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    //! This is the core implementation that doesn't depend on the hasher at all.
    //!
    //! The methods of `IndexMapCore` don't use any Hash properties of K.
    //!
    //! It's cleaner to separate them out, then the compiler checks that we are not
    //! using Hash at all in these methods.
    //!
    //! However, we should probably not let this show in the public API or docs.

    mod raw;

    use hashbrown::raw::RawTable;

    use crate::vec::{Drain, Vec};
    use core::cmp;
    use core::fmt;
    use core::mem::replace;
    use core::ops::RangeBounds;

    use crate::equivalent::Equivalent;
    use crate::util::{enumerate, simplify_range};
    use crate::{Bucket, Entries, HashValue};

    /// Core of the map that does not depend on S
    pub(crate) struct IndexMapCore<K, V> {
        /// indices mapping from the entry hash to its index.
        indices: RawTable<usize>,
        /// entries is a dense vec of entries in their order.
        entries: Vec<Bucket<K, V>>,
    }

    #[inline(always)]
    fn get_hash<K, V>(entries: &[Bucket<K, V>]) -> impl Fn(&usize) -> u64 + '_ {
        move |&i| entries[i].hash.get()
    }

    #[inline]
    fn equivalent<'a, K, V, Q: ?Sized + Equivalent<K>>(
        key: &'a Q,
        entries: &'a [Bucket<K, V>],
    ) -> impl Fn(&usize) -> bool + 'a {
        move |&i| Q::equivalent(key, &entries[i].key)
    }

    #[inline]
    fn erase_index(table: &mut RawTable<usize>, hash: HashValue, index: usize) {
        table.erase_entry(hash.get(), move |&i| i == index);
    }

    #[inline]
    fn update_index(table: &mut RawTable<usize>, hash: HashValue, old: usize, new: usize) {
        let index = table
            .get_mut(hash.get(), move |&i| i == old)
            .expect("index not found");
        *index = new;
    }

    impl<K, V> Clone for IndexMapCore<K, V>
    where
        K: Clone,
        V: Clone,
    {
        fn clone(&self) -> Self {
            let indices = self.indices.clone();
            let mut entries = Vec::with_capacity(indices.capacity());
            entries.clone_from(&self.entries);
            IndexMapCore { indices, entries }
        }

        fn clone_from(&mut self, other: &Self) {
            let hasher = get_hash(&other.entries);
            self.indices.clone_from_with_hasher(&other.indices, hasher);
            if self.entries.capacity() < other.entries.len() {
                // If we must resize, match the indices capacity
                self.reserve_entries();
            }
            self.entries.clone_from(&other.entries);
        }
    }

    impl<K, V> fmt::Debug for IndexMapCore<K, V>
    where
        K: fmt::Debug,
        V: fmt::Debug,
    {
        fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
            f.debug_struct("IndexMapCore")
                .field("indices", &raw::DebugIndices(&self.indices))
                .field("entries", &self.entries)
                .finish()
        }
    }

    impl<K, V> Entries for IndexMapCore<K, V> {
        type Entry = Bucket<K, V>;

        #[inline]
        fn into_entries(self) -> Vec<Self::Entry> {
            self.entries
        }

        #[inline]
        fn as_entries(&self) -> &[Self::Entry] {
            &self.entries
        }

        #[inline]
        fn as_entries_mut(&mut self) -> &mut [Self::Entry] {
            &mut self.entries
        }

        fn with_entries<F>(&mut self, f: F)
        where
            F: FnOnce(&mut [Self::Entry]),
        {
            f(&mut self.entries);
            self.rebuild_hash_table();
        }
    }

    impl<K, V> IndexMapCore<K, V> {
        #[inline]
        pub(crate) fn new() -> Self {
            IndexMapCore {
                indices: RawTable::new(),
                entries: Vec::new(),
            }
        }

        #[inline]
        pub(crate) fn with_capacity(n: usize) -> Self {
            IndexMapCore {
                indices: RawTable::with_capacity(n),
                entries: Vec::with_capacity(n),
            }
        }

        #[inline]
        pub(crate) fn len(&self) -> usize {
            self.indices.len()
        }

