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    use core::{borrow::Borrow, fmt, iter::FromIterator, mem, num::NonZeroU32, ops, slice};

    use hash32::{BuildHasher, BuildHasherDefault, FnvHasher, Hash, Hasher};

    use crate::Vec;

    /// A [`heapless::IndexMap`](./struct.IndexMap.html) using the default FNV hasher
    ///
    /// A list of all Methods and Traits available for `FnvIndexMap` can be found in
    /// the [`heapless::IndexMap`](./struct.IndexMap.html) documentation.
    ///
    /// # Examples
    /// ```
    /// use heapless::FnvIndexMap;
    ///
    /// // A hash map with a capacity of 16 key-value pairs allocated on the stack
    /// let mut book_reviews = FnvIndexMap::<_, _, 16>::new();
    ///
    /// // review some books.
    /// book_reviews.insert("Adventures of Huckleberry Finn",    "My favorite book.").unwrap();
    /// book_reviews.insert("Grimms' Fairy Tales",               "Masterpiece.").unwrap();
    /// book_reviews.insert("Pride and Prejudice",               "Very enjoyable.").unwrap();
    /// book_reviews.insert("The Adventures of Sherlock Holmes", "Eye lyked it alot.").unwrap();
    ///
    /// // check for a specific one.
    /// if !book_reviews.contains_key("Les Misérables") {
    ///     println!("We've got {} reviews, but Les Misérables ain't one.",
    ///              book_reviews.len());
    /// }
    ///
    /// // oops, this review has a lot of spelling mistakes, let's delete it.
    /// book_reviews.remove("The Adventures of Sherlock Holmes");
    ///
    /// // look up the values associated with some keys.
    /// let to_find = ["Pride and Prejudice", "Alice's Adventure in Wonderland"];
    /// for book in &to_find {
    ///     match book_reviews.get(book) {
    ///         Some(review) => println!("{}: {}", book, review),
    ///         None => println!("{} is unreviewed.", book)
    ///     }
    /// }
    ///
    /// // iterate over everything.
    /// for (book, review) in &book_reviews {
    ///     println!("{}: \"{}\"", book, review);
    /// }
    /// ```
    pub type FnvIndexMap<K, V, const N: usize> = IndexMap<K, V, BuildHasherDefault<FnvHasher>, N>;

    #[derive(Clone, Copy, Eq, PartialEq)]
    struct HashValue(u16);

    impl HashValue {
        fn desired_pos(&self, mask: usize) -> usize {
            usize::from(self.0) & mask
        }

        fn probe_distance(&self, mask: usize, current: usize) -> usize {
            current.wrapping_sub(self.desired_pos(mask) as usize) & mask
        }
    }

    #[doc(hidden)]
    #[derive(Clone)]
    pub struct Bucket<K, V> {
        hash: HashValue,
        key: K,
        value: V,
    }

    #[doc(hidden)]
    #[derive(Clone, Copy, PartialEq)]
    pub struct Pos {
        // compact representation of `{ hash_value: u16, index: u16 }`
        // To get the most from `NonZero` we store the *value minus 1*. This way `None::Option<Pos>`
        // is equivalent to the very unlikely value of  `{ hash_value: 0xffff, index: 0xffff }` instead
        // the more likely of `{ hash_value: 0x00, index: 0x00 }`
        nz: NonZeroU32,
    }

    impl Pos {
        fn new(index: usize, hash: HashValue) -> Self {
            Pos {
                nz: unsafe {
                    NonZeroU32::new_unchecked(
                        ((u32::from(hash.0) << 16) + index as u32).wrapping_add(1),
                    )
                },
            }
        }

        fn hash(&self) -> HashValue {
            HashValue((self.nz.get().wrapping_sub(1) >> 16) as u16)
        }

        fn index(&self) -> usize {
            self.nz.get().wrapping_sub(1) as u16 as usize
        }
    }

    enum Insert<K, V> {
        Success(Inserted<V>),
        Full((K, V)),
    }
    struct Inserted<V> {
        index: usize,
        old_value: Option<V>,
    }

    macro_rules! probe_loop {
        ($probe_var: ident < $len: expr, $body: expr) => {
            loop {
                if $probe_var < $len {
                    $body
                        $probe_var += 1;
                } else {
                    $probe_var = 0;
                }
            }
        }
    }

    struct CoreMap<K, V, const N: usize> {
        entries: Vec<Bucket<K, V>, N>,
        indices: [Option<Pos>; N],
    }

    impl<K, V, const N: usize> CoreMap<K, V, N> {
        const fn new() -> Self {
            const INIT: Option<Pos> = None;

