Scala example source code file (Iterator.scala)

This example Scala source code file (Iterator.scala) is included in my "Source Code Warehouse" project. The intent of this project is to help you more easily find Scala source code examples by using tags.

All credit for the original source code belongs to scala-lang.org; I'm just trying to make examples easier to find. (For my Scala work, see my Scala examples and tutorials.)

Scala tags/keywords

a, abstractiterator, annotation, b, b1, boolean, collection, generics, gentraversableonce, groupediterator, int, iterator, t

The Iterator.scala Scala example source code

/*                     __                                               *\
**     ________ ___   / /  ___     Scala API                            **
**    / __/ __// _ | / /  / _ |    (c) 2003-2013, LAMP/EPFL             **
**  __\ \/ /__/ __ |/ /__/ __ |    http://scala-lang.org/               **
** /____/\___/_/ |_/____/_/ | |                                         **
**                          |/                                          **
\*                                                                      */

package scala
package collection

import mutable.ArrayBuffer
import scala.annotation.migration
import immutable.Stream
import scala.collection.generic.CanBuildFrom
import scala.annotation.unchecked.{ uncheckedVariance => uV }

/** The `Iterator` object provides various functions for creating specialized iterators.
 *
 *  @author  Martin Odersky
 *  @author  Matthias Zenger
 *  @version 2.8
 *  @since   2.8
 */
object Iterator {

  /** With the advent of `TraversableOnce` and `Iterator`, it can be useful to have a builder which
   *  operates on `Iterator`s so they can be treated uniformly along with the collections.
   *  See `scala.util.Random.shuffle` for an example.
   */
  implicit def IteratorCanBuildFrom[A] = new TraversableOnce.BufferedCanBuildFrom[A, Iterator] {
    def bufferToColl[B](coll: ArrayBuffer[B]) = coll.iterator
    def traversableToColl[B](t: GenTraversable[B]) = t.toIterator
  }

  /** The iterator which produces no values. */
  val empty: Iterator[Nothing] = new AbstractIterator[Nothing] {
    def hasNext: Boolean = false
    def next(): Nothing = throw new NoSuchElementException("next on empty iterator")
  }

  /** Creates an iterator which produces a single element.
   *  '''Note:''' Equivalent, but more efficient than Iterator(elem)
   *
   *  @param elem the element
   *  @return An iterator which produces `elem` on the first call to `next`,
   *          and which has no further elements.
   */
  def single[A](elem: A): Iterator[A] = new AbstractIterator[A] {
    private var hasnext = true
    def hasNext: Boolean = hasnext
    def next(): A =
      if (hasnext) { hasnext = false; elem }
      else empty.next()
  }

  /** Creates an iterator with given elements.
   *
   *  @param elems  The elements returned one-by-one from the iterator
   *  @return An iterator which produces the given elements on the
   *          first calls to `next`, and which has no further elements.
   */
  def apply[A](elems: A*): Iterator[A] = elems.iterator

  /** Creates iterator that produces the results of some element computation a number of times.
   *
   *  @param   len  the number of elements returned by the iterator.
   *  @param   elem the element computation
   *  @return  An iterator that produces the results of `n` evaluations of `elem`.
   */
  def fill[A](len: Int)(elem: => A): Iterator[A] = new AbstractIterator[A] {
    private var i = 0
    def hasNext: Boolean = i < len
    def next(): A =
      if (hasNext) { i += 1; elem }
      else empty.next()
  }

  /** Creates an iterator producing the values of a given function over a range of integer values starting from 0.
   *
   *  @param  end The number of elements returned by the iterator
   *  @param  f   The function computing element values
   *  @return An iterator that produces the values `f(0), ..., f(n -1)`.
   */
  def tabulate[A](end: Int)(f: Int => A): Iterator[A] = new AbstractIterator[A] {
    private var i = 0
    def hasNext: Boolean = i < end
    def next(): A =
      if (hasNext) { val result = f(i); i += 1; result }
      else empty.next()
  }

  /** Creates nn iterator returning successive values in some integer interval.
   *
   *  @param start the start value of the iterator
   *  @param end   the end value of the iterator (the first value NOT returned)
   *  @return      the iterator producing values `start, start + 1, ..., end - 1`
   */
  def range(start: Int, end: Int): Iterator[Int] = range(start, end, 1)

  /** An iterator producing equally spaced values in some integer interval.
   *
   *  @param start the start value of the iterator
   *  @param end   the end value of the iterator (the first value NOT returned)
   *  @param step  the increment value of the iterator (must be positive or negative)
   *  @return      the iterator producing values `start, start + step, ...` up to, but excluding `end`
   */
  def range(start: Int, end: Int, step: Int): Iterator[Int] = new AbstractIterator[Int] {
    if (step == 0) throw new IllegalArgumentException("zero step")
    private var i = start
    def hasNext: Boolean = (step <= 0 || i < end) && (step >= 0 || i > end)
    def next(): Int =
      if (hasNext) { val result = i; i += step; result }
      else empty.next()
  }

  /** Creates an infinite iterator that repeatedly applies a given function to the previous result.
   *
   *  @param start the start value of the iterator
   *  @param f     the function that's repeatedly applied
   *  @return      the iterator producing the infinite sequence of values `start, f(start), f(f(start)), ...`
   */
  def iterate[T](start: T)(f: T => T): Iterator[T] = new AbstractIterator[T] {
    private[this] var first = true
    private[this] var acc = start
    def hasNext: Boolean = true
    def next(): T = {
      if (first) first = false
      else acc = f(acc)

      acc
    }
  }

  /** Creates an infinite-length iterator which returns successive values from some start value.