        #[inline]
        pub(crate) fn capacity(&self) -> usize {
            cmp::min(self.indices.capacity(), self.entries.capacity())
        }

        pub(crate) fn clear(&mut self) {
            self.indices.clear();
            self.entries.clear();
        }

        pub(crate) fn truncate(&mut self, len: usize) {
            if len < self.len() {
                self.erase_indices(len, self.entries.len());
                self.entries.truncate(len);
            }
        }

        pub(crate) fn drain<R>(&mut self, range: R) -> Drain<'_, Bucket<K, V>>
        where
            R: RangeBounds<usize>,
        {
            let range = simplify_range(range, self.entries.len());
            self.erase_indices(range.start, range.end);
            self.entries.drain(range)
        }

        pub(crate) fn split_off(&mut self, at: usize) -> Self {
            assert!(at <= self.entries.len());
            self.erase_indices(at, self.entries.len());
            let entries = self.entries.split_off(at);

            let mut indices = RawTable::with_capacity(entries.len());
            for (i, entry) in enumerate(&entries) {
                indices.insert_no_grow(entry.hash.get(), i);
            }
            Self { indices, entries }
        }

        /// Reserve capacity for `additional` more key-value pairs.
        pub(crate) fn reserve(&mut self, additional: usize) {
            self.indices.reserve(additional, get_hash(&self.entries));
            self.reserve_entries();
        }

        /// Reserve entries capacity to match the indices
        fn reserve_entries(&mut self) {
            let additional = self.indices.capacity() - self.entries.len();
            self.entries.reserve_exact(additional);
        }

        /// Shrink the capacity of the map as much as possible.
        pub(crate) fn shrink_to_fit(&mut self) {
            self.indices.shrink_to(0, get_hash(&self.entries));
            self.entries.shrink_to_fit();
        }

        /// Remove the last key-value pair
        pub(crate) fn pop(&mut self) -> Option<(K, V)> {
            if let Some(entry) = self.entries.pop() {
                let last = self.entries.len();
                erase_index(&mut self.indices, entry.hash, last);
                Some((entry.key, entry.value))
            } else {
                None
            }
        }

        /// Append a key-value pair, *without* checking whether it already exists,
        /// and return the pair's new index.
        fn push(&mut self, hash: HashValue, key: K, value: V) -> usize {
            let i = self.entries.len();
            self.indices.insert(hash.get(), i, get_hash(&self.entries));
            if i == self.entries.capacity() {
                // Reserve our own capacity synced to the indices,
                // rather than letting `Vec::push` just double it.
                self.reserve_entries();
            }
            self.entries.push(Bucket { hash, key, value });
            i
        }

        /// Return the index in `entries` where an equivalent key can be found
        pub(crate) fn get_index_of<Q>(&self, hash: HashValue, key: &Q) -> Option<usize>
        where
            Q: ?Sized + Equivalent<K>,
        {
            let eq = equivalent(key, &self.entries);
            self.indices.get(hash.get(), eq).copied()
        }

        pub(crate) fn insert_full(&mut self, hash: HashValue, key: K, value: V) -> (usize, Option<V>)
        where
            K: Eq,
        {
            match self.get_index_of(hash, &key) {
                Some(i) => (i, Some(replace(&mut self.entries[i].value, value))),
                None => (self.push(hash, key, value), None),
            }
        }

        /// Remove an entry by shifting all entries that follow it
        pub(crate) fn shift_remove_full<Q>(&mut self, hash: HashValue, key: &Q) -> Option<(usize, K, V)>
        where
            Q: ?Sized + Equivalent<K>,
        {
            let eq = equivalent(key, &self.entries);
            match self.indices.remove_entry(hash.get(), eq) {
                Some(index) => {
                    let (key, value) = self.shift_remove_finish(index);
                    Some((index, key, value))
                }
                None => None,
            }
        }

        /// Remove an entry by shifting all entries that follow it
        pub(crate) fn shift_remove_index(&mut self, index: usize) -> Option<(K, V)> {
            match self.entries.get(index) {
                Some(entry) => {
                    erase_index(&mut self.indices, entry.hash, index);
                    Some(self.shift_remove_finish(index))
                }
                None => None,
            }
        }

