/**
 * 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] = {
    val it = iter.iterator
    new Iterator[A] {
      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
      }
    }
  }
}