            CoreMap {
                entries: Vec::new(),
                indices: [INIT; N],
            }
        }
    }

    impl<K, V, const N: usize> CoreMap<K, V, N>
    where
        K: Eq + Hash,
    {
        fn capacity() -> usize {
            N
        }

        fn mask() -> usize {
            Self::capacity() - 1
        }

        fn find<Q>(&self, hash: HashValue, query: &Q) -> Option<(usize, usize)>
        where
            K: Borrow<Q>,
            Q: ?Sized + Eq,
        {
            let mut probe = hash.desired_pos(Self::mask());
            let mut dist = 0;

            probe_loop!(probe < self.indices.len(), {
                if let Some(pos) = self.indices[probe] {
                    let entry_hash = pos.hash();
                    // NOTE(i) we use unchecked indexing below
                    let i = pos.index();
                    debug_assert!(i < self.entries.len());

                    if dist > entry_hash.probe_distance(Self::mask(), probe) {
                        // give up when probe distance is too long
                        return None;
                    } else if entry_hash == hash
                        && unsafe { self.entries.get_unchecked(i).key.borrow() == query }
                    {
                        return Some((probe, i));
                    }
                } else {
                    return None;
                }

                dist += 1;
            });
        }

        fn insert(&mut self, hash: HashValue, key: K, value: V) -> Insert<K, V> {
            let mut probe = hash.desired_pos(Self::mask());
            let mut dist = 0;

            probe_loop!(probe < self.indices.len(), {
                let pos = &mut self.indices[probe];

                if let Some(pos) = *pos {
                    let entry_hash = pos.hash();
                    // NOTE(i) we use unchecked indexing below
                    let i = pos.index();
                    debug_assert!(i < self.entries.len());

                    let their_dist = entry_hash.probe_distance(Self::mask(), probe);

                    if their_dist < dist {
                        if self.entries.is_full() {
                            return Insert::Full((key, value));
                        }
                        // robin hood: steal the spot if it's better for us
                        let index = self.entries.len();
                        unsafe { self.entries.push_unchecked(Bucket { hash, key, value }) };
                        return Insert::Success(Inserted {
                            index: self.insert_phase_2(probe, Pos::new(index, hash)),
                            old_value: None,
                        });
                    } else if entry_hash == hash && unsafe { self.entries.get_unchecked(i).key == key }
                    {
                        return Insert::Success(Inserted {
                            index: i,
                            old_value: Some(mem::replace(
                                unsafe { &mut self.entries.get_unchecked_mut(i).value },
                                value,
                            )),
                        });
                    }
                } else {
                    if self.entries.is_full() {
                        return Insert::Full((key, value));
                    }
                    // empty bucket, insert here
                    let index = self.entries.len();
                    *pos = Some(Pos::new(index, hash));
                    unsafe { self.entries.push_unchecked(Bucket { hash, key, value }) };
                    return Insert::Success(Inserted {
                        index,
                        old_value: None,
                    });
                }
                dist += 1;
            });
        }

        // phase 2 is post-insert where we forward-shift `Pos` in the indices.
        fn insert_phase_2(&mut self, mut probe: usize, mut old_pos: Pos) -> usize {
            probe_loop!(probe < self.indices.len(), {
                let pos = unsafe { self.indices.get_unchecked_mut(probe) };

                let mut is_none = true; // work around lack of NLL
                if let Some(pos) = pos.as_mut() {
                    old_pos = mem::replace(pos, old_pos);
                    is_none = false;
                }

                if is_none {
                    *pos = Some(old_pos);
                    return probe;
                }
            });
        }

        fn remove_found(&mut self, probe: usize, found: usize) -> (K, V) {
            // index `probe` and entry `found` is to be removed
            // use swap_remove, but then we need to update the index that points
            // to the other entry that has to move
            self.indices[probe] = None;
            let entry = unsafe { self.entries.swap_remove_unchecked(found) };

            // correct index that points to the entry that had to swap places
            if let Some(entry) = self.entries.get(found) {
                // was not last element
                // examine new element in `found` and find it in indices
                let mut probe = entry.hash.desired_pos(Self::mask());

                probe_loop!(probe < self.indices.len(), {
                    if let Some(pos) = self.indices[probe] {
                        if pos.index() >= self.entries.len() {
                            // found it
                            self.indices[probe] = Some(Pos::new(found, entry.hash));
                            break;
                        }
                    }
                });
            }

            self.backward_shift_after_removal(probe);

            (entry.key, entry.value)
        }

        fn backward_shift_after_removal(&mut self, probe_at_remove: usize) {
            // backward shift deletion in self.indices
            // after probe, shift all non-ideally placed indices backward
            let mut last_probe = probe_at_remove;
            let mut probe = probe_at_remove + 1;

            probe_loop!(probe < self.indices.len(), {
                if let Some(pos) = self.indices[probe] {
                    let entry_hash = pos.hash();

                    if entry_hash.probe_distance(Self::mask(), probe) > 0 {
                        unsafe { *self.indices.get_unchecked_mut(last_probe) = self.indices[probe] }
                        self.indices[probe] = None;
                    } else {
                        break;
                    }
                } else {
                    break;
                }
                last_probe = probe;
            });
        }
    }

    impl<K, V, const N: usize> Clone for CoreMap<K, V, N>
    where
        K: Eq + Hash + Clone,
        V: Clone,
    {
        fn clone(&self) -> Self {
            Self {
                entries: self.entries.clone(),
                indices: self.indices.clone(),
            }
        }
    }

    /// A view into an entry in the map
    pub enum Entry<'a, K, V, const N: usize> {
        /// The entry corresponding to the key `K` exists in the map
        Occupied(OccupiedEntry<'a, K, V, N>),
        /// The entry corresponding to the key `K` does not exist in the map
        Vacant(VacantEntry<'a, K, V, N>),
    }

    /// An occupied entry which can be manipulated
    pub struct OccupiedEntry<'a, K, V, const N: usize> {
        key: K,
        probe: usize,
        pos: usize,
        core: &'a mut CoreMap<K, V, N>,
    }

    impl<'a, K, V, const N: usize> OccupiedEntry<'a, K, V, N>
    where
        K: Eq + Hash,
    {
        /// Gets a reference to the key that this entity corresponds to
        pub fn key(&self) -> &K {
            &self.key
        }

