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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 values, or "elements", that can be accessed sequentially.
///
/// The elements of the iterable are accessed by getting an [Iterator]
/// using the [iterator] getter, and using it to step through the values.
/// Stepping with the iterator is done by calling [Iterator.moveNext],
/// and if the call returns `true`,
/// the iterator has now moved to the next element,
/// which is then available as [Iterator.current].
/// If the call returns `false`, there are no more elements.
/// The [Iterator.current] value must only be used when the most
/// recent call to [Iterator.moveNext] has returned `true`.
/// If it is used before calling [Iterator.moveNext] the first time
/// on an iterator, or after a call has returned false or has thrown an error,
/// reading [Iterator.current] may throw or may return an arbitrary value.
///
/// You can create more than one iterator from the same `Iterable`.
/// Each time `iterator` is read, it returns a new iterator,
/// and different iterators can be stepped through independently,
/// each giving access to all the elements of the iterable.
/// The iterators of the same iterable *should* provide the same values
/// in the same order (unless the underlying collection is modified between
/// the iterations, which some collections allow).
///
/// You can also iterate over the elements of an `Iterable`
/// using the for-in loop construct, which uses the `iterator` getter behind the
/// scenes.
/// For example, you can iterate over all of the keys of a [Map],
/// because `Map` keys are iterable.
/// ```dart
/// var kidsBooks = {'Matilda': 'Roald Dahl',
/// 'Green Eggs and Ham': 'Dr Seuss',
/// 'Where the Wild Things Are': 'Maurice Sendak'};
/// for (var book in kidsBooks.keys) {
/// print('$book was written by ${kidsBooks[book]}');
/// }
/// ```
/// The [List] and [Set] classes are both `Iterable`,
/// as are most classes in the `dart:collection` library.
///
/// Some [Iterable] collections can be modified.
/// Adding an element to a `List` or `Set` will change which elements it
/// contains, and adding a new key to a `Map` changes the elements of [Map.keys].
/// Iterators created after the change will provide the new elements, and may
/// or may not preserve the order of existing elements
/// (for example, a [HashSet] may completely change its order when a single
/// element is added).
///
/// Changing a collection *while* it is being iterated
/// is generally *not* allowed.
/// Doing so will break the iteration, which is typically signalled
/// by throwing a [ConcurrentModificationError]
/// the next time [Iterator.moveNext] is called.
/// The current value of [Iterator.current] getter
/// should not be affected by the change in the collection,
/// the `current` value was set by the previous call to [Iterator.moveNext].
///
/// Some iterables compute their elements dynamically every time they are
/// iterated, like the one returned by [Iterable.generate] or the iterable
/// returned by a `sync*` generator function. If the computation doesn't depend
/// on other objects that may change, then the generated sequence should be
/// the same one every time it's iterated.
///
/// The members of `Iterable`, other than `iterator` itself,
/// work by looking at the elements of the iterable.
/// This can be implemented by running through the [iterator], but some classes
/// may have more efficient ways of finding the result
/// (like [last] or [length] on a [List], or [contains] on a [Set]).
///
/// The methods that return another `Iterable` (like [map] and [where])
/// are all *lazy* - they will iterate the original (as necessary)
/// every time the returned iterable is iterated, and not before.
///
/// Since an iterable may be iterated more than once, it's not recommended to
/// have detectable side-effects in the iterator.
/// For methods like [map] and [where], the returned iterable will execute the
/// argument function on every iteration, so those functions should also not
/// have side effects.
abstract class Iterable<E> {
// TODO(lrn): When we allow forwarding const constructors through
// mixin applications, make this class implement [IterableMixin].
const Iterable();
/// Creates an `Iterable` which generates its elements dynamically.
///
/// The generated iterable has [count] elements,
/// and the element at index `n` is computed by calling `generator(n)`.
/// Values are not cached, so each iteration computes the values again.
///
/// If [generator] is omitted, it defaults to an identity function
/// on integers `(int x) => x`, so it may only be omitted if the type
/// parameter allows integer values. That is, if [E] is a super-type
/// of [int].
///
/// As an `Iterable`, `Iterable.generate(n, generator))` is equivalent to
/// `const [0, ..., n - 1].map(generator)`.
factory Iterable.generate(int count, [E generator(int index)?]) {
if (count <= 0) return EmptyIterable<E>();
return _GeneratorIterable<E>(count, generator);
}
/// Creates an empty iterable.
