comparator 0.3.0

//! 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
//! [`BinaryHeap`]: struct.BinaryHeap.html
//!
//! ```
//! use comparator::reverse_order;
//! use comparator::collections::BinaryHeap;
//! use std::cmp::Ordering;
//! use std::usize;
//!
//! #[derive(Copy, Clone, Eq, PartialEq, Ord, PartialOrd)]
//! struct State {
//!     cost: usize,
//!     position: usize,
//! }
//!
//! // 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::with_comparator(reverse_order());
//!
//!     // 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);
//! }
//! ```

use combinators::NaturalOrder;
use std::cmp::Ordering;
use std::fmt;
use std::iter::{FromIterator, FusedIterator};
use std::mem::{size_of, swap, ManuallyDrop};
use std::ops::{Deref, DerefMut};
use std::ptr;
use std::slice;
use std::vec;
use Comparator;

/// 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 comparator::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 {
///     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())
/// ```
pub struct BinaryHeap<T, U = NaturalOrder> {
    data: Vec<T>,
    comparator: U,
}

/// Structure wrapping a mutable reference to the greatest item on a
/// `BinaryHeap`.
///
/// This `struct` is created by the [`peek_mut`] method on [`BinaryHeap`]. See
/// its documentation for more.
///
/// [`peek_mut`]: struct.BinaryHeap.html#method.peek_mut
/// [`BinaryHeap`]: struct.BinaryHeap.html
pub struct PeekMut<'a, T, U>
where
    T: 'a,
    U: 'a + Comparator<T>,
{
    heap: &'a mut BinaryHeap<T, U>,
    sift: bool,
}

impl<'a, T, U> fmt::Debug for PeekMut<'a, T, U>
where
    T: fmt::Debug,
    U: Comparator<T>,
{
    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
        f.debug_tuple("PeekMut").field(&self.heap.data[0]).finish()
    }
}

impl<'a, T, U> Drop for PeekMut<'a, T, U>
where
    U: Comparator<T>,
{
    fn drop(&mut self) {
        if self.sift {
            self.heap.sift_down(0);
        }
    }
}

impl<'a, T, U> Deref for PeekMut<'a, T, U>
where
    U: Comparator<T>,
{
    type Target = T;
    fn deref(&self) -> &T {
        &self.heap.data[0]
    }
}

impl<'a, T, U> DerefMut for PeekMut<'a, T, U>
where
    U: Comparator<T>,
{
    fn deref_mut(&mut self) -> &mut T {
        &mut self.heap.data[0]
    }
}

impl<'a, T, U> PeekMut<'a, T, U>
where
    U: Comparator<T>,
{
    /// Removes the peeked value from the heap and returns it.
    pub fn pop(mut this: PeekMut<'a, T, U>) -> T {
        let value = this.heap.pop().unwrap();
        this.sift = false;
        value
    }
}

impl<T: Clone, U: Clone> Clone for BinaryHeap<T, U> {
    fn clone(&self) -> Self {
        Self {
            data: self.data.clone(),
            comparator: self.comparator.clone(),
        }
    }

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

impl<T, U> Default for BinaryHeap<T, U>
where
    U: Comparator<T> + Default,
{
    /// Creates an empty `BinaryHeap<T>`.
    fn default() -> Self {
        Self::with_comparator(U::default())
    }
}

impl<T, U> fmt::Debug for BinaryHeap<T, U>
where
    T: fmt::Debug + Ord,
    U: Comparator<T>,
{
    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
        f.debug_list().entries(self.iter()).finish()
    }
}

impl<T> BinaryHeap<T>
where
    T: Ord,
{
    /// Creates an empty `BinaryHeap` as a max-heap.
    ///
    /// # Examples
    ///
    /// Basic usage:
    ///
    /// ```
    /// use comparator::collections::BinaryHeap;
    /// let mut heap = BinaryHeap::new();
    /// heap.push(4);
    /// ```
    pub fn new() -> Self {
        Self::default()
    }
}

impl<T, U> BinaryHeap<T, U>
where
    U: Default,
{
    /// 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);
    /// ```
    pub fn with_capacity(capacity: usize) -> Self {
        Self {
            data: Vec::with_capacity(capacity),
            comparator: U::default(),
        }
    }
}

impl<T, U> BinaryHeap<T, U>
where
    U: Comparator<T>,
{
    /// Creates an empty `BinaryHeap` with a given comparator.
    ///
    /// # Examples
    ///
    /// Basic usage:
    ///
    /// ```
    /// use comparator::comparing;
    /// use comparator::collections::BinaryHeap;
    /// struct Action {
    ///     weight: u32,
    ///     name: &'static str,
    /// }
    /// let mut heap = BinaryHeap::with_comparator(comparing(|a: &Action| a.weight));
    /// heap.push(Action { weight: 2, name: "B" });
    /// heap.push(Action { weight: 3, name: "A" });
    /// heap.push(Action { weight: 1, name: "C" });
    /// assert_eq!(heap.pop().unwrap().name, "A");
    /// assert_eq!(heap.pop().unwrap().name, "B");
    /// assert_eq!(heap.pop().unwrap().name, "C");
    /// ```
    pub fn with_comparator(comparator: U) -> Self {
        Self {
            data: Vec::new(),
            comparator,
        }
    }

