// Copyright 2014 The Rust Project Developers. See the COPYRIGHT
// file at the top-level directory of this distribution and at
// http://rust-lang.org/COPYRIGHT.
//
// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
// option. This file may not be copied, modified, or distributed
// except according to those terms.

use core::borrow::Borrow;
use core::fmt;
use core::hash::{BuildHasher, Hash};
use core::iter::{Chain, FromIterator, FusedIterator};
#[cfg(feature = "serde")]
use core::marker::PhantomData;
use core::ops::{BitAnd, BitOr, BitXor, Sub};
#[cfg(feature = "serde")]
use serde::de::{Deserialize, Deserializer, SeqAccess, Visitor};
#[cfg(feature = "serde")]
use serde::ser::{Serialize, Serializer};

#[cfg(feature = "serde")]
use super::size_hint;

use super::map::{self, DefaultHashBuilder, HashMap, Keys};

// Future Optimization (FIXME!)
// =============================
//
// Iteration over zero sized values is a noop. There is no need
// for `bucket.val` in the case of HashSet. I suppose we would need HKT
// to get rid of it properly.

/// A hash set implemented as a `HashMap` where the value is `()`.
///
/// As with the [`HashMap`] type, a `HashSet` requires that the elements
/// implement the [`Eq`] and [`Hash`] traits. This can frequently be achieved by
/// using `#[derive(PartialEq, Eq, Hash)]`. If you implement these yourself,
/// it is important that the following property holds:
///
/// ```text
/// k1 == k2 -> hash(k1) == hash(k2)
/// ```
///
/// In other words, if two keys are equal, their hashes must be equal.
///
///
/// It is a logic error for an item to be modified in such a way that the
/// item's hash, as determined by the [`Hash`] trait, or its equality, as
/// determined by the [`Eq`] trait, changes while it is in the set. This is
/// normally only possible through [`Cell`], [`RefCell`], global state, I/O, or
/// unsafe code.
///
/// It is also a logic error for the [`Hash`] implementation of a key to panic.
/// This is generally only possible if the trait is implemented manually. If a
/// panic does occur then the contents of the `HashSet` may become corrupted and
/// some items may be dropped from the table.
///
/// # Examples
///
/// ```
/// use hashbrown::HashSet;
/// // Type inference lets us omit an explicit type signature (which
/// // would be `HashSet<&str>` in this example).
/// let mut books = HashSet::new();
///
/// // Add some books.
/// books.insert("A Dance With Dragons");
/// books.insert("To Kill a Mockingbird");
/// books.insert("The Odyssey");
/// books.insert("The Great Gatsby");
///
/// // 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);
/// }
/// ```
///
/// The easiest way to use `HashSet` with a custom type is to derive
/// [`Eq`] and [`Hash`]. We must also derive [`PartialEq`], this will in the
/// future be implied by [`Eq`].
///
/// ```
/// use hashbrown::HashSet;
/// #[derive(Hash, Eq, PartialEq, Debug)]
/// struct Viking<'a> {
///     name: &'a str,
///     power: usize,
/// }
///
/// let mut vikings = HashSet::new();
///
/// vikings.insert(Viking { name: "Einar", power: 9 });
/// vikings.insert(Viking { name: "Einar", power: 9 });
/// vikings.insert(Viking { name: "Olaf", power: 4 });
/// vikings.insert(Viking { name: "Harald", power: 8 });
///
/// // Use derived implementation to print the vikings.
/// for x in &vikings {
///     println!("{:?}", x);
/// }
/// ```
///
/// A `HashSet` with fixed list of elements can be initialized from an array:
///
/// ```
/// use hashbrown::HashSet;
///
/// fn main() {
///     let viking_names: HashSet<&str> =
///         [ "Einar", "Olaf", "Harald" ].iter().cloned().collect();
///     // use the values stored in the set
/// }
/// ```
///
/// [`Cell`]: ../../std/cell/struct.Cell.html
/// [`Eq`]: ../../std/cmp/trait.Eq.html
/// [`Hash`]: ../../std/hash/trait.Hash.html
/// [`HashMap`]: struct.HashMap.html
/// [`PartialEq`]: ../../std/cmp/trait.PartialEq.html
/// [`RefCell`]: ../../std/cell/struct.RefCell.html
#[derive(Clone)]
pub struct HashSet<T, S = DefaultHashBuilder> {
    map: HashMap<T, (), S>,
}

impl<T: Hash + Eq> HashSet<T, DefaultHashBuilder> {
    /// Creates an empty `HashSet`.
    ///
    /// The hash set is initially created with a capacity of 0, so it will not allocate until it
    /// is first inserted into.
    ///
    /// # Examples
    ///
    /// ```
    /// use hashbrown::HashSet;
    /// let set: HashSet<i32> = HashSet::new();
    /// ```
    #[inline]
    pub fn new() -> HashSet<T, DefaultHashBuilder> {
        HashSet {
            map: HashMap::new(),
        }
    }