        /// Removes this entry from the map and yields its corresponding key and value
        pub fn remove_entry(self) -> (K, V) {
            self.core.remove_found(self.probe, self.pos)
        }

        /// Gets a reference to the value associated with this entry
        pub fn get(&self) -> &V {
            // SAFETY: Already checked existence at instantiation and the only mutable reference
            // to the map is internally held.
            unsafe { &self.core.entries.get_unchecked(self.pos).value }
        }

        /// Gets a mutable reference to the value associated with this entry
        pub fn get_mut(&mut self) -> &mut V {
            // SAFETY: Already checked existence at instantiation and the only mutable reference
            // to the map is internally held.
            unsafe { &mut self.core.entries.get_unchecked_mut(self.pos).value }
        }

        /// Consumes this entry and yields a reference to the underlying value
        pub fn into_mut(self) -> &'a mut V {
            // SAFETY: Already checked existence at instantiation and the only mutable reference
            // to the map is internally held.
            unsafe { &mut self.core.entries.get_unchecked_mut(self.pos).value }
        }

        /// Overwrites the underlying map's value with this entry's value
        pub fn insert(self, value: V) -> V {
            // SAFETY: Already checked existence at instantiation and the only mutable reference
            // to the map is internally held.
            unsafe {
                mem::replace(
                    &mut self.core.entries.get_unchecked_mut(self.pos).value,
                    value,
                )
            }
        }

        /// Removes this entry from the map and yields its value
        pub fn remove(self) -> V {
            self.remove_entry().1
        }
    }

    /// A view into an empty slot in the underlying map
    pub struct VacantEntry<'a, K, V, const N: usize> {
        key: K,
        hash_val: HashValue,
        core: &'a mut CoreMap<K, V, N>,
    }
    impl<'a, K, V, const N: usize> VacantEntry<'a, K, V, N>
    where
        K: Eq + Hash,
    {
        /// Get the key associated with this entry
        pub fn key(&self) -> &K {
            &self.key
        }

        /// Consumes this entry to yield to key associated with it
        pub fn into_key(self) -> K {
            self.key
        }

        /// Inserts this entry into to underlying map, yields a mutable reference to the inserted value.
        /// If the map is at capacity the value is returned instead.
        pub fn insert(self, value: V) -> Result<&'a mut V, V> {
            if self.core.entries.is_full() {
                Err(value)
            } else {
                match self.core.insert(self.hash_val, self.key, value) {
                    Insert::Success(inserted) => {
                        unsafe {
                            // SAFETY: Already checked existence at instantiation and the only mutable reference
                            // to the map is internally held.
                            Ok(&mut (*self.core.entries.as_mut_ptr().add(inserted.index)).value)
                        }
                    }
                    Insert::Full((_, v)) => Err(v),
                }
            }
        }
    }

    /// Fixed capacity [`IndexMap`](https://docs.rs/indexmap/1/indexmap/map/struct.IndexMap.html)
    ///
    /// Note that you cannot use `IndexMap` directly, since it is generic around the hashing algorithm
    /// in use. Pick a concrete instantiation like [`FnvIndexMap`](./type.FnvIndexMap.html) instead
    /// or create your own.
    ///
    /// Note that the capacity of the `IndexMap` must be a power of 2.
    ///
    /// # Examples
    /// Since `IndexMap` cannot be used directly, we're using its `FnvIndexMap` instantiation
    /// for this example.
    ///
    /// ```
    /// use heapless::FnvIndexMap;
    ///
    /// // A hash map with a capacity of 16 key-value pairs allocated on the stack
    /// let mut book_reviews = FnvIndexMap::<_, _, 16>::new();
    ///
    /// // review some books.
    /// book_reviews.insert("Adventures of Huckleberry Finn",    "My favorite book.").unwrap();
    /// book_reviews.insert("Grimms' Fairy Tales",               "Masterpiece.").unwrap();
    /// book_reviews.insert("Pride and Prejudice",               "Very enjoyable.").unwrap();
    /// book_reviews.insert("The Adventures of Sherlock Holmes", "Eye lyked it alot.").unwrap();
    ///
    /// // check for a specific one.
    /// if !book_reviews.contains_key("Les Misérables") {
    ///     println!("We've got {} reviews, but Les Misérables ain't one.",
    ///              book_reviews.len());
    /// }
    ///
    /// // oops, this review has a lot of spelling mistakes, let's delete it.
    /// book_reviews.remove("The Adventures of Sherlock Holmes");
    ///
    /// // look up the values associated with some keys.
    /// let to_find = ["Pride and Prejudice", "Alice's Adventure in Wonderland"];
    /// for book in &to_find {
    ///     match book_reviews.get(book) {
    ///         Some(review) => println!("{}: {}", book, review),
    ///         None => println!("{} is unreviewed.", book)
    ///     }
    /// }
    ///
    /// // iterate over everything.
    /// for (book, review) in &book_reviews {
    ///     println!("{}: \"{}\"", book, review);
    /// }
    /// ```
    pub struct IndexMap<K, V, S, const N: usize> {
        core: CoreMap<K, V, N>,
        build_hasher: S,
    }

    impl<K, V, S, const N: usize> IndexMap<K, V, BuildHasherDefault<S>, N> {
        /// Creates an empty `IndexMap`.
        pub const fn new() -> Self {
            // Const assert
            crate::sealed::greater_than_1::<N>();
            crate::sealed::power_of_two::<N>();

