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    // Copyright 2015 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.

    use core::cmp::Ordering;
    use core::fmt::Debug;
    use core::hash::{Hash, Hasher};
    use core::iter::{FromIterator, Peekable, FusedIterator};
    use core::marker::PhantomData;
    use core::ops::Index;
    use core::{fmt, intrinsics, mem, ptr};

    use borrow::Borrow;
    use Bound::{Excluded, Included, Unbounded};
    use range::RangeArgument;

    use super::node::{self, Handle, NodeRef, marker};
    use super::search;

    use super::node::InsertResult::*;
    use super::node::ForceResult::*;
    use super::search::SearchResult::*;
    use self::UnderflowResult::*;
    use self::Entry::*;

    /// A map based on a B-Tree.
    ///
    /// B-Trees represent a fundamental compromise between cache-efficiency and actually minimizing
    /// the amount of work performed in a search. In theory, a binary search tree (BST) is the optimal
    /// choice for a sorted map, as a perfectly balanced BST performs the theoretical minimum amount of
    /// comparisons necessary to find an element (log<sub>2</sub>n). However, in practice the way this
    /// is done is *very* inefficient for modern computer architectures. In particular, every element
    /// is stored in its own individually heap-allocated node. This means that every single insertion
    /// triggers a heap-allocation, and every single comparison should be a cache-miss. Since these
    /// are both notably expensive things to do in practice, we are forced to at very least reconsider
    /// the BST strategy.
    ///
    /// A B-Tree instead makes each node contain B-1 to 2B-1 elements in a contiguous array. By doing
    /// this, we reduce the number of allocations by a factor of B, and improve cache efficiency in
    /// searches. However, this does mean that searches will have to do *more* comparisons on average.
    /// The precise number of comparisons depends on the node search strategy used. For optimal cache
    /// efficiency, one could search the nodes linearly. For optimal comparisons, one could search
    /// the node using binary search. As a compromise, one could also perform a linear search
    /// that initially only checks every i<sup>th</sup> element for some choice of i.
    ///
    /// Currently, our implementation simply performs naive linear search. This provides excellent
    /// performance on *small* nodes of elements which are cheap to compare. However in the future we
    /// would like to further explore choosing the optimal search strategy based on the choice of B,
    /// and possibly other factors. Using linear search, searching for a random element is expected
    /// to take O(B log<sub>B</sub>n) comparisons, which is generally worse than a BST. In practice,
    /// however, performance is excellent.
    ///
    /// It is a logic error for a key to be modified in such a way that the key's ordering relative to
    /// any other key, as determined by the [`Ord`] trait, changes while it is in the map. This is
    /// normally only possible through [`Cell`], [`RefCell`], global state, I/O, or unsafe code.
    ///
    /// [`Ord`]: ../../std/cmp/trait.Ord.html
    /// [`Cell`]: ../../std/cell/struct.Cell.html
    /// [`RefCell`]: ../../std/cell/struct.RefCell.html
    ///
    /// # Examples
    ///
    /// ```
    /// use std::collections::BTreeMap;
    ///
    /// // type inference lets us omit an explicit type signature (which
    /// // would be `BTreeMap<&str, &str>` in this example).
    /// let mut movie_reviews = BTreeMap::new();
    ///
    /// // review some movies.
    /// movie_reviews.insert("Office Space",       "Deals with real issues in the workplace.");
    /// movie_reviews.insert("Pulp Fiction",       "Masterpiece.");
    /// movie_reviews.insert("The Godfather",      "Very enjoyable.");
    /// movie_reviews.insert("The Blues Brothers", "Eye lyked it alot.");
    ///
    /// // check for a specific one.
    /// if !movie_reviews.contains_key("Les Misérables") {
    ///     println!("We've got {} reviews, but Les Misérables ain't one.",
    ///              movie_reviews.len());
    /// }
    ///
    /// // oops, this review has a lot of spelling mistakes, let's delete it.
    /// movie_reviews.remove("The Blues Brothers");
    ///
    /// // look up the values associated with some keys.
    /// let to_find = ["Up!", "Office Space"];
    /// for book in &to_find {
    ///     match movie_reviews.get(book) {
    ///        Some(review) => println!("{}: {}", book, review),
    ///        None => println!("{} is unreviewed.", book)
    ///     }
    /// }
    ///
    /// // iterate over everything.
    /// for (movie, review) in &movie_reviews {
    ///     println!("{}: \"{}\"", movie, review);
    /// }
    /// ```
    ///
    /// `BTreeMap` also implements an [`Entry API`](#method.entry), which allows
    /// for more complex methods of getting, setting, updating and removing keys and
    /// their values:
    ///
    /// ```
    /// use std::collections::BTreeMap;
    ///
    /// // type inference lets us omit an explicit type signature (which
    /// // would be `BTreeMap<&str, u8>` in this example).
    /// let mut player_stats = BTreeMap::new();
    ///
    /// fn random_stat_buff() -> u8 {
    ///     // could actually return some random value here - let's just return
    ///     // some fixed value for now
    ///     42
    /// }
    ///
    /// // insert a key only if it doesn't already exist
    /// player_stats.entry("health").or_insert(100);
    ///
    /// // insert a key using a function that provides a new value only if it
    /// // doesn't already exist
    /// player_stats.entry("defence").or_insert_with(random_stat_buff);
    ///
    /// // update a key, guarding against the key possibly not being set
    /// let stat = player_stats.entry("attack").or_insert(100);
    /// *stat += random_stat_buff();
    /// ```
    #[stable(feature = "rust1", since = "1.0.0")]
    pub struct BTreeMap<K, V> {
        root: node::Root<K, V>,
        length: usize,
    }

    #[stable(feature = "btree_drop", since = "1.7.0")]
    unsafe impl<#[may_dangle] K, #[may_dangle] V> Drop for BTreeMap<K, V> {
        fn drop(&mut self) {
            unsafe {
                drop(ptr::read(self).into_iter());
            }
        }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<K: Clone, V: Clone> Clone for BTreeMap<K, V> {
        fn clone(&self) -> BTreeMap<K, V> {
            fn clone_subtree<K: Clone, V: Clone>(node: node::NodeRef<marker::Immut,
                                                                     K,
                                                                     V,
                                                                     marker::LeafOrInternal>)
                                                 -> BTreeMap<K, V> {

                match node.force() {
                    Leaf(leaf) => {
                        let mut out_tree = BTreeMap {
                            root: node::Root::new_leaf(),
                            length: 0,
                        };

                        {
                            let mut out_node = match out_tree.root.as_mut().force() {
                                Leaf(leaf) => leaf,
                                Internal(_) => unreachable!(),
                            };

                            let mut in_edge = leaf.first_edge();
                            while let Ok(kv) = in_edge.right_kv() {
                                let (k, v) = kv.into_kv();
                                in_edge = kv.right_edge();

                                out_node.push(k.clone(), v.clone());
                                out_tree.length += 1;
                            }
                        }

                        out_tree
                    }
                    Internal(internal) => {
                        let mut out_tree = clone_subtree(internal.first_edge().descend());

                        {
                            let mut out_node = out_tree.root.push_level();
                            let mut in_edge = internal.first_edge();
                            while let Ok(kv) = in_edge.right_kv() {
                                let (k, v) = kv.into_kv();
                                in_edge = kv.right_edge();

                                let k = (*k).clone();
                                let v = (*v).clone();
                                let subtree = clone_subtree(in_edge.descend());

                                // We can't destructure subtree directly
                                // because BTreeMap implements Drop
                                let (subroot, sublength) = unsafe {
                                    let root = ptr::read(&subtree.root);
                                    let length = subtree.length;
                                    mem::forget(subtree);
                                    (root, length)
                                };

                                out_node.push(k, v, subroot);
                                out_tree.length += 1 + sublength;
                            }
                        }

                        out_tree
                    }
                }
            }

            clone_subtree(self.root.as_ref())
        }
    }

    impl<K, Q: ?Sized> super::Recover<Q> for BTreeMap<K, ()>
        where K: Borrow<Q> + Ord,
              Q: Ord
    {
        type Key = K;

        fn get(&self, key: &Q) -> Option<&K> {
            match search::search_tree(self.root.as_ref(), key) {
                Found(handle) => Some(handle.into_kv().0),
                GoDown(_) => None,
            }
        }

        fn take(&mut self, key: &Q) -> Option<K> {
            match search::search_tree(self.root.as_mut(), key) {
                Found(handle) => {
                    Some(OccupiedEntry {
                             handle,
                             length: &mut self.length,
                             _marker: PhantomData,
                         }
                         .remove_kv()
                         .0)
                }
                GoDown(_) => None,
            }
        }

        fn replace(&mut self, key: K) -> Option<K> {
            match search::search_tree::<marker::Mut, K, (), K>(self.root.as_mut(), &key) {
                Found(handle) => Some(mem::replace(handle.into_kv_mut().0, key)),
                GoDown(handle) => {
                    VacantEntry {
                        key,
                        handle,
                        length: &mut self.length,
                        _marker: PhantomData,
                    }
                    .insert(());
                    None
                }
            }
        }
    }

    /// An iterator over the entries of a `BTreeMap`.
    ///
    /// This `struct` is created by the [`iter`] method on [`BTreeMap`]. See its
    /// documentation for more.
    ///
    /// [`iter`]: struct.BTreeMap.html#method.iter
    /// [`BTreeMap`]: struct.BTreeMap.html
    #[stable(feature = "rust1", since = "1.0.0")]
    pub struct Iter<'a, K: 'a, V: 'a> {
        range: Range<'a, K, V>,
        length: usize,
    }

    #[stable(feature = "collection_debug", since = "1.17.0")]
    impl<'a, K: 'a + fmt::Debug, V: 'a + fmt::Debug> fmt::Debug for Iter<'a, K, V> {
        fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
            f.debug_list().entries(self.clone()).finish()
        }
    }