   *  @param start the start value of the iterator
   *  @return      the iterator producing the infinite sequence of values `start, start + 1, start + 2, ...`
   */
  def from(start: Int): Iterator[Int] = from(start, 1)

  /** Creates an infinite-length iterator returning values equally spaced apart.
   *
   *  @param start the start value of the iterator
   *  @param step  the increment between successive values
   *  @return      the iterator producing the infinite sequence of values `start, start + 1 * step, start + 2 * step, ...`
   */
  def from(start: Int, step: Int): Iterator[Int] = new AbstractIterator[Int] {
    private var i = start
    def hasNext: Boolean = true
    def next(): Int = { val result = i; i += step; result }
  }

  /** Creates an infinite-length iterator returning the results of evaluating an expression.
   *  The expression is recomputed for every element.
   *
   *  @param elem the element computation.
   *  @return the iterator containing an infinite number of results of evaluating `elem`.
   */
  def continually[A](elem: => A): Iterator[A] = new AbstractIterator[A] {
    def hasNext = true
    def next = elem
  }

  /** Avoid stack overflows when applying ++ to lots of iterators by
   *  flattening the unevaluated iterators out into a vector of closures.
   */
  private[scala] final class ConcatIterator[+A](private[this] var current: Iterator[A], initial: Vector[() => Iterator[A]]) extends Iterator[A] {
    @deprecated def this(initial: Vector[() => Iterator[A]]) = this(Iterator.empty, initial) // for binary compatibility
    private[this] var queue: Vector[() => Iterator[A]] = initial
    // Advance current to the next non-empty iterator
    // current is set to null when all iterators are exhausted
    private[this] def advance(): Boolean = {
      if (queue.isEmpty) {
        current = null
        false
      }
      else {
        current = queue.head()
        queue = queue.tail
        current.hasNext || advance()
      }
    }
    def hasNext = (current ne null) && (current.hasNext || advance())
    def next()  = if (hasNext) current.next else Iterator.empty.next

    override def ++[B >: A](that: => GenTraversableOnce[B]): Iterator[B] =
      new ConcatIterator(current, queue :+ (() => that.toIterator))
  }

  private[scala] final class JoinIterator[+A](lhs: Iterator[A], that: => GenTraversableOnce[A]) extends Iterator[A] {
    private[this] lazy val rhs: Iterator[A] = that.toIterator
    def hasNext = lhs.hasNext || rhs.hasNext
    def next    = if (lhs.hasNext) lhs.next else rhs.next

    override def ++[B >: A](that: => GenTraversableOnce[B]) =
      new ConcatIterator(this, Vector(() => that.toIterator))
  }
}

import Iterator.empty

/** Iterators are data structures that allow to iterate over a sequence
 *  of elements. They have a `hasNext` method for checking
 *  if there is a next element available, and a `next` method
 *  which returns the next element and discards it from the iterator.
 *
 *  An iterator is mutable: most operations on it change its state. While it is often used
 *  to iterate through the elements of a collection, it can also be used without
 *  being backed by any collection (see constructors on the companion object).
 *
 *  It is of particular importance to note that, unless stated otherwise, ''one should never
 *  use an iterator after calling a method on it''. The two most important exceptions
 *  are also the sole abstract methods: `next` and `hasNext`.
 *
 *  Both these methods can be called any number of times without having to discard the
 *  iterator. Note that even `hasNext` may cause mutation -- such as when iterating
 *  from an input stream, where it will block until the stream is closed or some
 *  input becomes available.
 *
 *  Consider this example for safe and unsafe use:
 *
 *  {{{
 *  def f[A](it: Iterator[A]) = {
 *    if (it.hasNext) {            // Safe to reuse "it" after "hasNext"
 *      it.next                    // Safe to reuse "it" after "next"
 *      val remainder = it.drop(2) // it is *not* safe to use "it" again after this line!
 *      remainder.take(2)          // it is *not* safe to use "remainder" after this line!
 *    } else it
 *  }
 *  }}}
 *
 *  @author  Martin Odersky, Matthias Zenger
 *  @version 2.8
 *  @since   1
 *  @define willNotTerminateInf
 *  Note: will not terminate for infinite iterators.
 *  @define mayNotTerminateInf
 *  Note: may not terminate for infinite iterators.
 *  @define preservesIterator
 *  The iterator remains valid for further use whatever result is returned.
 *  @define consumesIterator
 *  After calling this method, one should discard the iterator it was called
 *  on. Using it is undefined and subject to change.
 *  @define consumesAndProducesIterator
 *  After calling this method, one should discard the iterator it was called
 *  on, and use only the iterator that was returned. Using the old iterator
 *  is undefined, subject to change, and may result in changes to the new
 *  iterator as well.
 *  @define consumesTwoAndProducesOneIterator
 *  After calling this method, one should discard the iterator it was called
 *  on, as well as the one passed as a parameter, and use only the iterator
 *  that was returned. Using the old iterators is undefined, subject to change,
 *  and may result in changes to the new iterator as well.
 *  @define consumesOneAndProducesTwoIterators
 *  After calling this method, one should discard the iterator it was called
 *  on, and use only the iterators that were returned. Using the old iterator
 *  is undefined, subject to change, and may result in changes to the new
 *  iterators as well.
 *  @define consumesTwoIterators
 *  After calling this method, one should discard the iterator it was called
 *  on, as well as the one passed as parameter. Using the old iterators is
 *  undefined and subject to change.
 */
trait Iterator[+A] extends TraversableOnce[A] {
  self =>

  def seq: Iterator[A] = this

  /** Tests whether this iterator can provide another element.
   *
   *  @return  `true` if a subsequent call to `next` will yield an element,
   *           `false` otherwise.
   *  @note    Reuse: $preservesIterator
   */
  def hasNext: Boolean