    /// Creates an empty `HashSet` with the specified capacity.
    ///
    /// The hash set will be able to hold at least `capacity` elements without
    /// reallocating. If `capacity` is 0, the hash set will not allocate.
    ///
    /// # Examples
    ///
    /// ```
    /// use hashbrown::HashSet;
    /// let set: HashSet<i32> = HashSet::with_capacity(10);
    /// assert!(set.capacity() >= 10);
    /// ```
    #[inline]
    pub fn with_capacity(capacity: usize) -> HashSet<T, DefaultHashBuilder> {
        HashSet {
            map: HashMap::with_capacity(capacity),
        }
    }
}

impl<T, S> HashSet<T, S>
where
    T: Eq + Hash,
    S: BuildHasher,
{
    /// Creates a new empty hash set which will use the given hasher to hash
    /// keys.
    ///
    /// The hash set is also created with the default initial capacity.
    ///
    /// Warning: `hasher` is normally randomly generated, and
    /// is designed to allow `HashSet`s to be resistant to attacks that
    /// cause many collisions and very poor performance. Setting it
    /// manually using this function can expose a DoS attack vector.
    ///
    /// # Examples
    ///
    /// ```
    /// use hashbrown::HashSet;
    /// use hashbrown::hash_map::DefaultHashBuilder;
    ///
    /// let s = DefaultHashBuilder::default();
    /// let mut set = HashSet::with_hasher(s);
    /// set.insert(2);
    /// ```
    #[inline]
    pub fn with_hasher(hasher: S) -> HashSet<T, S> {
        HashSet {
            map: HashMap::with_hasher(hasher),
        }
    }

    /// Creates an empty `HashSet` with with the specified capacity, using
    /// `hasher` to hash the keys.
    ///
    /// The hash set will be able to hold at least `capacity` elements without
    /// reallocating. If `capacity` is 0, the hash set will not allocate.
    ///
    /// Warning: `hasher` is normally randomly generated, and
    /// is designed to allow `HashSet`s to be resistant to attacks that
    /// cause many collisions and very poor performance. Setting it
    /// manually using this function can expose a DoS attack vector.
    ///
    /// # Examples
    ///
    /// ```
    /// use hashbrown::HashSet;
    /// use hashbrown::hash_map::DefaultHashBuilder;
    ///
    /// let s = DefaultHashBuilder::default();
    /// let mut set = HashSet::with_capacity_and_hasher(10, s);
    /// set.insert(1);
    /// ```
    #[inline]
    pub fn with_capacity_and_hasher(capacity: usize, hasher: S) -> HashSet<T, S> {
        HashSet {
            map: HashMap::with_capacity_and_hasher(capacity, hasher),
        }
    }

    /// Returns a reference to the set's [`BuildHasher`].
    ///
    /// [`BuildHasher`]: ../../std/hash/trait.BuildHasher.html
    ///
    /// # Examples
    ///
    /// ```
    /// use hashbrown::HashSet;
    /// use hashbrown::hash_map::DefaultHashBuilder;
    ///
    /// let hasher = DefaultHashBuilder::default();
    /// let set: HashSet<i32> = HashSet::with_hasher(hasher);
    /// let hasher: &DefaultHashBuilder = set.hasher();
    /// ```
    #[inline]
    pub fn hasher(&self) -> &S {
        self.map.hasher()
    }

    /// Returns the number of elements the set can hold without reallocating.
    ///
    /// # Examples
    ///
    /// ```
    /// use hashbrown::HashSet;
    /// let set: HashSet<i32> = HashSet::with_capacity(100);
    /// assert!(set.capacity() >= 100);
    /// ```
    #[inline]
    pub fn capacity(&self) -> usize {
        self.map.capacity()
    }

    /// Reserves capacity for at least `additional` more elements to be inserted
    /// in the `HashSet`. The collection may reserve more space to avoid
    /// frequent reallocations.
    ///
    /// # Panics
    ///
    /// Panics if the new allocation size overflows `usize`.
    ///
    /// # Examples
    ///
    /// ```
    /// use hashbrown::HashSet;
    /// let mut set: HashSet<i32> = HashSet::new();
    /// set.reserve(10);
    /// assert!(set.capacity() >= 10);
    /// ```
    #[inline]
    pub fn reserve(&mut self, additional: usize) {
        self.map.reserve(additional)
    }

    /// Shrinks the capacity of the set as much as possible. It will drop
    /// down as much as possible while maintaining the internal rules
    /// and possibly leaving some space in accordance with the resize policy.
    ///
    /// # Examples
    ///
    /// ```
    /// use hashbrown::HashSet;
    ///
    /// let mut set = HashSet::with_capacity(100);
    /// set.insert(1);
    /// set.insert(2);
    /// assert!(set.capacity() >= 100);
    /// set.shrink_to_fit();
    /// assert!(set.capacity() >= 2);
    /// ```
    #[inline]
    pub fn shrink_to_fit(&mut self) {
        self.map.shrink_to_fit()
    }

    /// Shrinks the capacity of the set with a lower limit. It will drop
    /// down no lower than the supplied limit while maintaining the internal rules
    /// and possibly leaving some space in accordance with the resize policy.
    ///
    /// Panics if the current capacity is smaller than the supplied
    /// minimum capacity.
    ///
    /// # Examples
    ///
    /// ```
    /// use hashbrown::HashSet;
    ///
    /// let mut set = HashSet::with_capacity(100);
    /// set.insert(1);
    /// set.insert(2);
    /// assert!(set.capacity() >= 100);
    /// set.shrink_to(10);
    /// assert!(set.capacity() >= 10);
    /// set.shrink_to(0);
    /// assert!(set.capacity() >= 2);
    /// ```
    #[inline]
    pub fn shrink_to(&mut self, min_capacity: usize) {
        self.map.shrink_to(min_capacity)
    }

