use core::borrow::Borrow;
use core::cmp::Ordering;
use core::fmt::{self, Debug};
use core::hash::{Hash, Hasher};
use core::iter::{FromIterator, FusedIterator};
use core::marker::PhantomData;
use core::mem::{self, ManuallyDrop};
use core::ops::{Index, RangeBounds};
use core::ptr;

use super::borrow::DormantMutRef;
use super::merge_iter::MergeIterInner;
use super::node::{self, marker, ForceResult::*, Handle, NodeRef};
use super::search::{self, SearchResult::*};
use super::unwrap_unchecked;

mod entry;
pub use entry::{Entry, OccupiedEntry, VacantEntry};
use Entry::*;

/// Minimum number of elements in nodes that are not a root.
/// We might temporarily have fewer elements during methods.
pub(super) const MIN_LEN: usize = node::MIN_LEN_AFTER_SPLIT;

/// 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(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.
///
/// [`Cell`]: core::cell::Cell
/// [`RefCell`]: core::cell::RefCell
///
/// # 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 a lot.");
///
/// // 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 movie in &to_find {
///     match movie_reviews.get(movie) {
///        Some(review) => println!("{}: {}", movie, review),
///        None => println!("{} is unreviewed.", movie)
///     }
/// }
///
/// // Look up the value for a key (will panic if the key is not found).
/// println!("Movie review: {}", movie_reviews["Office Space"]);
///
/// // iterate over everything.
/// for (movie, review) in &movie_reviews {
///     println!("{}: \"{}\"", movie, review);
/// }
/// ```
///
/// `BTreeMap` also implements an [`Entry API`], which allows for more complex
/// methods of getting, setting, updating and removing keys and their values:
///
/// [`Entry API`]: BTreeMap::entry
///
/// ```
/// 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: Option<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<'a, K: Clone, V: Clone>(
            node: node::NodeRef<marker::Immut<'a>, K, V, marker::LeafOrInternal>,
        ) -> BTreeMap<K, V>
        where
            K: 'a,
            V: 'a,
        {
            match node.force() {
                Leaf(leaf) => {
                    let mut out_tree = BTreeMap { root: Some(node::Root::new_leaf()), length: 0 };

                    {
                        let root = out_tree.root.as_mut().unwrap(); // unwrap succeeds because we just wrapped
                        let mut out_node = match root.node_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 out_root = BTreeMap::ensure_is_owned(&mut out_tree.root);
                        let mut out_node = out_root.push_internal_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 subtree = ManuallyDrop::new(subtree);
                                let root = ptr::read(&subtree.root);
                                let length = subtree.length;
                                (root, length)
                            };

                            out_node.push(k, v, subroot.unwrap_or_else(node::Root::new_leaf));
                            out_tree.length += 1 + sublength;
                        }
                    }

                    out_tree
                }
            }
        }

        if self.is_empty() {
            // Ideally we'd call `BTreeMap::new` here, but that has the `K:
            // Ord` constraint, which this method lacks.
            BTreeMap { root: None, length: 0 }
        } else {
            clone_subtree(self.root.as_ref().unwrap().node_as_ref()) // unwrap succeeds because not empty
        }
    }
}

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> {
        let root_node = self.root.as_ref()?.node_as_ref();
        match search::search_tree(root_node, key) {
            Found(handle) => Some(handle.into_kv().0),
            GoDown(_) => None,
        }
    }

    fn take(&mut self, key: &Q) -> Option<K> {
        let (map, dormant_map) = DormantMutRef::new(self);
        let root_node = map.root.as_mut()?.node_as_mut();
        match search::search_tree(root_node, key) {
            Found(handle) => {
                Some(OccupiedEntry { handle, dormant_map, _marker: PhantomData }.remove_kv().0)
            }
            GoDown(_) => None,
        }
    }

    fn replace(&mut self, key: K) -> Option<K> {
        let (map, dormant_map) = DormantMutRef::new(self);
        let root_node = Self::ensure_is_owned(&mut map.root).node_as_mut();
        match search::search_tree::<marker::Mut<'_>, K, (), K>(root_node, &key) {
            Found(handle) => Some(mem::replace(handle.into_key_mut(), key)),
            GoDown(handle) => {
                VacantEntry { key, handle, dormant_map, _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`]: BTreeMap::iter
#[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<K: fmt::Debug, V: fmt::Debug> fmt::Debug for Iter<'_, 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`]: BTreeMap::iter_mut
#[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`]
/// (provided by the `IntoIterator` trait). See its documentation for more.
///
/// [`into_iter`]: IntoIterator::into_iter
#[stable(feature = "rust1", since = "1.0.0")]
pub struct IntoIter<K, V> {
    front: Option<Handle<NodeRef<marker::Owned, K, V, marker::Leaf>, marker::Edge>>,
    back: Option<Handle<NodeRef<marker::Owned, K, V, marker::Leaf>, marker::Edge>>,
    length: usize,
}

impl<K, V> IntoIter<K, V> {
    /// Returns an iterator of references over the remaining items.
    #[inline]
    pub(super) fn iter(&self) -> Iter<'_, K, V> {
        let range = Range {
            front: self.front.as_ref().map(|f| f.reborrow()),
            back: self.back.as_ref().map(|b| b.reborrow()),
        };