            IndexMap {
                build_hasher: BuildHasherDefault::new(),
                core: CoreMap::new(),
            }
        }
    }

    impl<K, V, S, const N: usize> IndexMap<K, V, S, N>
    where
        K: Eq + Hash,
        S: BuildHasher,
    {
        /* Public API */
        /// Returns the number of elements the map can hold
        pub fn capacity(&self) -> usize {
            N
        }

        /// Return an iterator over the keys of the map, in their order
        ///
        /// ```
        /// use heapless::FnvIndexMap;
        ///
        /// let mut map = FnvIndexMap::<_, _, 16>::new();
        /// map.insert("a", 1).unwrap();
        /// map.insert("b", 2).unwrap();
        /// map.insert("c", 3).unwrap();
        ///
        /// for key in map.keys() {
        ///     println!("{}", key);
        /// }
        /// ```
        pub fn keys(&self) -> impl Iterator<Item = &K> {
            self.core.entries.iter().map(|bucket| &bucket.key)
        }

        /// Return an iterator over the values of the map, in their order
        ///
        /// ```
        /// use heapless::FnvIndexMap;
        ///
        /// let mut map = FnvIndexMap::<_, _, 16>::new();
        /// map.insert("a", 1).unwrap();
        /// map.insert("b", 2).unwrap();
        /// map.insert("c", 3).unwrap();
        ///
        /// for val in map.values() {
        ///     println!("{}", val);
        /// }
        /// ```
        pub fn values(&self) -> impl Iterator<Item = &V> {
            self.core.entries.iter().map(|bucket| &bucket.value)
        }

        /// Return an iterator over mutable references to the the values of the map, in their order
        ///
        /// ```
        /// use heapless::FnvIndexMap;
        ///
        /// let mut map = FnvIndexMap::<_, _, 16>::new();
        /// map.insert("a", 1).unwrap();
        /// map.insert("b", 2).unwrap();
        /// map.insert("c", 3).unwrap();
        ///
        /// for val in map.values_mut() {
        ///     *val += 10;
        /// }
        ///
        /// for val in map.values() {
        ///     println!("{}", val);
        /// }
        /// ```
        pub fn values_mut(&mut self) -> impl Iterator<Item = &mut V> {
            self.core.entries.iter_mut().map(|bucket| &mut bucket.value)
        }

        /// Return an iterator over the key-value pairs of the map, in their order
        ///
        /// ```
        /// use heapless::FnvIndexMap;
        ///
        /// let mut map = FnvIndexMap::<_, _, 16>::new();
        /// map.insert("a", 1).unwrap();
        /// map.insert("b", 2).unwrap();
        /// map.insert("c", 3).unwrap();
        ///
        /// for (key, val) in map.iter() {
        ///     println!("key: {} val: {}", key, val);
        /// }
        /// ```
        pub fn iter(&self) -> Iter<'_, K, V> {
            Iter {
                iter: self.core.entries.iter(),
            }
        }

        /// Return an iterator over the key-value pairs of the map, in their order
        ///
        /// ```
        /// use heapless::FnvIndexMap;
        ///
        /// let mut map = FnvIndexMap::<_, _, 16>::new();
        /// map.insert("a", 1).unwrap();
        /// map.insert("b", 2).unwrap();
        /// map.insert("c", 3).unwrap();
        ///
        /// for (_, val) in map.iter_mut() {
        ///     *val = 2;
        /// }
        ///
        /// for (key, val) in &map {
        ///     println!("key: {} val: {}", key, val);
        /// }
        /// ```
        pub fn iter_mut(&mut self) -> IterMut<'_, K, V> {
            IterMut {
                iter: self.core.entries.iter_mut(),
            }
        }

        /// Get the first key-value pair
        ///
        /// Computes in **O(1)** time
        pub fn first(&self) -> Option<(&K, &V)> {
            self.core
                .entries
                .first()
                .map(|bucket| (&bucket.key, &bucket.value))
        }

        /// Get the first key-value pair, with mutable access to the value
        ///
        /// Computes in **O(1)** time
        pub fn first_mut(&mut self) -> Option<(&K, &mut V)> {
            self.core
                .entries
                .first_mut()
                .map(|bucket| (&bucket.key, &mut bucket.value))
        }

        /// Get the last key-value pair
        ///
        /// Computes in **O(1)** time
        pub fn last(&self) -> Option<(&K, &V)> {
            self.core
                .entries
                .last()
                .map(|bucket| (&bucket.key, &bucket.value))
        }

        /// Get the last key-value pair, with mutable access to the value
        ///
        /// Computes in **O(1)** time
        pub fn last_mut(&mut self) -> Option<(&K, &mut V)> {
            self.core
                .entries
                .last_mut()
                .map(|bucket| (&bucket.key, &mut bucket.value))
        }