///
/// The empty iterable has no elements, and iterating it always stops
/// immediately.
const factory Iterable.empty() = EmptyIterable<E>;
/// Adapts [source] to be an `Iterable<T>`.
///
/// Any time the iterable would produce an element that is not a [T],
/// the element access will throw. If all elements of [source] are actually
/// instances of [T], or if only elements that are actually instances of [T]
/// are accessed, then the resulting iterable can be used as an `Iterable<T>`.
static Iterable<T> castFrom<S, T>(Iterable<S> source) =>
CastIterable<S, T>(source);
/// Returns a new `Iterator` that allows iterating the elements of this
/// `Iterable`.
///
/// Iterable classes may specify the iteration order of their elements
/// (for example [List] always iterate in index order),
/// or they may leave it unspecified (for example a hash-based [Set]
/// may iterate in any order).
///
/// Each time `iterator` is read, it returns a new iterator,
/// which can be used to iterate through all the elements again.
/// The iterators of the same iterable can be stepped through independently,
/// but should return the same elements in the same order,
/// as long as the underlying collection isn't changed.
///
/// Modifying the collection may cause new iterators to produce
/// different elements, and may change the order of existing elements.
/// A [List] specifies its iteration order precisely,
/// so modifying the list changes the iteration order predictably.
/// A hash-based [Set] may change its iteration order completely
/// when adding a new element to the set.
///
/// Modifying the underlying collection after creating the new iterator
/// may cause an error the next time [Iterator.moveNext] is called
/// on that iterator.
/// Any *modifiable* iterable class should specify which operations will
/// break iteration.
Iterator<E> get iterator;
/// Provides a view of this iterable as an iterable of [R] instances.
///
/// If this iterable only contains instances of [R], all operations
/// will work correctly. If any operation tries to access an element
/// that is not an instance of [R], the access will throw instead.
///
/// When the returned iterable creates a new object that depends on
/// the type [R], e.g., from [toList], it will have exactly the type [R].
Iterable<R> cast<R>() => Iterable.castFrom<E, R>(this);
/// Returns the lazy concatenation of this iterable and [other].
///
/// The returned iterable will provide the same elements as this iterable,
/// and, after that, the elements of [other], in the same order as in the
/// original iterables.
Iterable<E> followedBy(Iterable<E> other) {
var self = this; // TODO(lrn): Remove when we can promote `this`.
if (self is EfficientLengthIterable<E>) {
return FollowedByIterable<E>.firstEfficient(self, other);
}
return FollowedByIterable<E>(this, other);
}
/// The current elements of this iterable modified by [toElement].
///
/// Returns a new lazy [Iterable] with elements that are created by
/// calling `toElement` on each element of this `Iterable` in
/// iteration order.
///
/// The returned iterable is lazy, so it won't iterate the elements of
/// this iterable until it is itself iterated, and then it will apply
/// [toElement] to create one element at a time.
/// The converted elements are not cached.
/// Iterating multiple times over the returned [Iterable]
/// will invoke the supplied [toElement] function once per element
/// for on each iteration.
///
/// Methods on the returned iterable are allowed to omit calling `toElement`
/// on any element where the result isn't needed.
/// For example, [elementAt] may call `toElement` only once.
///
/// Equivalent to:
/// ```dart
/// Iterable<T> map<T>(T toElement(E e)) sync* {
/// for (var value in this) {
/// yield toElement(value);
/// }
/// }
/// ```
Iterable<T> map<T>(T toElement(E e)) => MappedIterable<E, T>(this, toElement);
/// Returns a new lazy [Iterable] with all elements that satisfy the
/// predicate [test].
///
/// The matching elements have the same order in the returned iterable
/// as they have in [iterator].
///
/// This method returns a view of the mapped elements.
/// As long as the returned [Iterable] is not iterated over,
/// the supplied function [test] will not be invoked.
/// Iterating will not cache results, and thus iterating multiple times over
/// the returned [Iterable] may invoke the supplied
/// function [test] multiple times on the same element.
Iterable<E> where(bool test(E element)) => WhereIterable<E>(this, test);
/// Returns a new lazy [Iterable] with all elements that have type [T].
///
/// The matching elements have the same order in the returned iterable
/// as they have in [iterator].
///
/// This method returns a view of the mapped elements.
/// Iterating will not cache results, and thus iterating multiple times over
/// the returned [Iterable] may yield different results,
/// if the underlying elements change between iterations.
