linked_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;

/// A [LinkedHashSet] is a hash-table based [Set] implementation.
///
/// The `LinkedHashSet` also keep track of the order that elements were inserted
/// in, and iteration happens in first-to-last insertion order.
///
/// The elements of a `LinkedHashSet` must have consistent [Object.==]
/// and [Object.hashCode] implementations. This means that the `==` operator
/// must define a stable equivalence relation on the elements (reflexive,
/// symmetric, transitive, and consistent over time), and that `hashCode`
/// must be the same for objects that are considered equal by `==`.
///
/// The set allows `null` as an element.
///
/// Iteration of elements is done in element insertion order.
/// An element that was added after another will occur later in the iteration.
/// Adding an element that is already in the set
/// does not change its position in the iteration order,
/// but removing an element and adding it again,
/// will make it the last element of an iteration.
///
/// 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..
abstract class LinkedHashSet<E> implements Set<E> {
  /// Create an insertion-ordered hash set using the provided
  /// [equals] and [hashCode].
  ///
  /// 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],
  /// and [isValidKey] is ignored since these operations are assumed
  /// to work on all objects.
  ///
  /// 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 LinkedHashSet<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 [LinkedHashSet.identity].
  external factory LinkedHashSet(
      {bool equals(E e1, E e2),
      int hashCode(E e),
      bool isValidKey(potentialKey)});

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

  /// Create a linked hash set containing all [elements].
  ///
  /// Creates a linked hash set as by `new LinkedHashSet<E>()` and adds each
  /// element of `elements` to this set in the order they are iterated.
  ///
  /// 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:
  ///
  ///     Set<SuperType> superSet = ...;
  ///     Iterable<SuperType> tmp = superSet.where((e) => e is SubType);
  ///     Set<SubType> subSet = new LinkedHashSet<SubType>.from(tmp);
  factory LinkedHashSet.from(Iterable elements) {
    LinkedHashSet<E> result = LinkedHashSet<E>();
    for (final element in elements) {
      result.add(element);
    }
    return result;
  }

  /// Create a linked hash set from [elements].
  ///
  /// Creates a linked hash set as by `new LinkedHashSet<E>()` and adds each
  /// element of `elements` to this set in the order they are iterated.
  factory LinkedHashSet.of(Iterable<E> elements) =>
      LinkedHashSet<E>()..addAll(elements);

  /// Executes a function on each element of the set.
  ///
  /// The elements are iterated in insertion order.
  void forEach(void action(E element));

  /// Provides an iterator that iterates over the elements in insertion order.
  Iterator<E> get iterator;
}