/**
 * Author: Evgeny Vanslov (evans@yandex-team.ru)
 * Created: 28.11.14
 */
class Filter[A](iterable: Iterable[A], filter: (A) => Boolean) extends Iterable[A] {
  private val iter = iterable
  private val f = filter

  override def iterator: Iterator[A] = new FilterIterator(iter.iterator, f)
}

class FilterIterator[A](iterator: Iterator[A], filter: (A) => Boolean) extends Iterator[A] {
  val it = iterator
  val f = filter
  var nextEl: Option[A] = None

  private def tryNext(): Unit = {
    while (nextEl == None && it.hasNext) {
      val res = it.next()
      if (f(res)) {
        nextEl = Some(res)
      }
    }
  }

  override def hasNext: Boolean = {
    tryNext()
    nextEl.isDefined
  }

  override def next(): A = {
    tryNext()
    if (nextEl == None) {
      throw new NoSuchElementException
    }
    val Some(res) = nextEl
    nextEl = None
    res
  }
}