  /** Produces the next element of this iterator.
   *
   *  @return  the next element of this iterator, if `hasNext` is `true`,
   *           undefined behavior otherwise.
   *  @note    Reuse: $preservesIterator
   */
  def next(): A

  /** Tests whether this iterator is empty.
   *
   *  @return   `true` if hasNext is false, `false` otherwise.
   *  @note     Reuse: $preservesIterator
   */
  def isEmpty: Boolean = !hasNext

  /** Tests whether this Iterator can be repeatedly traversed.
   *
   *  @return   `false`
   *  @note     Reuse: $preservesIterator
   */
  def isTraversableAgain = false

  /** Tests whether this Iterator has a known size.
   *
   *  @return   `true` for empty Iterators, `false` otherwise.
   *  @note     Reuse: $preservesIterator
   */
  def hasDefiniteSize = isEmpty

  /** Selects first ''n'' values of this iterator.
   *
   *  @param  n    the number of values to take
   *  @return an iterator producing only of the first `n` values of this iterator, or else the
   *          whole iterator, if it produces fewer than `n` values.
   *  @note   Reuse: $consumesAndProducesIterator
   */
  def take(n: Int): Iterator[A] = slice(0, n)

  /** Advances this iterator past the first ''n'' elements, or the length of the iterator, whichever is smaller.
   *
   *  @param n the number of elements to drop
   *  @return  an iterator which produces all values of the current iterator, except
   *           it omits the first `n` values.
   *  @note    Reuse: $consumesAndProducesIterator
   */
  def drop(n: Int): Iterator[A] = slice(n, Int.MaxValue)

  /** Creates an iterator returning an interval of the values produced by this iterator.
   *
   *  @param from   the index of the first element in this iterator which forms part of the slice.
   *  @param until  the index of the first element following the slice.
   *  @return an iterator which advances this iterator past the first `from` elements using `drop`,
   *  and then takes `until - from` elements, using `take`.
   *  @note         Reuse: $consumesAndProducesIterator
   */
  def slice(from: Int, until: Int): Iterator[A] = {
    val lo = from max 0
    var toDrop = lo
    while (toDrop > 0 && self.hasNext) {
      self.next()
      toDrop -= 1
    }

    new AbstractIterator[A] {
      private var remaining = until - lo
      def hasNext = remaining > 0 && self.hasNext
      def next(): A =
        if (remaining > 0) {
          remaining -= 1
          self.next()
        }
        else empty.next()
    }
  }

  /** Creates a new iterator that maps all produced values of this iterator
   *  to new values using a transformation function.
   *
   *  @param f  the transformation function
   *  @return a new iterator which transforms every value produced by this
   *          iterator by applying the function `f` to it.
   *  @note   Reuse: $consumesAndProducesIterator
   */
  def map[B](f: A => B): Iterator[B] = new AbstractIterator[B] {
    def hasNext = self.hasNext
    def next() = f(self.next())
  }

  /** Concatenates this iterator with another.
   *
   *  @param   that   the other iterator
   *  @return  a new iterator that first yields the values produced by this
   *  iterator followed by the values produced by iterator `that`.
   *  @note    Reuse: $consumesTwoAndProducesOneIterator
   *
   *  @usecase def ++(that: => Iterator[A]): Iterator[A]
   *    @inheritdoc
   */
  def ++[B >: A](that: => GenTraversableOnce[B]): Iterator[B] = new Iterator.JoinIterator(self, that)

  /** Creates a new iterator by applying a function to all values produced by this iterator
   *  and concatenating the results.
   *
   *  @param f the function to apply on each element.
   *  @return  the iterator resulting from applying the given iterator-valued function
   *           `f` to each value produced by this iterator and concatenating the results.
   *  @note    Reuse: $consumesAndProducesIterator
   */
  def flatMap[B](f: A => GenTraversableOnce[B]): Iterator[B] = new AbstractIterator[B] {
    private var cur: Iterator[B] = empty
    def hasNext: Boolean =
      cur.hasNext || self.hasNext && { cur = f(self.next()).toIterator; hasNext }
    def next(): B = (if (hasNext) cur else empty).next()
  }