        Iter { range: range, length: self.length }
    }
}

#[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 {
        f.debug_list().entries(self.iter()).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`]: BTreeMap::keys
#[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<K: fmt::Debug, V> fmt::Debug for Keys<'_, 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`]: BTreeMap::values
#[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<K, V: fmt::Debug> fmt::Debug for Values<'_, 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`]: BTreeMap::values_mut
#[stable(feature = "map_values_mut", since = "1.10.0")]
pub struct ValuesMut<'a, K: 'a, V: 'a> {
    inner: IterMut<'a, K, V>,
}

#[stable(feature = "map_values_mut", since = "1.10.0")]
impl<K, V: fmt::Debug> fmt::Debug for ValuesMut<'_, K, V> {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        f.debug_list().entries(self.inner.iter().map(|(_, val)| val)).finish()
    }
}

/// An owning iterator over the keys of a `BTreeMap`.
///
/// This `struct` is created by the [`into_keys`] method on [`BTreeMap`].
/// See its documentation for more.
///
/// [`into_keys`]: BTreeMap::into_keys
#[unstable(feature = "map_into_keys_values", issue = "75294")]
pub struct IntoKeys<K, V> {
    inner: IntoIter<K, V>,
}

#[unstable(feature = "map_into_keys_values", issue = "75294")]
impl<K: fmt::Debug, V> fmt::Debug for IntoKeys<K, V> {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        f.debug_list().entries(self.inner.iter().map(|(key, _)| key)).finish()
    }
}

/// An owning iterator over the values of a `BTreeMap`.
///
/// This `struct` is created by the [`into_values`] method on [`BTreeMap`].
/// See its documentation for more.
///
/// [`into_values`]: BTreeMap::into_values
#[unstable(feature = "map_into_keys_values", issue = "75294")]
pub struct IntoValues<K, V> {
    inner: IntoIter<K, V>,
}

#[unstable(feature = "map_into_keys_values", issue = "75294")]
impl<K, V: fmt::Debug> fmt::Debug for IntoValues<K, V> {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        f.debug_list().entries(self.inner.iter().map(|(_, val)| val)).finish()
    }
}

/// 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`]: BTreeMap::range
#[stable(feature = "btree_range", since = "1.17.0")]
pub struct Range<'a, K: 'a, V: 'a> {
    front: Option<Handle<NodeRef<marker::Immut<'a>, K, V, marker::Leaf>, marker::Edge>>,
    back: Option<Handle<NodeRef<marker::Immut<'a>, K, V, marker::Leaf>, marker::Edge>>,
}

#[stable(feature = "collection_debug", since = "1.17.0")]
impl<K: fmt::Debug, V: fmt::Debug> fmt::Debug for Range<'_, 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`]: BTreeMap::range_mut
#[stable(feature = "btree_range", since = "1.17.0")]
pub struct RangeMut<'a, K: 'a, V: 'a> {
    front: Option<Handle<NodeRef<marker::ValMut<'a>, K, V, marker::Leaf>, marker::Edge>>,
    back: Option<Handle<NodeRef<marker::ValMut<'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<K: fmt::Debug, V: fmt::Debug> fmt::Debug for RangeMut<'_, K, V> {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        let range = Range {
            front: self.front.as_ref().map(|f| f.reborrow()),
            back: self.back.as_ref().map(|b| b.reborrow()),
        };
        f.debug_list().entries(range).finish()
    }
}

// An iterator for merging two sorted sequences into one
struct MergeIter<K, V, I: Iterator<Item = (K, V)>>(MergeIterInner<I>);

impl<K: Ord, V> BTreeMap<K, V> {
    /// Makes a new empty BTreeMap.
    ///
    /// Does not allocate anything on its own.
    ///
    /// # 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")]
    #[rustc_const_unstable(feature = "const_btree_new", issue = "71835")]
    pub const fn new() -> BTreeMap<K, V> {
        BTreeMap { root: None, length: 0 }
    }

    /// Clears the map, removing all elements.
    ///
    /// # 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) {
        *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,
    {
        let root_node = self.root.as_ref()?.node_as_ref();
        match search::search_tree(root_node, key) {
            Found(handle) => Some(handle.into_kv().1),
            GoDown(_) => None,
        }
    }

