scala.collection.AbstractIterator

abstract class AbstractIterator[+A] extends Iterator[A]

Explicit instantiation of the Iterator trait to reduce class file size in subclasses.

Type Members

class GroupedIterator[B >: A] extends AbstractIterator[Seq[B]] with Iterator[Seq[B]]

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 .

  • Definition Classes
    • Iterator

Instance Constructors From scala.collection.AbstractIterator

new AbstractIterator()

(defined at scala.collection.AbstractIterator)

Concrete Value Members From scala.collection.Iterator

abstract def hasNext: Boolean

Tests whether this iterator can provide another element.

  • returns
    • true if a subsequent call to next will yield an element, false otherwise.
  • Definition Classes
    • Iterator
  • Note
    • Reuse: The iterator remains valid for further use whatever result is returned.

(defined at scala.collection.Iterator)

def ++[B >: A](that: ⇒ GenTraversableOnce[B]): Iterator[B]

[use case]

Concatenates this iterator with another.

  • that
    • the other iterator
  • returns
    • a new iterator that first yields the values produced by this iterator followed by the values produced by iterator that .
  • Definition Classes
    • Iterator

(defined at scala.collection.Iterator)

def buffered: BufferedIterator[A]

Creates a buffered iterator from this iterator.

  • returns
    • a buffered iterator producing the same values as this iterator.
  • Definition Classes
    • Iterator
  • Note
    • Reuse: 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.
  • See also
    • scala.collection.BufferedIterator

(defined at scala.collection.Iterator)

def collect[B](pf: PartialFunction[A, B]): Iterator[B]

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.

  • pf
    • the partial function which filters and maps the iterator.
  • returns
    • a new iterator which yields each value x produced by this iterator for which pf is defined the image pf(x) .
  • Definition Classes
    • Iterator
  • Annotations
    • @migration
  • Migration
    • (Changed in version 2.8.0) collect has changed. The previous behavior can be reproduced with toSeq .
  • Note
    • Reuse: 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.

(defined at scala.collection.Iterator)

def contains(elem: Any): Boolean

Tests whether this iterator contains a given value as an element.

Note: may not terminate for infinite iterators.

  • elem
    • the element to test.
  • returns
    • true if this iterator produces some value that is is equal (as determined by == ) to elem , false otherwise.
  • Definition Classes
    • Iterator
  • Note
    • Reuse: After calling this method, one should discard the iterator it was called on. Using it is undefined and subject to change.

(defined at scala.collection.Iterator)

def copyToArray[B >: A](xs: Array[B], start: Int, len: Int): Unit

[use case]

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.

Note: will not terminate for infinite iterators.

  • xs
    • the array to fill.
  • start
    • the starting index.
  • len
    • the maximal number of elements to copy.
  • Definition Classes
    • Iterator → TraversableOnce → GenTraversableOnce

(defined at scala.collection.Iterator)

def corresponds[B](that: GenTraversableOnce[B])(p: (A, B) ⇒ Boolean): Boolean

Tests whether every element of this iterator relates to the corresponding element of another collection by satisfying a test predicate.

  • B
    • the type of the elements of that
  • that
    • the other collection
  • p
    • the test predicate, which relates elements from both collections
  • returns
    • 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
  • Definition Classes
    • Iterator

(defined at scala.collection.Iterator)

def drop(n: Int): Iterator[A]

Advances this iterator past the first n elements, or the length of the iterator, whichever is smaller.

  • n
    • the number of elements to drop
  • returns
    • an iterator which produces all values of the current iterator, except it omits the first n values.
  • Definition Classes
    • Iterator
  • Note
    • Reuse: 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.

(defined at scala.collection.Iterator)

def dropWhile(p: (A) ⇒ Boolean): Iterator[A]

Skips longest sequence of elements of this iterator which satisfy given predicate p , and returns an iterator of the remaining elements.

  • p
    • the predicate used to skip elements.
  • returns
    • an iterator consisting of the remaining elements
  • Definition Classes
    • Iterator
  • Note
    • Reuse: 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.

(defined at scala.collection.Iterator)

def exists(p: (A) ⇒ Boolean): Boolean

Tests whether a predicate holds for some of the values produced by this iterator.

Note: may not terminate for infinite iterators.

  • p
    • the predicate used to test elements.
  • returns
    • true if the given predicate p holds for some of the values produced by this iterator, otherwise false .
  • Definition Classes
    • Iterator → TraversableOnce → GenTraversableOnce
  • Note
    • Reuse: After calling this method, one should discard the iterator it was called on. Using it is undefined and subject to change.