    /// A mutable iterator over the entries of a `BTreeMap`.
    ///
    /// This `struct` is created by the [`iter_mut`] method on [`BTreeMap`]. See its
    /// documentation for more.
    ///
    /// [`iter_mut`]: struct.BTreeMap.html#method.iter_mut
    /// [`BTreeMap`]: struct.BTreeMap.html
    #[stable(feature = "rust1", since = "1.0.0")]
    #[derive(Debug)]
    pub struct IterMut<'a, K: 'a, V: 'a> {
        range: RangeMut<'a, K, V>,
        length: usize,
    }

    /// An owning iterator over the entries of a `BTreeMap`.
    ///
    /// This `struct` is created by the [`into_iter`] method on [`BTreeMap`][`BTreeMap`]
    /// (provided by the `IntoIterator` trait). See its documentation for more.
    ///
    /// [`into_iter`]: struct.BTreeMap.html#method.into_iter
    /// [`BTreeMap`]: struct.BTreeMap.html
    #[stable(feature = "rust1", since = "1.0.0")]
    pub struct IntoIter<K, V> {
        front: Handle<NodeRef<marker::Owned, K, V, marker::Leaf>, marker::Edge>,
        back: Handle<NodeRef<marker::Owned, K, V, marker::Leaf>, marker::Edge>,
        length: usize,
    }

    #[stable(feature = "collection_debug", since = "1.17.0")]
    impl<K: fmt::Debug, V: fmt::Debug> fmt::Debug for IntoIter<K, V> {
        fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
            let range = Range {
                front: self.front.reborrow(),
                back: self.back.reborrow(),
            };
            f.debug_list().entries(range).finish()
        }
    }

    /// An iterator over the keys of a `BTreeMap`.
    ///
    /// This `struct` is created by the [`keys`] method on [`BTreeMap`]. See its
    /// documentation for more.
    ///
    /// [`keys`]: struct.BTreeMap.html#method.keys
    /// [`BTreeMap`]: struct.BTreeMap.html
    #[stable(feature = "rust1", since = "1.0.0")]
    pub struct Keys<'a, K: 'a, V: 'a> {
        inner: Iter<'a, K, V>,
    }

    #[stable(feature = "collection_debug", since = "1.17.0")]
    impl<'a, K: 'a + fmt::Debug, V: 'a> fmt::Debug for Keys<'a, K, V> {
        fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
            f.debug_list().entries(self.clone()).finish()
        }
    }

    /// An iterator over the values of a `BTreeMap`.
    ///
    /// This `struct` is created by the [`values`] method on [`BTreeMap`]. See its
    /// documentation for more.
    ///
    /// [`values`]: struct.BTreeMap.html#method.values
    /// [`BTreeMap`]: struct.BTreeMap.html
    #[stable(feature = "rust1", since = "1.0.0")]
    pub struct Values<'a, K: 'a, V: 'a> {
        inner: Iter<'a, K, V>,
    }

    #[stable(feature = "collection_debug", since = "1.17.0")]
    impl<'a, K: 'a, V: 'a + fmt::Debug> fmt::Debug for Values<'a, K, V> {
        fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
            f.debug_list().entries(self.clone()).finish()
        }
    }

    /// A mutable iterator over the values of a `BTreeMap`.
    ///
    /// This `struct` is created by the [`values_mut`] method on [`BTreeMap`]. See its
    /// documentation for more.
    ///
    /// [`values_mut`]: struct.BTreeMap.html#method.values_mut
    /// [`BTreeMap`]: struct.BTreeMap.html
    #[stable(feature = "map_values_mut", since = "1.10.0")]
    #[derive(Debug)]
    pub struct ValuesMut<'a, K: 'a, V: 'a> {
        inner: IterMut<'a, K, V>,
    }

    /// An iterator over a sub-range of entries in a `BTreeMap`.
    ///
    /// This `struct` is created by the [`range`] method on [`BTreeMap`]. See its
    /// documentation for more.
    ///
    /// [`range`]: struct.BTreeMap.html#method.range
    /// [`BTreeMap`]: struct.BTreeMap.html
    #[stable(feature = "btree_range", since = "1.17.0")]
    pub struct Range<'a, K: 'a, V: 'a> {
        front: Handle<NodeRef<marker::Immut<'a>, K, V, marker::Leaf>, marker::Edge>,
        back: Handle<NodeRef<marker::Immut<'a>, K, V, marker::Leaf>, marker::Edge>,
    }

    #[stable(feature = "collection_debug", since = "1.17.0")]
    impl<'a, K: 'a + fmt::Debug, V: 'a + fmt::Debug> fmt::Debug for Range<'a, K, V> {
        fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
            f.debug_list().entries(self.clone()).finish()
        }
    }

    /// A mutable iterator over a sub-range of entries in a `BTreeMap`.
    ///
    /// This `struct` is created by the [`range_mut`] method on [`BTreeMap`]. See its
    /// documentation for more.
    ///
    /// [`range_mut`]: struct.BTreeMap.html#method.range_mut
    /// [`BTreeMap`]: struct.BTreeMap.html
    #[stable(feature = "btree_range", since = "1.17.0")]
    pub struct RangeMut<'a, K: 'a, V: 'a> {
        front: Handle<NodeRef<marker::Mut<'a>, K, V, marker::Leaf>, marker::Edge>,
        back: Handle<NodeRef<marker::Mut<'a>, K, V, marker::Leaf>, marker::Edge>,

        // Be invariant in `K` and `V`
        _marker: PhantomData<&'a mut (K, V)>,
    }

    #[stable(feature = "collection_debug", since = "1.17.0")]
    impl<'a, K: 'a + fmt::Debug, V: 'a + fmt::Debug> fmt::Debug for RangeMut<'a, K, V> {
        fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
            let range = Range {
                front: self.front.reborrow(),
                back: self.back.reborrow(),
            };
            f.debug_list().entries(range).finish()
        }
    }

    /// A view into a single entry in a map, which may either be vacant or occupied.
    ///
    /// This `enum` is constructed from the [`entry`] method on [`BTreeMap`].
    ///
    /// [`BTreeMap`]: struct.BTreeMap.html
    /// [`entry`]: struct.BTreeMap.html#method.entry
    #[stable(feature = "rust1", since = "1.0.0")]
    pub enum Entry<'a, K: 'a, V: 'a> {
        /// A vacant entry.
        #[stable(feature = "rust1", since = "1.0.0")]
        Vacant(#[stable(feature = "rust1", since = "1.0.0")]
               VacantEntry<'a, K, V>),

        /// An occupied entry.
        #[stable(feature = "rust1", since = "1.0.0")]
        Occupied(#[stable(feature = "rust1", since = "1.0.0")]
                 OccupiedEntry<'a, K, V>),
    }

    #[stable(feature= "debug_btree_map", since = "1.12.0")]
    impl<'a, K: 'a + Debug + Ord, V: 'a + Debug> Debug for Entry<'a, K, V> {
        fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
            match *self {
                Vacant(ref v) => f.debug_tuple("Entry")
                                  .field(v)
                                  .finish(),
                Occupied(ref o) => f.debug_tuple("Entry")
                                    .field(o)
                                    .finish(),
            }
        }
    }

    /// A view into a vacant entry in a `BTreeMap`.
    /// It is part of the [`Entry`] enum.
    ///
    /// [`Entry`]: enum.Entry.html
    #[stable(feature = "rust1", since = "1.0.0")]
    pub struct VacantEntry<'a, K: 'a, V: 'a> {
        key: K,
        handle: Handle<NodeRef<marker::Mut<'a>, K, V, marker::Leaf>, marker::Edge>,
        length: &'a mut usize,

        // Be invariant in `K` and `V`
        _marker: PhantomData<&'a mut (K, V)>,
    }

    #[stable(feature= "debug_btree_map", since = "1.12.0")]
    impl<'a, K: 'a + Debug + Ord, V: 'a> Debug for VacantEntry<'a, K, V> {
        fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
            f.debug_tuple("VacantEntry")
             .field(self.key())
             .finish()
        }
    }

    /// A view into an occupied entry in a `BTreeMap`.
    /// It is part of the [`Entry`] enum.
    ///
    /// [`Entry`]: enum.Entry.html
    #[stable(feature = "rust1", since = "1.0.0")]
    pub struct OccupiedEntry<'a, K: 'a, V: 'a> {
        handle: Handle<NodeRef<marker::Mut<'a>, K, V, marker::LeafOrInternal>, marker::KV>,

        length: &'a mut usize,

        // Be invariant in `K` and `V`
        _marker: PhantomData<&'a mut (K, V)>,
    }

    #[stable(feature= "debug_btree_map", since = "1.12.0")]
    impl<'a, K: 'a + Debug + Ord, V: 'a + Debug> Debug for OccupiedEntry<'a, K, V> {
        fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
            f.debug_struct("OccupiedEntry")
             .field("key", self.key())
             .field("value", self.get())
             .finish()
        }
    }

    // An iterator for merging two sorted sequences into one
    struct MergeIter<K, V, I: Iterator<Item = (K, V)>> {
        left: Peekable<I>,
        right: Peekable<I>,
    }

    impl<K: Ord, V> BTreeMap<K, V> {
        /// Makes a new empty BTreeMap with a reasonable choice for B.
        ///
        /// # Examples
        ///
        /// Basic usage:
        ///
        /// ```
        /// use std::collections::BTreeMap;
        ///
        /// let mut map = BTreeMap::new();
        ///
        /// // entries can now be inserted into the empty map
        /// map.insert(1, "a");
        /// ```
        #[stable(feature = "rust1", since = "1.0.0")]
        pub fn new() -> BTreeMap<K, V> {
            BTreeMap {
                root: node::Root::new_leaf(),
                length: 0,
            }
        }