    /// An iterator visiting all elements in arbitrary order.
    /// The iterator element type is `&'a T`.
    ///
    /// # Examples
    ///
    /// ```
    /// use hashbrown::HashSet;
    /// let mut set = HashSet::new();
    /// set.insert("a");
    /// set.insert("b");
    ///
    /// // Will print in an arbitrary order.
    /// for x in set.iter() {
    ///     println!("{}", x);
    /// }
    /// ```
    #[inline]
    pub fn iter(&self) -> Iter<T> {
        Iter {
            iter: self.map.keys(),
        }
    }

    /// Visits the values representing the difference,
    /// i.e. the values that are in `self` but not in `other`.
    ///
    /// # Examples
    ///
    /// ```
    /// use hashbrown::HashSet;
    /// let a: HashSet<_> = [1, 2, 3].iter().cloned().collect();
    /// let b: HashSet<_> = [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: HashSet<_> = a.difference(&b).collect();
    /// assert_eq!(diff, [1].iter().collect());
    ///
    /// // Note that difference is not symmetric,
    /// // and `b - a` means something else:
    /// let diff: HashSet<_> = b.difference(&a).collect();
    /// assert_eq!(diff, [4].iter().collect());
    /// ```
    #[inline]
    pub fn difference<'a>(&'a self, other: &'a HashSet<T, S>) -> Difference<'a, T, S> {
        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 hashbrown::HashSet;
    /// let a: HashSet<_> = [1, 2, 3].iter().cloned().collect();
    /// let b: HashSet<_> = [4, 2, 3, 4].iter().cloned().collect();
    ///
    /// // Print 1, 4 in arbitrary order.
    /// for x in a.symmetric_difference(&b) {
    ///     println!("{}", x);
    /// }
    ///
    /// let diff1: HashSet<_> = a.symmetric_difference(&b).collect();
    /// let diff2: HashSet<_> = b.symmetric_difference(&a).collect();
    ///
    /// assert_eq!(diff1, diff2);
    /// assert_eq!(diff1, [1, 4].iter().collect());
    /// ```
    #[inline]
    pub fn symmetric_difference<'a>(
        &'a self,
        other: &'a HashSet<T, S>,
    ) -> SymmetricDifference<'a, T, S> {
        SymmetricDifference {
            iter: 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 hashbrown::HashSet;
    /// let a: HashSet<_> = [1, 2, 3].iter().cloned().collect();
    /// let b: HashSet<_> = [4, 2, 3, 4].iter().cloned().collect();
    ///
    /// // Print 2, 3 in arbitrary order.
    /// for x in a.intersection(&b) {
    ///     println!("{}", x);
    /// }
    ///
    /// let intersection: HashSet<_> = a.intersection(&b).collect();
    /// assert_eq!(intersection, [2, 3].iter().collect());
    /// ```
    #[inline]
    pub fn intersection<'a>(&'a self, other: &'a HashSet<T, S>) -> Intersection<'a, T, S> {
        Intersection {
            iter: self.iter(),
            other,
        }
    }

    /// Visits the values representing the union,
    /// i.e. all the values in `self` or `other`, without duplicates.
    ///
    /// # Examples
    ///
    /// ```
    /// use hashbrown::HashSet;
    /// let a: HashSet<_> = [1, 2, 3].iter().cloned().collect();
    /// let b: HashSet<_> = [4, 2, 3, 4].iter().cloned().collect();
    ///
    /// // Print 1, 2, 3, 4 in arbitrary order.
    /// for x in a.union(&b) {
    ///     println!("{}", x);
    /// }
    ///
    /// let union: HashSet<_> = a.union(&b).collect();
    /// assert_eq!(union, [1, 2, 3, 4].iter().collect());
    /// ```
    #[inline]
    pub fn union<'a>(&'a self, other: &'a HashSet<T, S>) -> Union<'a, T, S> {
        Union {
            iter: self.iter().chain(other.difference(self)),
        }
    }

    /// Returns the number of elements in the set.
    ///
    /// # Examples
    ///
    /// ```
    /// use hashbrown::HashSet;
    ///
    /// let mut v = HashSet::new();
    /// assert_eq!(v.len(), 0);
    /// v.insert(1);
    /// assert_eq!(v.len(), 1);
    /// ```
    #[inline]
    pub fn len(&self) -> usize {
        self.map.len()
    }

    /// Returns true if the set contains no elements.
    ///
    /// # Examples
    ///
    /// ```
    /// use hashbrown::HashSet;
    ///
    /// let mut v = HashSet::new();
    /// assert!(v.is_empty());
    /// v.insert(1);
    /// assert!(!v.is_empty());
    /// ```
    #[inline]
    pub fn is_empty(&self) -> bool {
        self.map.is_empty()
    }

    /// Clears the set, returning all elements in an iterator.
    ///
    /// # Examples
    ///
    /// ```
    /// use hashbrown::HashSet;
    ///
    /// let mut set: HashSet<_> = [1, 2, 3].iter().cloned().collect();
    /// assert!(!set.is_empty());
    ///
    /// // print 1, 2, 3 in an arbitrary order
    /// for i in set.drain() {
    ///     println!("{}", i);
    /// }
    ///
    /// assert!(set.is_empty());
    /// ```
    #[inline]
    pub fn drain(&mut self) -> Drain<T> {
        Drain {
            iter: self.map.drain(),
        }
    }

