaryHeap<T> {
        fn extend<I: IntoIterator<Item = T>>(&mut self, iterable: I) {
            let iter = iterable.into_iter();
            let (lower, _) = iter.size_hint();

            self.reserve(lower);

            for elem in iter {
                self.push(elem);
            }
        }
    }

    #[stable(feature = "extend_ref", since = "1.2.0")]
    impl<'a, T: 'a + Ord + Copy> Extend<&'a T> for BinaryHeap<T> {
        fn extend<I: IntoIterator<Item = &'a T>>(&mut self, iter: I) {
            self.extend(iter.into_iter().cloned());
        }
    }

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