    /// Returns the key-value pair corresponding to the supplied key.
    ///
    /// The supplied 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
    ///
    /// ```
    /// use std::collections::BTreeMap;
    ///
    /// let mut map = BTreeMap::new();
    /// map.insert(1, "a");
    /// assert_eq!(map.get_key_value(&1), Some((&1, &"a")));
    /// assert_eq!(map.get_key_value(&2), None);
    /// ```
    #[stable(feature = "map_get_key_value", since = "1.40.0")]
    pub fn get_key_value<Q: ?Sized>(&self, k: &Q) -> Option<(&K, &V)>
    where
        K: Borrow<Q>,
        Q: Ord,
    {
        let root_node = self.root.as_ref()?.node_as_ref();
        match search::search_tree(root_node, k) {
            Found(handle) => Some(handle.into_kv()),
            GoDown(_) => None,
        }
    }

    /// Returns the first key-value pair in the map.
    /// The key in this pair is the minimum key in the map.
    ///
    /// # Examples
    ///
    /// Basic usage:
    ///
    /// ```
    /// #![feature(map_first_last)]
    /// use std::collections::BTreeMap;
    ///
    /// let mut map = BTreeMap::new();
    /// assert_eq!(map.first_key_value(), None);
    /// map.insert(1, "b");
    /// map.insert(2, "a");
    /// assert_eq!(map.first_key_value(), Some((&1, &"b")));
    /// ```
    #[unstable(feature = "map_first_last", issue = "62924")]
    pub fn first_key_value(&self) -> Option<(&K, &V)> {
        let root_node = self.root.as_ref()?.node_as_ref();
        root_node.first_leaf_edge().right_kv().ok().map(Handle::into_kv)
    }

    /// Returns the first entry in the map for in-place manipulation.
    /// The key of this entry is the minimum key in the map.
    ///
    /// # Examples
    ///
    /// ```
    /// #![feature(map_first_last)]
    /// use std::collections::BTreeMap;
    ///
    /// let mut map = BTreeMap::new();
    /// map.insert(1, "a");
    /// map.insert(2, "b");
    /// if let Some(mut entry) = map.first_entry() {
    ///     if *entry.key() > 0 {
    ///         entry.insert("first");
    ///     }
    /// }
    /// assert_eq!(*map.get(&1).unwrap(), "first");
    /// assert_eq!(*map.get(&2).unwrap(), "b");
    /// ```
    #[unstable(feature = "map_first_last", issue = "62924")]
    pub fn first_entry(&mut self) -> Option<OccupiedEntry<'_, K, V>> {
        let (map, dormant_map) = DormantMutRef::new(self);
        let root_node = map.root.as_mut()?.node_as_mut();
        let kv = root_node.first_leaf_edge().right_kv().ok()?;
        Some(OccupiedEntry { handle: kv.forget_node_type(), dormant_map, _marker: PhantomData })
    }

    /// Removes and returns the first element in the map.
    /// The key of this element is the minimum key that was in the map.
    ///
    /// # Examples
    ///
    /// Draining elements in ascending order, while keeping a usable map each iteration.
    ///
    /// ```
    /// #![feature(map_first_last)]
    /// use std::collections::BTreeMap;
    ///
    /// let mut map = BTreeMap::new();
    /// map.insert(1, "a");
    /// map.insert(2, "b");
    /// while let Some((key, _val)) = map.pop_first() {
    ///     assert!(map.iter().all(|(k, _v)| *k > key));
    /// }
    /// assert!(map.is_empty());
    /// ```
    #[unstable(feature = "map_first_last", issue = "62924")]
    pub fn pop_first(&mut self) -> Option<(K, V)> {
        self.first_entry().map(|entry| entry.remove_entry())
    }

    /// Returns the last key-value pair in the map.
    /// The key in this pair is the maximum key in the map.
    ///
    /// # Examples
    ///
    /// Basic usage:
    ///
    /// ```
    /// #![feature(map_first_last)]
    /// use std::collections::BTreeMap;
    ///
    /// let mut map = BTreeMap::new();
    /// map.insert(1, "b");
    /// map.insert(2, "a");
    /// assert_eq!(map.last_key_value(), Some((&2, &"a")));
    /// ```
    #[unstable(feature = "map_first_last", issue = "62924")]
    pub fn last_key_value(&self) -> Option<(&K, &V)> {
        let root_node = self.root.as_ref()?.node_as_ref();
        root_node.last_leaf_edge().left_kv().ok().map(Handle::into_kv)
    }

    /// Returns the last entry in the map for in-place manipulation.
    /// The key of this entry is the maximum key in the map.
    ///
    /// # Examples
    ///
    /// ```
    /// #![feature(map_first_last)]
    /// use std::collections::BTreeMap;
    ///
    /// let mut map = BTreeMap::new();
    /// map.insert(1, "a");
    /// map.insert(2, "b");
    /// if let Some(mut entry) = map.last_entry() {
    ///     if *entry.key() > 0 {
    ///         entry.insert("last");
    ///     }
    /// }
    /// assert_eq!(*map.get(&1).unwrap(), "a");
    /// assert_eq!(*map.get(&2).unwrap(), "last");
    /// ```
    #[unstable(feature = "map_first_last", issue = "62924")]
    pub fn last_entry(&mut self) -> Option<OccupiedEntry<'_, K, V>> {
        let (map, dormant_map) = DormantMutRef::new(self);
        let root_node = map.root.as_mut()?.node_as_mut();
        let kv = root_node.last_leaf_edge().left_kv().ok()?;
        Some(OccupiedEntry { handle: kv.forget_node_type(), dormant_map, _marker: PhantomData })
    }

