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/* __ *\
** ________ ___ / / ___ Scala API **
** / __/ __// _ | / / / _ | (c) 2003-2011, LAMP/EPFL **
** __\ \/ /__/ __ |/ /__/ __ | http://scala-lang.org/ **
** /____/\___/_/ |_/____/_/ | | **
** |/ **
\* */

package scala.collection

import mutable.ArrayBuffer
import annotation.migration
import immutable.Stream

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

  /** The iterator which produces no values. */
  val empty = new Iterator[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) = new Iterator[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) = new Iterator[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) = new Iterator[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) = new Iterator[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 Iterator[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 Iterator[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 Iterator[A] {
    def hasNext = true
    def next = elem
  }

  @deprecated("use `xs.iterator' or `Iterator(xs)' instead", "2.8.0")
  def fromValues[a](xs: a*) = xs.iterator

  /** @param xs the array of elements
* @see also: IndexedSeq.iterator and slice
*/
  @deprecated("use `xs.iterator' instead", "2.8.0")
  def fromArray[a](xs: Array[a]): Iterator[a] =
    fromArray(xs, 0, xs.length)

  /**
* @param xs the array of elements
* @param start the start index
* @param length the length
* @see also: IndexedSeq.iterator and slice
*/
  @deprecated("use `xs.slice(start, start + length).iterator' instead", "2.8.0")
  def fromArray[a](xs: Array[a], start: Int, length: Int): Iterator[a] =
    xs.slice(start, start + length).iterator

  /**
* @param n the product arity
* @return the iterator on `Product&lt;n&gt;`.
*/
  @deprecated("use product.productIterator instead", "2.8.0")
  def fromProduct(n: Product): Iterator[Any] = new Iterator[Any] {
    private var c: Int = 0
    private val cmax = n.productArity
    def hasNext = c < cmax
    def next() = { val a = n productElement c; c += 1; a }
  }

  /** Create an iterator with elements
* `e<sub>n+1</sub> = step(e<sub>n</sub>)`
* where `e<sub>0</sub> = start`
* and elements are in the range between `start` (inclusive)
* and `end` (exclusive)
*
* @param start the start value of the iterator
* @param end the end value of the iterator
* @param step the increment function of the iterator, must be monotonically increasing or decreasing
* @return the iterator with values in range `[start;end)`.
*/
  @deprecated("use Iterator.iterate(start, end - start)(step) instead", "2.8.0")
  def range(start: Int, end: Int, step: Int => Int) = new Iterator[Int] {
    private val up = step(start) > start
    private val down = step(start) < start
    private var i = start
    def hasNext: Boolean = (!up || i < end) && (!down || i > end)
    def next(): Int =
      if (hasNext) { val j = i; i = step(i); j }
      else empty.next()
  }

  /** Create an iterator with elements
* `e<sub>n+1</sub> = step(e<sub>n</sub>)`
* where `e<sub>0</sub> = start`.
*
* @param start the start value of the iterator
* @param step the increment function of the iterator
* @return the iterator starting at value `start`.
*/
  @deprecated("use iterate(start)(step) instead", "2.8.0")
  def from(start: Int, step: Int => Int): Iterator[Int] = new Iterator[Int] {
    private var i = start
    override def hasNext: Boolean = true
    def next(): Int = { val j = i; i = step(i); j }
  }

  /** Create an iterator that is the concatenation of all iterators
* returned by a given iterator of iterators.
* @param its The iterator which returns on each call to next
* a new iterator whose elements are to be concatenated to the result.
*/
  @deprecated("use its.flatten instead", "2.8.0")
  def flatten[T](its: Iterator[Iterator[T]]): Iterator[T] = new Iterator[T] {
    private var cur = its.next
    def hasNext: Boolean = {
      while (!cur.hasNext && its.hasNext) cur = its.next
      cur.hasNext
    }
    def next(): T =
      (if (hasNext) cur else empty).next()
  }
}