        /// Returns an entry for the corresponding key
        /// ```
        /// use heapless::FnvIndexMap;
        /// use heapless::Entry;
        /// let mut map = FnvIndexMap::<_, _, 16>::new();
        /// if let Entry::Vacant(v) = map.entry("a") {
        ///     v.insert(1).unwrap();
        /// }
        /// if let Entry::Occupied(mut o) = map.entry("a") {
        ///     println!("found {}", *o.get()); // Prints 1
        ///     o.insert(2);
        /// }
        /// // Prints 2
        /// println!("val: {}", *map.get("a").unwrap());
        /// ```
        pub fn entry(&mut self, key: K) -> Entry<'_, K, V, N> {
            let hash_val = hash_with(&key, &self.build_hasher);
            if let Some((probe, pos)) = self.core.find(hash_val, &key) {
                Entry::Occupied(OccupiedEntry {
                    key,
                    probe,
                    pos,
                    core: &mut self.core,
                })
            } else {
                Entry::Vacant(VacantEntry {
                    key,
                    hash_val,
                    core: &mut self.core,
                })
            }
        }

        /// Return the number of key-value pairs in the map.
        ///
        /// Computes in **O(1)** time.
        ///
        /// ```
        /// use heapless::FnvIndexMap;
        ///
        /// let mut a = FnvIndexMap::<_, _, 16>::new();
        /// assert_eq!(a.len(), 0);
        /// a.insert(1, "a").unwrap();
        /// assert_eq!(a.len(), 1);
        /// ```
        pub fn len(&self) -> usize {
            self.core.entries.len()
        }

        /// Returns true if the map contains no elements.
        ///
        /// Computes in **O(1)** time.
        ///
        /// ```
        /// use heapless::FnvIndexMap;
        ///
        /// let mut a = FnvIndexMap::<_, _, 16>::new();
        /// assert!(a.is_empty());
        /// a.insert(1, "a");
        /// assert!(!a.is_empty());
        /// ```
        pub fn is_empty(&self) -> bool {
            self.len() == 0
        }

        /// Remove all key-value pairs in the map, while preserving its capacity.
        ///
        /// Computes in **O(n)** time.
        ///
        /// ```
        /// use heapless::FnvIndexMap;
        ///
        /// let mut a = FnvIndexMap::<_, _, 16>::new();
        /// a.insert(1, "a");
        /// a.clear();
        /// assert!(a.is_empty());
        /// ```
        pub fn clear(&mut self) {
            self.core.entries.clear();
            for pos in self.core.indices.iter_mut() {
                *pos = None;
            }
        }

        /// Returns a reference to the value corresponding to the key.
        ///
        /// The key may be any borrowed form of the map's key type, but `Hash` and `Eq` on the borrowed
        /// form *must* match those for the key type.
        ///
        /// Computes in **O(1)** time (average).
        ///
        /// ```
        /// use heapless::FnvIndexMap;
        ///
        /// let mut map = FnvIndexMap::<_, _, 16>::new();
        /// map.insert(1, "a").unwrap();
        /// assert_eq!(map.get(&1), Some(&"a"));
        /// assert_eq!(map.get(&2), None);
        /// ```
        pub fn get<Q>(&self, key: &Q) -> Option<&V>
        where
            K: Borrow<Q>,
            Q: ?Sized + Hash + Eq,
        {
            self.find(key)
                .map(|(_, found)| unsafe { &self.core.entries.get_unchecked(found).value })
        }

        /// 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 `Hash` and `Eq` on the borrowed
        /// form *must* match those for the key type.
        ///
        /// Computes in **O(1)** time (average).
        ///
        /// # Examples
        ///
        /// ```
        /// use heapless::FnvIndexMap;
        ///
        /// let mut map = FnvIndexMap::<_, _, 8>::new();
        /// map.insert(1, "a").unwrap();
        /// assert_eq!(map.contains_key(&1), true);
        /// assert_eq!(map.contains_key(&2), false);
        /// ```
        pub fn contains_key<Q>(&self, key: &Q) -> bool
        where
            K: Borrow<Q>,
            Q: ?Sized + Eq + Hash,
        {
            self.find(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 `Hash` and `Eq` on the borrowed
        /// form *must* match those for the key type.
        ///
        /// Computes in **O(1)** time (average).
        ///
        /// # Examples
        ///
        /// ```
        /// use heapless::FnvIndexMap;
        ///
        /// let mut map = FnvIndexMap::<_, _, 8>::new();
        /// map.insert(1, "a").unwrap();
        /// if let Some(x) = map.get_mut(&1) {
        ///     *x = "b";
        /// }
        /// assert_eq!(map[&1], "b");
        /// ```
        pub fn get_mut<'v, Q>(&'v mut self, key: &Q) -> Option<&'v mut V>
        where
            K: Borrow<Q>,
            Q: ?Sized + Hash + Eq,
        {
            if let Some((_, found)) = self.find(key) {
                Some(unsafe { &mut self.core.entries.get_unchecked_mut(found).value })
            } else {
                None
            }
        }

        /// Inserts a key-value pair into the map.
        ///
        /// If an equivalent key already exists in the map: the key remains and retains in its place in
        /// the order, its corresponding value is updated with `value` and the older value is returned
        /// inside `Some(_)`.
        ///
        /// If no equivalent key existed in the map: the new key-value pair is inserted, last in order,
        /// and `None` is returned.
        ///
        /// Computes in **O(1)** time (average).
        ///
        /// See also entry if you you want to insert or modify or if you need to get the index of the
        /// corresponding key-value pair.
        ///
        /// # Examples
        ///
        /// ```
        /// use heapless::FnvIndexMap;
        ///
        /// let mut map = FnvIndexMap::<_, _, 8>::new();
        /// assert_eq!(map.insert(37, "a"), Ok(None));
        /// assert_eq!(map.is_empty(), false);
        ///
        /// map.insert(37, "b");
        /// assert_eq!(map.insert(37, "c"), Ok(Some("b")));
        /// assert_eq!(map[&37], "c");
        /// ```
        pub fn insert(&mut self, key: K, value: V) -> Result<Option<V>, (K, V)> {
            let hash = hash_with(&key, &self.build_hasher);
            match self.core.insert(hash, key, value) {
                Insert::Success(inserted) => Ok(inserted.old_value),
                Insert::Full((k, v)) => Err((k, v)),
            }
        }