    /// Removes and returns the last element in the map.
    /// The key of this element is the maximum key that was in the map.
    ///
    /// # Examples
    ///
    /// Draining elements in descending order, while keeping a usable map each iteration.
    ///
    /// ```
    /// #![feature(map_first_last)]
    /// use std::collections::BTreeMap;
    ///
    /// let mut map = BTreeMap::new();
    /// map.insert(1, "a");
    /// map.insert(2, "b");
    /// while let Some((key, _val)) = map.pop_last() {
    ///     assert!(map.iter().all(|(k, _v)| *k < key));
    /// }
    /// assert!(map.is_empty());
    /// ```
    #[unstable(feature = "map_first_last", issue = "62924")]
    pub fn pop_last(&mut self) -> Option<(K, V)> {
        self.last_entry().map(|entry| entry.remove_entry())
    }

    /// 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,
    {
        let root_node = self.root.as_mut()?.node_as_mut();
        match search::search_tree(root_node, key) {
            Found(handle) => Some(handle.into_val_mut()),
            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,
    {
        self.remove_entry(key).map(|(_, v)| v)
    }

    /// Removes a key from the map, returning the stored key and value 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_entry(&1), Some((1, "a")));
    /// assert_eq!(map.remove_entry(&1), None);
    /// ```
    #[stable(feature = "btreemap_remove_entry", since = "1.45.0")]
    pub fn remove_entry<Q: ?Sized>(&mut self, key: &Q) -> Option<(K, V)>
    where
        K: Borrow<Q>,
        Q: Ord,
    {
        let (map, dormant_map) = DormantMutRef::new(self);
        let root_node = map.root.as_mut()?.node_as_mut();
        match search::search_tree(root_node, key) {
            Found(handle) => {
                Some(OccupiedEntry { handle, dormant_map, _marker: PhantomData }.remove_entry())
            }
            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.is_empty() {
            return;
        }

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

        // First, we merge `self` and `other` into a sorted sequence in linear time.
        let self_iter = mem::take(self).into_iter();
        let other_iter = mem::take(other).into_iter();
        let iter = MergeIter(MergeIterInner::new(self_iter, other_iter));

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

    /// 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::ops::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: RangeBounds<T>,
    {
        if let Some(root) = &self.root {
            let (f, b) = root.node_as_ref().range_search(range);

            Range { front: Some(f), back: Some(b) }
        } else {
            Range { front: None, back: None }
        }
    }

    /// 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: RangeBounds<T>,
    {
        if let Some(root) = &mut self.root {
            let (f, b) = root.node_as_valmut().range_search(range);

            RangeMut { front: Some(f), back: Some(b), _marker: PhantomData }
        } else {
            RangeMut { front: None, back: None, _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> {
        // FIXME(@porglezomp) Avoid allocating if we don't insert
        let (map, dormant_map) = DormantMutRef::new(self);
        let root_node = Self::ensure_is_owned(&mut map.root).node_as_mut();
        match search::search_tree(root_node, &key) {
            Found(handle) => Occupied(OccupiedEntry { handle, dormant_map, _marker: PhantomData }),
            GoDown(handle) => {
                Vacant(VacantEntry { key, handle, dormant_map, _marker: PhantomData })
            }
        }
    }

    fn from_sorted_iter<I: Iterator<Item = (K, V)>>(&mut self, iter: I) {
        let root = Self::ensure_is_owned(&mut self.root);
        let mut cur_node = root.node_as_mut().last_leaf_edge().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(_) => {
                            // We are at the top, create a new root node and push there.
                            open_node = root.push_internal_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_internal_level();
                }
                open_node.push(key, value, right_tree);

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

            self.length += 1;
        }
        Self::fix_right_edge(root)
    }

    fn fix_right_edge(root: &mut node::Root<K, V>) {
        // Handle underfull nodes, start from the top.
        let mut cur_node = root.node_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 < 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(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 left_root = self.root.as_mut().unwrap(); // unwrap succeeds because not empty

        let mut right = Self::new();
        let right_root = Self::ensure_is_owned(&mut right.root);

        left_root.split_off(right_root, key);

        if left_root.height() < right_root.height() {
            self.length = left_root.node_as_ref().calc_length();
            right.length = total_num - self.len();
        } else {
            right.length = right_root.node_as_ref().calc_length();
            self.length = total_num - right.len();
        }