        /// Remove an entry by shifting all entries that follow it
        ///
        /// The index should already be removed from `self.indices`.
        fn shift_remove_finish(&mut self, index: usize) -> (K, V) {
            // use Vec::remove, but then we need to update the indices that point
            // to all of the other entries that have to move
            let entry = self.entries.remove(index);

            // correct indices that point to the entries that followed the removed entry.
            // use a heuristic between a full sweep vs. a `find()` for every shifted item.
            let raw_capacity = self.indices.buckets();
            let shifted_entries = &self.entries[index..];
            if shifted_entries.len() > raw_capacity / 2 {
                // shift all indices greater than `index`
                for i in self.indices_mut() {
                    if *i > index {
                        *i -= 1;
                    }
                }
            } else {
                // find each following entry to shift its index
                for (i, entry) in (index + 1..).zip(shifted_entries) {
                    update_index(&mut self.indices, entry.hash, i, i - 1);
                }
            }

            (entry.key, entry.value)
        }

        /// Remove an entry by swapping it with the last
        pub(crate) fn swap_remove_full<Q>(&mut self, hash: HashValue, key: &Q) -> Option<(usize, K, V)>
        where
            Q: ?Sized + Equivalent<K>,
        {
            let eq = equivalent(key, &self.entries);
            match self.indices.remove_entry(hash.get(), eq) {
                Some(index) => {
                    let (key, value) = self.swap_remove_finish(index);
                    Some((index, key, value))
                }
                None => None,
            }
        }

        /// Remove an entry by swapping it with the last
        pub(crate) fn swap_remove_index(&mut self, index: usize) -> Option<(K, V)> {
            match self.entries.get(index) {
                Some(entry) => {
                    erase_index(&mut self.indices, entry.hash, index);
                    Some(self.swap_remove_finish(index))
                }
                None => None,
            }
        }

        /// Finish removing an entry by swapping it with the last
        ///
        /// The index should already be removed from `self.indices`.
        fn swap_remove_finish(&mut self, index: usize) -> (K, V) {
            // use swap_remove, but then we need to update the index that points
            // to the other entry that has to move
            let entry = self.entries.swap_remove(index);

            // correct index that points to the entry that had to swap places
            if let Some(entry) = self.entries.get(index) {
                // was not last element
                // examine new element in `index` and find it in indices
                let last = self.entries.len();
                update_index(&mut self.indices, entry.hash, last, index);
            }

            (entry.key, entry.value)
        }

        /// Erase `start..end` from `indices`, and shift `end..` indices down to `start..`
        ///
        /// All of these items should still be at their original location in `entries`.
        /// This is used by `drain`, which will let `Vec::drain` do the work on `entries`.
        fn erase_indices(&mut self, start: usize, end: usize) {
            let (init, shifted_entries) = self.entries.split_at(end);
            let (start_entries, erased_entries) = init.split_at(start);

            let erased = erased_entries.len();
            let shifted = shifted_entries.len();
            let half_capacity = self.indices.buckets() / 2;

            // Use a heuristic between different strategies
            if erased == 0 {
                // Degenerate case, nothing to do
            } else if start + shifted < half_capacity && start < erased {
                // Reinsert everything, as there are few kept indices
                self.indices.clear();

                // Reinsert stable indices
                for (i, entry) in enumerate(start_entries) {
                    self.indices.insert_no_grow(entry.hash.get(), i);
                }

                // Reinsert shifted indices
                for (i, entry) in (start..).zip(shifted_entries) {
                    self.indices.insert_no_grow(entry.hash.get(), i);
                }
            } else if erased + shifted < half_capacity {
                // Find each affected index, as there are few to adjust

                // Find erased indices
                for (i, entry) in (start..).zip(erased_entries) {
                    erase_index(&mut self.indices, entry.hash, i);
                }

