LookAheadIterator

LookAheadIterator.scala

/**
 * An iterator that allows retrieving elements ahead without advancing the iterator.
 */
class LookAheadIterator[T](underlying:Iterator[T]) extends BufferedIterator[T] {
  
  private val queue = collection.mutable.Queue[T]()
  
  /**
   * Returns a new iterator to read ahead element. 
   * Elements from underlying are queued so that they can be returned by the
   * current iterator.   
   */
  def lookahead() = queue.iterator ++ new Iterator[T] {
    def hasNext = underlying.hasNext
    def next() = { val x = underlying.next(); queue += x; x }
  }
  
  def hasNext = underlying.hasNext || !queue.isEmpty
  def next() = if (!queue.isEmpty) queue.dequeue() else underlying.next()
  
  def head = if (queue.isEmpty) { 
    val x = underlying.next(); queue += x; x 
  } else {
    queue.head
  }
  
  def headOption = if (hasNext) Some(head) else None
  
  def takeUntil(p: T => Boolean): Iterator[T] = {
    val buf = collection.mutable.ArrayBuffer[T]()
    val elems = lookahead().takeWhile(t => !p(t))
    buf ++= elems
    (1 to buf.size) foreach { _ => next() }
    buf.iterator
  }
  
}