        right
    }

    /// Creates an iterator which uses a closure to determine if an element should be removed.
    ///
    /// If the closure returns true, the element is removed from the map and yielded.
    /// If the closure returns false, or panics, the element remains in the map and will not be
    /// yielded.
    ///
    /// Note that `drain_filter` lets you mutate every value in the filter closure, regardless of
    /// whether you choose to keep or remove it.
    ///
    /// If the iterator is only partially consumed or not consumed at all, each of the remaining
    /// elements will still be subjected to the closure and removed and dropped if it returns true.
    ///
    /// It is unspecified how many more elements will be subjected to the closure
    /// if a panic occurs in the closure, or a panic occurs while dropping an element,
    /// or if the `DrainFilter` value is leaked.
    ///
    /// # Examples
    ///
    /// Splitting a map into even and odd keys, reusing the original map:
    ///
    /// ```
    /// #![feature(btree_drain_filter)]
    /// use std::collections::BTreeMap;
    ///
    /// let mut map: BTreeMap<i32, i32> = (0..8).map(|x| (x, x)).collect();
    /// let evens: BTreeMap<_, _> = map.drain_filter(|k, _v| k % 2 == 0).collect();
    /// let odds = map;
    /// assert_eq!(evens.keys().copied().collect::<Vec<_>>(), vec![0, 2, 4, 6]);
    /// assert_eq!(odds.keys().copied().collect::<Vec<_>>(), vec![1, 3, 5, 7]);
    /// ```
    #[unstable(feature = "btree_drain_filter", issue = "70530")]
    pub fn drain_filter<F>(&mut self, pred: F) -> DrainFilter<'_, K, V, F>
    where
        F: FnMut(&K, &mut V) -> bool,
    {
        DrainFilter { pred, inner: self.drain_filter_inner() }
    }

    pub(super) fn drain_filter_inner(&mut self) -> DrainFilterInner<'_, K, V> {
        if let Some(root) = self.root.as_mut() {
            let (root, dormant_root) = DormantMutRef::new(root);
            let front = root.node_as_mut().first_leaf_edge();
            DrainFilterInner {
                length: &mut self.length,
                dormant_root: Some(dormant_root),
                cur_leaf_edge: Some(front),
            }
        } else {
            DrainFilterInner { length: &mut self.length, dormant_root: None, cur_leaf_edge: None }
        }
    }

    /// Creates a consuming iterator visiting all the keys, in sorted order.
    /// The map cannot be used after calling this.
    /// The iterator element type is `K`.
    ///
    /// # Examples
    ///
    /// ```
    /// #![feature(map_into_keys_values)]
    /// use std::collections::BTreeMap;
    ///
    /// let mut a = BTreeMap::new();
    /// a.insert(2, "b");
    /// a.insert(1, "a");
    ///
    /// let keys: Vec<i32> = a.into_keys().collect();
    /// assert_eq!(keys, [1, 2]);
    /// ```
    #[inline]
    #[unstable(feature = "map_into_keys_values", issue = "75294")]
    pub fn into_keys(self) -> IntoKeys<K, V> {
        IntoKeys { inner: self.into_iter() }
    }

    /// Creates a consuming iterator visiting all the values, in order by key.
    /// The map cannot be used after calling this.
    /// The iterator element type is `V`.
    ///
    /// # Examples
    ///
    /// ```
    /// #![feature(map_into_keys_values)]
    /// use std::collections::BTreeMap;
    ///
    /// let mut a = BTreeMap::new();
    /// a.insert(1, "hello");
    /// a.insert(2, "goodbye");
    ///
    /// let values: Vec<&str> = a.into_values().collect();
    /// assert_eq!(values, ["hello", "goodbye"]);
    /// ```
    #[inline]
    #[unstable(feature = "map_into_keys_values", issue = "75294")]
    pub fn into_values(self) -> IntoValues<K, V> {
        IntoValues { inner: self.into_iter() }
    }
}

#[stable(feature = "rust1", since = "1.0.0")]
impl<'a, K, V> 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))
    }

    fn last(mut self) -> Option<(&'a K, &'a V)> {
        self.next_back()
    }

    fn min(mut self) -> Option<(&'a K, &'a V)> {
        self.next()
    }

    fn max(mut self) -> Option<(&'a K, &'a V)> {
        self.next_back()
    }
}

#[stable(feature = "fused", since = "1.26.0")]
impl<K, V> FusedIterator for Iter<'_, 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<K, V> ExactSizeIterator for Iter<'_, K, V> {
    fn len(&self) -> usize {
        self.length
    }
}

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

#[stable(feature = "rust1", since = "1.0.0")]
impl<'a, K, V> 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;
            let (k, v) = unsafe { self.range.next_unchecked() };
            Some((k, v)) // coerce k from `&mut K` to `&K`
        }
    }

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

    fn last(mut self) -> Option<(&'a K, &'a mut V)> {
        self.next_back()
    }

    fn min(mut self) -> Option<(&'a K, &'a mut V)> {
        self.next()
    }

    fn max(mut self) -> Option<(&'a K, &'a mut V)> {
        self.next_back()
    }
}