        /// Clears the map, removing all values.
        ///
        /// # Examples
        ///
        /// Basic usage:
        ///
        /// ```
        /// use std::collections::BTreeMap;
        ///
        /// let mut a = BTreeMap::new();
        /// a.insert(1, "a");
        /// a.clear();
        /// assert!(a.is_empty());
        /// ```
        #[stable(feature = "rust1", since = "1.0.0")]
        pub fn clear(&mut self) {
            // FIXME(gereeter) .clear() allocates
            *self = BTreeMap::new();
        }

        /// Returns a reference to the value corresponding to the key.
        ///
        /// The key may be any borrowed form of the map's key type, but the ordering
        /// on the borrowed form *must* match the ordering on the key type.
        ///
        /// # Examples
        ///
        /// Basic usage:
        ///
        /// ```
        /// use std::collections::BTreeMap;
        ///
        /// let mut map = BTreeMap::new();
        /// map.insert(1, "a");
        /// assert_eq!(map.get(&1), Some(&"a"));
        /// assert_eq!(map.get(&2), None);
        /// ```
        #[stable(feature = "rust1", since = "1.0.0")]
        pub fn get<Q: ?Sized>(&self, key: &Q) -> Option<&V>
            where K: Borrow<Q>,
                  Q: Ord
        {
            match search::search_tree(self.root.as_ref(), key) {
                Found(handle) => Some(handle.into_kv().1),
                GoDown(_) => None,
            }
        }

        /// Returns `true` if the map contains a value for the specified key.
        ///
        /// The key may be any borrowed form of the map's key type, but the ordering
        /// on the borrowed form *must* match the ordering on the key type.
        ///
        /// # Examples
        ///
        /// Basic usage:
        ///
        /// ```
        /// use std::collections::BTreeMap;
        ///
        /// let mut map = BTreeMap::new();
        /// map.insert(1, "a");
        /// assert_eq!(map.contains_key(&1), true);
        /// assert_eq!(map.contains_key(&2), false);
        /// ```
        #[stable(feature = "rust1", since = "1.0.0")]
        pub fn contains_key<Q: ?Sized>(&self, key: &Q) -> bool
            where K: Borrow<Q>,
                  Q: Ord
        {
            self.get(key).is_some()
        }

        /// Returns a mutable reference to the value corresponding to the key.
        ///
        /// The key may be any borrowed form of the map's key type, but the ordering
        /// on the borrowed form *must* match the ordering on the key type.
        ///
        /// # Examples
        ///
        /// Basic usage:
        ///
        /// ```
        /// use std::collections::BTreeMap;
        ///
        /// let mut map = BTreeMap::new();
        /// map.insert(1, "a");
        /// if let Some(x) = map.get_mut(&1) {
        ///     *x = "b";
        /// }
        /// assert_eq!(map[&1], "b");
        /// ```
        // See `get` for implementation notes, this is basically a copy-paste with mut's added
        #[stable(feature = "rust1", since = "1.0.0")]
        pub fn get_mut<Q: ?Sized>(&mut self, key: &Q) -> Option<&mut V>
            where K: Borrow<Q>,
                  Q: Ord
        {
            match search::search_tree(self.root.as_mut(), key) {
                Found(handle) => Some(handle.into_kv_mut().1),
                GoDown(_) => None,
            }
        }

        /// Inserts a key-value pair into the map.
        ///
        /// If the map did not have this key present, `None` is returned.
        ///
        /// If the map did have this key present, the value is updated, and the old
        /// value is returned. The key is not updated, though; this matters for
        /// types that can be `==` without being identical. See the [module-level
        /// documentation] for more.
        ///
        /// [module-level documentation]: index.html#insert-and-complex-keys
        ///
        /// # Examples
        ///
        /// Basic usage:
        ///
        /// ```
        /// use std::collections::BTreeMap;
        ///
        /// let mut map = BTreeMap::new();
        /// assert_eq!(map.insert(37, "a"), None);
        /// assert_eq!(map.is_empty(), false);
        ///
        /// map.insert(37, "b");
        /// assert_eq!(map.insert(37, "c"), Some("b"));
        /// assert_eq!(map[&37], "c");
        /// ```
        #[stable(feature = "rust1", since = "1.0.0")]
        pub fn insert(&mut self, key: K, value: V) -> Option<V> {
            match self.entry(key) {
                Occupied(mut entry) => Some(entry.insert(value)),
                Vacant(entry) => {
                    entry.insert(value);
                    None
                }
            }
        }

        /// Removes a key from the map, returning the value at the key if the key
        /// was previously in the map.
        ///
        /// The key may be any borrowed form of the map's key type, but the ordering
        /// on the borrowed form *must* match the ordering on the key type.
        ///
        /// # Examples
        ///
        /// Basic usage:
        ///
        /// ```
        /// use std::collections::BTreeMap;
        ///
        /// let mut map = BTreeMap::new();
        /// map.insert(1, "a");
        /// assert_eq!(map.remove(&1), Some("a"));
        /// assert_eq!(map.remove(&1), None);
        /// ```
        #[stable(feature = "rust1", since = "1.0.0")]
        pub fn remove<Q: ?Sized>(&mut self, key: &Q) -> Option<V>
            where K: Borrow<Q>,
                  Q: Ord
        {
            match search::search_tree(self.root.as_mut(), key) {
                Found(handle) => {
                    Some(OccupiedEntry {
                             handle,
                             length: &mut self.length,
                             _marker: PhantomData,
                         }
                         .remove())
                }
                GoDown(_) => None,
            }
        }

        /// Moves all elements from `other` into `Self`, leaving `other` empty.
        ///
        /// # Examples
        ///
        /// ```
        /// use std::collections::BTreeMap;
        ///
        /// let mut a = BTreeMap::new();
        /// a.insert(1, "a");
        /// a.insert(2, "b");
        /// a.insert(3, "c");
        ///
        /// let mut b = BTreeMap::new();
        /// b.insert(3, "d");
        /// b.insert(4, "e");
        /// b.insert(5, "f");
        ///
        /// a.append(&mut b);
        ///
        /// assert_eq!(a.len(), 5);
        /// assert_eq!(b.len(), 0);
        ///
        /// assert_eq!(a[&1], "a");
        /// assert_eq!(a[&2], "b");
        /// assert_eq!(a[&3], "d");
        /// assert_eq!(a[&4], "e");
        /// assert_eq!(a[&5], "f");
        /// ```
        #[stable(feature = "btree_append", since = "1.11.0")]
        pub fn append(&mut self, other: &mut Self) {
            // Do we have to append anything at all?
            if other.len() == 0 {
                return;
            }

            // We can just swap `self` and `other` if `self` is empty.
            if self.len() == 0 {
                mem::swap(self, other);
                return;
            }

            // First, we merge `self` and `other` into a sorted sequence in linear time.
            let self_iter = mem::replace(self, BTreeMap::new()).into_iter();
            let other_iter = mem::replace(other, BTreeMap::new()).into_iter();
            let iter = MergeIter {
                left: self_iter.peekable(),
                right: other_iter.peekable(),
            };

            // Second, we build a tree from the sorted sequence in linear time.
            self.from_sorted_iter(iter);
            self.fix_right_edge();
        }

        /// Constructs a double-ended iterator over a sub-range of elements in the map.
        /// The simplest way is to use the range syntax `min..max`, thus `range(min..max)` will
        /// yield elements from min (inclusive) to max (exclusive).
        /// The range may also be entered as `(Bound<T>, Bound<T>)`, so for example
        /// `range((Excluded(4), Included(10)))` will yield a left-exclusive, right-inclusive
        /// range from 4 to 10.
        ///
        /// # Panics
        ///
        /// Panics if range `start > end`.
        /// Panics if range `start == end` and both bounds are `Excluded`.
        ///
        /// # Examples
        ///
        /// Basic usage:
        ///
        /// ```
        /// use std::collections::BTreeMap;
        /// use std::collections::Bound::Included;
        ///
        /// let mut map = BTreeMap::new();
        /// map.insert(3, "a");
        /// map.insert(5, "b");
        /// map.insert(8, "c");
        /// for (&key, &value) in map.range((Included(&4), Included(&8))) {
        ///     println!("{}: {}", key, value);
        /// }
        /// assert_eq!(Some((&5, &"b")), map.range(4..).next());
        /// ```
        #[stable(feature = "btree_range", since = "1.17.0")]
        pub fn range<T: ?Sized, R>(&self, range: R) -> Range<K, V>
            where T: Ord, K: Borrow<T>, R: RangeArgument<T>
        {
            let root1 = self.root.as_ref();
            let root2 = self.root.as_ref();
            let (f, b) = range_search(root1, root2, range);

            Range { front: f, back: b}
        }

        /// Constructs a mutable double-ended iterator over a sub-range of elements in the map.
        /// The simplest way is to use the range syntax `min..max`, thus `range(min..max)` will
        /// yield elements from min (inclusive) to max (exclusive).
        /// The range may also be entered as `(Bound<T>, Bound<T>)`, so for example
        /// `range((Excluded(4), Included(10)))` will yield a left-exclusive, right-inclusive
        /// range from 4 to 10.
        ///
        /// # Panics
        ///
        /// Panics if range `start > end`.
        /// Panics if range `start == end` and both bounds are `Excluded`.
        ///
        /// # Examples
        ///
        /// Basic usage:
        ///
        /// ```
        /// use std::collections::BTreeMap;
        ///
        /// let mut map: BTreeMap<&str, i32> = ["Alice", "Bob", "Carol", "Cheryl"].iter()
        ///                                                                       .map(|&s| (s, 0))
        ///                                                                       .collect();
        /// for (_, balance) in map.range_mut("B".."Cheryl") {
        ///     *balance += 100;
        /// }
        /// for (name, balance) in &map {
        ///     println!("{} => {}", name, balance);
        /// }
        /// ```
        #[stable(feature = "btree_range", since = "1.17.0")]
        pub fn range_mut<T: ?Sized, R>(&mut self, range: R) -> RangeMut<K, V>
            where T: Ord, K: Borrow<T>, R: RangeArgument<T>
        {
            let root1 = self.root.as_mut();
            let root2 = unsafe { ptr::read(&root1) };
            let (f, b) = range_search(root1, root2, range);

