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// Copyright (c) 2017, 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.
import 'dart:collection';
/// A view of several iterables combined sequentially into a single iterable.
/// All methods and accessors treat the [CombinedIterableView] as if it were a
/// single concatenated iterable, but the underlying implementation is based on
/// lazily accessing individual iterable instances. This means that if the
/// underlying iterables change, the [CombinedIterableView] will reflect those
/// changes.
class CombinedIterableView<T> extends IterableBase<T> {
/// The iterables that this combines.
final Iterable<Iterable<T>> _iterables;
/// Creates a combined view of [iterables].
const CombinedIterableView(this._iterables);
Iterator<T> get iterator =>
_CombinedIterator<T>( => i.iterator).iterator);
// Special cased contains/isEmpty/length since many iterables have an
// efficient implementation instead of running through the entire iterator.
bool contains(Object? element) => _iterables.any((i) => i.contains(element));
bool get isEmpty => _iterables.every((i) => i.isEmpty);
int get length => _iterables.fold(0, (length, i) => length + i.length);
/// The iterator for [CombinedIterableView].
/// This moves through each iterable's iterators in sequence.
class _CombinedIterator<T> implements Iterator<T> {
/// The iterators that this combines.
/// Because this comes from a call to [], it's lazy and will
/// avoid instantiating unnecessary iterators.
final Iterator<Iterator<T>> _iterators;
/// The current iterator in [_iterators], or `null` if done iterating.
Iterator<T>? _currentItr;
_CombinedIterator(this._iterators) {
T get current => _iterators.current.current;
bool moveNext() {
if (_currentItr == null) return false;
if (_currentItr!.moveNext()) {
return true;
} else {
return moveNext();
/// Advances [_currentItr] or sets it to `null` if there are no more entries
/// in [_iterators].
void _advance() {
_currentItr = _iterators.moveNext() ? _iterators.current : null;