#[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;
            let (k, v) = unsafe { self.range.next_back_unchecked() };
            Some((k, v)) // coerce k from `&mut K` to `&K`
        }
    }
}

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

#[stable(feature = "fused", since = "1.26.0")]
impl<K, V> FusedIterator for IterMut<'_, K, V> {}

impl<'a, K, V> IterMut<'a, K, V> {
    /// Returns an iterator of references over the remaining items.
    #[inline]
    pub(super) fn iter(&self) -> Iter<'_, K, V> {
        Iter { range: self.range.iter(), length: self.length }
    }
}

#[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 mut me = ManuallyDrop::new(self);
        if let Some(root) = me.root.take() {
            let (f, b) = root.into_ref().full_range();

            IntoIter { front: Some(f), back: Some(b), length: me.length }
        } else {
            IntoIter { front: None, back: None, length: 0 }
        }
    }
}

#[stable(feature = "btree_drop", since = "1.7.0")]
impl<K, V> Drop for IntoIter<K, V> {
    fn drop(&mut self) {
        struct DropGuard<'a, K, V>(&'a mut IntoIter<K, V>);

        impl<'a, K, V> Drop for DropGuard<'a, K, V> {
            fn drop(&mut self) {
                // Continue the same loop we perform below. This only runs when unwinding, so we
                // don't have to care about panics this time (they'll abort).
                while let Some(_) = self.0.next() {}

                unsafe {
                    let mut node =
                        unwrap_unchecked(ptr::read(&self.0.front)).into_node().forget_type();
                    while let Some(parent) = node.deallocate_and_ascend() {
                        node = parent.into_node().forget_type();
                    }
                }
            }
        }

        while let Some(pair) = self.next() {
            let guard = DropGuard(self);
            drop(pair);
            mem::forget(guard);
        }

        unsafe {
            if let Some(front) = ptr::read(&self.front) {
                let mut node = front.into_node().forget_type();
                // Most of the nodes have been deallocated while traversing
                // but one pile from a leaf up to the root is left standing.
                while let Some(parent) = node.deallocate_and_ascend() {
                    node = parent.into_node().forget_type();
                }
            }
        }
    }
}

#[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 {
            None
        } else {
            self.length -= 1;
            Some(unsafe { self.front.as_mut().unwrap().next_unchecked() })
        }
    }

    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 {
            None
        } else {
            self.length -= 1;
            Some(unsafe { self.back.as_mut().unwrap().next_back_unchecked() })
        }
    }
}

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

#[stable(feature = "fused", since = "1.26.0")]
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()
    }

    fn last(mut self) -> Option<&'a K> {
        self.next_back()
    }

    fn min(mut self) -> Option<&'a K> {
        self.next()
    }

    fn max(mut self) -> Option<&'a K> {
        self.next_back()
    }
}

#[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<K, V> ExactSizeIterator for Keys<'_, K, V> {
    fn len(&self) -> usize {
        self.inner.len()
    }
}

#[stable(feature = "fused", since = "1.26.0")]
impl<K, V> FusedIterator for Keys<'_, K, V> {}

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

    fn last(mut self) -> Option<&'a V> {
        self.next_back()
    }
}

#[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<K, V> ExactSizeIterator for Values<'_, K, V> {
    fn len(&self) -> usize {
        self.inner.len()
    }
}

#[stable(feature = "fused", since = "1.26.0")]
impl<K, V> FusedIterator for Values<'_, K, V> {}

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

/// An iterator produced by calling `drain_filter` on BTreeMap.
#[unstable(feature = "btree_drain_filter", issue = "70530")]
pub struct DrainFilter<'a, K, V, F>
where
    K: 'a,
    V: 'a,
    F: 'a + FnMut(&K, &mut V) -> bool,
{
    pred: F,
    inner: DrainFilterInner<'a, K, V>,
}
/// Most of the implementation of DrainFilter, independent of the type
/// of the predicate, thus also serving for BTreeSet::DrainFilter.
pub(super) struct DrainFilterInner<'a, K: 'a, V: 'a> {
    /// Reference to the length field in the borrowed map, updated live.
    length: &'a mut usize,
    /// Burried reference to the root field in the borrowed map.
    /// Wrapped in `Option` to allow drop handler to `take` it.
    dormant_root: Option<DormantMutRef<'a, node::Root<K, V>>>,
    /// Contains a leaf edge preceding the next element to be returned, or the last leaf edge.
    /// Empty if the map has no root, if iteration went beyond the last leaf edge,
    /// or if a panic occurred in the predicate.
    cur_leaf_edge: Option<Handle<NodeRef<marker::Mut<'a>, K, V, marker::Leaf>, marker::Edge>>,
}

#[unstable(feature = "btree_drain_filter", issue = "70530")]
impl<K, V, F> Drop for DrainFilter<'_, K, V, F>
where
    F: FnMut(&K, &mut V) -> bool,
{
    fn drop(&mut self) {
        self.for_each(drop);
    }
}