            RangeMut {
                front: f,
                back: b,
                _marker: PhantomData,
            }
        }

        /// Gets the given key's corresponding entry in the map for in-place manipulation.
        ///
        /// # Examples
        ///
        /// Basic usage:
        ///
        /// ```
        /// use std::collections::BTreeMap;
        ///
        /// let mut count: BTreeMap<&str, usize> = BTreeMap::new();
        ///
        /// // count the number of occurrences of letters in the vec
        /// for x in vec!["a","b","a","c","a","b"] {
        ///     *count.entry(x).or_insert(0) += 1;
        /// }
        ///
        /// assert_eq!(count["a"], 3);
        /// ```
        #[stable(feature = "rust1", since = "1.0.0")]
        pub fn entry(&mut self, key: K) -> Entry<K, V> {
            match search::search_tree(self.root.as_mut(), &key) {
                Found(handle) => {
                    Occupied(OccupiedEntry {
                        handle,
                        length: &mut self.length,
                        _marker: PhantomData,
                    })
                }
                GoDown(handle) => {
                    Vacant(VacantEntry {
                        key,
                        handle,
                        length: &mut self.length,
                        _marker: PhantomData,
                    })
                }
            }
        }

        fn from_sorted_iter<I: Iterator<Item = (K, V)>>(&mut self, iter: I) {
            let mut cur_node = last_leaf_edge(self.root.as_mut()).into_node();
            // Iterate through all key-value pairs, pushing them into nodes at the right level.
            for (key, value) in iter {
                // Try to push key-value pair into the current leaf node.
                if cur_node.len() < node::CAPACITY {
                    cur_node.push(key, value);
                } else {
                    // No space left, go up and push there.
                    let mut open_node;
                    let mut test_node = cur_node.forget_type();
                    loop {
                        match test_node.ascend() {
                            Ok(parent) => {
                                let parent = parent.into_node();
                                if parent.len() < node::CAPACITY {
                                    // Found a node with space left, push here.
                                    open_node = parent;
                                    break;
                                } else {
                                    // Go up again.
                                    test_node = parent.forget_type();
                                }
                            }
                            Err(node) => {
                                // We are at the top, create a new root node and push there.
                                open_node = node.into_root_mut().push_level();
                                break;
                            }
                        }
                    }

                    // Push key-value pair and new right subtree.
                    let tree_height = open_node.height() - 1;
                    let mut right_tree = node::Root::new_leaf();
                    for _ in 0..tree_height {
                        right_tree.push_level();
                    }
                    open_node.push(key, value, right_tree);

                    // Go down to the right-most leaf again.
                    cur_node = last_leaf_edge(open_node.forget_type()).into_node();
                }

                self.length += 1;
            }
        }

        fn fix_right_edge(&mut self) {
            // Handle underfull nodes, start from the top.
            let mut cur_node = self.root.as_mut();
            while let Internal(internal) = cur_node.force() {
                // Check if right-most child is underfull.
                let mut last_edge = internal.last_edge();
                let right_child_len = last_edge.reborrow().descend().len();
                if right_child_len < node::MIN_LEN {
                    // We need to steal.
                    let mut last_kv = match last_edge.left_kv() {
                        Ok(left) => left,
                        Err(_) => unreachable!(),
                    };
                    last_kv.bulk_steal_left(node::MIN_LEN - right_child_len);
                    last_edge = last_kv.right_edge();
                }

                // Go further down.
                cur_node = last_edge.descend();
            }
        }

        /// Splits the collection into two at the given key. Returns everything after the given key,
        /// including the key.
        ///
        /// # Examples
        ///
        /// Basic usage:
        ///
        /// ```
        /// use std::collections::BTreeMap;
        ///
        /// let mut a = BTreeMap::new();
        /// a.insert(1, "a");
        /// a.insert(2, "b");
        /// a.insert(3, "c");
        /// a.insert(17, "d");
        /// a.insert(41, "e");
        ///
        /// let b = a.split_off(&3);
        ///
        /// assert_eq!(a.len(), 2);
        /// assert_eq!(b.len(), 3);
        ///
        /// assert_eq!(a[&1], "a");
        /// assert_eq!(a[&2], "b");
        ///
        /// assert_eq!(b[&3], "c");
        /// assert_eq!(b[&17], "d");
        /// assert_eq!(b[&41], "e");
        /// ```
        #[stable(feature = "btree_split_off", since = "1.11.0")]
        pub fn split_off<Q: ?Sized + Ord>(&mut self, key: &Q) -> Self
            where K: Borrow<Q>
        {
            if self.is_empty() {
                return Self::new();
            }

            let total_num = self.len();

            let mut right = Self::new();
            for _ in 0..(self.root.as_ref().height()) {
                right.root.push_level();
            }

            {
                let mut left_node = self.root.as_mut();
                let mut right_node = right.root.as_mut();

                loop {
                    let mut split_edge = match search::search_node(left_node, key) {
                        // key is going to the right tree
                        Found(handle) => handle.left_edge(),
                        GoDown(handle) => handle,
                    };

                    split_edge.move_suffix(&mut right_node);

                    match (split_edge.force(), right_node.force()) {
                        (Internal(edge), Internal(node)) => {
                            left_node = edge.descend();
                            right_node = node.first_edge().descend();
                        }
                        (Leaf(_), Leaf(_)) => {
                            break;
                        }
                        _ => {
                            unreachable!();
                        }
                    }
                }
            }

            self.fix_right_border();
            right.fix_left_border();

            if self.root.as_ref().height() < right.root.as_ref().height() {
                self.recalc_length();
                right.length = total_num - self.len();
            } else {
                right.recalc_length();
                self.length = total_num - right.len();
            }

            right
        }

        /// Calculates the number of elements if it is incorrect.
        fn recalc_length(&mut self) {
            fn dfs<K, V>(node: NodeRef<marker::Immut, K, V, marker::LeafOrInternal>) -> usize {
                let mut res = node.len();

                if let Internal(node) = node.force() {
                    let mut edge = node.first_edge();
                    loop {
                        res += dfs(edge.reborrow().descend());
                        match edge.right_kv() {
                            Ok(right_kv) => {
                                edge = right_kv.right_edge();
                            }
                            Err(_) => {
                                break;
                            }
                        }
                    }
                }

                res
            }

            self.length = dfs(self.root.as_ref());
        }

        /// Removes empty levels on the top.
        fn fix_top(&mut self) {
            loop {
                {
                    let node = self.root.as_ref();
                    if node.height() == 0 || node.len() > 0 {
                        break;
                    }
                }
                self.root.pop_level();
            }
        }

        fn fix_right_border(&mut self) {
            self.fix_top();

            {
                let mut cur_node = self.root.as_mut();

                while let Internal(node) = cur_node.force() {
                    let mut last_kv = node.last_kv();

                    if last_kv.can_merge() {
                        cur_node = last_kv.merge().descend();
                    } else {
                        let right_len = last_kv.reborrow().right_edge().descend().len();
                        // `MINLEN + 1` to avoid readjust if merge happens on the next level.
                        if right_len < node::MIN_LEN + 1 {
                            last_kv.bulk_steal_left(node::MIN_LEN + 1 - right_len);
                        }
                        cur_node = last_kv.right_edge().descend();
                    }
                }
            }

            self.fix_top();
        }

        /// The symmetric clone of `fix_right_border`.
        fn fix_left_border(&mut self) {
            self.fix_top();

            {
                let mut cur_node = self.root.as_mut();

                while let Internal(node) = cur_node.force() {
                    let mut first_kv = node.first_kv();

                    if first_kv.can_merge() {
                        cur_node = first_kv.merge().descend();
                    } else {
                        let left_len = first_kv.reborrow().left_edge().descend().len();
                        if left_len < node::MIN_LEN + 1 {
                            first_kv.bulk_steal_right(node::MIN_LEN + 1 - left_len);
                        }
                        cur_node = first_kv.left_edge().descend();
                    }
                }
            }

            self.fix_top();
        }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<'a, K: 'a, V: 'a> IntoIterator for &'a BTreeMap<K, V> {
        type Item = (&'a K, &'a V);
        type IntoIter = Iter<'a, K, V>;

        fn into_iter(self) -> Iter<'a, K, V> {
            self.iter()
        }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<'a, K: 'a, V: 'a> Iterator for Iter<'a, K, V> {
        type Item = (&'a K, &'a V);

        fn next(&mut self) -> Option<(&'a K, &'a V)> {
            if self.length == 0 {
                None
            } else {
                self.length -= 1;
                unsafe { Some(self.range.next_unchecked()) }
            }
        }

        fn size_hint(&self) -> (usize, Option<usize>) {
            (self.length, Some(self.length))
        }
    }

    #[unstable(feature = "fused", issue = "35602")]
    impl<'a, K, V> FusedIterator for Iter<'a, K, V> {}

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<'a, K: 'a, V: 'a> DoubleEndedIterator for Iter<'a, K, V> {
        fn next_back(&mut self) -> Option<(&'a K, &'a V)> {
            if self.length == 0 {
                None
            } else {
                self.length -= 1;
                unsafe { Some(self.range.next_back_unchecked()) }
            }
        }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<'a, K: 'a, V: 'a> ExactSizeIterator for Iter<'a, K, V> {
        fn len(&self) -> usize {
            self.length
        }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<'a, K, V> Clone for Iter<'a, K, V> {
        fn clone(&self) -> Iter<'a, K, V> {
            Iter {
                range: self.range.clone(),
                length: self.length,
            }
        }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<'a, K: 'a, V: 'a> IntoIterator for &'a mut BTreeMap<K, V> {
        type Item = (&'a K, &'a mut V);
        type IntoIter = IterMut<'a, K, V>;

        fn into_iter(self) -> IterMut<'a, K, V> {
            self.iter_mut()
        }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<'a, K: 'a, V: 'a> Iterator for IterMut<'a, K, V> {
        type Item = (&'a K, &'a mut V);

        fn next(&mut self) -> Option<(&'a K, &'a mut V)> {
            if self.length == 0 {
                None
            } else {
                self.length -= 1;
                unsafe { Some(self.range.next_unchecked()) }
            }
        }

        fn size_hint(&self) -> (usize, Option<usize>) {
            (self.length, Some(self.length))
        }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<'a, K: 'a, V: 'a> DoubleEndedIterator for IterMut<'a, K, V> {
        fn next_back(&mut self) -> Option<(&'a K, &'a mut V)> {
            if self.length == 0 {
                None
            } else {
                self.length -= 1;
                unsafe { Some(self.range.next_back_unchecked()) }
            }
        }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<'a, K: 'a, V: 'a> ExactSizeIterator for IterMut<'a, K, V> {
        fn len(&self) -> usize {
            self.length
        }
    }

