hash_set.dart

// Copyright (c) 2013, the Dart project authors.  Please see the AUTHORS file
// for details. All rights reserved. Use of this source code is governed by a
// BSD-style license that can be found in the LICENSE file.

// @dart = 2.6

part of dart.collection;

/// An unordered hash-table based [Set] implementation.
///
/// The elements of a `HashSet` must have consistent equality
/// and hashCode implementations. This means that the equals operation
/// must define a stable equivalence relation on the elements (reflexive,
/// symmetric, transitive, and consistent over time), and that the hashCode
/// must consistent with equality, so that the same for objects that are
/// considered equal.
///
/// The set allows `null` as an element.
///
/// Most simple operations on `HashSet` are done in (potentially amortized)
/// constant time: [add], [contains], [remove], and [length], provided the hash
/// codes of objects are well distributed.
///
/// The iteration order of the set is not specified and depends on
/// the hashcodes of the provided elements. However, the order is stable:
/// multiple iterations over the same set produce the same order, as long as
/// the set is not modified.
abstract class HashSet<E> implements Set<E> {
  /// Create a hash set using the provided [equals] as equality.
  ///
  /// The provided [equals] must define a stable equivalence relation, and
  /// [hashCode] must be consistent with [equals]. If the [equals] or [hashCode]
  /// methods won't work on all objects, but only on some instances of E, the
  /// [isValidKey] predicate can be used to restrict the keys that the functions
  /// are applied to.
  /// Any key for which [isValidKey] returns false is automatically assumed
  /// to not be in the set when asking `contains`.
  ///
  /// If [equals] or [hashCode] are omitted, the set uses
  /// the elements' intrinsic [Object.==] and [Object.hashCode].
  ///
  /// If you supply one of [equals] and [hashCode],
  /// you should generally also to supply the other.
  ///
  /// If the supplied `equals` or `hashCode` functions won't work on all [E]
  /// objects, and the map will be used in a setting where a non-`E` object
  /// is passed to, e.g., `contains`, then the [isValidKey] function should
  /// also be supplied.
  ///
  /// If [isValidKey] is omitted, it defaults to testing if the object is an
  /// [E] instance. That means that:
  ///
  ///     new HashSet<int>(equals: (int e1, int e2) => (e1 - e2) % 5 == 0,
  ///                      hashCode: (int e) => e % 5)
  ///
  /// does not need an `isValidKey` argument, because it defaults to only
  /// accepting `int` values which are accepted by both `equals` and `hashCode`.
  ///
  /// If neither `equals`, `hashCode`, nor `isValidKey` is provided,
  /// the default `isValidKey` instead accepts all values.
  /// The default equality and hashcode operations are assumed to work on all
  /// objects.
  ///
  /// Likewise, if `equals` is [identical], `hashCode` is [identityHashCode]
  /// and `isValidKey` is omitted, the resulting set is identity based,
  /// and the `isValidKey` defaults to accepting all keys.
  /// Such a map can be created directly using [HashSet.identity].
  external factory HashSet(
      {bool equals(E e1, E e2),
      int hashCode(E e),
      bool isValidKey(potentialKey)});

  /// Creates an unordered identity-based set.
  ///
  /// Effectively a shorthand for:
  ///
  ///     new HashSet<E>(equals: identical,
  ///                    hashCode: identityHashCode)
  external factory HashSet.identity();

  /// Create a hash set containing all [elements].
  ///
  /// Creates a hash set as by `new HashSet<E>()` and adds all given [elements]
  /// to the set. The elements are added in order. If [elements] contains
  /// two entries that are equal, but not identical, then the first one is
  /// the one in the resulting set.
  ///
  /// All the [elements] should be instances of [E].
  /// The `elements` iterable itself may have any element type, so this
  /// constructor can be used to down-cast a `Set`, for example as:
  /// ```dart
  /// Set<SuperType> superSet = ...;
  /// Set<SubType> subSet =
  ///     new HashSet<SubType>.from(superSet.whereType<SubType>());
  /// ```
  factory HashSet.from(Iterable elements) {
    HashSet<E> result = HashSet<E>();
    for (final e in elements) {
      result.add(e);
    }
    return result;
  }

  /// Create a hash set containing all [elements].
  ///
  /// Creates a hash set as by `new HashSet<E>()` and adds all given [elements]
  /// to the set. The elements are added in order. If [elements] contains
  /// two entries that are equal, but not identical, then the first one is
  /// the one in the resulting set.
  factory HashSet.of(Iterable<E> elements) => HashSet<E>()..addAll(elements);

  /// Provides an iterator that iterates over the elements of this set.
  ///
  /// The order of iteration is unspecified,
  /// but consistent between changes to the set.
  Iterator<E> get iterator;
}