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    use crate::indexmap::{self, IndexMap};
    use core::{borrow::Borrow, fmt, iter::FromIterator};
    use hash32::{BuildHasher, BuildHasherDefault, FnvHasher, Hash};

    /// A [`heapless::IndexSet`](./struct.IndexSet.html) using the
    /// default FNV hasher.
    /// A list of all Methods and Traits available for `FnvIndexSet` can be found in
    /// the [`heapless::IndexSet`](./struct.IndexSet.html) documentation.
    ///
    /// # Examples
    /// ```
    /// use heapless::FnvIndexSet;
    ///
    /// // A hash set with a capacity of 16 elements allocated on the stack
    /// let mut books = FnvIndexSet::<_, 16>::new();
    ///
    /// // Add some books.
    /// books.insert("A Dance With Dragons").unwrap();
    /// books.insert("To Kill a Mockingbird").unwrap();
    /// books.insert("The Odyssey").unwrap();
    /// books.insert("The Great Gatsby").unwrap();
    ///
    /// // Check for a specific one.
    /// if !books.contains("The Winds of Winter") {
    ///     println!("We have {} books, but The Winds of Winter ain't one.",
    ///              books.len());
    /// }
    ///
    /// // Remove a book.
    /// books.remove("The Odyssey");
    ///
    /// // Iterate over everything.
    /// for book in &books {
    ///     println!("{}", book);
    /// }
    /// ```
    pub type FnvIndexSet<T, const N: usize> = IndexSet<T, BuildHasherDefault<FnvHasher>, N>;

    /// Fixed capacity [`IndexSet`](https://docs.rs/indexmap/1/indexmap/set/struct.IndexSet.html).
    ///
    /// Note that you cannot use `IndexSet` directly, since it is generic around the hashing algorithm
    /// in use. Pick a concrete instantiation like [`FnvIndexSet`](./type.FnvIndexSet.html) instead
    /// or create your own.
    ///
    /// Note that the capacity of the `IndexSet` must be a power of 2.
    ///
    /// # Examples
    /// Since `IndexSet` cannot be used directly, we're using its `FnvIndexSet` instantiation
    /// for this example.
    ///
    /// ```
    /// use heapless::FnvIndexSet;
    ///
    /// // A hash set with a capacity of 16 elements allocated on the stack
    /// let mut books = FnvIndexSet::<_, 16>::new();
    ///
    /// // Add some books.
    /// books.insert("A Dance With Dragons").unwrap();
    /// books.insert("To Kill a Mockingbird").unwrap();
    /// books.insert("The Odyssey").unwrap();
    /// books.insert("The Great Gatsby").unwrap();
    ///
    /// // Check for a specific one.
    /// if !books.contains("The Winds of Winter") {
    ///     println!("We have {} books, but The Winds of Winter ain't one.",
    ///              books.len());
    /// }
    ///
    /// // Remove a book.
    /// books.remove("The Odyssey");
    ///
    /// // Iterate over everything.
    /// for book in &books {
    ///     println!("{}", book);
    /// }
    /// ```
    pub struct IndexSet<T, S, const N: usize> {
        map: IndexMap<T, (), S, N>,
    }

    impl<T, S, const N: usize> IndexSet<T, BuildHasherDefault<S>, N> {
        /// Creates an empty `IndexSet`
        pub const fn new() -> Self {
            IndexSet {
                map: IndexMap::new(),
            }
        }
    }

    impl<T, S, const N: usize> IndexSet<T, S, N>
    where
        T: Eq + Hash,
        S: BuildHasher,
    {
        /// Returns the number of elements the set can hold
        ///
        /// # Examples
        ///
        /// ```
        /// use heapless::FnvIndexSet;
        ///
        /// let set = FnvIndexSet::<i32, 16>::new();
        /// assert_eq!(set.capacity(), 16);
        /// ```
        pub fn capacity(&self) -> usize {
            self.map.capacity()
        }