    /// Clears the set, removing all values.
    ///
    /// # Examples
    ///
    /// ```
    /// use hashbrown::HashSet;
    ///
    /// let mut v = HashSet::new();
    /// v.insert(1);
    /// v.clear();
    /// assert!(v.is_empty());
    /// ```
    #[inline]
    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 hashbrown::HashSet;
    ///
    /// let set: HashSet<_> = [1, 2, 3].iter().cloned().collect();
    /// assert_eq!(set.contains(&1), true);
    /// assert_eq!(set.contains(&4), false);
    /// ```
    ///
    /// [`Eq`]: ../../std/cmp/trait.Eq.html
    /// [`Hash`]: ../../std/hash/trait.Hash.html
    #[inline]
    pub fn contains<Q: ?Sized>(&self, value: &Q) -> bool
    where
        T: Borrow<Q>,
        Q: Hash + Eq,
    {
        self.map.contains_key(value)
    }

    /// Returns a reference to the value in the set, if any, that is equal to the given 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 hashbrown::HashSet;
    ///
    /// let set: HashSet<_> = [1, 2, 3].iter().cloned().collect();
    /// assert_eq!(set.get(&2), Some(&2));
    /// assert_eq!(set.get(&4), None);
    /// ```
    ///
    /// [`Eq`]: ../../std/cmp/trait.Eq.html
    /// [`Hash`]: ../../std/hash/trait.Hash.html
    #[inline]
    pub fn get<Q: ?Sized>(&self, value: &Q) -> Option<&T>
    where
        T: Borrow<Q>,
        Q: Hash + Eq,
    {
        self.map.get_key_value(value).map(|(k, _)| k)
    }

    /// Returns `true` if `self` has no elements in common with `other`.
    /// This is equivalent to checking for an empty intersection.
    ///
    /// # Examples
    ///
    /// ```
    /// use hashbrown::HashSet;
    ///
    /// let a: HashSet<_> = [1, 2, 3].iter().cloned().collect();
    /// let mut b = HashSet::new();
    ///
    /// assert_eq!(a.is_disjoint(&b), true);
    /// b.insert(4);
    /// assert_eq!(a.is_disjoint(&b), true);
    /// b.insert(1);
    /// assert_eq!(a.is_disjoint(&b), false);
    /// ```
    pub fn is_disjoint(&self, other: &HashSet<T, S>) -> bool {
        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 hashbrown::HashSet;
    ///
    /// let sup: HashSet<_> = [1, 2, 3].iter().cloned().collect();
    /// let mut set = HashSet::new();
    ///
    /// assert_eq!(set.is_subset(&sup), true);
    /// set.insert(2);
    /// assert_eq!(set.is_subset(&sup), true);
    /// set.insert(4);
    /// assert_eq!(set.is_subset(&sup), false);
    /// ```
    pub fn is_subset(&self, other: &HashSet<T, S>) -> bool {
        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 hashbrown::HashSet;
    ///
    /// let sub: HashSet<_> = [1, 2].iter().cloned().collect();
    /// let mut set = HashSet::new();
    ///
    /// assert_eq!(set.is_superset(&sub), false);
    ///
    /// set.insert(0);
    /// set.insert(1);
    /// assert_eq!(set.is_superset(&sub), false);
    ///
    /// set.insert(2);
    /// assert_eq!(set.is_superset(&sub), true);
    /// ```
    #[inline]
    pub fn is_superset(&self, other: &HashSet<T, S>) -> bool {
        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 hashbrown::HashSet;
    ///
    /// let mut set = HashSet::new();
    ///
    /// assert_eq!(set.insert(2), true);
    /// assert_eq!(set.insert(2), false);
    /// assert_eq!(set.len(), 1);
    /// ```
    #[inline]
    pub fn insert(&mut self, value: T) -> bool {
        self.map.insert(value, ()).is_none()
    }

    /// Adds a value to the set, replacing the existing value, if any, that is equal to the given
    /// one. Returns the replaced value.
    ///
    /// # Examples
    ///
    /// ```
    /// use hashbrown::HashSet;
    ///
    /// let mut set = HashSet::new();
    /// set.insert(Vec::<i32>::new());
    ///
    /// assert_eq!(set.get(&[][..]).unwrap().capacity(), 0);
    /// set.replace(Vec::with_capacity(10));
    /// assert_eq!(set.get(&[][..]).unwrap().capacity(), 10);
    /// ```
    #[inline]
    pub fn replace(&mut self, value: T) -> Option<T> {
        match self.map.entry(value) {
            map::Entry::Occupied(occupied) => Some(occupied.replace_key()),
            map::Entry::Vacant(vacant) => {
                vacant.insert(());
                None
            }
        }
    }

    /// 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 hashbrown::HashSet;
    ///
    /// let mut set = HashSet::new();
    ///
    /// set.insert(2);
    /// assert_eq!(set.remove(&2), true);
    /// assert_eq!(set.remove(&2), false);
    /// ```
    ///
    /// [`Eq`]: ../../std/cmp/trait.Eq.html
    /// [`Hash`]: ../../std/hash/trait.Hash.html
    #[inline]
    pub fn remove<Q: ?Sized>(&mut self, value: &Q) -> bool
    where
        T: Borrow<Q>,
        Q: Hash + Eq,
    {
        self.map.remove(value).is_some()
    }