        /// Same as [`swap_remove`](struct.IndexMap.html#method.swap_remove)
        ///
        /// Computes in **O(1)** time (average).
        ///
        /// # Examples
        ///
        /// ```
        /// use heapless::FnvIndexMap;
        ///
        /// let mut map = FnvIndexMap::<_, _, 8>::new();
        /// map.insert(1, "a").unwrap();
        /// assert_eq!(map.remove(&1), Some("a"));
        /// assert_eq!(map.remove(&1), None);
        /// ```
        pub fn remove<Q>(&mut self, key: &Q) -> Option<V>
        where
            K: Borrow<Q>,
            Q: ?Sized + Hash + Eq,
        {
            self.swap_remove(key)
        }

        /// Remove the key-value pair equivalent to `key` and return its 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 postion of what used to be the last element!**
        ///
        /// Return `None` if `key` is not in map.
        ///
        /// Computes in **O(1)** time (average).
        pub fn swap_remove<Q>(&mut self, key: &Q) -> Option<V>
        where
            K: Borrow<Q>,
            Q: ?Sized + Hash + Eq,
        {
            self.find(key)
                .map(|(probe, found)| self.core.remove_found(probe, found).1)
        }

        /* Private API */
        /// Return probe (indices) and position (entries)
        fn find<Q>(&self, key: &Q) -> Option<(usize, usize)>
        where
            K: Borrow<Q>,
            Q: ?Sized + Hash + Eq,
        {
            if self.len() == 0 {
                return None;
            }
            let h = hash_with(key, &self.build_hasher);
            self.core.find(h, key)
        }
    }

    impl<'a, K, Q, V, S, const N: usize> ops::Index<&'a Q> for IndexMap<K, V, S, N>
    where
        K: Eq + Hash + Borrow<Q>,
        Q: ?Sized + Eq + Hash,
        S: BuildHasher,
    {
        type Output = V;

        fn index(&self, key: &Q) -> &V {
            self.get(key).expect("key not found")
        }
    }

    impl<'a, K, Q, V, S, const N: usize> ops::IndexMut<&'a Q> for IndexMap<K, V, S, N>
    where
        K: Eq + Hash + Borrow<Q>,
        Q: ?Sized + Eq + Hash,
        S: BuildHasher,
    {
        fn index_mut(&mut self, key: &Q) -> &mut V {
            self.get_mut(key).expect("key not found")
        }
    }

    impl<K, V, S, const N: usize> Clone for IndexMap<K, V, S, N>
    where
        K: Eq + Hash + Clone,
        V: Clone,
        S: Clone,
    {
        fn clone(&self) -> Self {
            Self {
                core: self.core.clone(),
                build_hasher: self.build_hasher.clone(),
            }
        }
    }

    impl<K, V, S, const N: usize> fmt::Debug for IndexMap<K, V, S, N>
    where
        K: Eq + Hash + fmt::Debug,
        V: fmt::Debug,
        S: BuildHasher,
    {
        fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
            f.debug_map().entries(self.iter()).finish()
        }
    }

    impl<K, V, S, const N: usize> Default for IndexMap<K, V, S, N>
    where
        K: Eq + Hash,
        S: BuildHasher + Default,
    {
        fn default() -> Self {
            // Const assert
            crate::sealed::greater_than_1::<N>();
            crate::sealed::power_of_two::<N>();

            IndexMap {
                build_hasher: <_>::default(),
                core: CoreMap::new(),
            }
        }
    }

    impl<K, V, S, S2, const N: usize, const N2: usize> PartialEq<IndexMap<K, V, S2, N2>>
        for IndexMap<K, V, S, N>
    where
        K: Eq + Hash,
        V: Eq,
        S: BuildHasher,
        S2: BuildHasher,
    {
        fn eq(&self, other: &IndexMap<K, V, S2, N2>) -> bool {
            self.len() == other.len()
                && self
                    .iter()
                    .all(|(key, value)| other.get(key).map_or(false, |v| *value == *v))
        }
    }

    impl<K, V, S, const N: usize> Eq for IndexMap<K, V, S, N>
    where
        K: Eq + Hash,
        V: Eq,
        S: BuildHasher,
    {
    }

    impl<K, V, S, const N: usize> Extend<(K, V)> for IndexMap<K, V, S, N>
    where
        K: Eq + Hash,
        S: BuildHasher,
    {
        fn extend<I>(&mut self, iterable: I)
        where
            I: IntoIterator<Item = (K, V)>,
        {
            for (k, v) in iterable {
                self.insert(k, v).ok().unwrap();
            }
        }
    }

    impl<'a, K, V, S, const N: usize> Extend<(&'a K, &'a V)> for IndexMap<K, V, S, N>
    where
        K: Eq + Hash + Copy,
        V: Copy,
        S: BuildHasher,
    {
        fn extend<I>(&mut self, iterable: I)
        where
            I: IntoIterator<Item = (&'a K, &'a V)>,
        {
            self.extend(iterable.into_iter().map(|(&key, &value)| (key, value)))
        }
    }

    impl<K, V, S, const N: usize> FromIterator<(K, V)> for IndexMap<K, V, S, N>
    where
        K: Eq + Hash,
        S: BuildHasher + Default,
    {
        fn from_iter<I>(iterable: I) -> Self
        where
            I: IntoIterator<Item = (K, V)>,
        {
            let mut map = IndexMap::default();
            map.extend(iterable);
            map
        }
    }