    /// 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);
    /// }
    /// ```
    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));
    ///
    /// ```
    pub fn peek(&self) -> Option<&T> {
        self.data.get(0)
    }

    /// Returns a mutable reference to the greatest item in the binary heap, or
    /// `None` if it is empty.
    ///
    /// Note: If the `PeekMut` value is leaked, the heap may be in an
    /// inconsistent state.
    ///
    /// # Examples
    ///
    /// Basic usage:
    ///
    /// ```
    /// use std::collections::BinaryHeap;
    /// let mut heap = BinaryHeap::new();
    /// assert!(heap.peek_mut().is_none());
    ///
    /// heap.push(1);
    /// heap.push(5);
    /// heap.push(2);
    /// {
    ///     let mut val = heap.peek_mut().unwrap();
    ///     *val = 0;
    /// }
    /// assert_eq!(heap.peek(), Some(&2));
    /// ```
    pub fn peek_mut(&mut self) -> Option<PeekMut<T, U>> {
        if self.is_empty() {
            None
        } else {
            Some(PeekMut {
                heap: self,
                sift: true,
            })
        }
    }

    /// 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);
    /// ```
    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);
    /// ```
    ///
    /// [`reserve`]: #method.reserve
    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);
    /// ```
    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);
    /// ```
    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);
    /// ```
    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));
    /// ```
    pub fn push(&mut self, item: T) {
        let old_len = self.len();
        self.data.push(item);
        self.sift_up(0, old_len);
    }

    /// 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);
    /// }
    /// ```
    pub fn into_vec(self) -> Vec<T> {
        self.data
    }

    /// 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]);
    /// ```
    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()
    }

    fn sift_up(&mut self, start: usize, pos: usize) -> 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 self.comparator.compare(&hole.element(), &hole.get(parent)) != Ordering::Greater
                {
                    break;
                }
                hole.move_to(parent);
            }
            hole.pos()
        }
    }

    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
                    && self.comparator.compare(&hole.get(child), hole.get(right))
                        != Ordering::Greater
                {
                    child = right;
                }
                // if we are already in order, stop.
                if self.comparator.compare(&hole.element(), &hole.get(child)) != Ordering::Less {
                    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);
    }

    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
                    && self.comparator.compare(&hole.get(child), &hole.get(right))
                        != Ordering::Greater
                {
                    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);
    /// ```
    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());
    /// ```
    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());
    /// ```
    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());
    /// ```
    pub fn clear(&mut self) {
        self.drain();
    }

    fn rebuild(&mut self) {
        let mut n = self.len() / 2;
        while n > 0 {
            n -= 1;
            self.sift_down(n);
        }
    }

    /// Moves all the elements of `other` into `self`, leaving `other` empty.
    ///
    /// # Examples
    ///
    /// Basic usage:
    ///
    /// ```
    /// use std::collections::BinaryHeap;
    ///
    /// let v = vec![-10, 1, 2, 3, 3];
    /// let mut a = BinaryHeap::from(v);
    ///
    /// let v = vec![-20, 5, 43];
    /// let mut b = BinaryHeap::from(v);
    ///
    /// a.append(&mut b);
    ///
    /// assert_eq!(a.into_sorted_vec(), [-20, -10, 1, 2, 3, 3, 5, 43]);
    /// assert!(b.is_empty());
    /// ```
    pub fn append<W>(&mut self, other: &mut BinaryHeap<T, W>)
    where
        W: Comparator<T>,
    {
        #[inline(always)]
        fn log2_fast(x: usize) -> usize {
            8 * size_of::<usize>() - (x.leading_zeros() as usize) - 1
        }

        #[inline]
        fn better_to_rebuild(len1: usize, len2: usize) -> bool {
            2 * (len1 + len2) < len2 * log2_fast(len1)
        }

        if other.is_empty() {
            return;
        }

        if better_to_rebuild(self.len(), other.len()) {
            self.data.append(&mut other.data);
            self.rebuild();
        } else {
            self.extend(other.drain());
        }
    }
}

struct Hole<'a, T: 'a> {
    data: &'a mut [T],
    elt: ManuallyDrop<T>,
    pos: usize,
}

impl<'a, T> Hole<'a, T> {
    #[inline]
    unsafe fn new(data: &'a mut [T], pos: usize) -> Self {
        debug_assert!(pos < data.len());
        let elt = ptr::read(&data[pos]);
        Hole {
            data,
            elt: ManuallyDrop::new(elt),
            pos,
        }
    }

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

    #[inline]
    fn element(&self) -> &T {
        &self.elt
    }

    #[inline]
    unsafe fn get(&self, index: usize) -> &T {
        debug_assert!(index != self.pos);
        debug_assert!(index < self.data.len());
        self.data.get_unchecked(index)
    }