    /// Removes and returns the value in the set, if any, that is equal to the given one.
    ///
    /// 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 hashbrown::HashSet;
    ///
    /// let mut set: HashSet<_> = [1, 2, 3].iter().cloned().collect();
    /// assert_eq!(set.take(&2), Some(2));
    /// assert_eq!(set.take(&2), None);
    /// ```
    ///
    /// [`Eq`]: ../../std/cmp/trait.Eq.html
    /// [`Hash`]: ../../std/hash/trait.Hash.html
    #[inline]
    pub fn take<Q: ?Sized>(&mut self, value: &Q) -> Option<T>
    where
        T: Borrow<Q>,
        Q: Hash + Eq,
    {
        self.map.remove_entry(value).map(|(k, _)| k)
    }

    /// Retains only the elements specified by the predicate.
    ///
    /// In other words, remove all elements `e` such that `f(&e)` returns `false`.
    ///
    /// # Examples
    ///
    /// ```
    /// use hashbrown::HashSet;
    ///
    /// let xs = [1,2,3,4,5,6];
    /// let mut set: HashSet<i32> = xs.iter().cloned().collect();
    /// set.retain(|&k| k % 2 == 0);
    /// assert_eq!(set.len(), 3);
    /// ```
    pub fn retain<F>(&mut self, mut f: F)
    where
        F: FnMut(&T) -> bool,
    {
        self.map.retain(|k, _| f(k));
    }
}

impl<T, S> PartialEq for HashSet<T, S>
where
    T: Eq + Hash,
    S: BuildHasher,
{
    fn eq(&self, other: &HashSet<T, S>) -> bool {
        if self.len() != other.len() {
            return false;
        }

        self.iter().all(|key| other.contains(key))
    }
}

impl<T, S> Eq for HashSet<T, S>
where
    T: Eq + Hash,
    S: BuildHasher,
{
}

impl<T, S> fmt::Debug for HashSet<T, S>
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> FromIterator<T> for HashSet<T, S>
where
    T: Eq + Hash,
    S: BuildHasher + Default,
{
    fn from_iter<I: IntoIterator<Item = T>>(iter: I) -> HashSet<T, S> {
        let mut set = HashSet::with_hasher(Default::default());
        set.extend(iter);
        set
    }
}

impl<T, S> Extend<T> for HashSet<T, S>
where
    T: Eq + Hash,
    S: BuildHasher,
{
    #[inline]
    fn extend<I: IntoIterator<Item = T>>(&mut self, iter: I) {
        self.map.extend(iter.into_iter().map(|k| (k, ())));
    }
}

impl<'a, T, S> Extend<&'a T> for HashSet<T, S>
where
    T: 'a + Eq + Hash + Copy,
    S: BuildHasher,
{
    #[inline]
    fn extend<I: IntoIterator<Item = &'a T>>(&mut self, iter: I) {
        self.extend(iter.into_iter().cloned());
    }
}

impl<T, S> Default for HashSet<T, S>
where
    T: Eq + Hash,
    S: BuildHasher + Default,
{
    /// Creates an empty `HashSet<T, S>` with the `Default` value for the hasher.
    #[inline]
    fn default() -> HashSet<T, S> {
        HashSet {
            map: HashMap::default(),
        }
    }
}

impl<'a, 'b, T, S> BitOr<&'b HashSet<T, S>> for &'a HashSet<T, S>
where
    T: Eq + Hash + Clone,
    S: BuildHasher + Default,
{
    type Output = HashSet<T, S>;

    /// Returns the union of `self` and `rhs` as a new `HashSet<T, S>`.
    ///
    /// # Examples
    ///
    /// ```
    /// use hashbrown::HashSet;
    ///
    /// let a: HashSet<_> = vec![1, 2, 3].into_iter().collect();
    /// let b: HashSet<_> = vec![3, 4, 5].into_iter().collect();
    ///
    /// let set = &a | &b;
    ///
    /// let mut i = 0;
    /// let expected = [1, 2, 3, 4, 5];
    /// for x in &set {
    ///     assert!(expected.contains(x));
    ///     i += 1;
    /// }
    /// assert_eq!(i, expected.len());
    /// ```
    fn bitor(self, rhs: &HashSet<T, S>) -> HashSet<T, S> {
        self.union(rhs).cloned().collect()
    }
}

impl<'a, 'b, T, S> BitAnd<&'b HashSet<T, S>> for &'a HashSet<T, S>
where
    T: Eq + Hash + Clone,
    S: BuildHasher + Default,
{
    type Output = HashSet<T, S>;

    /// Returns the intersection of `self` and `rhs` as a new `HashSet<T, S>`.
    ///
    /// # Examples
    ///
    /// ```
    /// use hashbrown::HashSet;
    ///
    /// let a: HashSet<_> = vec![1, 2, 3].into_iter().collect();
    /// let b: HashSet<_> = vec![2, 3, 4].into_iter().collect();
    ///
    /// let set = &a & &b;
    ///
    /// let mut i = 0;
    /// let expected = [2, 3];
    /// for x in &set {
    ///     assert!(expected.contains(x));
    ///     i += 1;
    /// }
    /// assert_eq!(i, expected.len());
    /// ```
    fn bitand(self, rhs: &HashSet<T, S>) -> HashSet<T, S> {
        self.intersection(rhs).cloned().collect()
    }
}

impl<'a, 'b, T, S> BitXor<&'b HashSet<T, S>> for &'a HashSet<T, S>
where
    T: Eq + Hash + Clone,
    S: BuildHasher + Default,
{
    type Output = HashSet<T, S>;

