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// Copyright (c) 2011, 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.core;
/// A collection of objects in which each object can occur only once.
///
/// That is, for each object of the element type, the object is either considered
/// to be in the set, or to _not_ be in the set.
///
/// Set implementations may consider some elements indistinguishable. These
/// elements are treated as being the same for any operation on the set.
///
/// The default [Set] implementation, [LinkedHashSet], considers objects
/// indistinguishable if they are equal with regard to
/// operator [Object.==].
///
/// Iterating over elements of a set may be either unordered
/// or ordered in some way. Examples:
///
/// * A [HashSet] is unordered, which means that its iteration order is
/// unspecified,
/// * [LinkedHashSet] iterates in the insertion order of its elements, and
/// * a sorted set like [SplayTreeSet] iterates the elements in sorted order.
///
/// It is generally not allowed to modify the set (add or remove elements) while
/// an operation on the set is being performed, for example during a call to
/// [forEach] or [containsAll]. Nor is it allowed to modify the set while
/// iterating either the set itself or any [Iterable] that is backed by the set,
/// such as the ones returned by methods like [where] and [map].
///
/// It is generally not allowed to modify the equality of elements (and thus not
/// their hashcode) while they are in the set. Some specialized subtypes may be
/// more permissive, in which case they should document this behavior.
abstract class Set<E> extends EfficientLengthIterable<E> {
/// Creates an empty [Set].
///
/// The created [Set] is a plain [LinkedHashSet].
/// As such, it considers elements that are equal (using [operator ==]) to be
/// indistinguishable, and requires them to have a compatible
/// [Object.hashCode] implementation.
///
/// The set is equivalent to one created by `LinkedHashSet<E>()`.
factory Set() = LinkedHashSet<E>;
/// Creates an empty identity [Set].
///
/// The created [Set] is a [LinkedHashSet] that uses identity as equality
/// relation.
///
/// The set is equivalent to one created by `LinkedHashSet<E>.identity()`.
factory Set.identity() = LinkedHashSet<E>.identity;
/// Creates a [Set] that contains all [elements].
///
/// All the [elements] should be instances of [E].
/// The `elements` iterable itself can have any type,
/// so this constructor can be used to down-cast a `Set`, for example as:
/// ```dart
/// Set<SuperType> superSet = ...;
/// Set<SubType> subSet =
/// Set<SubType>.from(superSet.where((e) => e is SubType));
/// ```
/// The created [Set] is a [LinkedHashSet]. As such, it considers elements that
/// are equal (using [operator ==]) to be indistinguishable, and requires them to
/// have a compatible [Object.hashCode] implementation.
///
/// The set is equivalent to one created by
/// `LinkedHashSet<E>.from(elements)`.
factory Set.from(Iterable elements) = LinkedHashSet<E>.from;
/// Creates a [Set] from [elements].
///
/// The created [Set] is a [LinkedHashSet]. As such, it considers elements that
/// are equal (using [operator ==]) to be indistinguishable, and requires them to
/// have a compatible [Object.hashCode] implementation.
///
/// The set is equivalent to one created by
/// `LinkedHashSet<E>.of(elements)`.
factory Set.of(Iterable<E> elements) = LinkedHashSet<E>.of;
/// Creates an unmodifiable [Set] from [elements].
///
/// The new set behaves like the result of [Set.of],
/// except that the set returned by this constructor is not modifiable.
@Since("2.12")
factory Set.unmodifiable(Iterable<E> elements) =>
UnmodifiableSetView<E>(<E>{...elements});
/// Adapts [source] to be a `Set<T>`.
///
/// If [newSet] is provided, it is used to create the new sets returned
/// by [toSet], [union], and is also used for [intersection] and [difference].
/// If [newSet] is omitted, it defaults to creating a new set using the
/// default [Set] constructor, and [intersection] and [difference]
/// returns an adapted version of calling the same method on the source.
///
/// Any time the set would produce an element that is not a [T],
/// the element access will throw.
///
/// Any time a [T] value is attempted added into the adapted set,
/// the store will throw unless the value is also an instance of [S].