        /// Return an iterator over the values of the set, in their order
        ///
        /// # Examples
        ///
        /// ```
        /// use heapless::FnvIndexSet;
        ///
        /// let mut set = FnvIndexSet::<_, 16>::new();
        /// set.insert("a").unwrap();
        /// set.insert("b").unwrap();
        ///
        /// // Will print in an arbitrary order.
        /// for x in set.iter() {
        ///     println!("{}", x);
        /// }
        /// ```
        pub fn iter(&self) -> Iter<'_, T> {
            Iter {
                iter: self.map.iter(),
            }
        }

        /// Get the first value
        ///
        /// Computes in **O(1)** time
        pub fn first(&self) -> Option<&T> {
            self.map.first().map(|(k, _v)| k)
        }

        /// Get the last value
        ///
        /// Computes in **O(1)** time
        pub fn last(&self) -> Option<&T> {
            self.map.last().map(|(k, _v)| k)
        }

        /// Visits the values representing the difference, i.e. the values that are in `self` but not in
        /// `other`.
        ///
        /// # Examples
        ///
        /// ```
        /// use heapless::FnvIndexSet;
        ///
        /// let mut a: FnvIndexSet<_, 16> = [1, 2, 3].iter().cloned().collect();
        /// let mut b: FnvIndexSet<_, 16> = [4, 2, 3, 4].iter().cloned().collect();
        ///
        /// // Can be seen as `a - b`.
        /// for x in a.difference(&b) {
        ///     println!("{}", x); // Print 1
        /// }
        ///
        /// let diff: FnvIndexSet<_, 16> = a.difference(&b).collect();
        /// assert_eq!(diff, [1].iter().collect::<FnvIndexSet<_, 16>>());
        ///
        /// // Note that difference is not symmetric,
        /// // and `b - a` means something else:
        /// let diff: FnvIndexSet<_, 16> = b.difference(&a).collect();
        /// assert_eq!(diff, [4].iter().collect::<FnvIndexSet<_, 16>>());
        /// ```
        pub fn difference<'a, S2, const N2: usize>(
            &'a self,
            other: &'a IndexSet<T, S2, N2>,
        ) -> Difference<'a, T, S2, N2>
        where
            S2: BuildHasher,
        {
            Difference {
                iter: self.iter(),
                other,
            }
        }

        /// Visits the values representing the symmetric difference, i.e. the values that are in `self`
        /// or in `other` but not in both.
        ///
        /// # Examples
        ///
        /// ```
        /// use heapless::FnvIndexSet;
        ///
        /// let mut a: FnvIndexSet<_, 16> = [1, 2, 3].iter().cloned().collect();
        /// let mut b: FnvIndexSet<_, 16> = [4, 2, 3, 4].iter().cloned().collect();
        ///
        /// // Print 1, 4 in that order order.
        /// for x in a.symmetric_difference(&b) {
        ///     println!("{}", x);
        /// }
        ///
        /// let diff1: FnvIndexSet<_, 16> = a.symmetric_difference(&b).collect();
        /// let diff2: FnvIndexSet<_, 16> = b.symmetric_difference(&a).collect();
        ///
        /// assert_eq!(diff1, diff2);
        /// assert_eq!(diff1, [1, 4].iter().collect::<FnvIndexSet<_, 16>>());
        /// ```
        pub fn symmetric_difference<'a, S2, const N2: usize>(
            &'a self,
            other: &'a IndexSet<T, S2, N2>,
        ) -> impl Iterator<Item = &'a T>
        where
            S2: BuildHasher,
        {
            self.difference(other).chain(other.difference(self))
        }

        /// Visits the values representing the intersection, i.e. the values that are both in `self` and
        /// `other`.
        ///
        /// # Examples
        ///
        /// ```
        /// use heapless::FnvIndexSet;
        ///
        /// let mut a: FnvIndexSet<_, 16> = [1, 2, 3].iter().cloned().collect();
        /// let mut b: FnvIndexSet<_, 16> = [4, 2, 3, 4].iter().cloned().collect();
        ///
        /// // Print 2, 3 in that order.
        /// for x in a.intersection(&b) {
        ///     println!("{}", x);
        /// }
        ///
        /// let intersection: FnvIndexSet<_, 16> = a.intersection(&b).collect();
        /// assert_eq!(intersection, [2, 3].iter().collect::<FnvIndexSet<_, 16>>());
        /// ```
        pub fn intersection<'a, S2, const N2: usize>(
            &'a self,
            other: &'a IndexSet<T, S2, N2>,
        ) -> Intersection<'a, T, S2, N2>
        where
            S2: BuildHasher,
        {
            Intersection {
                iter: self.iter(),
                other,
            }
        }