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 Iterator[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 Iterator[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]
*/
  def ++[B >: A](that: => GenTraversableOnce[B]): Iterator[B] = new Iterator[B] {
    // optimize a little bit to prevent n log n behavior.
    private var cur : Iterator[B] = self
    // since that is by-name, make sure it's only referenced once -
    // if "val it = that" is inside the block, then hasNext on an empty
    // iterator will continually reevaluate it. (ticket #3269)
    lazy val it = that.toIterator
    // the eq check is to avoid an infinite loop on "x ++ x"
    def hasNext = cur.hasNext || ((cur eq self) && {
      it.hasNext && {
        cur = it
        true
      }
    })
    def next() = { hasNext; cur.next() }
  }

  /** 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 Iterator[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 Iterator[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()
  }

  /** 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 Iterator[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 Iterator[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 Iterator[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 Iterator[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 Iterator[A] {
      private var isDone = false
      val lookahead = new mutable.Queue[A]
      def advance() = {
        self.hasNext && p(self.head) && {
          lookahead += self.next
          true
        }
      }
      def finish() = {
        while (advance()) ()
        isDone = true
      }
      def hasNext = lookahead.nonEmpty || advance()
      def next() = {
        if (lookahead.isEmpty)
          advance()

        lookahead.dequeue()
      }
    }
    val leading = new Leading
    val trailing = new Iterator[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 Iterator[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]) = new Iterator[(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]
*/
  def padTo[A1 >: A](len: Int, elem: A1) = new Iterator[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 = new Iterator[(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)]
*/
  def zipAll[B, A1 >: A, B1 >: B](that: Iterator[B], thisElem: A1, thatElem: B1) = new Iterator[(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
*/
  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 (wrt `==`) 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 BufferedIterator
* @return a buffered iterator producing the same values as this iterator.
* @note Reuse: $consumesAndProducesIterator
*/
  def buffered = new 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 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 Iterator[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) = new Iterator[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 = {
      val result: B =
        if (i < from || !patchElems.hasNext) origElems.next()
        else patchElems.next()
      i += 1
      if (i == from) origElems = origElems drop replaced
      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.
*
* $willNotTerminateInf
*
* @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
*/
  def copyToArray[B >: A](xs: Array[B], start: Int, len: Int): Unit = {
    var i = start
    val end = start + math.min(len, xs.length)
    while (hasNext && i < end) {
      xs(i) = next()
      i += 1
    }
  }

  /** 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"

  /** Returns a new iterator that first yields the elements of this
* iterator followed by the elements provided by iterator `that`.
*/
  @deprecated("use `++`", "2.3.2")
  def append[B >: A](that: Iterator[B]) = self ++ that

  /** Returns index of the first element satisfying a predicate, or -1. */
  @deprecated("use `indexWhere` instead", "2.8.0")
  def findIndexOf(p: A => Boolean): Int = indexWhere(p)

  /** Returns a counted iterator from this iterator.
*/
  @deprecated("use zipWithIndex in Iterator", "2.8.0")
  def counted = new CountedIterator[A] {
    private var cnt = 0
    def count = cnt
    def hasNext: Boolean = self.hasNext
    def next(): A = { cnt += 1; self.next }
  }

  /** Fills the given array `xs` with the elements of
* this sequence starting at position `start`. Like `copyToArray`,
* but designed to accomodate IO stream operations.
*
* '''Note:''' the array must be large enough to hold `sz` elements.
* @param xs the array to fill.
* @param start the starting index.
* @param sz the maximum number of elements to be read.
*/
  @deprecated("use copyToArray instead", "2.8.0")
  def readInto[B >: A](xs: Array[B], start: Int, sz: Int) {
    var i = start
    while (hasNext && i - start < sz) {
      xs(i) = next
      i += 1
    }
  }

  @deprecated("use copyToArray instead", "2.8.0")
  def readInto[B >: A](xs: Array[B], start: Int) {
    readInto(xs, start, xs.length - start)
  }

  @deprecated("use copyToArray instead", "2.8.0")
  def readInto[B >: A](xs: Array[B]) {
    readInto(xs, 0, xs.length)
  }
}
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