                // Find shifted indices
                for ((new, old), entry) in (start..).zip(end..).zip(shifted_entries) {
                    update_index(&mut self.indices, entry.hash, old, new);
                }
            } else {
                // Sweep the whole table for adjustments
                self.erase_indices_sweep(start, end);
            }

            debug_assert_eq!(self.indices.len(), start + shifted);
        }

        pub(crate) fn retain_in_order<F>(&mut self, mut keep: F)
        where
            F: FnMut(&mut K, &mut V) -> bool,
        {
            // Like Vec::retain in self.entries, but with mutable K and V.
            // We swap-shift all the items we want to keep, truncate the rest,
            // then rebuild the raw hash table with the new indexes.
            let len = self.entries.len();
            let mut n_deleted = 0;
            for i in 0..len {
                let will_keep = {
                    let entry = &mut self.entries[i];
                    keep(&mut entry.key, &mut entry.value)
                };
                if !will_keep {
                    n_deleted += 1;
                } else if n_deleted > 0 {
                    self.entries.swap(i - n_deleted, i);
                }
            }
            if n_deleted > 0 {
                self.entries.truncate(len - n_deleted);
                self.rebuild_hash_table();
            }
        }

        fn rebuild_hash_table(&mut self) {
            self.indices.clear();
            debug_assert!(self.indices.capacity() >= self.entries.len());
            for (i, entry) in enumerate(&self.entries) {
                // We should never have to reallocate, so there's no need for a real hasher.
                self.indices.insert_no_grow(entry.hash.get(), i);
            }
        }

        pub(crate) fn reverse(&mut self) {
            self.entries.reverse();

            // No need to save hash indices, can easily calculate what they should
            // be, given that this is an in-place reversal.
            let len = self.entries.len();
            for i in self.indices_mut() {
                *i = len - *i - 1;
            }
        }
    }

    /// Entry for an existing key-value pair or a vacant location to
    /// insert one.
    pub enum Entry<'a, K, V> {
        /// Existing slot with equivalent key.
        Occupied(OccupiedEntry<'a, K, V>),
        /// Vacant slot (no equivalent key in the map).
        Vacant(VacantEntry<'a, K, V>),
    }

    impl<'a, K, V> Entry<'a, K, V> {
        /// Inserts the given default value in the entry if it is vacant and returns a mutable
        /// reference to it. Otherwise a mutable reference to an already existent value is returned.
        ///
        /// Computes in **O(1)** time (amortized average).
        pub fn or_insert(self, default: V) -> &'a mut V {
            match self {
                Entry::Occupied(entry) => entry.into_mut(),
                Entry::Vacant(entry) => entry.insert(default),
            }
        }

        /// Inserts the result of the `call` function in the entry if it is vacant and returns a mutable
        /// reference to it. Otherwise a mutable reference to an already existent value is returned.
        ///
        /// Computes in **O(1)** time (amortized average).
        pub fn or_insert_with<F>(self, call: F) -> &'a mut V
        where
            F: FnOnce() -> V,
        {
            match self {
                Entry::Occupied(entry) => entry.into_mut(),
                Entry::Vacant(entry) => entry.insert(call()),
            }
        }

        /// Inserts the result of the `call` function with a reference to the entry's key if it is
        /// vacant, and returns a mutable reference to the new value. Otherwise a mutable reference to
        /// an already existent value is returned.
        ///
        /// Computes in **O(1)** time (amortized average).
        pub fn or_insert_with_key<F>(self, call: F) -> &'a mut V
        where
            F: FnOnce(&K) -> V,
        {
            match self {
                Entry::Occupied(entry) => entry.into_mut(),
                Entry::Vacant(entry) => {
                    let value = call(&entry.key);
                    entry.insert(value)
                }
            }
        }

        /// Gets a reference to the entry's key, either within the map if occupied,
        /// or else the new key that was used to find the entry.
        pub fn key(&self) -> &K {
            match *self {
                Entry::Occupied(ref entry) => entry.key(),
                Entry::Vacant(ref entry) => entry.key(),
            }
        }

        /// Return the index where the key-value pair exists or will be inserted.
        pub fn index(&self) -> usize {
            match *self {
                Entry::Occupied(ref entry) => entry.index(),
                Entry::Vacant(ref entry) => entry.index(),
            }
        }

        /// Modifies the entry if it is occupied.
        pub fn and_modify<F>(self, f: F) -> Self
        where
            F: FnOnce(&mut V),
        {
            match self {
                Entry::Occupied(mut o) => {
                    f(o.get_mut());
                    Entry::Occupied(o)
                }
                x => x,
            }
        }