  /** Returns an iterator over all the elements of this iterator that satisfy the predicate `p`.
   *  The order of the elements is preserved.
   *
   *  @param p the predicate used to test values.
   *  @return  an iterator which produces those values of this iterator which satisfy the predicate `p`.
   *  @note    Reuse: $consumesAndProducesIterator
   */
  def filter(p: A => Boolean): Iterator[A] = new AbstractIterator[A] {
    private var hd: A = _
    private var hdDefined: Boolean = false

    def hasNext: Boolean = hdDefined || {
      do {
        if (!self.hasNext) return false
        hd = self.next()
      } while (!p(hd))
      hdDefined = true
      true
    }

    def next() = if (hasNext) { hdDefined = false; hd } else empty.next()
  }

  /** Tests whether every element of this iterator relates to the
   *  corresponding element of another collection by satisfying a test predicate.
   *
   *  @param   that    the other collection
   *  @param   p       the test predicate, which relates elements from both collections
   *  @tparam  B       the type of the elements of `that`
   *  @return          `true` if both collections have the same length and
   *                   `p(x, y)` is `true` for all corresponding elements `x` of this iterator
   *                   and `y` of `that`, otherwise `false`
   */
  def corresponds[B](that: GenTraversableOnce[B])(p: (A, B) => Boolean): Boolean = {
    val that0 = that.toIterator
    while (hasNext && that0.hasNext)
      if (!p(next(), that0.next())) return false

    hasNext == that0.hasNext
  }

  /** Creates an iterator over all the elements of this iterator that
   *  satisfy the predicate `p`. The order of the elements
   *  is preserved.
   *
   *  '''Note:''' `withFilter` is the same as `filter` on iterators. It exists so that
   *  for-expressions with filters work over iterators.
   *
   *  @param p the predicate used to test values.
   *  @return  an iterator which produces those values of this iterator which satisfy the predicate `p`.
   *  @note    Reuse: $consumesAndProducesIterator
   */
  def withFilter(p: A => Boolean): Iterator[A] = filter(p)

  /** Creates an iterator over all the elements of this iterator which
   *  do not satisfy a predicate p.
   *
   *  @param p the predicate used to test values.
   *  @return  an iterator which produces those values of this iterator which do not satisfy the predicate `p`.
   *  @note    Reuse: $consumesAndProducesIterator
   */
  def filterNot(p: A => Boolean): Iterator[A] = filter(!p(_))

 /** Creates an iterator by transforming values
  *  produced by this iterator with a partial function, dropping those
  *  values for which the partial function is not defined.
  *
  *  @param pf the partial function which filters and maps the iterator.
  *  @return   a new iterator which yields each value `x` produced by this iterator for
  *  which `pf` is defined the image `pf(x)`.
  *  @note     Reuse: $consumesAndProducesIterator
  */
  @migration("`collect` has changed. The previous behavior can be reproduced with `toSeq`.", "2.8.0")
  def collect[B](pf: PartialFunction[A, B]): Iterator[B] = {
    val self = buffered
    new AbstractIterator[B] {
      private def skip() = while (self.hasNext && !pf.isDefinedAt(self.head)) self.next()
      def hasNext = { skip(); self.hasNext }
      def next() = { skip(); pf(self.next()) }
    }
  }

  /** Produces a collection containing cummulative results of applying the
   *  operator going left to right.
   *
   *  $willNotTerminateInf
   *  $orderDependent
   *
   *  @tparam B      the type of the elements in the resulting collection
   *  @param z       the initial value
   *  @param op      the binary operator applied to the intermediate result and the element
   *  @return        iterator with intermediate results
   *  @note          Reuse: $consumesAndProducesIterator
   */
  def scanLeft[B](z: B)(op: (B, A) => B): Iterator[B] = new AbstractIterator[B] {
    var hasNext = true
    var elem = z
    def next() = if (hasNext) {
      val res = elem
      if (self.hasNext) elem = op(elem, self.next())
      else hasNext = false
      res
    } else Iterator.empty.next()
  }

  /** Produces a collection containing cummulative results of applying the operator going right to left.
   *  The head of the collection is the last cummulative result.
   *
   *  $willNotTerminateInf
   *  $orderDependent
   *
   *  @tparam B      the type of the elements in the resulting collection
   *  @param z       the initial value
   *  @param op      the binary operator applied to the intermediate result and the element
   *  @return        iterator with intermediate results
   *  @example       {{{
   *    Iterator(1, 2, 3, 4).scanRight(0)(_ + _).toList == List(10, 9, 7, 4, 0)
   *  }}}
   *  @note          Reuse: $consumesAndProducesIterator
   */
  def scanRight[B](z: B)(op: (A, B) => B): Iterator[B] = toBuffer.scanRight(z)(op).iterator

  /** Takes longest prefix of values produced by this iterator that satisfy a predicate.
   *
   *  @param   p  The predicate used to test elements.
   *  @return  An iterator returning the values produced by this iterator, until
   *           this iterator produces a value that does not satisfy
   *           the predicate `p`.
   *  @note    Reuse: $consumesAndProducesIterator
   */
  def takeWhile(p: A => Boolean): Iterator[A] = new AbstractIterator[A] {
    private var hd: A = _
    private var hdDefined: Boolean = false
    private var tail: Iterator[A] = self

    def hasNext = hdDefined || tail.hasNext && {
      hd = tail.next()
      if (p(hd)) hdDefined = true
      else tail = Iterator.empty
      hdDefined
    }
    def next() = if (hasNext) { hdDefined = false; hd } else empty.next()
  }

  /** Partitions this iterator in two iterators according to a predicate.
   *
   *  @param p the predicate on which to partition
   *  @return  a pair of iterators: the iterator that satisfies the predicate
   *           `p` and the iterator that does not.
   *           The relative order of the elements in the resulting iterators
   *           is the same as in the original iterator.
   *  @note    Reuse: $consumesOneAndProducesTwoIterators
   */
  def partition(p: A => Boolean): (Iterator[A], Iterator[A]) = {
    val self = buffered
    class PartitionIterator(p: A => Boolean) extends AbstractIterator[A] {
      var other: PartitionIterator = _
      val lookahead = new mutable.Queue[A]
      def skip() =
        while (self.hasNext && !p(self.head)) {
          other.lookahead += self.next
        }
      def hasNext = !lookahead.isEmpty || { skip(); self.hasNext }
      def next() = if (!lookahead.isEmpty) lookahead.dequeue()
                   else { skip(); self.next() }
    }
    val l = new PartitionIterator(p)
    val r = new PartitionIterator(!p(_))
    l.other = r
    r.other = l
    (l, r)
  }