    /// Returns the symmetric difference of `self` and `rhs` as a new `HashSet<T, S>`.
    ///
    /// # Examples
    ///
    /// ```
    /// use hashbrown::HashSet;
    ///
    /// let a: HashSet<_> = vec![1, 2, 3].into_iter().collect();
    /// let b: HashSet<_> = vec![3, 4, 5].into_iter().collect();
    ///
    /// let set = &a ^ &b;
    ///
    /// let mut i = 0;
    /// let expected = [1, 2, 4, 5];
    /// for x in &set {
    ///     assert!(expected.contains(x));
    ///     i += 1;
    /// }
    /// assert_eq!(i, expected.len());
    /// ```
    fn bitxor(self, rhs: &HashSet<T, S>) -> HashSet<T, S> {
        self.symmetric_difference(rhs).cloned().collect()
    }
}

impl<'a, 'b, T, S> Sub<&'b HashSet<T, S>> for &'a HashSet<T, S>
where
    T: Eq + Hash + Clone,
    S: BuildHasher + Default,
{
    type Output = HashSet<T, S>;

    /// Returns the difference of `self` and `rhs` as a new `HashSet<T, S>`.
    ///
    /// # Examples
    ///
    /// ```
    /// use hashbrown::HashSet;
    ///
    /// let a: HashSet<_> = vec![1, 2, 3].into_iter().collect();
    /// let b: HashSet<_> = vec![3, 4, 5].into_iter().collect();
    ///
    /// let set = &a - &b;
    ///
    /// let mut i = 0;
    /// let expected = [1, 2];
    /// for x in &set {
    ///     assert!(expected.contains(x));
    ///     i += 1;
    /// }
    /// assert_eq!(i, expected.len());
    /// ```
    fn sub(self, rhs: &HashSet<T, S>) -> HashSet<T, S> {
        self.difference(rhs).cloned().collect()
    }
}

/// An iterator over the items of a `HashSet`.
///
/// This `struct` is created by the [`iter`] method on [`HashSet`].
/// See its documentation for more.
///
/// [`HashSet`]: struct.HashSet.html
/// [`iter`]: struct.HashSet.html#method.iter
pub struct Iter<'a, K: 'a> {
    iter: Keys<'a, K, ()>,
}

/// An owning iterator over the items of a `HashSet`.
///
/// This `struct` is created by the [`into_iter`] method on [`HashSet`][`HashSet`]
/// (provided by the `IntoIterator` trait). See its documentation for more.
///
/// [`HashSet`]: struct.HashSet.html
/// [`into_iter`]: struct.HashSet.html#method.into_iter
pub struct IntoIter<K> {
    iter: map::IntoIter<K, ()>,
}

/// A draining iterator over the items of a `HashSet`.
///
/// This `struct` is created by the [`drain`] method on [`HashSet`].
/// See its documentation for more.
///
/// [`HashSet`]: struct.HashSet.html
/// [`drain`]: struct.HashSet.html#method.drain
pub struct Drain<'a, K: 'a> {
    iter: map::Drain<'a, K, ()>,
}

/// A lazy iterator producing elements in the intersection of `HashSet`s.
///
/// This `struct` is created by the [`intersection`] method on [`HashSet`].
/// See its documentation for more.
///
/// [`HashSet`]: struct.HashSet.html
/// [`intersection`]: struct.HashSet.html#method.intersection
pub struct Intersection<'a, T: 'a, S: 'a> {
    // iterator of the first set
    iter: Iter<'a, T>,
    // the second set
    other: &'a HashSet<T, S>,
}

/// A lazy iterator producing elements in the difference of `HashSet`s.
///
/// This `struct` is created by the [`difference`] method on [`HashSet`].
/// See its documentation for more.
///
/// [`HashSet`]: struct.HashSet.html
/// [`difference`]: struct.HashSet.html#method.difference
pub struct Difference<'a, T: 'a, S: 'a> {
    // iterator of the first set
    iter: Iter<'a, T>,
    // the second set
    other: &'a HashSet<T, S>,
}

/// A lazy iterator producing elements in the symmetric difference of `HashSet`s.
///
/// This `struct` is created by the [`symmetric_difference`] method on
/// [`HashSet`]. See its documentation for more.
///
/// [`HashSet`]: struct.HashSet.html
/// [`symmetric_difference`]: struct.HashSet.html#method.symmetric_difference
pub struct SymmetricDifference<'a, T: 'a, S: 'a> {
    iter: Chain<Difference<'a, T, S>, Difference<'a, T, S>>,
}

/// A lazy iterator producing elements in the union of `HashSet`s.
///
/// This `struct` is created by the [`union`] method on [`HashSet`].
/// See its documentation for more.
///
/// [`HashSet`]: struct.HashSet.html
/// [`union`]: struct.HashSet.html#method.union
pub struct Union<'a, T: 'a, S: 'a> {
    iter: Chain<Iter<'a, T>, Difference<'a, T, S>>,
}

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

    #[inline]
    fn into_iter(self) -> Iter<'a, T> {
        self.iter()
    }
}

impl<T, S> IntoIterator for HashSet<T, S>
where
    T: Eq + Hash,
    S: BuildHasher,
{
    type Item = T;
    type IntoIter = IntoIter<T>;