        /// Visits the values representing the union, i.e. all the values in `self` or `other`, without
        /// duplicates.
        ///
        /// # Examples
        ///
        /// ```
        /// use heapless::FnvIndexSet;
        ///
        /// let mut a: FnvIndexSet<_, 16> = [1, 2, 3].iter().cloned().collect();
        /// let mut b: FnvIndexSet<_, 16> = [4, 2, 3, 4].iter().cloned().collect();
        ///
        /// // Print 1, 2, 3, 4 in that order.
        /// for x in a.union(&b) {
        ///     println!("{}", x);
        /// }
        ///
        /// let union: FnvIndexSet<_, 16> = a.union(&b).collect();
        /// assert_eq!(union, [1, 2, 3, 4].iter().collect::<FnvIndexSet<_, 16>>());
        /// ```
        pub fn union<'a, S2, const N2: usize>(
            &'a self,
            other: &'a IndexSet<T, S2, N2>,
        ) -> impl Iterator<Item = &'a T>
        where
            S2: BuildHasher,
        {
            self.iter().chain(other.difference(self))
        }

        /// Returns the number of elements in the set.
        ///
        /// # Examples
        ///
        /// ```
        /// use heapless::FnvIndexSet;
        ///
        /// let mut v: FnvIndexSet<_, 16> = FnvIndexSet::new();
        /// assert_eq!(v.len(), 0);
        /// v.insert(1).unwrap();
        /// assert_eq!(v.len(), 1);
        /// ```
        pub fn len(&self) -> usize {
            self.map.len()
        }

        /// Returns `true` if the set contains no elements.
        ///
        /// # Examples
        ///
        /// ```
        /// use heapless::FnvIndexSet;
        ///
        /// let mut v: FnvIndexSet<_, 16> = FnvIndexSet::new();
        /// assert!(v.is_empty());
        /// v.insert(1).unwrap();
        /// assert!(!v.is_empty());
        /// ```
        pub fn is_empty(&self) -> bool {
            self.map.is_empty()
        }

        /// Clears the set, removing all values.
        ///
        /// # Examples
        ///
        /// ```
        /// use heapless::FnvIndexSet;
        ///
        /// let mut v: FnvIndexSet<_, 16> = FnvIndexSet::new();
        /// v.insert(1).unwrap();
        /// v.clear();
        /// assert!(v.is_empty());
        /// ```
        pub fn clear(&mut self) {
            self.map.clear()
        }

        /// Returns `true` if the set contains a value.
        ///
        /// The value may be any borrowed form of the set's value type, but `Hash` and `Eq` on the
        /// borrowed form must match those for the value type.
        ///
        /// # Examples
        ///
        /// ```
        /// use heapless::FnvIndexSet;
        ///
        /// let set: FnvIndexSet<_, 16> = [1, 2, 3].iter().cloned().collect();
        /// assert_eq!(set.contains(&1), true);
        /// assert_eq!(set.contains(&4), false);
        /// ```
        pub fn contains<Q>(&self, value: &Q) -> bool
        where
            T: Borrow<Q>,
            Q: ?Sized + Eq + Hash,
        {
            self.map.contains_key(value)
        }