    #[unstable(feature = "fused", issue = "35602")]
    impl<'a, K, V> FusedIterator for IterMut<'a, K, V> {}

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<K, V> IntoIterator for BTreeMap<K, V> {
        type Item = (K, V);
        type IntoIter = IntoIter<K, V>;

        fn into_iter(self) -> IntoIter<K, V> {
            let root1 = unsafe { ptr::read(&self.root).into_ref() };
            let root2 = unsafe { ptr::read(&self.root).into_ref() };
            let len = self.length;
            mem::forget(self);

            IntoIter {
                front: first_leaf_edge(root1),
                back: last_leaf_edge(root2),
                length: len,
            }
        }
    }

    #[stable(feature = "btree_drop", since = "1.7.0")]
    impl<K, V> Drop for IntoIter<K, V> {
        fn drop(&mut self) {
            for _ in &mut *self {
            }
            unsafe {
                let leaf_node = ptr::read(&self.front).into_node();
                if let Some(first_parent) = leaf_node.deallocate_and_ascend() {
                    let mut cur_node = first_parent.into_node();
                    while let Some(parent) = cur_node.deallocate_and_ascend() {
                        cur_node = parent.into_node()
                    }
                }
            }
        }
    }

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

        fn next(&mut self) -> Option<(K, V)> {
            if self.length == 0 {
                return None;
            } else {
                self.length -= 1;
            }

            let handle = unsafe { ptr::read(&self.front) };

            let mut cur_handle = match handle.right_kv() {
                Ok(kv) => {
                    let k = unsafe { ptr::read(kv.reborrow().into_kv().0) };
                    let v = unsafe { ptr::read(kv.reborrow().into_kv().1) };
                    self.front = kv.right_edge();
                    return Some((k, v));
                }
                Err(last_edge) => unsafe {
                    unwrap_unchecked(last_edge.into_node().deallocate_and_ascend())
                },
            };

            loop {
                match cur_handle.right_kv() {
                    Ok(kv) => {
                        let k = unsafe { ptr::read(kv.reborrow().into_kv().0) };
                        let v = unsafe { ptr::read(kv.reborrow().into_kv().1) };
                        self.front = first_leaf_edge(kv.right_edge().descend());
                        return Some((k, v));
                    }
                    Err(last_edge) => unsafe {
                        cur_handle = unwrap_unchecked(last_edge.into_node().deallocate_and_ascend());
                    },
                }
            }
        }

        fn size_hint(&self) -> (usize, Option<usize>) {
            (self.length, Some(self.length))
        }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<K, V> DoubleEndedIterator for IntoIter<K, V> {
        fn next_back(&mut self) -> Option<(K, V)> {
            if self.length == 0 {
                return None;
            } else {
                self.length -= 1;
            }

            let handle = unsafe { ptr::read(&self.back) };

            let mut cur_handle = match handle.left_kv() {
                Ok(kv) => {
                    let k = unsafe { ptr::read(kv.reborrow().into_kv().0) };
                    let v = unsafe { ptr::read(kv.reborrow().into_kv().1) };
                    self.back = kv.left_edge();
                    return Some((k, v));
                }
                Err(last_edge) => unsafe {
                    unwrap_unchecked(last_edge.into_node().deallocate_and_ascend())
                },
            };

            loop {
                match cur_handle.left_kv() {
                    Ok(kv) => {
                        let k = unsafe { ptr::read(kv.reborrow().into_kv().0) };
                        let v = unsafe { ptr::read(kv.reborrow().into_kv().1) };
                        self.back = last_leaf_edge(kv.left_edge().descend());
                        return Some((k, v));
                    }
                    Err(last_edge) => unsafe {
                        cur_handle = unwrap_unchecked(last_edge.into_node().deallocate_and_ascend());
                    },
                }
            }
        }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<K, V> ExactSizeIterator for IntoIter<K, V> {
        fn len(&self) -> usize {
            self.length
        }
    }

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

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<'a, K, V> Iterator for Keys<'a, K, V> {
        type Item = &'a K;

        fn next(&mut self) -> Option<&'a K> {
            self.inner.next().map(|(k, _)| k)
        }

        fn size_hint(&self) -> (usize, Option<usize>) {
            self.inner.size_hint()
        }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<'a, K, V> DoubleEndedIterator for Keys<'a, K, V> {
        fn next_back(&mut self) -> Option<&'a K> {
            self.inner.next_back().map(|(k, _)| k)
        }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<'a, K, V> ExactSizeIterator for Keys<'a, K, V> {
        fn len(&self) -> usize {
            self.inner.len()
        }
    }

    #[unstable(feature = "fused", issue = "35602")]
    impl<'a, K, V> FusedIterator for Keys<'a, K, V> {}

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<'a, K, V> Clone for Keys<'a, K, V> {
        fn clone(&self) -> Keys<'a, K, V> {
            Keys { inner: self.inner.clone() }
        }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<'a, K, V> Iterator for Values<'a, K, V> {
        type Item = &'a V;

        fn next(&mut self) -> Option<&'a V> {
            self.inner.next().map(|(_, v)| v)
        }

        fn size_hint(&self) -> (usize, Option<usize>) {
            self.inner.size_hint()
        }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<'a, K, V> DoubleEndedIterator for Values<'a, K, V> {
        fn next_back(&mut self) -> Option<&'a V> {
            self.inner.next_back().map(|(_, v)| v)
        }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<'a, K, V> ExactSizeIterator for Values<'a, K, V> {
        fn len(&self) -> usize {
            self.inner.len()
        }
    }

    #[unstable(feature = "fused", issue = "35602")]
    impl<'a, K, V> FusedIterator for Values<'a, K, V> {}

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<'a, K, V> Clone for Values<'a, K, V> {
        fn clone(&self) -> Values<'a, K, V> {
            Values { inner: self.inner.clone() }
        }
    }

    #[stable(feature = "btree_range", since = "1.17.0")]
    impl<'a, K, V> Iterator for Range<'a, K, V> {
        type Item = (&'a K, &'a V);

        fn next(&mut self) -> Option<(&'a K, &'a V)> {
            if self.front == self.back {
                None
            } else {
                unsafe { Some(self.next_unchecked()) }
            }
        }
    }

    #[stable(feature = "map_values_mut", since = "1.10.0")]
    impl<'a, K, V> Iterator for ValuesMut<'a, K, V> {
        type Item = &'a mut V;

        fn next(&mut self) -> Option<&'a mut V> {
            self.inner.next().map(|(_, v)| v)
        }

        fn size_hint(&self) -> (usize, Option<usize>) {
            self.inner.size_hint()
        }
    }

    #[stable(feature = "map_values_mut", since = "1.10.0")]
    impl<'a, K, V> DoubleEndedIterator for ValuesMut<'a, K, V> {
        fn next_back(&mut self) -> Option<&'a mut V> {
            self.inner.next_back().map(|(_, v)| v)
        }
    }

    #[stable(feature = "map_values_mut", since = "1.10.0")]
    impl<'a, K, V> ExactSizeIterator for ValuesMut<'a, K, V> {
        fn len(&self) -> usize {
            self.inner.len()
        }
    }

    #[unstable(feature = "fused", issue = "35602")]
    impl<'a, K, V> FusedIterator for ValuesMut<'a, K, V> {}

    impl<'a, K, V> Range<'a, K, V> {
        unsafe fn next_unchecked(&mut self) -> (&'a K, &'a V) {
            let handle = self.front;

            let mut cur_handle = match handle.right_kv() {
                Ok(kv) => {
                    let ret = kv.into_kv();
                    self.front = kv.right_edge();
                    return ret;
                }
                Err(last_edge) => {
                    let next_level = last_edge.into_node().ascend().ok();
                    unwrap_unchecked(next_level)
                }
            };

            loop {
                match cur_handle.right_kv() {
                    Ok(kv) => {
                        let ret = kv.into_kv();
                        self.front = first_leaf_edge(kv.right_edge().descend());
                        return ret;
                    }
                    Err(last_edge) => {
                        let next_level = last_edge.into_node().ascend().ok();
                        cur_handle = unwrap_unchecked(next_level);
                    }
                }
            }
        }
    }

    #[stable(feature = "btree_range", since = "1.17.0")]
    impl<'a, K, V> DoubleEndedIterator for Range<'a, K, V> {
        fn next_back(&mut self) -> Option<(&'a K, &'a V)> {
            if self.front == self.back {
                None
            } else {
                unsafe { Some(self.next_back_unchecked()) }
            }
        }
    }

    impl<'a, K, V> Range<'a, K, V> {
        unsafe fn next_back_unchecked(&mut self) -> (&'a K, &'a V) {
            let handle = self.back;

            let mut cur_handle = match handle.left_kv() {
                Ok(kv) => {
                    let ret = kv.into_kv();
                    self.back = kv.left_edge();
                    return ret;
                }
                Err(last_edge) => {
                    let next_level = last_edge.into_node().ascend().ok();
                    unwrap_unchecked(next_level)
                }
            };

            loop {
                match cur_handle.left_kv() {
                    Ok(kv) => {
                        let ret = kv.into_kv();
                        self.back = last_leaf_edge(kv.left_edge().descend());
                        return ret;
                    }
                    Err(last_edge) => {
                        let next_level = last_edge.into_node().ascend().ok();
                        cur_handle = unwrap_unchecked(next_level);
                    }
                }
            }
        }
    }