Iterable<T> whereType<T>() => WhereTypeIterable<T>(this);
/// Expands each element of this [Iterable] into zero or more elements.
///
/// The resulting Iterable runs through the elements returned
/// by [toElements] for each element of this, in iteration order.
///
/// The returned [Iterable] is lazy, and calls [toElements] for each element
/// of this iterable every time the returned iterable is iterated.
///
/// Example:
/// ```dart
/// var pairs = [[1, 2], [3, 4]];
/// var flattened = pairs.expand((pair) => pair).toList();
/// print(flattened); // => [1, 2, 3, 4];
///
/// var input = [1, 2, 3];
/// var duplicated = input.expand((i) => [i, i]).toList();
/// print(duplicated); // => [1, 1, 2, 2, 3, 3]
/// ```
///
/// Equivalent to:
/// ```dart
/// Iterable<T> expand<T>(Iterable<T> toElements(E e)) sync* {
/// for (var value in this) {
/// yield* toElements(value);
/// }
/// }
/// ```
Iterable<T> expand<T>(Iterable<T> toElements(E element)) =>
ExpandIterable<E, T>(this, toElements);
/// Whether the collection contains an element equal to [element].
///
/// This operation will check each element in order for being equal to
/// [element], unless it has a more efficient way to find an element
/// equal to [element].
///
/// The equality used to determine whether [element] is equal to an element of
/// the iterable defaults to the [Object.==] of the element.
///
/// Some types of iterable may have a different equality used for its elements.
/// For example, a [Set] may have a custom equality
/// (see [Set.identity]) that its `contains` uses.
/// Likewise the `Iterable` returned by a [Map.keys] call
/// should use the same equality that the `Map` uses for keys.
bool contains(Object? element) {
for (E e in this) {
if (e == element) return true;
}
return false;
}
/// Invokes [action] on each element of this iterable in iteration order.
void forEach(void action(E element)) {
for (E element in this) action(element);
}
/// Reduces a collection to a single value by iteratively combining elements
/// of the collection using the provided function.
///
/// The iterable must have at least one element.
/// If it has only one element, that element is returned.
///
/// Otherwise this method starts with the first element from the iterator,
/// and then combines it with the remaining elements in iteration order,
/// as if by:
/// ```dart
/// E value = iterable.first;
/// iterable.skip(1).forEach((element) {
/// value = combine(value, element);
/// });
/// return value;
/// ```
/// Example of calculating the sum of an iterable:
/// ```dart
/// iterable.reduce((value, element) => value + element);
/// ```
E reduce(E combine(E value, E element)) {
Iterator<E> iterator = this.iterator;
if (!iterator.moveNext()) {
throw IterableElementError.noElement();
}
E value = iterator.current;
while (iterator.moveNext()) {
value = combine(value, iterator.current);
}
return value;
}
/// Reduces a collection to a single value by iteratively combining each
/// element of the collection with an existing value
///
/// Uses [initialValue] as the initial value,
/// then iterates through the elements and updates the value with
/// each element using the [combine] function, as if by:
/// ```dart
/// var value = initialValue;
/// for (E element in this) {
/// value = combine(value, element);
/// }
/// return value;
/// ```
/// Example of calculating the sum of an iterable:
/// ```dart
/// iterable.fold(0, (prev, element) => prev + element);
/// ```
T fold<T>(T initialValue, T combine(T previousValue, E element)) {
var value = initialValue;
for (E element in this) value = combine(value, element);
return value;
}
/// Checks whether every element of this iterable satisfies [test].
///
/// Checks every element in iteration order, and returns `false` if
/// any of them make [test] return `false`, otherwise returns `true`.
bool every(bool test(E element)) {
for (E element in this) {
if (!test(element)) return false;
}
return true;
}
/// Converts each element to a [String] and concatenates the strings.
///
/// Iterates through elements of this iterable,
/// converts each one to a [String] by calling [Object.toString],
/// and then concatenates the strings, with the
/// [separator] string interleaved between the elements.
String join([String separator = ""]) {
Iterator<E> iterator = this.iterator;
if (!iterator.moveNext()) return "";
StringBuffer buffer = StringBuffer();
if (separator == null || separator == "") {
do {
buffer.write(iterator.current.toString());
} while (iterator.moveNext());
} else {
buffer.write(iterator.current.toString());
while (iterator.moveNext()) {
buffer.write(separator);
buffer.write(iterator.current.toString());
}
}
return buffer.toString();
}
/// Checks whether any element of this iterable satisfies [test].