  /** Splits this Iterator into a prefix/suffix pair according to a predicate.
   *
   *  @param p the test predicate
   *  @return  a pair of Iterators consisting of the longest prefix of this
   *           whose elements all satisfy `p`, and the rest of the Iterator.
   *  @note    Reuse: $consumesOneAndProducesTwoIterators
   */
  def span(p: A => Boolean): (Iterator[A], Iterator[A]) = {
    val self = buffered

    /*
     * Giving a name to following iterator (as opposed to trailing) because
     * anonymous class is represented as a structural type that trailing
     * iterator is referring (the finish() method) and thus triggering
     * handling of structural calls. It's not what's intended here.
     */
    class Leading extends AbstractIterator[A] {
      val lookahead = new mutable.Queue[A]
      def advance() = {
        self.hasNext && p(self.head) && {
          lookahead += self.next
          true
        }
      }
      def finish() = {
        while (advance()) ()
      }
      def hasNext = lookahead.nonEmpty || advance()
      def next() = {
        if (lookahead.isEmpty)
          advance()

        lookahead.dequeue()
      }
    }
    val leading = new Leading
    val trailing = new AbstractIterator[A] {
      private lazy val it = {
        leading.finish()
        self
      }
      def hasNext = it.hasNext
      def next() = it.next()
      override def toString = "unknown-if-empty iterator"
    }

    (leading, trailing)
  }

  /** Skips longest sequence of elements of this iterator which satisfy given
   *  predicate `p`, and returns an iterator of the remaining elements.
   *
   *  @param p the predicate used to skip elements.
   *  @return  an iterator consisting of the remaining elements
   *  @note    Reuse: $consumesAndProducesIterator
   */
  def dropWhile(p: A => Boolean): Iterator[A] = {
    val self = buffered
    new AbstractIterator[A] {
      var dropped = false
      private def skip() =
        if (!dropped) {
          while (self.hasNext && p(self.head)) self.next()
          dropped = true
        }
      def hasNext = { skip(); self.hasNext }
      def next() = { skip(); self.next() }
    }
  }

  /** Creates an iterator formed from this iterator and another iterator
   *  by combining corresponding values in pairs.
   *  If one of the two iterators is longer than the other, its remaining
   *  elements are ignored.
   *
   *  @param   that  The iterator providing the second half of each result pair
   *  @return        a new iterator containing pairs consisting of
   *                 corresponding elements of this iterator and `that`. The number
   *                 of elements returned by the new iterator is the
   *                 minimum of the number of elements returned by this
   *                 iterator and `that`.
   *  @note          Reuse: $consumesTwoAndProducesOneIterator
   */
  def zip[B](that: Iterator[B]): Iterator[(A, B)] = new AbstractIterator[(A, B)] {
    def hasNext = self.hasNext && that.hasNext
    def next = (self.next(), that.next())
  }

  /** Appends an element value to this iterator until a given target length is reached.
   *
   *  @param   len   the target length
   *  @param   elem  the padding value
   *  @return a new iterator consisting of producing all values of this iterator,
   *          followed by the minimal number of occurrences of `elem` so
   *          that the number of produced values is at least `len`.
   *  @note    Reuse: $consumesAndProducesIterator
   *
   *  @usecase def padTo(len: Int, elem: A): Iterator[A]
   *    @inheritdoc
   */
  def padTo[A1 >: A](len: Int, elem: A1): Iterator[A1] = new AbstractIterator[A1] {
    private var count = 0
    def hasNext = self.hasNext || count < len
    def next = {
      count += 1
      if (self.hasNext) self.next()
      else if (count <= len) elem
      else empty.next()
    }
  }

  /** Creates an iterator that pairs each element produced by this iterator
   *  with its index, counting from 0.
   *
   *  @return        a new iterator containing pairs consisting of
   *                 corresponding elements of this iterator and their indices.
   *  @note          Reuse: $consumesAndProducesIterator
   */
  def zipWithIndex: Iterator[(A, Int)] = new AbstractIterator[(A, Int)] {
    var idx = 0
    def hasNext = self.hasNext
    def next = {
      val ret = (self.next(), idx)
      idx += 1
      ret
    }
  }

  /** Creates an iterator formed from this iterator and another iterator
   *  by combining corresponding elements in pairs.
   *  If one of the two iterators is shorter than the other,
   *  placeholder elements are used to extend the shorter iterator to the length of the longer.
   *
   *  @param that     iterator `that` may have a different length
   *                  as the self iterator.
   *  @param thisElem element `thisElem` is used to fill up the
   *                  resulting iterator if the self iterator is shorter than
   *                  `that`
   *  @param thatElem element `thatElem` is used to fill up the
   *                  resulting iterator if `that` is shorter than
   *                  the self iterator
   *  @return         a new iterator containing pairs consisting of
   *                  corresponding values of this iterator and `that`. The length
   *                  of the returned iterator is the maximum of the lengths of this iterator and `that`.
   *                  If this iterator is shorter than `that`, `thisElem` values are used to pad the result.
   *                  If `that` is shorter than this iterator, `thatElem` values are used to pad the result.
   *  @note           Reuse: $consumesTwoAndProducesOneIterator
   *
   *  @usecase def zipAll[B](that: Iterator[B], thisElem: A, thatElem: B): Iterator[(A, B)]
   *    @inheritdoc
   */
  def zipAll[B, A1 >: A, B1 >: B](that: Iterator[B], thisElem: A1, thatElem: B1): Iterator[(A1, B1)] = new AbstractIterator[(A1, B1)] {
    def hasNext = self.hasNext || that.hasNext
    def next(): (A1, B1) =
      if (self.hasNext) {
        if (that.hasNext) (self.next(), that.next())
        else (self.next(), thatElem)
      } else {
        if (that.hasNext) (thisElem, that.next())
        else empty.next()
      }
  }