    #[unstable(feature = "fused", issue = "35602")]
    impl<'a, K, V> FusedIterator for Range<'a, K, V> {}

    #[stable(feature = "btree_range", since = "1.17.0")]
    impl<'a, K, V> Clone for Range<'a, K, V> {
        fn clone(&self) -> Range<'a, K, V> {
            Range {
                front: self.front,
                back: self.back,
            }
        }
    }

    #[stable(feature = "btree_range", since = "1.17.0")]
    impl<'a, K, V> Iterator for RangeMut<'a, K, V> {
        type Item = (&'a K, &'a mut V);

        fn next(&mut self) -> Option<(&'a K, &'a mut V)> {
            if self.front == self.back {
                None
            } else {
                unsafe { Some(self.next_unchecked()) }
            }
        }
    }

    impl<'a, K, V> RangeMut<'a, K, V> {
        unsafe fn next_unchecked(&mut self) -> (&'a K, &'a mut V) {
            let handle = ptr::read(&self.front);

            let mut cur_handle = match handle.right_kv() {
                Ok(kv) => {
                    let (k, v) = ptr::read(&kv).into_kv_mut();
                    self.front = kv.right_edge();
                    return (k, v);
                }
                Err(last_edge) => {
                    let next_level = last_edge.into_node().ascend().ok();
                    unwrap_unchecked(next_level)
                }
            };

            loop {
                match cur_handle.right_kv() {
                    Ok(kv) => {
                        let (k, v) = ptr::read(&kv).into_kv_mut();
                        self.front = first_leaf_edge(kv.right_edge().descend());
                        return (k, v);
                    }
                    Err(last_edge) => {
                        let next_level = last_edge.into_node().ascend().ok();
                        cur_handle = unwrap_unchecked(next_level);
                    }
                }
            }
        }
    }

    #[stable(feature = "btree_range", since = "1.17.0")]
    impl<'a, K, V> DoubleEndedIterator for RangeMut<'a, K, V> {
        fn next_back(&mut self) -> Option<(&'a K, &'a mut V)> {
            if self.front == self.back {
                None
            } else {
                unsafe { Some(self.next_back_unchecked()) }
            }
        }
    }

    #[unstable(feature = "fused", issue = "35602")]
    impl<'a, K, V> FusedIterator for RangeMut<'a, K, V> {}

    impl<'a, K, V> RangeMut<'a, K, V> {
        unsafe fn next_back_unchecked(&mut self) -> (&'a K, &'a mut V) {
            let handle = ptr::read(&self.back);

            let mut cur_handle = match handle.left_kv() {
                Ok(kv) => {
                    let (k, v) = ptr::read(&kv).into_kv_mut();
                    self.back = kv.left_edge();
                    return (k, v);
                }
                Err(last_edge) => {
                    let next_level = last_edge.into_node().ascend().ok();
                    unwrap_unchecked(next_level)
                }
            };

            loop {
                match cur_handle.left_kv() {
                    Ok(kv) => {
                        let (k, v) = ptr::read(&kv).into_kv_mut();
                        self.back = last_leaf_edge(kv.left_edge().descend());
                        return (k, v);
                    }
                    Err(last_edge) => {
                        let next_level = last_edge.into_node().ascend().ok();
                        cur_handle = unwrap_unchecked(next_level);
                    }
                }
            }
        }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<K: Ord, V> FromIterator<(K, V)> for BTreeMap<K, V> {
        fn from_iter<T: IntoIterator<Item = (K, V)>>(iter: T) -> BTreeMap<K, V> {
            let mut map = BTreeMap::new();
            map.extend(iter);
            map
        }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<K: Ord, V> Extend<(K, V)> for BTreeMap<K, V> {
        #[inline]
        fn extend<T: IntoIterator<Item = (K, V)>>(&mut self, iter: T) {
            for (k, v) in iter {
                self.insert(k, v);
            }
        }
    }

    #[stable(feature = "extend_ref", since = "1.2.0")]
    impl<'a, K: Ord + Copy, V: Copy> Extend<(&'a K, &'a V)> for BTreeMap<K, V> {
        fn extend<I: IntoIterator<Item = (&'a K, &'a V)>>(&mut self, iter: I) {
            self.extend(iter.into_iter().map(|(&key, &value)| (key, value)));
        }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<K: Hash, V: Hash> Hash for BTreeMap<K, V> {
        fn hash<H: Hasher>(&self, state: &mut H) {
            for elt in self {
                elt.hash(state);
            }
        }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<K: Ord, V> Default for BTreeMap<K, V> {
        /// Creates an empty `BTreeMap<K, V>`.
        fn default() -> BTreeMap<K, V> {
            BTreeMap::new()
        }
    }

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

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

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

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<K: Ord, V: Ord> Ord for BTreeMap<K, V> {
        #[inline]
        fn cmp(&self, other: &BTreeMap<K, V>) -> Ordering {
            self.iter().cmp(other.iter())
        }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<K: Debug, V: Debug> Debug for BTreeMap<K, V> {
        fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
            f.debug_map().entries(self.iter()).finish()
        }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<'a, K: Ord, Q: ?Sized, V> Index<&'a Q> for BTreeMap<K, V>
        where K: Borrow<Q>,
              Q: Ord
    {
        type Output = V;

        /// Returns a reference to the value corresponding to the supplied key.
        ///
        /// # Panics
        ///
        /// Panics if the key is not present in the `BTreeMap`.
        #[inline]
        fn index(&self, key: &Q) -> &V {
            self.get(key).expect("no entry found for key")
        }
    }

    fn first_leaf_edge<BorrowType, K, V>
        (mut node: NodeRef<BorrowType, K, V, marker::LeafOrInternal>)
         -> Handle<NodeRef<BorrowType, K, V, marker::Leaf>, marker::Edge> {
        loop {
            match node.force() {
                Leaf(leaf) => return leaf.first_edge(),
                Internal(internal) => {
                    node = internal.first_edge().descend();
                }
            }
        }
    }

    fn last_leaf_edge<BorrowType, K, V>
        (mut node: NodeRef<BorrowType, K, V, marker::LeafOrInternal>)
         -> Handle<NodeRef<BorrowType, K, V, marker::Leaf>, marker::Edge> {
        loop {
            match node.force() {
                Leaf(leaf) => return leaf.last_edge(),
                Internal(internal) => {
                    node = internal.last_edge().descend();
                }
            }
        }
    }

    fn range_search<BorrowType, K, V, Q: ?Sized, R: RangeArgument<Q>>(
        root1: NodeRef<BorrowType, K, V, marker::LeafOrInternal>,
        root2: NodeRef<BorrowType, K, V, marker::LeafOrInternal>,
        range: R
    )-> (Handle<NodeRef<BorrowType, K, V, marker::Leaf>, marker::Edge>,
         Handle<NodeRef<BorrowType, K, V, marker::Leaf>, marker::Edge>)
            where Q: Ord, K: Borrow<Q>
    {
        match (range.start(), range.end()) {
            (Excluded(s), Excluded(e)) if s==e =>
                panic!("range start and end are equal and excluded in BTreeMap"),
            (Included(s), Included(e)) |
            (Included(s), Excluded(e)) |
            (Excluded(s), Included(e)) |
            (Excluded(s), Excluded(e)) if s>e =>
                panic!("range start is greater than range end in BTreeMap"),
            _ => {},
        };

        let mut min_node = root1;
        let mut max_node = root2;
        let mut min_found = false;
        let mut max_found = false;
        let mut diverged = false;

        loop {
            let min_edge = match (min_found, range.start()) {
                (false, Included(key)) => match search::search_linear(&min_node, key) {
                    (i, true) => { min_found = true; i },
                    (i, false) => i,
                },
                (false, Excluded(key)) => match search::search_linear(&min_node, key) {
                    (i, true) => { min_found = true; i+1 },
                    (i, false) => i,
                },
                (_, Unbounded) => 0,
                (true, Included(_)) => min_node.keys().len(),
                (true, Excluded(_)) => 0,
            };

            let max_edge = match (max_found, range.end()) {
                (false, Included(key)) => match search::search_linear(&max_node, key) {
                    (i, true) => { max_found = true; i+1 },
                    (i, false) => i,
                },
                (false, Excluded(key)) => match search::search_linear(&max_node, key) {
                    (i, true) => { max_found = true; i },
                    (i, false) => i,
                },
                (_, Unbounded) => max_node.keys().len(),
                (true, Included(_)) => 0,
                (true, Excluded(_)) => max_node.keys().len(),
            };

            if !diverged {
                if max_edge < min_edge { panic!("Ord is ill-defined in BTreeMap range") }
                if min_edge != max_edge { diverged = true; }
            }

            let front = Handle::new_edge(min_node, min_edge);
            let back = Handle::new_edge(max_node, max_edge);
            match (front.force(), back.force()) {
                (Leaf(f), Leaf(b)) => {
                    return (f, b);
                },
                (Internal(min_int), Internal(max_int)) => {
                    min_node = min_int.descend();
                    max_node = max_int.descend();
                },
                _ => unreachable!("BTreeMap has different depths"),
            };
        }
    }