#[unstable(feature = "btree_drain_filter", issue = "70530")]
impl<K, V, F> fmt::Debug for DrainFilter<'_, K, V, F>
where
    K: fmt::Debug,
    V: fmt::Debug,
    F: FnMut(&K, &mut V) -> bool,
{
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        f.debug_tuple("DrainFilter").field(&self.inner.peek()).finish()
    }
}

#[unstable(feature = "btree_drain_filter", issue = "70530")]
impl<K, V, F> Iterator for DrainFilter<'_, K, V, F>
where
    F: FnMut(&K, &mut V) -> bool,
{
    type Item = (K, V);

    fn next(&mut self) -> Option<(K, V)> {
        self.inner.next(&mut self.pred)
    }

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

impl<'a, K: 'a, V: 'a> DrainFilterInner<'a, K, V> {
    /// Allow Debug implementations to predict the next element.
    pub(super) fn peek(&self) -> Option<(&K, &V)> {
        let edge = self.cur_leaf_edge.as_ref()?;
        edge.reborrow().next_kv().ok().map(Handle::into_kv)
    }

    /// Implementation of a typical `DrainFilter::next` method, given the predicate.
    pub(super) fn next<F>(&mut self, pred: &mut F) -> Option<(K, V)>
    where
        F: FnMut(&K, &mut V) -> bool,
    {
        while let Ok(mut kv) = self.cur_leaf_edge.take()?.next_kv() {
            let (k, v) = kv.kv_mut();
            if pred(k, v) {
                *self.length -= 1;
                let (kv, pos) = kv.remove_kv_tracking(|| {
                    // SAFETY: we will touch the root in a way that will not
                    // invalidate the position returned.
                    let root = unsafe { self.dormant_root.take().unwrap().awaken() };
                    root.pop_internal_level();
                    self.dormant_root = Some(DormantMutRef::new(root).1);
                });
                self.cur_leaf_edge = Some(pos);
                return Some(kv);
            }
            self.cur_leaf_edge = Some(kv.next_leaf_edge());
        }
        None
    }

    /// Implementation of a typical `DrainFilter::size_hint` method.
    pub(super) fn size_hint(&self) -> (usize, Option<usize>) {
        // In most of the btree iterators, `self.length` is the number of elements
        // yet to be visited. Here, it includes elements that were visited and that
        // the predicate decided not to drain. Making this upper bound more accurate
        // requires maintaining an extra field and is not worth while.
        (0, Some(*self.length))
    }
}

#[unstable(feature = "btree_drain_filter", issue = "70530")]
impl<K, V, F> FusedIterator for DrainFilter<'_, K, V, F> where F: FnMut(&K, &mut V) -> bool {}

#[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.is_empty() { None } else { unsafe { Some(self.next_unchecked()) } }
    }

    fn last(mut self) -> Option<(&'a K, &'a V)> {
        self.next_back()
    }

    fn min(mut self) -> Option<(&'a K, &'a V)> {
        self.next()
    }

    fn max(mut self) -> Option<(&'a K, &'a V)> {
        self.next_back()
    }
}

#[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()
    }

    fn last(mut self) -> Option<&'a mut V> {
        self.next_back()
    }
}

#[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<K, V> ExactSizeIterator for ValuesMut<'_, K, V> {
    fn len(&self) -> usize {
        self.inner.len()
    }
}

#[stable(feature = "fused", since = "1.26.0")]
impl<K, V> FusedIterator for ValuesMut<'_, K, V> {}

impl<'a, K, V> Range<'a, K, V> {
    fn is_empty(&self) -> bool {
        self.front == self.back
    }

    unsafe fn next_unchecked(&mut self) -> (&'a K, &'a V) {
        unsafe { unwrap_unchecked(self.front.as_mut()).next_unchecked() }
    }
}

#[unstable(feature = "map_into_keys_values", issue = "75294")]
impl<K, V> Iterator for IntoKeys<K, V> {
    type Item = K;

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

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

    fn last(mut self) -> Option<K> {
        self.next_back()
    }

    fn min(mut self) -> Option<K> {
        self.next()
    }

    fn max(mut self) -> Option<K> {
        self.next_back()
    }
}

#[unstable(feature = "map_into_keys_values", issue = "75294")]
impl<K, V> DoubleEndedIterator for IntoKeys<K, V> {
    fn next_back(&mut self) -> Option<K> {
        self.inner.next_back().map(|(k, _)| k)
    }
}

#[unstable(feature = "map_into_keys_values", issue = "75294")]
impl<K, V> ExactSizeIterator for IntoKeys<K, V> {
    fn len(&self) -> usize {
        self.inner.len()
    }
}

#[unstable(feature = "map_into_keys_values", issue = "75294")]
impl<K, V> FusedIterator for IntoKeys<K, V> {}

#[unstable(feature = "map_into_keys_values", issue = "75294")]
impl<K, V> Iterator for IntoValues<K, V> {
    type Item = V;