  /** Applies a function `f` to all values produced by this iterator.
   *
   *  @param  f   the function that is applied for its side-effect to every element.
   *              The result of function `f` is discarded.
   *
   *  @tparam  U  the type parameter describing the result of function `f`.
   *              This result will always be ignored. Typically `U` is `Unit`,
   *              but this is not necessary.
   *
   *  @note    Reuse: $consumesIterator
   *
   *  @usecase def foreach(f: A => Unit): Unit
   *    @inheritdoc
   */
  def foreach[U](f: A =>  U) { while (hasNext) f(next()) }

  /** Tests whether a predicate holds for all values produced by this iterator.
   *  $mayNotTerminateInf
   *
   *  @param   p     the predicate used to test elements.
   *  @return        `true` if the given predicate `p` holds for all values
   *                 produced by this iterator, otherwise `false`.
   *  @note          Reuse: $consumesIterator
   */
  def forall(p: A => Boolean): Boolean = {
    var res = true
    while (res && hasNext) res = p(next())
    res
  }

  /** Tests whether a predicate holds for some of the values produced by this iterator.
   *  $mayNotTerminateInf
   *
   *  @param   p     the predicate used to test elements.
   *  @return        `true` if the given predicate `p` holds for some of the values
   *                 produced by this iterator, otherwise `false`.
   *  @note          Reuse: $consumesIterator
   */
  def exists(p: A => Boolean): Boolean = {
    var res = false
    while (!res && hasNext) res = p(next())
    res
  }

  /** Tests whether this iterator contains a given value as an element.
   *  $mayNotTerminateInf
   *
   *  @param elem  the element to test.
   *  @return     `true` if this iterator produces some value that is
   *               is equal (as determined by `==`) to `elem`, `false` otherwise.
   *  @note        Reuse: $consumesIterator
   */
  def contains(elem: Any): Boolean = exists(_ == elem)

  /** Finds the first value produced by the iterator satisfying a
   *  predicate, if any.
   *  $mayNotTerminateInf
   *
   *  @param p the predicate used to test values.
   *  @return  an option value containing the first value produced by the iterator that satisfies
   *           predicate `p`, or `None` if none exists.
   *  @note    Reuse: $consumesIterator
   */
  def find(p: A => Boolean): Option[A] = {
    var res: Option[A] = None
    while (res.isEmpty && hasNext) {
      val e = next()
      if (p(e)) res = Some(e)
    }
    res
  }

  /** Returns the index of the first produced value satisfying a predicate, or -1.
   *  $mayNotTerminateInf
   *
   *  @param  p the predicate to test values
   *  @return   the index of the first produced value satisfying `p`,
   *           or -1 if such an element does not exist until the end of the iterator is reached.
   *  @note    Reuse: $consumesIterator
   */
  def indexWhere(p: A => Boolean): Int = {
    var i = 0
    var found = false
    while (!found && hasNext) {
      if (p(next())) {
        found = true
      } else {
        i += 1
      }
    }
    if (found) i else -1
  }

  /** Returns the index of the first occurrence of the specified
   *  object in this iterable object.
   *  $mayNotTerminateInf
   *
   *  @param  elem  element to search for.
   *  @return the index of the first occurrence of `elem` in the values produced by this iterator,
   *          or -1 if such an element does not exist until the end of the iterator is reached.
   *  @note   Reuse: $consumesIterator
   */
  def indexOf[B >: A](elem: B): Int = {
    var i = 0
    var found = false
    while (!found && hasNext) {
      if (next() == elem) {
        found = true
      } else {
        i += 1
      }
    }
    if (found) i else -1
  }

  /** Creates a buffered iterator from this iterator.
   *
   *  @see [[scala.collection.BufferedIterator]]
   *  @return  a buffered iterator producing the same values as this iterator.
   *  @note    Reuse: $consumesAndProducesIterator
   */
  def buffered: BufferedIterator[A] = new AbstractIterator[A] with BufferedIterator[A] {
    private var hd: A = _
    private var hdDefined: Boolean = false

    def head: A = {
      if (!hdDefined) {
        hd = next()
        hdDefined = true
      }
      hd
    }

    def hasNext =
      hdDefined || self.hasNext

    def next() =
      if (hdDefined) {
        hdDefined = false
        hd
      } else self.next()
  }