    #[inline(always)]
    unsafe fn unwrap_unchecked<T>(val: Option<T>) -> T {
        val.unwrap_or_else(|| {
            if cfg!(debug_assertions) {
                panic!("'unchecked' unwrap on None in BTreeMap");
            } else {
                intrinsics::unreachable();
            }
        })
    }

    impl<K, V> BTreeMap<K, V> {
        /// Gets an iterator over the entries of the map, sorted by key.
        ///
        /// # Examples
        ///
        /// Basic usage:
        ///
        /// ```
        /// use std::collections::BTreeMap;
        ///
        /// let mut map = BTreeMap::new();
        /// map.insert(3, "c");
        /// map.insert(2, "b");
        /// map.insert(1, "a");
        ///
        /// for (key, value) in map.iter() {
        ///     println!("{}: {}", key, value);
        /// }
        ///
        /// let (first_key, first_value) = map.iter().next().unwrap();
        /// assert_eq!((*first_key, *first_value), (1, "a"));
        /// ```
        #[stable(feature = "rust1", since = "1.0.0")]
        pub fn iter(&self) -> Iter<K, V> {
            Iter {
                range: Range {
                    front: first_leaf_edge(self.root.as_ref()),
                    back: last_leaf_edge(self.root.as_ref()),
                },
                length: self.length,
            }
        }

        /// Gets a mutable iterator over the entries of the map, sorted by key.
        ///
        /// # Examples
        ///
        /// Basic usage:
        ///
        /// ```
        /// use std::collections::BTreeMap;
        ///
        /// let mut map = BTreeMap::new();
        /// map.insert("a", 1);
        /// map.insert("b", 2);
        /// map.insert("c", 3);
        ///
        /// // add 10 to the value if the key isn't "a"
        /// for (key, value) in map.iter_mut() {
        ///     if key != &"a" {
        ///         *value += 10;
        ///     }
        /// }
        /// ```
        #[stable(feature = "rust1", since = "1.0.0")]
        pub fn iter_mut(&mut self) -> IterMut<K, V> {
            let root1 = self.root.as_mut();
            let root2 = unsafe { ptr::read(&root1) };
            IterMut {
                range: RangeMut {
                    front: first_leaf_edge(root1),
                    back: last_leaf_edge(root2),
                    _marker: PhantomData,
                },
                length: self.length,
            }
        }

        /// Gets an iterator over the keys of the map, in sorted order.
        ///
        /// # Examples
        ///
        /// Basic usage:
        ///
        /// ```
        /// use std::collections::BTreeMap;
        ///
        /// let mut a = BTreeMap::new();
        /// a.insert(2, "b");
        /// a.insert(1, "a");
        ///
        /// let keys: Vec<_> = a.keys().cloned().collect();
        /// assert_eq!(keys, [1, 2]);
        /// ```
        #[stable(feature = "rust1", since = "1.0.0")]
        pub fn keys<'a>(&'a self) -> Keys<'a, K, V> {
            Keys { inner: self.iter() }
        }

        /// Gets an iterator over the values of the map, in order by key.
        ///
        /// # Examples
        ///
        /// Basic usage:
        ///
        /// ```
        /// use std::collections::BTreeMap;
        ///
        /// let mut a = BTreeMap::new();
        /// a.insert(1, "hello");
        /// a.insert(2, "goodbye");
        ///
        /// let values: Vec<&str> = a.values().cloned().collect();
        /// assert_eq!(values, ["hello", "goodbye"]);
        /// ```
        #[stable(feature = "rust1", since = "1.0.0")]
        pub fn values<'a>(&'a self) -> Values<'a, K, V> {
            Values { inner: self.iter() }
        }

        /// Gets a mutable iterator over the values of the map, in order by key.
        ///
        /// # Examples
        ///
        /// Basic usage:
        ///
        /// ```
        /// use std::collections::BTreeMap;
        ///
        /// let mut a = BTreeMap::new();
        /// a.insert(1, String::from("hello"));
        /// a.insert(2, String::from("goodbye"));
        ///
        /// for value in a.values_mut() {
        ///     value.push_str("!");
        /// }
        ///
        /// let values: Vec<String> = a.values().cloned().collect();
        /// assert_eq!(values, [String::from("hello!"),
        ///                     String::from("goodbye!")]);
        /// ```
        #[stable(feature = "map_values_mut", since = "1.10.0")]
        pub fn values_mut(&mut self) -> ValuesMut<K, V> {
            ValuesMut { inner: self.iter_mut() }
        }

        /// Returns the number of elements in the map.
        ///
        /// # Examples
        ///
        /// Basic usage:
        ///
        /// ```
        /// use std::collections::BTreeMap;
        ///
        /// let mut a = BTreeMap::new();
        /// assert_eq!(a.len(), 0);
        /// a.insert(1, "a");
        /// assert_eq!(a.len(), 1);
        /// ```
        #[stable(feature = "rust1", since = "1.0.0")]
        pub fn len(&self) -> usize {
            self.length
        }

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

    impl<'a, K: Ord, V> Entry<'a, K, V> {
        /// Ensures a value is in the entry by inserting the default if empty, and returns
        /// a mutable reference to the value in the entry.
        ///
        /// # Examples
        ///
        /// ```
        /// use std::collections::BTreeMap;
        ///
        /// let mut map: BTreeMap<&str, usize> = BTreeMap::new();
        /// map.entry("poneyland").or_insert(12);
        ///
        /// assert_eq!(map["poneyland"], 12);
        /// ```
        #[stable(feature = "rust1", since = "1.0.0")]
        pub fn or_insert(self, default: V) -> &'a mut V {
            match self {
                Occupied(entry) => entry.into_mut(),
                Vacant(entry) => entry.insert(default),
            }
        }

        /// Ensures a value is in the entry by inserting the result of the default function if empty,
        /// and returns a mutable reference to the value in the entry.
        ///
        /// # Examples
        ///
        /// ```
        /// use std::collections::BTreeMap;
        ///
        /// let mut map: BTreeMap<&str, String> = BTreeMap::new();
        /// let s = "hoho".to_string();
        ///
        /// map.entry("poneyland").or_insert_with(|| s);
        ///
        /// assert_eq!(map["poneyland"], "hoho".to_string());
        /// ```
        #[stable(feature = "rust1", since = "1.0.0")]
        pub fn or_insert_with<F: FnOnce() -> V>(self, default: F) -> &'a mut V {
            match self {
                Occupied(entry) => entry.into_mut(),
                Vacant(entry) => entry.insert(default()),
            }
        }

        /// Returns a reference to this entry's key.
        ///
        /// # Examples
        ///
        /// ```
        /// use std::collections::BTreeMap;
        ///
        /// let mut map: BTreeMap<&str, usize> = BTreeMap::new();
        /// assert_eq!(map.entry("poneyland").key(), &"poneyland");
        /// ```
        #[stable(feature = "map_entry_keys", since = "1.10.0")]
        pub fn key(&self) -> &K {
            match *self {
                Occupied(ref entry) => entry.key(),
                Vacant(ref entry) => entry.key(),
            }
        }

        /// Provides in-place mutable access to an occupied entry before any
        /// potential inserts into the map.
        ///
        /// # Examples
        ///
        /// ```
        /// #![feature(entry_and_modify)]
        /// use std::collections::BTreeMap;
        ///
        /// let mut map: BTreeMap<&str, usize> = BTreeMap::new();
        ///
        /// map.entry("poneyland")
        ///    .and_modify(|e| { *e += 1 })
        ///    .or_insert(42);
        /// assert_eq!(map["poneyland"], 42);
        ///
        /// map.entry("poneyland")
        ///    .and_modify(|e| { *e += 1 })
        ///    .or_insert(42);
        /// assert_eq!(map["poneyland"], 43);
        /// ```
        #[unstable(feature = "entry_and_modify", issue = "44733")]
        pub fn and_modify<F>(self, mut f: F) -> Self
            where F: FnMut(&mut V)
        {
            match self {
                Occupied(mut entry) => {
                    f(entry.get_mut());
                    Occupied(entry)
                },
                Vacant(entry) => Vacant(entry),
            }
        }
    }

    impl<'a, K: Ord, V: Default> Entry<'a, K, V> {
        #[unstable(feature = "entry_or_default", issue = "44324")]
        /// Ensures a value is in the entry by inserting the default value if empty,
        /// and returns a mutable reference to the value in the entry.
        ///
        /// # Examples
        ///
        /// ```
        /// #![feature(entry_or_default)]
        /// # fn main() {
        /// use std::collections::BTreeMap;
        ///
        /// let mut map: BTreeMap<&str, Option<usize>> = BTreeMap::new();
        /// map.entry("poneyland").or_default();
        ///
        /// assert_eq!(map["poneyland"], None);
        /// # }
        /// ```
        pub fn or_default(self) -> &'a mut V {
            match self {
                Occupied(entry) => entry.into_mut(),
                Vacant(entry) => entry.insert(Default::default()),
            }
        }

    }

    impl<'a, K: Ord, V> VacantEntry<'a, K, V> {
        /// Gets a reference to the key that would be used when inserting a value
        /// through the VacantEntry.
        ///
        /// # Examples
        ///
        /// ```
        /// use std::collections::BTreeMap;
        ///
        /// let mut map: BTreeMap<&str, usize> = BTreeMap::new();
        /// assert_eq!(map.entry("poneyland").key(), &"poneyland");
        /// ```
        #[stable(feature = "map_entry_keys", since = "1.10.0")]
        pub fn key(&self) -> &K {
            &self.key
        }

        /// Take ownership of the key.
        ///
        /// # Examples
        ///
        /// ```
        /// use std::collections::BTreeMap;
        /// use std::collections::btree_map::Entry;
        ///
        /// let mut map: BTreeMap<&str, usize> = BTreeMap::new();
        ///
        /// if let Entry::Vacant(v) = map.entry("poneyland") {
        ///     v.into_key();
        /// }
        /// ```
        #[stable(feature = "map_entry_recover_keys2", since = "1.12.0")]
        pub fn into_key(self) -> K {
            self.key
        }