///
/// Checks every element in iteration order, and returns `true` if
/// any of them make [test] return `true`, otherwise returns false.
bool any(bool test(E element)) {
for (E element in this) {
if (test(element)) return true;
}
return false;
}
/// Creates a [List] containing the elements of this [Iterable].
///
/// The elements are in iteration order.
/// The list is fixed-length if [growable] is false.
List<E> toList({bool growable = true}) {
return List<E>.of(this, growable: growable);
}
/// Creates a [Set] containing the same elements as this iterable.
///
/// The set may contain fewer elements than the iterable,
/// if the iterable contains an element more than once,
/// or it contains one or more elements that are equal.
/// The order of the elements in the set is not guaranteed to be the same
/// as for the iterable.
Set<E> toSet() => Set<E>.of(this);
/// Returns the number of elements in [this].
///
/// Counting all elements may involve iterating through all elements and can
/// therefore be slow.
/// Some iterables have a more efficient way to find the number of elements.
int get length {
assert(this is! EfficientLengthIterable);
int count = 0;
Iterator it = iterator;
while (it.moveNext()) {
count++;
}
return count;
}
/// Returns `true` if there are no elements in this collection.
///
/// May be computed by checking if `iterator.moveNext()` returns `false`.
bool get isEmpty => !iterator.moveNext();
/// Returns true if there is at least one element in this collection.
///
/// May be computed by checking if `iterator.moveNext()` returns `true`.
bool get isNotEmpty => !isEmpty;
/// Returns a lazy iterable of the [count] first elements of this iterable.
///
/// The returned `Iterable` may contain fewer than `count` elements, if `this`
/// contains fewer than `count` elements.
///
/// The elements can be computed by stepping through [iterator] until [count]
/// elements have been seen.
///
/// The `count` must not be negative.
Iterable<E> take(int count) {
return TakeIterable<E>(this, count);
}
/// Returns a lazy iterable of the leading elements satisfying [test].
///
/// The filtering happens lazily. Every new iterator of the returned
/// iterable starts iterating over the elements of `this`.
///
/// The elements can be computed by stepping through [iterator] until an
/// element is found where `test(element)` is false. At that point,
/// the returned iterable stops (its `moveNext()` returns false).
Iterable<E> takeWhile(bool test(E value)) {
return TakeWhileIterable<E>(this, test);
}
/// Returns an [Iterable] that provides all but the first [count] elements.
///
/// When the returned iterable is iterated, it starts iterating over `this`,
/// first skipping past the initial [count] elements.
/// If `this` has fewer than `count` elements, then the resulting Iterable is
/// empty.
/// After that, the remaining elements are iterated in the same order as
/// in this iterable.
///
/// Some iterables may be able to find later elements without first iterating
/// through earlier elements, for example when iterating a [List].
/// Such iterables are allowed to ignore the initial skipped elements.
///
/// The [count] must not be negative.
Iterable<E> skip(int count) {
return SkipIterable<E>(this, count);
}
/// Returns an `Iterable` that skips leading elements while [test] is satisfied.
///
/// The filtering happens lazily. Every new [Iterator] of the returned
/// iterable iterates over all elements of `this`.
///
/// The returned iterable provides elements by iterating this iterable,
/// but skipping over all initial elements where `test(element)` returns
/// true. If all elements satisfy `test` the resulting iterable is empty,
/// otherwise it iterates the remaining elements in their original order,
/// starting with the first element for which `test(element)` returns `false`.
Iterable<E> skipWhile(bool test(E value)) {
return SkipWhileIterable<E>(this, test);
}
/// Returns the first element.
///
/// Throws a [StateError] if `this` is empty.
/// Otherwise returns the first element in the iteration order,
/// equivalent to `this.elementAt(0)`.
E get first {
Iterator<E> it = iterator;
if (!it.moveNext()) {
throw IterableElementError.noElement();
}
return it.current;
}
/// Returns the last element.
///
/// Throws a [StateError] if `this` is empty.
/// Otherwise may iterate through the elements and returns the last one
/// seen.
/// Some iterables may have more efficient ways to find the last element
/// (for example a list can directly access the last element,
/// without iterating through the previous ones).
E get last {
Iterator<E> it = iterator;
if (!it.moveNext()) {
throw IterableElementError.noElement();
}
E result;
do {
result = it.current;
} while (it.moveNext());
return result;
}
/// Checks that this iterable has only one element, and returns that element.