    /// Creates a consuming iterator, that is, one that moves each value out
    /// of the set in arbitrary order. The set cannot be used after calling
    /// this.
    ///
    /// # Examples
    ///
    /// ```
    /// use hashbrown::HashSet;
    /// let mut set = HashSet::new();
    /// set.insert("a".to_string());
    /// set.insert("b".to_string());
    ///
    /// // Not possible to collect to a Vec<String> with a regular `.iter()`.
    /// let v: Vec<String> = set.into_iter().collect();
    ///
    /// // Will print in an arbitrary order.
    /// for x in &v {
    ///     println!("{}", x);
    /// }
    /// ```
    #[inline]
    fn into_iter(self) -> IntoIter<T> {
        IntoIter {
            iter: self.map.into_iter(),
        }
    }
}

impl<'a, K> Clone for Iter<'a, K> {
    #[inline]
    fn clone(&self) -> Iter<'a, K> {
        Iter {
            iter: self.iter.clone(),
        }
    }
}
impl<'a, K> Iterator for Iter<'a, K> {
    type Item = &'a K;

    #[inline]
    fn next(&mut self) -> Option<&'a K> {
        self.iter.next()
    }
    #[inline]
    fn size_hint(&self) -> (usize, Option<usize>) {
        self.iter.size_hint()
    }
}
impl<'a, K> ExactSizeIterator for Iter<'a, K> {
    #[inline]
    fn len(&self) -> usize {
        self.iter.len()
    }
}
impl<'a, K> FusedIterator for Iter<'a, K> {}

impl<'a, K: fmt::Debug> fmt::Debug for Iter<'a, K> {
    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
        f.debug_list().entries(self.clone()).finish()
    }
}

impl<K> Iterator for IntoIter<K> {
    type Item = K;

    #[inline]
    fn next(&mut self) -> Option<K> {
        self.iter.next().map(|(k, _)| k)
    }
    #[inline]
    fn size_hint(&self) -> (usize, Option<usize>) {
        self.iter.size_hint()
    }
}
impl<K> ExactSizeIterator for IntoIter<K> {
    #[inline]
    fn len(&self) -> usize {
        self.iter.len()
    }
}
impl<K> FusedIterator for IntoIter<K> {}

impl<K: fmt::Debug> fmt::Debug for IntoIter<K> {
    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
        let entries_iter = self.iter.iter().map(|(k, _)| k);
        f.debug_list().entries(entries_iter).finish()
    }
}

impl<'a, K> Iterator for Drain<'a, K> {
    type Item = K;

    #[inline]
    fn next(&mut self) -> Option<K> {
        self.iter.next().map(|(k, _)| k)
    }
    #[inline]
    fn size_hint(&self) -> (usize, Option<usize>) {
        self.iter.size_hint()
    }
}
impl<'a, K> ExactSizeIterator for Drain<'a, K> {
    #[inline]
    fn len(&self) -> usize {
        self.iter.len()
    }
}
impl<'a, K> FusedIterator for Drain<'a, K> {}

impl<'a, K: fmt::Debug> fmt::Debug for Drain<'a, K> {
    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
        let entries_iter = self.iter.iter().map(|(k, _)| k);
        f.debug_list().entries(entries_iter).finish()
    }
}

impl<'a, T, S> Clone for Intersection<'a, T, S> {
    #[inline]
    fn clone(&self) -> Intersection<'a, T, S> {
        Intersection {
            iter: self.iter.clone(),
            ..*self
        }
    }
}

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

    #[inline]
    fn next(&mut self) -> Option<&'a T> {
        loop {
            let elt = self.iter.next()?;
            if self.other.contains(elt) {
                return Some(elt);
            }
        }
    }

    #[inline]
    fn size_hint(&self) -> (usize, Option<usize>) {
        let (_, upper) = self.iter.size_hint();
        (0, upper)
    }
}

impl<'a, T, S> fmt::Debug for Intersection<'a, T, S>
where
    T: fmt::Debug + Eq + Hash,
    S: BuildHasher,
{
    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
        f.debug_list().entries(self.clone()).finish()
    }
}

impl<'a, T, S> FusedIterator for Intersection<'a, T, S>
where
    T: Eq + Hash,
    S: BuildHasher,
{
}

impl<'a, T, S> Clone for Difference<'a, T, S> {
    #[inline]
    fn clone(&self) -> Difference<'a, T, S> {
        Difference {
            iter: self.iter.clone(),
            ..*self
        }
    }
}

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

    #[inline]
    fn next(&mut self) -> Option<&'a T> {
        loop {
            let elt = self.iter.next()?;
            if !self.other.contains(elt) {
                return Some(elt);
            }
        }
    }

    #[inline]
    fn size_hint(&self) -> (usize, Option<usize>) {
        let (_, upper) = self.iter.size_hint();
        (0, upper)
    }
}

impl<'a, T, S> FusedIterator for Difference<'a, T, S>
where
    T: Eq + Hash,
    S: BuildHasher,
{
}

impl<'a, T, S> fmt::Debug for Difference<'a, T, S>
where
    T: fmt::Debug + Eq + Hash,
    S: BuildHasher,
{
    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
        f.debug_list().entries(self.clone()).finish()
    }
}

impl<'a, T, S> Clone for SymmetricDifference<'a, T, S> {
    #[inline]
    fn clone(&self) -> SymmetricDifference<'a, T, S> {
        SymmetricDifference {
            iter: self.iter.clone(),
        }
    }
}