        /// Returns `true` if `self` has no elements in common with `other`. This is equivalent to
        /// checking for an empty intersection.
        ///
        /// # Examples
        ///
        /// ```
        /// use heapless::FnvIndexSet;
        ///
        /// let a: FnvIndexSet<_, 16> = [1, 2, 3].iter().cloned().collect();
        /// let mut b = FnvIndexSet::<_, 16>::new();
        ///
        /// assert_eq!(a.is_disjoint(&b), true);
        /// b.insert(4).unwrap();
        /// assert_eq!(a.is_disjoint(&b), true);
        /// b.insert(1).unwrap();
        /// assert_eq!(a.is_disjoint(&b), false);
        /// ```
        pub fn is_disjoint<S2, const N2: usize>(&self, other: &IndexSet<T, S2, N2>) -> bool
        where
            S2: BuildHasher,
        {
            self.iter().all(|v| !other.contains(v))
        }

        /// Returns `true` if the set is a subset of another, i.e. `other` contains at least all the
        /// values in `self`.
        ///
        /// # Examples
        ///
        /// ```
        /// use heapless::FnvIndexSet;
        ///
        /// let sup: FnvIndexSet<_, 16> = [1, 2, 3].iter().cloned().collect();
        /// let mut set = FnvIndexSet::<_, 16>::new();
        ///
        /// assert_eq!(set.is_subset(&sup), true);
        /// set.insert(2).unwrap();
        /// assert_eq!(set.is_subset(&sup), true);
        /// set.insert(4).unwrap();
        /// assert_eq!(set.is_subset(&sup), false);
        /// ```
        pub fn is_subset<S2, const N2: usize>(&self, other: &IndexSet<T, S2, N2>) -> bool
        where
            S2: BuildHasher,
        {
            self.iter().all(|v| other.contains(v))
        }

        // Returns `true` if the set is a superset of another, i.e. `self` contains at least all the
        // values in `other`.
        ///
        /// # Examples
        ///
        /// ```
        /// use heapless::FnvIndexSet;
        ///
        /// let sub: FnvIndexSet<_, 16> = [1, 2].iter().cloned().collect();
        /// let mut set = FnvIndexSet::<_, 16>::new();
        ///
        /// assert_eq!(set.is_superset(&sub), false);
        ///
        /// set.insert(0).unwrap();
        /// set.insert(1).unwrap();
        /// assert_eq!(set.is_superset(&sub), false);
        ///
        /// set.insert(2).unwrap();
        /// assert_eq!(set.is_superset(&sub), true);
        /// ```
        pub fn is_superset<S2, const N2: usize>(&self, other: &IndexSet<T, S2, N2>) -> bool
        where
            S2: BuildHasher,
        {
            other.is_subset(self)
        }

        /// Adds a value to the set.
        ///
        /// If the set did not have this value present, `true` is returned.
        ///
        /// If the set did have this value present, `false` is returned.
        ///
        /// # Examples
        ///
        /// ```
        /// use heapless::FnvIndexSet;
        ///
        /// let mut set = FnvIndexSet::<_, 16>::new();
        ///
        /// assert_eq!(set.insert(2).unwrap(), true);
        /// assert_eq!(set.insert(2).unwrap(), false);
        /// assert_eq!(set.len(), 1);
        /// ```
        pub fn insert(&mut self, value: T) -> Result<bool, T> {
            self.map
                .insert(value, ())
                .map(|old| old.is_none())
                .map_err(|(k, _)| k)
        }

        /// Removes a value from the set. Returns `true` if the value was present in the set.
        ///
        /// The value may be any borrowed form of the set's value type, but `Hash` and `Eq` on the
        /// borrowed form must match those for the value type.
        ///
        /// # Examples
        ///
        /// ```
        /// use heapless::FnvIndexSet;
        ///
        /// let mut set = FnvIndexSet::<_, 16>::new();
        ///
        /// set.insert(2).unwrap();
        /// assert_eq!(set.remove(&2), true);
        /// assert_eq!(set.remove(&2), false);
        /// ```
        pub fn remove<Q>(&mut self, value: &Q) -> bool
        where
            T: Borrow<Q>,
            Q: ?Sized + Eq + Hash,
        {
            self.map.remove(value).is_some()
        }
    }