  /** A flexible iterator for transforming an `Iterator[A]` into an
   *  Iterator[Seq[A]], with configurable sequence size, step, and
   *  strategy for dealing with elements which don't fit evenly.
   *
   *  Typical uses can be achieved via methods `grouped` and `sliding`.
   */
  class GroupedIterator[B >: A](self: Iterator[A], size: Int, step: Int)
  extends AbstractIterator[Seq[B]]
     with Iterator[Seq[B]] {

    require(size >= 1 && step >= 1, "size=%d and step=%d, but both must be positive".format(size, step))

    private[this] var buffer: ArrayBuffer[B] = ArrayBuffer()  // the buffer
    private[this] var filled = false                          // whether the buffer is "hot"
    private[this] var _partial = true                         // whether we deliver short sequences
    private[this] var pad: Option[() => B] = None             // what to pad short sequences with

    /** Public functions which can be used to configure the iterator before use.
	 *
	 *  Pads the last segment if necessary so that all segments will
	 *  have the same size.
	 *
	 *  @param x The element that will be appended to the last segment, if necessary.
	 *  @return  The same iterator, and ''not'' a new iterator.
	 *  @note    This method mutates the iterator it is called on, which can be safely used afterwards.
	 *  @note    This method is mutually exclusive with `withPartial(true)`.
 	 */
    def withPadding(x: => B): this.type = {
      pad = Some(() => x)
      this
    }
	/** Public functions which can be used to configure the iterator before use.
  	 *
	 *  Select whether the last segment may be returned with less than `size`
	 *  elements. If not, some elements of the original iterator may not be
	 *  returned at all.
	 *
	 *  @param x `true` if partial segments may be returned, `false` otherwise.
	 *  @return  The same iterator, and ''not'' a new iterator.
	 *  @note    This method mutates the iterator it is called on, which can be safely used afterwards.
	 *  @note    This method is mutually exclusive with `withPadding`.
	 */
    def withPartial(x: Boolean): this.type = {
      _partial = x
      if (_partial == true) // reset pad since otherwise it will take precedence
        pad = None

      this
    }

    /** For reasons which remain to be determined, calling
     *  self.take(n).toSeq cause an infinite loop, so we have
     *  a slight variation on take for local usage.
     */
    private def takeDestructively(size: Int): Seq[A] = {
      val buf = new ArrayBuffer[A]
      var i = 0
      while (self.hasNext && i < size) {
        buf += self.next
        i += 1
      }
      buf
    }

    private def padding(x: Int) = List.fill(x)(pad.get())
    private def gap = (step - size) max 0

    private def go(count: Int) = {
      val prevSize = buffer.size
      def isFirst = prevSize == 0
      // If there is padding defined we insert it immediately
      // so the rest of the code can be oblivious
      val xs: Seq[B] = {
        val res = takeDestructively(count)
        // extra checks so we don't calculate length unless there's reason
        if (pad.isDefined && !self.hasNext) {
          val shortBy = count - res.length
          if (shortBy > 0) res ++ padding(shortBy) else res
        }
        else res
      }
      lazy val len = xs.length
      lazy val incomplete = len < count

      // if 0 elements are requested, or if the number of newly obtained
      // elements is less than the gap between sequences, we are done.
      def deliver(howMany: Int) = {
        (howMany > 0 && (isFirst || len > gap)) && {
          if (!isFirst)
            buffer trimStart (step min prevSize)

          val available =
            if (isFirst) len
            else howMany min (len - gap)

          buffer ++= (xs takeRight available)
          filled = true
          true
        }
      }

      if (xs.isEmpty) false                         // self ran out of elements
      else if (_partial) deliver(len min size)      // if _partial is true, we deliver regardless
      else if (incomplete) false                    // !_partial && incomplete means no more seqs
      else if (isFirst) deliver(len)                // first element
      else deliver(step min size)                   // the typical case
    }

    // fill() returns false if no more sequences can be produced
    private def fill(): Boolean = {
      if (!self.hasNext) false
      // the first time we grab size, but after that we grab step
      else if (buffer.isEmpty) go(size)
      else go(step)
    }

    def hasNext = filled || fill()
    def next = {
      if (!filled)
        fill()

      if (!filled)
        throw new NoSuchElementException("next on empty iterator")
      filled = false
      buffer.toList
    }
  }

  /** Returns an iterator which groups this iterator into fixed size
   *  blocks.  Example usages:
   *  {{{
   *    // Returns List(List(1, 2, 3), List(4, 5, 6), List(7)))
   *    (1 to 7).iterator grouped 3 toList
   *    // Returns List(List(1, 2, 3), List(4, 5, 6))
   *    (1 to 7).iterator grouped 3 withPartial false toList
   *    // Returns List(List(1, 2, 3), List(4, 5, 6), List(7, 20, 25)
   *    // Illustrating that withPadding's argument is by-name.
   *    val it2 = Iterator.iterate(20)(_ + 5)
   *    (1 to 7).iterator grouped 3 withPadding it2.next toList
   *  }}}
   *
   *  @note Reuse: $consumesAndProducesIterator
   */
  def grouped[B >: A](size: Int): GroupedIterator[B] =
    new GroupedIterator[B](self, size, size)