        /// Sets the value of the entry with the `VacantEntry`'s key,
        /// and returns a mutable reference to it.
        ///
        /// # Examples
        ///
        /// ```
        /// use std::collections::BTreeMap;
        ///
        /// let mut count: BTreeMap<&str, usize> = BTreeMap::new();
        ///
        /// // count the number of occurrences of letters in the vec
        /// for x in vec!["a","b","a","c","a","b"] {
        ///     *count.entry(x).or_insert(0) += 1;
        /// }
        ///
        /// assert_eq!(count["a"], 3);
        /// ```
        #[stable(feature = "rust1", since = "1.0.0")]
        pub fn insert(self, value: V) -> &'a mut V {
            *self.length += 1;

            let out_ptr;

            let mut ins_k;
            let mut ins_v;
            let mut ins_edge;

            let mut cur_parent = match self.handle.insert(self.key, value) {
                (Fit(handle), _) => return handle.into_kv_mut().1,
                (Split(left, k, v, right), ptr) => {
                    ins_k = k;
                    ins_v = v;
                    ins_edge = right;
                    out_ptr = ptr;
                    left.ascend().map_err(|n| n.into_root_mut())
                }
            };

            loop {
                match cur_parent {
                    Ok(parent) => {
                        match parent.insert(ins_k, ins_v, ins_edge) {
                            Fit(_) => return unsafe { &mut *out_ptr },
                            Split(left, k, v, right) => {
                                ins_k = k;
                                ins_v = v;
                                ins_edge = right;
                                cur_parent = left.ascend().map_err(|n| n.into_root_mut());
                            }
                        }
                    }
                    Err(root) => {
                        root.push_level().push(ins_k, ins_v, ins_edge);
                        return unsafe { &mut *out_ptr };
                    }
                }
            }
        }
    }

    impl<'a, K: Ord, V> OccupiedEntry<'a, K, V> {
        /// Gets a reference to the key in the entry.
        ///
        /// # Examples
        ///
        /// ```
        /// use std::collections::BTreeMap;
        ///
        /// let mut map: BTreeMap<&str, usize> = BTreeMap::new();
        /// map.entry("poneyland").or_insert(12);
        /// assert_eq!(map.entry("poneyland").key(), &"poneyland");
        /// ```
        #[stable(feature = "map_entry_keys", since = "1.10.0")]
        pub fn key(&self) -> &K {
            self.handle.reborrow().into_kv().0
        }

        /// Take ownership of the key and value from the map.
        ///
        /// # Examples
        ///
        /// ```
        /// use std::collections::BTreeMap;
        /// use std::collections::btree_map::Entry;
        ///
        /// let mut map: BTreeMap<&str, usize> = BTreeMap::new();
        /// map.entry("poneyland").or_insert(12);
        ///
        /// if let Entry::Occupied(o) = map.entry("poneyland") {
        ///     // We delete the entry from the map.
        ///     o.remove_entry();
        /// }
        ///
        /// // If now try to get the value, it will panic:
        /// // println!("{}", map["poneyland"]);
        /// ```
        #[stable(feature = "map_entry_recover_keys2", since = "1.12.0")]
        pub fn remove_entry(self) -> (K, V) {
            self.remove_kv()
        }

        /// Gets a reference to the value in the entry.
        ///
        /// # Examples
        ///
        /// ```
        /// use std::collections::BTreeMap;
        /// use std::collections::btree_map::Entry;
        ///
        /// let mut map: BTreeMap<&str, usize> = BTreeMap::new();
        /// map.entry("poneyland").or_insert(12);
        ///
        /// if let Entry::Occupied(o) = map.entry("poneyland") {
        ///     assert_eq!(o.get(), &12);
        /// }
        /// ```
        #[stable(feature = "rust1", since = "1.0.0")]
        pub fn get(&self) -> &V {
            self.handle.reborrow().into_kv().1
        }

        /// Gets a mutable reference to the value in the entry.
        ///
        /// # Examples
        ///
        /// ```
        /// use std::collections::BTreeMap;
        /// use std::collections::btree_map::Entry;
        ///
        /// let mut map: BTreeMap<&str, usize> = BTreeMap::new();
        /// map.entry("poneyland").or_insert(12);
        ///
        /// assert_eq!(map["poneyland"], 12);
        /// if let Entry::Occupied(mut o) = map.entry("poneyland") {
        ///      *o.get_mut() += 10;
        /// }
        /// assert_eq!(map["poneyland"], 22);
        /// ```
        #[stable(feature = "rust1", since = "1.0.0")]
        pub fn get_mut(&mut self) -> &mut V {
            self.handle.kv_mut().1
        }

        /// Converts the entry into a mutable reference to its value.
        ///
        /// # Examples
        ///
        /// ```
        /// use std::collections::BTreeMap;
        /// use std::collections::btree_map::Entry;
        ///
        /// let mut map: BTreeMap<&str, usize> = BTreeMap::new();
        /// map.entry("poneyland").or_insert(12);
        ///
        /// assert_eq!(map["poneyland"], 12);
        /// if let Entry::Occupied(o) = map.entry("poneyland") {
        ///     *o.into_mut() += 10;
        /// }
        /// assert_eq!(map["poneyland"], 22);
        /// ```
        #[stable(feature = "rust1", since = "1.0.0")]
        pub fn into_mut(self) -> &'a mut V {
            self.handle.into_kv_mut().1
        }

        /// Sets the value of the entry with the `OccupiedEntry`'s key,
        /// and returns the entry's old value.
        ///
        /// # Examples
        ///
        /// ```
        /// use std::collections::BTreeMap;
        /// use std::collections::btree_map::Entry;
        ///
        /// let mut map: BTreeMap<&str, usize> = BTreeMap::new();
        /// map.entry("poneyland").or_insert(12);
        ///
        /// if let Entry::Occupied(mut o) = map.entry("poneyland") {
        ///     assert_eq!(o.insert(15), 12);
        /// }
        /// assert_eq!(map["poneyland"], 15);
        /// ```
        #[stable(feature = "rust1", since = "1.0.0")]
        pub fn insert(&mut self, value: V) -> V {
            mem::replace(self.get_mut(), value)
        }

        /// Takes the value of the entry out of the map, and returns it.
        ///
        /// # Examples
        ///
        /// ```
        /// use std::collections::BTreeMap;
        /// use std::collections::btree_map::Entry;
        ///
        /// let mut map: BTreeMap<&str, usize> = BTreeMap::new();
        /// map.entry("poneyland").or_insert(12);
        ///
        /// if let Entry::Occupied(o) = map.entry("poneyland") {
        ///     assert_eq!(o.remove(), 12);
        /// }
        /// // If we try to get "poneyland"'s value, it'll panic:
        /// // println!("{}", map["poneyland"]);
        /// ```
        #[stable(feature = "rust1", since = "1.0.0")]
        pub fn remove(self) -> V {
            self.remove_kv().1
        }

        fn remove_kv(self) -> (K, V) {
            *self.length -= 1;

            let (small_leaf, old_key, old_val) = match self.handle.force() {
                Leaf(leaf) => {
                    let (hole, old_key, old_val) = leaf.remove();
                    (hole.into_node(), old_key, old_val)
                }
                Internal(mut internal) => {
                    let key_loc = internal.kv_mut().0 as *mut K;
                    let val_loc = internal.kv_mut().1 as *mut V;

                    let to_remove = first_leaf_edge(internal.right_edge().descend()).right_kv().ok();
                    let to_remove = unsafe { unwrap_unchecked(to_remove) };

                    let (hole, key, val) = to_remove.remove();

                    let old_key = unsafe { mem::replace(&mut *key_loc, key) };
                    let old_val = unsafe { mem::replace(&mut *val_loc, val) };

                    (hole.into_node(), old_key, old_val)
                }
            };

            // Handle underflow
            let mut cur_node = small_leaf.forget_type();
            while cur_node.len() < node::CAPACITY / 2 {
                match handle_underfull_node(cur_node) {
                    AtRoot => break,
                    EmptyParent(_) => unreachable!(),
                    Merged(parent) => {
                        if parent.len() == 0 {
                            // We must be at the root
                            parent.into_root_mut().pop_level();
                            break;
                        } else {
                            cur_node = parent.forget_type();
                        }
                    }
                    Stole(_) => break,
                }
            }

            (old_key, old_val)
        }
    }

    enum UnderflowResult<'a, K, V> {
        AtRoot,
        EmptyParent(NodeRef<marker::Mut<'a>, K, V, marker::Internal>),
        Merged(NodeRef<marker::Mut<'a>, K, V, marker::Internal>),
        Stole(NodeRef<marker::Mut<'a>, K, V, marker::Internal>),
    }

    fn handle_underfull_node<'a, K, V>(node: NodeRef<marker::Mut<'a>, K, V, marker::LeafOrInternal>)
                                       -> UnderflowResult<'a, K, V> {
        let parent = if let Ok(parent) = node.ascend() {
            parent
        } else {
            return AtRoot;
        };

        let (is_left, mut handle) = match parent.left_kv() {
            Ok(left) => (true, left),
            Err(parent) => {
                match parent.right_kv() {
                    Ok(right) => (false, right),
                    Err(parent) => {
                        return EmptyParent(parent.into_node());
                    }
                }
            }
        };

        if handle.can_merge() {
            Merged(handle.merge().into_node())
        } else {
            if is_left {
                handle.steal_left();
            } else {
                handle.steal_right();
            }
            Stole(handle.into_node())
        }
    }

    impl<K: Ord, V, I: Iterator<Item = (K, V)>> Iterator for MergeIter<K, V, I> {
        type Item = (K, V);

        fn next(&mut self) -> Option<(K, V)> {
            let res = match (self.left.peek(), self.right.peek()) {
                (Some(&(ref left_key, _)), Some(&(ref right_key, _))) => left_key.cmp(right_key),
                (Some(_), None) => Ordering::Less,
                (None, Some(_)) => Ordering::Greater,
                (None, None) => return None,
            };

            // Check which elements comes first and only advance the corresponding iterator.
            // If two keys are equal, take the value from `right`.
            match res {
                Ordering::Less => self.left.next(),
                Ordering::Greater => self.right.next(),
                Ordering::Equal => {
                    self.left.next();
                    self.right.next()
                }
            }
        }
    }

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