    impl<T, S, const N: usize> Clone for IndexSet<T, S, N>
    where
        T: Eq + Hash + Clone,
        S: Clone,
    {
        fn clone(&self) -> Self {
            Self {
                map: self.map.clone(),
            }
        }
    }

    impl<T, S, const N: usize> fmt::Debug for IndexSet<T, S, N>
    where
        T: Eq + Hash + fmt::Debug,
        S: BuildHasher,
    {
        fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
            f.debug_set().entries(self.iter()).finish()
        }
    }

    impl<T, S, const N: usize> Default for IndexSet<T, S, N>
    where
        T: Eq + Hash,
        S: BuildHasher + Default,
    {
        fn default() -> Self {
            IndexSet {
                map: <_>::default(),
            }
        }
    }

    impl<T, S1, S2, const N1: usize, const N2: usize> PartialEq<IndexSet<T, S2, N2>>
        for IndexSet<T, S1, N1>
    where
        T: Eq + Hash,
        S1: BuildHasher,
        S2: BuildHasher,
    {
        fn eq(&self, other: &IndexSet<T, S2, N2>) -> bool {
            self.len() == other.len() && self.is_subset(other)
        }
    }

    impl<T, S, const N: usize> Extend<T> for IndexSet<T, S, N>
    where
        T: Eq + Hash,
        S: BuildHasher,
    {
        fn extend<I>(&mut self, iterable: I)
        where
            I: IntoIterator<Item = T>,
        {
            self.map.extend(iterable.into_iter().map(|k| (k, ())))
        }
    }

    impl<'a, T, S, const N: usize> Extend<&'a T> for IndexSet<T, S, N>
    where
        T: 'a + Eq + Hash + Copy,
        S: BuildHasher,
    {
        fn extend<I>(&mut self, iterable: I)
        where
            I: IntoIterator<Item = &'a T>,
        {
            self.extend(iterable.into_iter().cloned())
        }
    }

    impl<T, S, const N: usize> FromIterator<T> for IndexSet<T, S, N>
    where
        T: Eq + Hash,
        S: BuildHasher + Default,
    {
        fn from_iter<I>(iter: I) -> Self
        where
            I: IntoIterator<Item = T>,
        {
            let mut set = IndexSet::default();
            set.extend(iter);
            set
        }
    }

    impl<'a, T, S, const N: usize> IntoIterator for &'a IndexSet<T, S, N>
    where
        T: Eq + Hash,
        S: BuildHasher,
    {
        type Item = &'a T;
        type IntoIter = Iter<'a, T>;

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

    pub struct Iter<'a, T> {
        iter: indexmap::Iter<'a, T, ()>,
    }

    impl<'a, T> Iterator for Iter<'a, T> {
        type Item = &'a T;

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

    impl<'a, T> Clone for Iter<'a, T> {
        fn clone(&self) -> Self {
            Self {
                iter: self.iter.clone(),
            }
        }
    }

    pub struct Difference<'a, T, S, const N: usize>
    where
        S: BuildHasher,
        T: Eq + Hash,
    {
        iter: Iter<'a, T>,
        other: &'a IndexSet<T, S, N>,
    }

    impl<'a, T, S, const N: usize> Iterator for Difference<'a, T, S, N>
    where
        S: BuildHasher,
        T: Eq + Hash,
    {
        type Item = &'a T;

        fn next(&mut self) -> Option<Self::Item> {
            loop {
                let elt = self.iter.next()?;
                if !self.other.contains(elt) {
                    return Some(elt);
                }
            }
        }
    }

    pub struct Intersection<'a, T, S, const N: usize>
    where
        S: BuildHasher,
        T: Eq + Hash,
    {
        iter: Iter<'a, T>,
        other: &'a IndexSet<T, S, N>,
    }

    impl<'a, T, S, const N: usize> Iterator for Intersection<'a, T, S, N>
    where
        S: BuildHasher,
        T: Eq + Hash,
    {
        type Item = &'a T;

        fn next(&mut self) -> Option<Self::Item> {
            loop {
                let elt = self.iter.next()?;
                if self.other.contains(elt) {
                    return Some(elt);
                }
            }
        }
    }