        /// Inserts a default-constructed value in the entry if it is vacant and returns a mutable
        /// reference to it. Otherwise a mutable reference to an already existent value is returned.
        ///
        /// Computes in **O(1)** time (amortized average).
        pub fn or_default(self) -> &'a mut V
        where
            V: Default,
        {
            match self {
                Entry::Occupied(entry) => entry.into_mut(),
                Entry::Vacant(entry) => entry.insert(V::default()),
            }
        }
    }

    impl<K: fmt::Debug, V: fmt::Debug> fmt::Debug for Entry<'_, K, V> {
        fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
            match *self {
                Entry::Vacant(ref v) => f.debug_tuple(stringify!(Entry)).field(v).finish(),
                Entry::Occupied(ref o) => f.debug_tuple(stringify!(Entry)).field(o).finish(),
            }
        }
    }

    pub use self::raw::OccupiedEntry;

    // Extra methods that don't threaten the unsafe encapsulation.
    impl<K, V> OccupiedEntry<'_, K, V> {
        /// Sets the value of the entry to `value`, and returns the entry's old value.
        pub fn insert(&mut self, value: V) -> V {
            replace(self.get_mut(), value)
        }

        /// Remove the key, value pair stored in the map for this entry, and return the value.
        ///
        /// **NOTE:** This is equivalent to `.swap_remove()`.
        pub fn remove(self) -> V {
            self.swap_remove()
        }

        /// Remove the key, value pair stored in the map for this entry, and return the value.
        ///
        /// Like `Vec::swap_remove`, the pair is removed by swapping it with the
        /// last element of the map and popping it off. **This perturbs
        /// the position of what used to be the last element!**
        ///
        /// Computes in **O(1)** time (average).
        pub fn swap_remove(self) -> V {
            self.swap_remove_entry().1
        }

        /// Remove the key, value pair stored in the map for this entry, and return the value.
        ///
        /// Like `Vec::remove`, the pair is removed by shifting all of the
        /// elements that follow it, preserving their relative order.
        /// **This perturbs the index of all of those elements!**
        ///
        /// Computes in **O(n)** time (average).
        pub fn shift_remove(self) -> V {
            self.shift_remove_entry().1
        }

        /// Remove and return the key, value pair stored in the map for this entry
        ///
        /// **NOTE:** This is equivalent to `.swap_remove_entry()`.
        pub fn remove_entry(self) -> (K, V) {
            self.swap_remove_entry()
        }
    }

    impl<K: fmt::Debug, V: fmt::Debug> fmt::Debug for OccupiedEntry<'_, K, V> {
        fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
            f.debug_struct(stringify!(OccupiedEntry))
                .field("key", self.key())
                .field("value", self.get())
                .finish()
        }
    }

    /// A view into a vacant entry in a `IndexMap`.
    /// It is part of the [`Entry`] enum.
    ///
    /// [`Entry`]: enum.Entry.html
    pub struct VacantEntry<'a, K, V> {
        map: &'a mut IndexMapCore<K, V>,
        hash: HashValue,
        key: K,
    }

    impl<'a, K, V> VacantEntry<'a, K, V> {
        /// Gets a reference to the key that was used to find the entry.
        pub fn key(&self) -> &K {
            &self.key
        }

        /// Takes ownership of the key, leaving the entry vacant.
        pub fn into_key(self) -> K {
            self.key
        }

        /// Return the index where the key-value pair will be inserted.
        pub fn index(&self) -> usize {
            self.map.len()
        }

        /// Inserts the entry's key and the given value into the map, and returns a mutable reference
        /// to the value.
        pub fn insert(self, value: V) -> &'a mut V {
            let i = self.map.push(self.hash, self.key, value);
            &mut self.map.entries[i].value
        }
    }

    impl<K: fmt::Debug, V> fmt::Debug for VacantEntry<'_, K, V> {
        fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
            f.debug_tuple(stringify!(VacantEntry))
                .field(self.key())
                .finish()
        }
    }

    #[test]
    fn assert_send_sync() {
        fn assert_send_sync<T: Send + Sync>() {}
        assert_send_sync::<IndexMapCore<i32, i32>>();
        assert_send_sync::<Entry<'_, i32, i32>>();
    }