Cached iterator

cached_iterator.dart

/// Proxy iterator for [origin] that caches the result so that it can be
/// iterated again using [rewind]. Multiple rewinded copies of this iterator
/// share the [sharedCache] so it doesn't matter which one reads from [origin].
class CachedIterator<T> implements Iterator<T> {
  var _i = -1;
  var _terminated = false;
  final Iterator<T> origin;
  final List<T> sharedCache;

  CachedIterator(this.origin, [List<T> cache])
      : sharedCache = cache ?? new List<T>();

  @override
  T get current => _i != -1 && _i < sharedCache.length ? sharedCache[_i] : null;

  @override
  bool moveNext() {
    if (_terminated) {
      return false;
    }
    _i++;
    if (_i < sharedCache.length) {
      return true;
    } else {
      assert(_i == sharedCache.length);
      if (origin.moveNext()) {
        sharedCache.add(origin.current);
        return true;
      } else {
        _terminated = true;
        return false;
      }
    }
  }

  /// Get rewinded copy of this iterator.
  CachedIterator<T> rewind() => new CachedIterator(origin, sharedCache);
}