    #[derive(Clone)]
    pub struct IntoIter<K, V, const N: usize> {
        entries: Vec<Bucket<K, V>, N>,
    }

    impl<K, V, const N: usize> Iterator for IntoIter<K, V, N> {
        type Item = (K, V);

        fn next(&mut self) -> Option<Self::Item> {
            self.entries.pop().map(|bucket| (bucket.key, bucket.value))
        }
    }

    impl<K, V, S, const N: usize> IntoIterator for IndexMap<K, V, S, N>
    where
        K: Eq + Hash,
        S: BuildHasher,
    {
        type Item = (K, V);
        type IntoIter = IntoIter<K, V, N>;

        fn into_iter(self) -> Self::IntoIter {
            IntoIter {
                entries: self.core.entries,
            }
        }
    }

    impl<'a, K, V, S, const N: usize> IntoIterator for &'a IndexMap<K, V, S, N>
    where
        K: Eq + Hash,
        S: BuildHasher,
    {
        type Item = (&'a K, &'a V);
        type IntoIter = Iter<'a, K, V>;

        fn into_iter(self) -> Self::IntoIter {
            self.iter()
        }
    }

    impl<'a, K, V, S, const N: usize> IntoIterator for &'a mut IndexMap<K, V, S, N>
    where
        K: Eq + Hash,
        S: BuildHasher,
    {
        type Item = (&'a K, &'a mut V);
        type IntoIter = IterMut<'a, K, V>;

        fn into_iter(self) -> Self::IntoIter {
            self.iter_mut()
        }
    }

    pub struct Iter<'a, K, V> {
        iter: slice::Iter<'a, Bucket<K, V>>,
    }

    impl<'a, K, V> Iterator for Iter<'a, K, V> {
        type Item = (&'a K, &'a V);

        fn next(&mut self) -> Option<Self::Item> {
            self.iter.next().map(|bucket| (&bucket.key, &bucket.value))
        }
    }

    impl<'a, K, V> Clone for Iter<'a, K, V> {
        fn clone(&self) -> Self {
            Self {
                iter: self.iter.clone(),
            }
        }
    }

    pub struct IterMut<'a, K, V> {
        iter: slice::IterMut<'a, Bucket<K, V>>,
    }

    impl<'a, K, V> Iterator for IterMut<'a, K, V> {
        type Item = (&'a K, &'a mut V);

        fn next(&mut self) -> Option<Self::Item> {
            self.iter
                .next()
                .map(|bucket| (&bucket.key, &mut bucket.value))
        }
    }

    fn hash_with<K, S>(key: &K, build_hasher: &S) -> HashValue
    where
        K: ?Sized + Hash,
        S: BuildHasher,
    {
        let mut h = build_hasher.build_hasher();
        key.hash(&mut h);
        HashValue(h.finish() as u16)
    }

    #[cfg(test)]
    mod tests {
        use crate::{indexmap::Entry, FnvIndexMap};

        use core::mem;

        #[test]
        fn size() {
            const CAP: usize = 4;
            assert_eq!(
                mem::size_of::<FnvIndexMap<i16, u16, CAP>>(),
                CAP * mem::size_of::<u32>() + // indices
                    CAP * (mem::size_of::<i16>() + // key
                         mem::size_of::<u16>() + // value
                         mem::size_of::<u16>() // hash
                    ) + // buckets
                    mem::size_of::<usize>() // entries.length
            )
        }

        #[test]
        fn partial_eq() {
            {
                let mut a: FnvIndexMap<_, _, 4> = FnvIndexMap::new();
                a.insert("k1", "v1").unwrap();

                let mut b: FnvIndexMap<_, _, 4> = FnvIndexMap::new();
                b.insert("k1", "v1").unwrap();

                assert!(a == b);

                b.insert("k2", "v2").unwrap();

                assert!(a != b);
            }

            {
                let mut a: FnvIndexMap<_, _, 4> = FnvIndexMap::new();
                a.insert("k1", "v1").unwrap();
                a.insert("k2", "v2").unwrap();

                let mut b: FnvIndexMap<_, _, 4> = FnvIndexMap::new();
                b.insert("k2", "v2").unwrap();
                b.insert("k1", "v1").unwrap();

                assert!(a == b);
            }
        }

        #[test]
        fn into_iter() {
            let mut src: FnvIndexMap<_, _, 4> = FnvIndexMap::new();
            src.insert("k1", "v1").unwrap();
            src.insert("k2", "v2").unwrap();
            src.insert("k3", "v3").unwrap();
            src.insert("k4", "v4").unwrap();
            let clone = src.clone();
            for (k, v) in clone.into_iter() {
                assert_eq!(v, *src.get(k).unwrap());
            }
        }

