sdk/pkg/sequence_zip/lib/iterable_zip.dart

// 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.

/**
 * Help for combining multiple Iterables into a single Iterable.
 *
 * This API is also available as part of the
 * [sequence_zip](#sequence_zip) library.
 */
library iterable_zip;

import "dart:collection";

/**
 * Iterable that iterates over lists of values from other iterables.
 *
 * When [iterator] is read, an [Iterator] is created for each [Iterable] in
 * the [Iterable] passed to the constructor.
 *
 * As long as all these iterators have a next value, those next values are
 * combined into a single list, which becomes the next value of this
 * [Iterable]'s [Iterator]. As soon as any of the iterators run out,
 * the zipped iterator also stops.
 */
class IterableZip extends IterableBase<List> {
  final Iterable<Iterable> _iterables;
  IterableZip(Iterable<Iterable> iterables)
      : this._iterables = iterables;

  /**
   * Returns an iterator that combines values of the iterables' iterators
   * as long as they all have values.
   */
  Iterator<List> get iterator {
    List iterators = _iterables.map((x) => x.iterator).toList(growable: false);
    // TODO(lrn): Return an empty iterator directly if iterators is empty?
    return new _IteratorZip(iterators);
  }
}

class _IteratorZip implements Iterator<List> {
  final List<Iterator> _iterators;
  List _current;
  _IteratorZip(List iterators) : _iterators = iterators;
  bool moveNext() {
    if (_iterators.isEmpty) return false;
    for (int i = 0; i < _iterators.length; i++) {
      if (!_iterators[i].moveNext()) {
        _current = null;
        return false;
      }
    }
    _current = new List(_iterators.length);
    for (int i = 0; i < _iterators.length; i++) {
      _current[i] = _iterators[i].current;
    }
    return true;
  }

  List get current => _current;
}