  /** Returns an iterator which presents a "sliding window" view of
   *  another iterator.  The first argument is the window size, and
   *  the second is how far to advance the window on each iteration;
   *  defaults to `1`.  Example usages:
   *  {{{
   *    // Returns List(List(1, 2, 3), List(2, 3, 4), List(3, 4, 5))
   *    (1 to 5).iterator.sliding(3).toList
   *    // Returns List(List(1, 2, 3, 4), List(4, 5))
   *    (1 to 5).iterator.sliding(4, 3).toList
   *    // Returns List(List(1, 2, 3, 4))
   *    (1 to 5).iterator.sliding(4, 3).withPartial(false).toList
   *    // Returns List(List(1, 2, 3, 4), List(4, 5, 20, 25))
   *    // Illustrating that withPadding's argument is by-name.
   *    val it2 = Iterator.iterate(20)(_ + 5)
   *    (1 to 5).iterator.sliding(4, 3).withPadding(it2.next).toList
   *  }}}
   *
   *  @note Reuse: $consumesAndProducesIterator
   */
  def sliding[B >: A](size: Int, step: Int = 1): GroupedIterator[B] =
    new GroupedIterator[B](self, size, step)

  /** Returns the number of elements in this iterator.
   *  $willNotTerminateInf
   *
   *  @note Reuse: $consumesIterator
   */
  def length: Int = this.size

  /** Creates two new iterators that both iterate over the same elements
   *  as this iterator (in the same order).  The duplicate iterators are
   *  considered equal if they are positioned at the same element.
   *
   *  Given that most methods on iterators will make the original iterator
   *  unfit for further use, this methods provides a reliable way of calling
   *  multiple such methods on an iterator.
   *
   *  @return a pair of iterators
   *  @note   The implementation may allocate temporary storage for elements
   *          iterated by one iterator but not yet by the other.
   *  @note   Reuse: $consumesOneAndProducesTwoIterators
   */
  def duplicate: (Iterator[A], Iterator[A]) = {
    val gap = new scala.collection.mutable.Queue[A]
    var ahead: Iterator[A] = null
    class Partner extends AbstractIterator[A] {
      def hasNext: Boolean = self.synchronized {
        (this ne ahead) && !gap.isEmpty || self.hasNext
      }
      def next(): A = self.synchronized {
        if (gap.isEmpty) ahead = this
        if (this eq ahead) {
          val e = self.next()
          gap enqueue e
          e
        } else gap.dequeue()
      }
      // to verify partnerhood we use reference equality on gap because
      // type testing does not discriminate based on origin.
      private def compareGap(queue: scala.collection.mutable.Queue[A]) = gap eq queue
      override def hashCode = gap.hashCode()
      override def equals(other: Any) = other match {
        case x: Partner   => x.compareGap(gap) && gap.isEmpty
        case _            => super.equals(other)
      }
    }
    (new Partner, new Partner)
  }

  /** Returns this iterator with patched values.
   *
   *  @param from       The start index from which to patch
   *  @param patchElems The iterator of patch values
   *  @param replaced   The number of values in the original iterator that are replaced by the patch.
   *  @note           Reuse: $consumesTwoAndProducesOneIterator
   */
  def patch[B >: A](from: Int, patchElems: Iterator[B], replaced: Int): Iterator[B] = new AbstractIterator[B] {
    private var origElems = self
    private var i = 0
    def hasNext: Boolean =
      if (i < from) origElems.hasNext
      else patchElems.hasNext || origElems.hasNext
    def next(): B = {
      // We have to do this *first* just in case from = 0.
      if (i == from) origElems = origElems drop replaced
      val result: B =
        if (i < from || !patchElems.hasNext) origElems.next()
        else patchElems.next()
      i += 1
      result
    }
  }

  /** Copies selected values produced by this iterator to an array.
   *  Fills the given array `xs` starting at index `start` with at most
   *  `len` values produced by this iterator.
   *  Copying will stop once either the end of the current iterator is reached,
   *  or the end of the array is reached, or `len` elements have been copied.
   *
   *  @param  xs     the array to fill.
   *  @param  start  the starting index.
   *  @param  len    the maximal number of elements to copy.
   *  @tparam B      the type of the elements of the array.
   *
   *  @note    Reuse: $consumesIterator
   *
   *  @usecase def copyToArray(xs: Array[A], start: Int, len: Int): Unit
   *    @inheritdoc
   *
   *    $willNotTerminateInf
   */
  def copyToArray[B >: A](xs: Array[B], start: Int, len: Int): Unit = {
    require(start >= 0 && (start < xs.length || xs.length == 0), s"start $start out of range ${xs.length}")
    var i = start
    val end = start + math.min(len, xs.length - start)
    while (i < end && hasNext) {
      xs(i) = next()
      i += 1
    }
    // TODO: return i - start so the caller knows how many values read?
  }

  /** Tests if another iterator produces the same values as this one.
   *
   *  $willNotTerminateInf
   *
   *  @param that  the other iterator
   *  @return      `true`, if both iterators produce the same elements in the same order, `false` otherwise.
   *
   *  @note        Reuse: $consumesTwoIterators
   */
  def sameElements(that: Iterator[_]): Boolean = {
    while (hasNext && that.hasNext)
      if (next != that.next)
        return false

    !hasNext && !that.hasNext
  }

  def toTraversable: Traversable[A] = toStream
  def toIterator: Iterator[A] = self
  def toStream: Stream[A] =
    if (self.hasNext) Stream.cons(self.next(), self.toStream)
    else Stream.empty[A]


  /** Converts this iterator to a string.
   *
   *  @return `"empty iterator"` or `"non-empty iterator"`, depending on
   *           whether or not the iterator is empty.
   *  @note    Reuse: $preservesIterator
   */
  override def toString = (if (hasNext) "non-empty" else "empty")+" iterator"
}

/** Explicit instantiation of the `Iterator` trait to reduce class file size in subclasses. */
abstract class AbstractIterator[+A] extends Iterator[A]