///
/// If all accessed elements of [source] are actually instances of [T],
/// and if all elements added to the returned set are actually instance
/// of [S],
/// then the returned set can be used as a `Set<T>`.
static Set<T> castFrom<S, T>(Set<S> source, {Set<R> Function<R>()? newSet}) =>
CastSet<S, T>(source, newSet);
/// Provides a view of this set as a set of [R] instances.
///
/// If this set contains only instances of [R], all read operations
/// will work correctly. If any operation tries to access an element
/// that is not an instance of [R], the access will throw instead.
///
/// Elements added to the set (e.g., by using [add] or [addAll])
/// must be instance of [R] to be valid arguments to the adding function,
/// and they must be instances of [E] as well to be accepted by
/// this set as well.
Set<R> cast<R>();
/// An iterator that iterates over the elements of this set.
///
/// The order of iteration is defined by the individual `Set` implementation,
/// but must be consistent between changes to the set.
Iterator<E> get iterator;
/// Whether [value] is in the set.
bool contains(Object? value);
/// Adds [value] to the set.
///
/// Returns `true` if [value] (or an equal value) was not yet in the set.
/// Otherwise returns `false` and the set is not changed.
///
/// Example:
/// ```dart
/// var set = Set();
/// var time1 = DateTime.fromMillisecondsSinceEpoch(0);
/// var time2 = DateTime.fromMillisecondsSinceEpoch(0);
/// // time1 and time2 are equal, but not identical.
/// assert(time1 == time2);
/// assert(!identical(time1, time2));
/// set.add(time1); // => true.
/// // A value equal to time2 exists already in the set, and the call to
/// // add doesn't change the set.
/// set.add(time2); // => false.
/// assert(set.length == 1);
/// assert(identical(time1, set.first));
/// ```
bool add(E value);
/// Adds all [elements] to this set.
///
/// Equivalent to adding each element in [elements] using [add],
/// but some collections may be able to optimize it.
void addAll(Iterable<E> elements);
/// Removes [value] from the set.
///
/// Returns `true` if [value] was in the set, and `false` if not.
/// The method has no effect if [value] was not in the set.
bool remove(Object? value);
/// If an object equal to [object] is in the set, return it.
///
/// Checks whether [object] is in the set, like [contains], and if so,
/// returns the object in the set, otherwise returns `null`.
///
/// If the equality relation used by the set is not identity,
/// then the returned object may not be *identical* to [object].
/// Some set implementations may not be able to implement this method.
/// If the [contains] method is computed,
/// rather than being based on an actual object instance,
/// then there may not be a specific object instance representing the
/// set element.
E? lookup(Object? object);
/// Removes each element of [elements] from this set.
void removeAll(Iterable<Object?> elements);
/// Removes all elements of this set that are not elements in [elements].
///
/// Checks for each element of [elements] whether there is an element in this
/// set that is equal to it (according to `this.contains`), and if so, the
/// equal element in this set is retained, and elements that are not equal
/// to any element in [elements] are removed.
void retainAll(Iterable<Object?> elements);
/// Removes all elements of this set that satisfy [test].
void removeWhere(bool test(E element));
/// Removes all elements of this set that fail to satisfy [test].
void retainWhere(bool test(E element));
/// Whether this set contains all the elements of [other].
bool containsAll(Iterable<Object?> other);
/// Creates a new set which is the intersection between this set and [other].
///
/// That is, the returned set contains all the elements of this [Set] that
/// are also elements of [other] according to `other.contains`.
Set<E> intersection(Set<Object?> other);
/// Creates a new set which contains all the elements of this set and [other].
///
/// That is, the returned set contains all the elements of this [Set] and
/// all the elements of [other].
Set<E> union(Set<E> other);
/// Creates a new set with the elements of this that are not in [other].
///
/// That is, the returned set contains all the elements of this [Set] that
/// are not elements of [other] according to `other.contains`.
Set<E> difference(Set<Object?> other);
/// Removes all elements from the set.
void clear();
/// Creates a [Set] with the same elements and behavior as this `Set`.
///
/// The returned set behaves the same as this set
/// with regard to adding and removing elements.
/// It initially contains the same elements.
/// If this set specifies an ordering of the elements,
/// the returned set will have the same order.
Set<E> toSet();
}