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

    #[inline]
    fn next(&mut self) -> Option<&'a T> {
        self.iter.next()
    }
    #[inline]
    fn size_hint(&self) -> (usize, Option<usize>) {
        self.iter.size_hint()
    }
}

impl<'a, T, S> FusedIterator for SymmetricDifference<'a, T, S>
where
    T: Eq + Hash,
    S: BuildHasher,
{
}

impl<'a, T, S> fmt::Debug for SymmetricDifference<'a, T, S>
where
    T: fmt::Debug + Eq + Hash,
    S: BuildHasher,
{
    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
        f.debug_list().entries(self.clone()).finish()
    }
}

impl<'a, T, S> Clone for Union<'a, T, S> {
    #[inline]
    fn clone(&self) -> Union<'a, T, S> {
        Union {
            iter: self.iter.clone(),
        }
    }
}

impl<'a, T, S> FusedIterator for Union<'a, T, S>
where
    T: Eq + Hash,
    S: BuildHasher,
{
}

impl<'a, T, S> fmt::Debug for Union<'a, T, S>
where
    T: fmt::Debug + Eq + Hash,
    S: BuildHasher,
{
    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
        f.debug_list().entries(self.clone()).finish()
    }
}

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

    #[inline]
    fn next(&mut self) -> Option<&'a T> {
        self.iter.next()
    }
    #[inline]
    fn size_hint(&self) -> (usize, Option<usize>) {
        self.iter.size_hint()
    }
}

#[cfg(feature = "serde")]
impl<T, H> Serialize for HashSet<T, H>
where
    T: Serialize + Eq + Hash,
    H: BuildHasher,
{
    #[inline]
    fn serialize<S>(&self, serializer: S) -> Result<S::Ok, S::Error>
    where
        S: Serializer,
    {
        serializer.collect_seq(self)
    }
}

#[cfg(feature = "serde")]
impl<'de, T, S> Deserialize<'de> for HashSet<T, S>
where
    T: Deserialize<'de> + Eq + Hash,
    S: BuildHasher + Default,
{
    fn deserialize<D>(deserializer: D) -> Result<Self, D::Error>
    where
        D: Deserializer<'de>,
    {
        struct SeqVisitor<T, S> {
            marker: PhantomData<HashSet<T, S>>,
        }

        impl<'de, T, S> Visitor<'de> for SeqVisitor<T, S>
        where
            T: Deserialize<'de> + Eq + Hash,
            S: BuildHasher + Default,
        {
            type Value = HashSet<T, S>;

            fn expecting(&self, formatter: &mut fmt::Formatter) -> fmt::Result {
                formatter.write_str("a sequence")
            }

            #[inline]
            fn visit_seq<A>(self, mut seq: A) -> Result<Self::Value, A::Error>
            where
                A: SeqAccess<'de>,
            {
                let mut values = HashSet::with_capacity_and_hasher(
                    size_hint::cautious(seq.size_hint()),
                    S::default(),
                );

                while let Some(value) = seq.next_element()? {
                    values.insert(value);
                }

                Ok(values)
            }
        }

        let visitor = SeqVisitor { marker: PhantomData };
        deserializer.deserialize_seq(visitor)
    }

    fn deserialize_in_place<D>(deserializer: D, place: &mut Self) -> Result<(), D::Error>
    where
        D: Deserializer<'de>,
    {
        struct SeqInPlaceVisitor<'a, T: 'a, S: 'a>(&'a mut HashSet<T, S>);

        impl<'a, 'de, T, S> Visitor<'de> for SeqInPlaceVisitor<'a, T, S>
        where
            T: Deserialize<'de> + Eq + Hash,
            S: BuildHasher + Default,
        {
            type Value = ();

            fn expecting(&self, formatter: &mut fmt::Formatter) -> fmt::Result {
                formatter.write_str("a sequence")
            }

            #[inline]
            fn visit_seq<A>(self, mut seq: A) -> Result<Self::Value, A::Error>
            where
                A: SeqAccess<'de>,
            {
                self.0.clear();
                self.0.reserve(size_hint::cautious(seq.size_hint()));

                while let Some(value) = seq.next_element()? {
                    self.0.insert(value);
                }

                Ok(())
            }
        }

        deserializer.deserialize_seq(SeqInPlaceVisitor(place))
    }
}

#[allow(dead_code)]
fn assert_covariance() {
    fn set<'new>(v: HashSet<&'static str>) -> HashSet<&'new str> {
        v
    }
    fn iter<'a, 'new>(v: Iter<'a, &'static str>) -> Iter<'a, &'new str> {
        v
    }
    fn into_iter<'new>(v: IntoIter<&'static str>) -> IntoIter<&'new str> {
        v
    }
    fn difference<'a, 'new>(
        v: Difference<'a, &'static str, DefaultHashBuilder>,
    ) -> Difference<'a, &'new str, DefaultHashBuilder> {
        v
    }
    fn symmetric_difference<'a, 'new>(
        v: SymmetricDifference<'a, &'static str, DefaultHashBuilder>,
    ) -> SymmetricDifference<'a, &'new str, DefaultHashBuilder> {
        v
    }
    fn intersection<'a, 'new>(
        v: Intersection<'a, &'static str, DefaultHashBuilder>,
    ) -> Intersection<'a, &'new str, DefaultHashBuilder> {
        v
    }
    fn union<'a, 'new>(
        v: Union<'a, &'static str, DefaultHashBuilder>,
    ) -> Union<'a, &'new str, DefaultHashBuilder> {
        v
    }
    fn drain<'new>(d: Drain<'static, &'static str>) -> Drain<'new, &'new str> {
        d
    }
}