///
/// Throws a [StateError] if `this` is empty or has more than one element.
E get single {
Iterator<E> it = iterator;
if (!it.moveNext()) throw IterableElementError.noElement();
E result = it.current;
if (it.moveNext()) throw IterableElementError.tooMany();
return result;
}
/// Returns the first element that satisfies the given predicate [test].
///
/// Iterates through elements and returns the first to satisfy [test].
///
/// If no element satisfies [test], the result of invoking the [orElse]
/// function is returned.
/// If [orElse] is omitted, it defaults to throwing a [StateError].
E firstWhere(bool test(E element), {E orElse()?}) {
for (E element in this) {
if (test(element)) return element;
}
if (orElse != null) return orElse();
throw IterableElementError.noElement();
}
/// Returns the last element that satisfies the given predicate [test].
///
/// An iterable that can access its elements directly may check its
/// elements in any order (for example a list starts by checking the
/// last element and then moves towards the start of the list).
/// The default implementation iterates elements in iteration order,
/// checks `test(element)` for each,
/// and finally returns that last one that matched.
///
/// If no element satisfies [test], the result of invoking the [orElse]
/// function is returned.
/// If [orElse] is omitted, it defaults to throwing a [StateError].
E lastWhere(bool test(E element), {E orElse()?}) {
late E result;
bool foundMatching = false;
for (E element in this) {
if (test(element)) {
result = element;
foundMatching = true;
}
}
if (foundMatching) return result;
if (orElse != null) return orElse();
throw IterableElementError.noElement();
}
/// Returns the single element that satisfies [test].
///
/// Checks elements to see if `test(element)` returns true.
/// If exactly one element satisfies [test], that element is returned.
/// If more than one matching element is found, throws [StateError].
/// If no matching element is found, returns the result of [orElse].
/// If [orElse] is omitted, it defaults to throwing a [StateError].
E singleWhere(bool test(E element), {E orElse()?}) {
late E result;
bool foundMatching = false;
for (E element in this) {
if (test(element)) {
if (foundMatching) {
throw IterableElementError.tooMany();
}
result = element;
foundMatching = true;
}
}
if (foundMatching) return result;
if (orElse != null) return orElse();
throw IterableElementError.noElement();
}
/// Returns the [index]th element.
///
/// The [index] must be non-negative and less than [length].
/// Index zero represents the first element (so `iterable.elementAt(0)` is
/// equivalent to `iterable.first`).
///
/// May iterate through the elements in iteration order, ignoring the
/// first [index] elements and then returning the next.
/// Some iterables may have a more efficient way to find the element.
E elementAt(int index) {
RangeError.checkNotNegative(index, "index");
int elementIndex = 0;
for (E element in this) {
if (index == elementIndex) return element;
elementIndex++;
}
throw RangeError.index(index, this, "index", null, elementIndex);
}
/// Returns a string representation of (some of) the elements of `this`.
///
/// Elements are represented by their own `toString` results.
///
/// The default representation always contains the first three elements.
/// If there are less than a hundred elements in the iterable, it also
/// contains the last two elements.
///
/// If the resulting string isn't above 80 characters, more elements are
/// included from the start of the iterable.
///
/// The conversion may omit calling `toString` on some elements if they
/// are known to not occur in the output, and it may stop iterating after
/// a hundred elements.
String toString() => IterableBase.iterableToShortString(this, '(', ')');
}
class _GeneratorIterable<E> extends ListIterable<E> {
/// The length of the generated iterable.
final int length;
/// The function mapping indices to values.
final E Function(int) _generator;
/// Creates the generated iterable.
///
/// If [generator] is `null`, it is checked that `int` is assignable to [E].
_GeneratorIterable(this.length, E generator(int index)?)
: // The `as` below is used as check to make sure that `int` is assignable
// to [E].
_generator = generator ?? (_id as E Function(int));
E elementAt(int index) {
RangeError.checkValidIndex(index, this);
return _generator(index);
}
/// Helper function used as default _generator function.
static int _id(int n) => n;
}
/// An [Iterator] that allows moving backwards as well as forwards.
abstract class BidirectionalIterator<E> implements Iterator<E> {
/// Move back to the previous element.
///
/// Returns true and updates [current] if successful. Returns false
/// and updates [current] to an implementation defined state if there is no
/// previous element
bool movePrevious();
}