// 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>(_iterables.map((i) => 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 [Iterable.map], it's lazy and will
  /// avoid instantiating unnecessary iterators.
  final Iterator<Iterator<T>> _iterators;

  _CombinedIterator(this._iterators);

  T get current => _iterators.current?.current;

  bool moveNext() {
    var current = _iterators.current;
    if (current != null && current.moveNext()) {
      return true;
    }
    return _iterators.moveNext() && moveNext();
  }
}