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

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

    fn last(mut self) -> Option<V> {
        self.next_back()
    }
}

#[unstable(feature = "map_into_keys_values", issue = "75294")]
impl<K, V> DoubleEndedIterator for IntoValues<K, V> {
    fn next_back(&mut self) -> Option<V> {
        self.inner.next_back().map(|(_, v)| v)
    }
}

#[unstable(feature = "map_into_keys_values", issue = "75294")]
impl<K, V> ExactSizeIterator for IntoValues<K, V> {
    fn len(&self) -> usize {
        self.inner.len()
    }
}

#[unstable(feature = "map_into_keys_values", issue = "75294")]
impl<K, V> FusedIterator for IntoValues<K, V> {}

#[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.is_empty() { None } else { Some(unsafe { self.next_back_unchecked() }) }
    }
}

impl<'a, K, V> Range<'a, K, V> {
    unsafe fn next_back_unchecked(&mut self) -> (&'a K, &'a V) {
        unsafe { unwrap_unchecked(self.back.as_mut()).next_back_unchecked() }
    }
}

#[stable(feature = "fused", since = "1.26.0")]
impl<K, V> FusedIterator for Range<'_, K, V> {}

#[stable(feature = "btree_range", since = "1.17.0")]
impl<K, V> Clone for Range<'_, K, V> {
    fn clone(&self) -> Self {
        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.is_empty() {
            None
        } else {
            let (k, v) = unsafe { self.next_unchecked() };
            Some((k, v)) // coerce k from `&mut K` to `&K`
        }
    }

    fn last(mut self) -> Option<(&'a K, &'a mut V)> {
        self.next_back()
    }

    fn min(mut self) -> Option<(&'a K, &'a mut V)> {
        self.next()
    }

    fn max(mut self) -> Option<(&'a K, &'a mut V)> {
        self.next_back()
    }
}

impl<'a, K, V> RangeMut<'a, K, V> {
    fn is_empty(&self) -> bool {
        self.front == self.back
    }

    unsafe fn next_unchecked(&mut self) -> (&'a K, &'a mut V) {
        unsafe { unwrap_unchecked(self.front.as_mut()).next_unchecked() }
    }

    /// Returns an iterator of references over the remaining items.
    #[inline]
    pub(super) fn iter(&self) -> Range<'_, K, V> {
        Range {
            front: self.front.as_ref().map(|f| f.reborrow()),
            back: self.back.as_ref().map(|b| b.reborrow()),
        }
    }
}

#[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.is_empty() {
            None
        } else {
            let (k, v) = unsafe { self.next_back_unchecked() };
            Some((k, v)) // coerce k from `&mut K` to `&K`
        }
    }
}

#[stable(feature = "fused", since = "1.26.0")]
impl<K, V> FusedIterator for RangeMut<'_, K, V> {}

impl<'a, K, V> RangeMut<'a, K, V> {
    unsafe fn next_back_unchecked(&mut self) -> (&'a K, &'a mut V) {
        unsafe { unwrap_unchecked(self.back.as_mut()).next_back_unchecked() }
    }
}

#[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) {
        iter.into_iter().for_each(move |(k, v)| {
            self.insert(k, v);
        });
    }

    #[inline]
    fn extend_one(&mut self, (k, v): (K, V)) {
        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)));
    }

    #[inline]
    fn extend_one(&mut self, (&k, &v): (&'a K, &'a V)) {
        self.insert(k, v);
    }
}

#[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<K: Ord, Q: ?Sized, V> Index<&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")
    }
}

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> {
        if let Some(root) = &self.root {
            let (f, b) = root.node_as_ref().full_range();

            Iter { range: Range { front: Some(f), back: Some(b) }, length: self.length }
        } else {
            Iter { range: Range { front: None, back: None }, length: 0 }
        }
    }

    /// 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> {
        if let Some(root) = &mut self.root {
            let (f, b) = root.node_as_valmut().full_range();

            IterMut {
                range: RangeMut { front: Some(f), back: Some(b), _marker: PhantomData },
                length: self.length,
            }
        } else {
            IterMut { range: RangeMut { front: None, back: None, _marker: PhantomData }, length: 0 }
        }
    }

    /// 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(&self) -> Keys<'_, 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(&self) -> Values<'_, 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
    }

    /// If the root node is the empty (non-allocated) root node, allocate our
    /// own node. Is an associated function to avoid borrowing the entire BTreeMap.
    fn ensure_is_owned(root: &mut Option<node::Root<K, V>>) -> &mut node::Root<K, V> {
        root.get_or_insert_with(node::Root::new_leaf)
    }
}

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

    /// If two keys are equal, returns the key/value-pair from the right source.
    fn next(&mut self) -> Option<(K, V)> {
        let (a_next, b_next) = self.0.nexts(|a: &(K, V), b: &(K, V)| K::cmp(&a.0, &b.0));
        b_next.or(a_next)
    }
}

#[cfg(test)]
mod tests;
