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// 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.
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 `==`.
/// 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:
/// ```dart template:expression
/// 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 Function(E, E)? equals,
int Function(E)? hashCode,
bool Function(dynamic)? isValidKey});
/// Creates an insertion-ordered identity-based set.
/// Effectively a shorthand for:
/// ```dart
/// 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:
/// ```dart
/// Set<SuperType> superSet = ...;
/// Iterable<SuperType> tmp = superSet.where((e) => e is SubType);
/// Set<SubType> subSet = LinkedHashSet<SubType>.from(tmp);
/// ```
factory LinkedHashSet.from(Iterable<dynamic> elements) {
LinkedHashSet<E> result = LinkedHashSet<E>();
for (final element in elements) {
result.add(element as E);
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) =>
/// 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;