        #[test]
        fn insert_replaces_on_full_map() {
            let mut a: FnvIndexMap<_, _, 2> = FnvIndexMap::new();
            a.insert("k1", "v1").unwrap();
            a.insert("k2", "v2").unwrap();
            a.insert("k1", "v2").unwrap();
            assert_eq!(a.get("k1"), a.get("k2"));
        }

        const MAP_SLOTS: usize = 4096;
        fn almost_filled_map() -> FnvIndexMap<usize, usize, MAP_SLOTS> {
            let mut almost_filled = FnvIndexMap::new();
            for i in 1..MAP_SLOTS {
                almost_filled.insert(i, i).unwrap();
            }
            almost_filled
        }

        #[test]
        fn entry_find() {
            let key = 0;
            let value = 0;
            let mut src = almost_filled_map();
            let entry = src.entry(key);
            match entry {
                Entry::Occupied(_) => {
                    panic!("Found entry without inserting");
                }
                Entry::Vacant(v) => {
                    assert_eq!(&key, v.key());
                    assert_eq!(key, v.into_key());
                }
            }
            src.insert(key, value).unwrap();
            let entry = src.entry(key);
            match entry {
                Entry::Occupied(mut o) => {
                    assert_eq!(&key, o.key());
                    assert_eq!(&value, o.get());
                    assert_eq!(&value, o.get_mut());
                    assert_eq!(&value, o.into_mut());
                }
                Entry::Vacant(_) => {
                    panic!("Entry not found");
                }
            }
        }

        #[test]
        fn entry_vacant_insert() {
            let key = 0;
            let value = 0;
            let mut src = almost_filled_map();
            assert_eq!(MAP_SLOTS - 1, src.len());
            let entry = src.entry(key);
            match entry {
                Entry::Occupied(_) => {
                    panic!("Entry found when empty");
                }
                Entry::Vacant(v) => {
                    v.insert(value).unwrap();
                }
            };
            assert_eq!(value, *src.get(&key).unwrap())
        }

        #[test]
        fn entry_occupied_insert() {
            let key = 0;
            let value = 0;
            let value2 = 5;
            let mut src = almost_filled_map();
            assert_eq!(MAP_SLOTS - 1, src.len());
            src.insert(key, value).unwrap();
            let entry = src.entry(key);
            match entry {
                Entry::Occupied(o) => {
                    assert_eq!(value, o.insert(value2));
                }
                Entry::Vacant(_) => {
                    panic!("Entry not found");
                }
            };
            assert_eq!(value2, *src.get(&key).unwrap())
        }

        #[test]
        fn entry_remove_entry() {
            let key = 0;
            let value = 0;
            let mut src = almost_filled_map();
            src.insert(key, value).unwrap();
            assert_eq!(MAP_SLOTS, src.len());
            let entry = src.entry(key);
            match entry {
                Entry::Occupied(o) => {
                    assert_eq!((key, value), o.remove_entry());
                }
                Entry::Vacant(_) => {
                    panic!("Entry not found")
                }
            };
            assert_eq!(MAP_SLOTS - 1, src.len());
        }

        #[test]
        fn entry_remove() {
            let key = 0;
            let value = 0;
            let mut src = almost_filled_map();
            src.insert(key, value).unwrap();
            assert_eq!(MAP_SLOTS, src.len());
            let entry = src.entry(key);
            match entry {
                Entry::Occupied(o) => {
                    assert_eq!(value, o.remove());
                }
                Entry::Vacant(_) => {
                    panic!("Entry not found");
                }
            };
            assert_eq!(MAP_SLOTS - 1, src.len());
        }

        #[test]
        fn entry_roll_through_all() {
            let mut src: FnvIndexMap<usize, usize, MAP_SLOTS> = FnvIndexMap::new();
            for i in 0..MAP_SLOTS {
                match src.entry(i) {
                    Entry::Occupied(_) => {
                        panic!("Entry found before insert");
                    }
                    Entry::Vacant(v) => {
                        v.insert(i).unwrap();
                    }
                }
            }
            let add_mod = 99;
            for i in 0..MAP_SLOTS {
                match src.entry(i) {
                    Entry::Occupied(o) => {
                        assert_eq!(i, o.insert(i + add_mod));
                    }
                    Entry::Vacant(_) => {
                        panic!("Entry not found after insert");
                    }
                }
            }
            for i in 0..MAP_SLOTS {
                match src.entry(i) {
                    Entry::Occupied(o) => {
                        assert_eq!((i, i + add_mod), o.remove_entry());
                    }
                    Entry::Vacant(_) => {
                        panic!("Entry not found after insert");
                    }
                }
            }
            for i in 0..MAP_SLOTS {
                assert!(matches!(src.entry(i), Entry::Vacant(_)));
            }
            assert!(src.is_empty());
        }

        #[test]
        fn first_last() {
            let mut map = FnvIndexMap::<_, _, 4>::new();

            assert_eq!(None, map.first());
            assert_eq!(None, map.last());

            map.insert(0, 0).unwrap();
            map.insert(2, 2).unwrap();

            assert_eq!(Some((&0, &0)), map.first());
            assert_eq!(Some((&2, &2)), map.last());

            map.insert(1, 1).unwrap();

            assert_eq!(Some((&1, &1)), map.last());

            *map.first_mut().unwrap().1 += 1;
            *map.last_mut().unwrap().1 += 1;

            assert_eq!(Some((&0, &1)), map.first());
            assert_eq!(Some((&1, &2)), map.last());
        }
    }