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    use core::fmt::{self, Debug};
    use core::marker::PhantomData;
    use core::mem;

    use super::super::borrow::DormantMutRef;
    use super::super::node::{marker, Handle, InsertResult::*, NodeRef};
    use super::BTreeMap;

    use Entry::*;

    /// 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`].
    ///
    /// [`entry`]: BTreeMap::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<K: Debug + Ord, V: Debug> Debug for Entry<'_, 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.
    #[stable(feature = "rust1", since = "1.0.0")]
    pub struct VacantEntry<'a, K: 'a, V: 'a> {
        pub(super) key: K,
        pub(super) handle: Handle<NodeRef<marker::Mut<'a>, K, V, marker::Leaf>, marker::Edge>,
        pub(super) dormant_map: DormantMutRef<'a, BTreeMap<K, V>>,

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

    #[stable(feature = "debug_btree_map", since = "1.12.0")]
    impl<K: Debug + Ord, V> Debug for VacantEntry<'_, 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.
    #[stable(feature = "rust1", since = "1.0.0")]
    pub struct OccupiedEntry<'a, K: 'a, V: 'a> {
        pub(super) handle: Handle<NodeRef<marker::Mut<'a>, K, V, marker::LeafOrInternal>, marker::KV>,
        pub(super) dormant_map: DormantMutRef<'a, BTreeMap<K, V>>,

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

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

    /// The error returned by [`try_insert`](BTreeMap::try_insert) when the key already exists.
    ///
    /// Contains the occupied entry, and the value that was not inserted.
    #[unstable(feature = "map_try_insert", issue = "82766")]
    pub struct OccupiedError<'a, K: 'a, V: 'a> {
        /// The entry in the map that was already occupied.
        pub entry: OccupiedEntry<'a, K, V>,
        /// The value which was not inserted, because the entry was already occupied.
        pub value: V,
    }

    #[unstable(feature = "map_try_insert", issue = "82766")]
    impl<K: Debug + Ord, V: Debug> Debug for OccupiedError<'_, K, V> {
        fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
            f.debug_struct("OccupiedError")
                .field("key", self.entry.key())
                .field("old_value", self.entry.get())
                .field("new_value", &self.value)
                .finish()
        }
    }

    #[unstable(feature = "map_try_insert", issue = "82766")]
    impl<'a, K: Debug + Ord, V: Debug> fmt::Display for OccupiedError<'a, K, V> {
        fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
            write!(
                f,
                "failed to insert {:?}, key {:?} already exists with value {:?}",
                self.value,
                self.entry.key(),
                self.entry.get(),
            )
        }
    }

    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()),
            }
        }

        /// Ensures a value is in the entry by inserting, if empty, the result of the default function.
        /// This method allows for generating key-derived values for insertion by providing the default
        /// function a reference to the key that was moved during the `.entry(key)` method call.
        ///
        /// The reference to the moved key is provided so that cloning or copying the key is
        /// unnecessary, unlike with `.or_insert_with(|| ... )`.
        ///
        /// # Examples
        ///
        /// ```
        /// use std::collections::BTreeMap;
        ///
        /// let mut map: BTreeMap<&str, usize> = BTreeMap::new();
        ///
        /// map.entry("poneyland").or_insert_with_key(|key| key.chars().count());
        ///
        /// assert_eq!(map["poneyland"], 9);
        /// ```
        #[inline]
        #[stable(feature = "or_insert_with_key", since = "1.50.0")]
        pub fn or_insert_with_key<F: FnOnce(&K) -> V>(self, default: F) -> &'a mut V {
            match self {
                Occupied(entry) => entry.into_mut(),
                Vacant(entry) => {
                    let value = default(entry.key());
                    entry.insert(value)
                }
            }
        }

        /// 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
        ///
        /// ```
        /// 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);
        /// ```
        #[stable(feature = "entry_and_modify", since = "1.26.0")]
        pub fn and_modify<F>(self, f: F) -> Self
        where
            F: FnOnce(&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> {
        #[stable(feature = "entry_or_default", since = "1.28.0")]
        /// 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
        ///
        /// ```
        /// 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;
        /// use std::collections::btree_map::Entry;
        ///
        /// let mut map: BTreeMap<&str, u32> = BTreeMap::new();
        ///
        /// if let Entry::Vacant(o) = map.entry("poneyland") {
        ///     o.insert(37);
        /// }
        /// assert_eq!(map["poneyland"], 37);
        /// ```
        #[stable(feature = "rust1", since = "1.0.0")]
        pub fn insert(self, value: V) -> &'a mut V {
            let out_ptr = match self.handle.insert_recursing(self.key, value) {
                (Fit(_), val_ptr) => {
                    // SAFETY: We have consumed self.handle and the handle returned.
                    let map = unsafe { self.dormant_map.awaken() };
                    map.length += 1;
                    val_ptr
                }
                (Split(ins), val_ptr) => {
                    drop(ins.left);
                    // SAFETY: We have consumed self.handle and the reference returned.
                    let map = unsafe { self.dormant_map.awaken() };
                    let root = map.root.as_mut().unwrap();
                    root.push_internal_level().push(ins.kv.0, ins.kv.1, ins.right);
                    map.length += 1;
                    val_ptr
                }
            };
            // Now that we have finished growing the tree using borrowed references,
            // dereference the pointer to a part of it, that we picked up along the way.
            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.
        ///
        /// If you need a reference to the `OccupiedEntry` that may outlive the
        /// destruction of the `Entry` value, see [`into_mut`].
        ///
        /// [`into_mut`]: OccupiedEntry::into_mut
        ///
        /// # 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!(*o.get(), 22);
        ///
        ///     // We can use the same Entry multiple times.
        ///     *o.get_mut() += 2;
        /// }
        /// assert_eq!(map["poneyland"], 24);
        /// ```
        #[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.
        ///
        /// If you need multiple references to the `OccupiedEntry`, see [`get_mut`].
        ///
        /// [`get_mut`]: OccupiedEntry::get_mut
        ///
        /// # 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_val_mut()
        }

        /// 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
        }

        // Body of `remove_entry`, probably separate because the name reflects the returned pair.
        pub(super) fn remove_kv(self) -> (K, V) {
            let mut emptied_internal_root = false;
            let (old_kv, _) = self.handle.remove_kv_tracking(|| emptied_internal_root = true);
            // SAFETY: we consumed the intermediate root borrow, `self.handle`.
            let map = unsafe { self.dormant_map.awaken() };
            map.length -= 1;
            if emptied_internal_root {
                let root = map.root.as_mut().unwrap();
                root.pop_internal_level();
            }
            old_kv
        }
    }