    #[inline]
    unsafe fn move_to(&mut self, index: usize) {
        debug_assert!(index != self.pos);
        debug_assert!(index < self.data.len());
        let index_ptr: *const _ = self.data.get_unchecked(index);
        let hole_ptr = self.data.get_unchecked_mut(self.pos);
        ptr::copy_nonoverlapping(index_ptr, hole_ptr, 1);
        self.pos = index;
    }
}

impl<'a, T> Drop for Hole<'a, T> {
    #[inline]
    fn drop(&mut self) {
        // fill the hole again
        unsafe {
            let pos = self.pos;
            ptr::copy_nonoverlapping(&*self.elt, self.data.get_unchecked_mut(pos), 1);
        }
    }
}

/// An iterator over the elements of a `BinaryHeap`.
///
/// This `struct` is created by the [`iter`] method on [`BinaryHeap`]. See its
/// documentation for more.
///
/// [`iter`]: struct.BinaryHeap.html#method.iter
/// [`BinaryHeap`]: struct.BinaryHeap.html
pub struct Iter<'a, T: 'a> {
    iter: slice::Iter<'a, T>,
}

impl<'a, T: 'a + fmt::Debug> fmt::Debug for Iter<'a, T> {
    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
        f.debug_tuple("Iter").field(&self.iter.as_slice()).finish()
    }
}

impl<'a, T> Clone for Iter<'a, T> {
    fn clone(&self) -> Iter<'a, T> {
        Iter {
            iter: self.iter.clone(),
        }
    }
}

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()
    }
}

impl<'a, T> DoubleEndedIterator for Iter<'a, T> {
    #[inline]
    fn next_back(&mut self) -> Option<&'a T> {
        self.iter.next_back()
    }
}

impl<'a, T> ExactSizeIterator for Iter<'a, T> {}

impl<'a, T> FusedIterator for Iter<'a, T> {}

/// An owning iterator over the elements of a `BinaryHeap`.
///
/// This `struct` is created by the [`into_iter`] method on [`BinaryHeap`][`BinaryHeap`]
/// (provided by the `IntoIterator` trait). See its documentation for more.
///
/// [`into_iter`]: struct.BinaryHeap.html#method.into_iter
/// [`BinaryHeap`]: struct.BinaryHeap.html
pub struct IntoIter<T> {
    iter: vec::IntoIter<T>,
}

impl<T: fmt::Debug> fmt::Debug for IntoIter<T> {
    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
        f.debug_tuple("IntoIter")
            .field(&self.iter.as_slice())
            .finish()
    }
}

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()
    }
}

impl<T> DoubleEndedIterator for IntoIter<T> {
    #[inline]
    fn next_back(&mut self) -> Option<T> {
        self.iter.next_back()
    }
}

impl<T> ExactSizeIterator for IntoIter<T> {}

impl<T> FusedIterator for IntoIter<T> {}

/// A draining iterator over the elements of a `BinaryHeap`.
///
/// This `struct` is created by the [`drain`] method on [`BinaryHeap`]. See its
/// documentation for more.
///
/// [`drain`]: struct.BinaryHeap.html#method.drain
/// [`BinaryHeap`]: struct.BinaryHeap.html
pub struct Drain<'a, T: 'a> {
    iter: vec::Drain<'a, T>,
}

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()
    }
}

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

impl<'a, T: 'a> ExactSizeIterator for Drain<'a, T> {}

impl<'a, T: 'a> FusedIterator for Drain<'a, T> {}

impl<T, U> From<Vec<T>> for BinaryHeap<T, U>
where
    U: Comparator<T> + Default,
{
    fn from(vec: Vec<T>) -> Self {
        let mut heap = Self {
            data: vec,
            comparator: U::default(),
        };
        heap.rebuild();
        heap
    }
}

impl<T, U> FromIterator<T> for BinaryHeap<T, U>
where
    U: Comparator<T> + Default,
{
    fn from_iter<I: IntoIterator<Item = T>>(iter: I) -> Self {
        Self::from(iter.into_iter().collect::<Vec<_>>())
    }
}

impl<T, U> IntoIterator for BinaryHeap<T, U> {
    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(),
        }
    }
}

impl<'a, T, U> IntoIterator for &'a BinaryHeap<T, U>
where
    U: Comparator<T>,
{
    type Item = &'a T;
    type IntoIter = Iter<'a, T>;

    fn into_iter(self) -> Iter<'a, T> {
        self.iter()
    }
}

impl<T, U> Extend<T> for BinaryHeap<T, U>
where
    U: Comparator<T>,
{
    #[inline]
    fn extend<I: IntoIterator<Item = T>>(&mut self, iter: I) {
        let iterator = iter.into_iter();
        let (lower, _) = iterator.size_hint();

        self.reserve(lower);

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

impl<'a, T, U> Extend<&'a T> for BinaryHeap<T, U>
where
    T: 'a + Ord + Copy,
    U: Comparator<T>,
{
    fn extend<I: IntoIterator<Item = &'a T>>(&mut self, iter: I) {
        self.extend(iter.into_iter().cloned());
    }
}