(defined at scala.collection.Iterator)

def filter(p: (A) ⇒ Boolean): Iterator[A]

Returns an iterator over all the elements of this iterator that satisfy the predicate p . The order of the elements is preserved.

  • p
    • the predicate used to test values.
  • returns
    • an iterator which produces those values of this iterator which satisfy the predicate p .
  • Definition Classes
    • Iterator
  • Note
    • Reuse: 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.

(defined at scala.collection.Iterator)

def filterNot(p: (A) ⇒ Boolean): Iterator[A]

Creates an iterator over all the elements of this iterator which do not satisfy a predicate p.

  • p
    • the predicate used to test values.
  • returns
    • an iterator which produces those values of this iterator which do not satisfy the predicate p .
  • Definition Classes
    • Iterator
  • Note
    • Reuse: 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.

(defined at scala.collection.Iterator)

def find(p: (A) ⇒ Boolean): Option[A]

Finds the first value produced by the iterator satisfying a predicate, if any.

Note: may not terminate for infinite iterators.

  • p
    • the predicate used to test values.
  • returns
    • an option value containing the first value produced by the iterator that satisfies predicate p , or None if none exists.
  • Definition Classes
    • Iterator → TraversableOnce → GenTraversableOnce
  • Note
    • Reuse: After calling this method, one should discard the iterator it was called on. Using it is undefined and subject to change.

(defined at scala.collection.Iterator)

def flatMap[B](f: (A) ⇒ GenTraversableOnce[B]): Iterator[B]

Creates a new iterator by applying a function to all values produced by this iterator and concatenating the results.

  • f
    • the function to apply on each element.
  • returns
    • the iterator resulting from applying the given iterator-valued function f to each value produced by this iterator and concatenating the results.
  • Definition Classes
    • Iterator
  • Note
    • Reuse: 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.

(defined at scala.collection.Iterator)

def forall(p: (A) ⇒ Boolean): Boolean

Tests whether a predicate holds for all values produced by this iterator.

Note: may not terminate for infinite iterators.

  • p
    • the predicate used to test elements.
  • returns
    • true if the given predicate p holds for all values produced by this iterator, otherwise false .
  • Definition Classes
    • Iterator → TraversableOnce → GenTraversableOnce
  • Note
    • Reuse: After calling this method, one should discard the iterator it was called on. Using it is undefined and subject to change.

(defined at scala.collection.Iterator)

def foreach[U](f: (A) ⇒ U): Unit

[use case]

Applies a function f to all values produced by this iterator.

  • f
    • the function that is applied for its side-effect to every element. The result of function f is discarded.
  • Definition Classes
    • Iterator → TraversableOnce → GenTraversableOnce

(defined at scala.collection.Iterator)

def grouped[B >: A](size: Int): GroupedIterator[B]

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
  • Definition Classes
    • Iterator
  • Note
    • Reuse: 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.

(defined at scala.collection.Iterator)

def indexOf[B >: A](elem: B): Int

Returns the index of the first occurrence of the specified object in this iterable object.

Note: may not terminate for infinite iterators.

  • elem
    • element to search for.
  • returns
    • 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.
  • Definition Classes
    • Iterator
  • Note
    • Reuse: After calling this method, one should discard the iterator it was called on. Using it is undefined and subject to change.

(defined at scala.collection.Iterator)

def indexOf[B >: A](elem: B, from: Int): Int

Returns the index of the first occurrence of the specified object in this iterable object after or at some start index.

Note: may not terminate for infinite iterators.

  • elem
    • element to search for.
  • from
    • the start index
  • returns
    • the index >= from 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.
  • Definition Classes
    • Iterator
  • Note
    • Reuse: After calling this method, one should discard the iterator it was called on. Using it is undefined and subject to change.

(defined at scala.collection.Iterator)

def indexWhere(p: (A) ⇒ Boolean): Int

Returns the index of the first produced value satisfying a predicate, or -1.

Note: may not terminate for infinite iterators.

  • p
    • the predicate to test values
  • returns
    • 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.
  • Definition Classes
    • Iterator
  • Note
    • Reuse: After calling this method, one should discard the iterator it was called on. Using it is undefined and subject to change.

(defined at scala.collection.Iterator)

def indexWhere(p: (A) ⇒ Boolean, from: Int): Int

Returns the index of the first produced value satisfying a predicate, or -1, after or at some start index.

Note: may not terminate for infinite iterators.

  • p
    • the predicate to test values
  • from
    • the start index
  • returns
    • the index >= from of the first produced value satisfying p , or -1 if such an element does not exist until the end of the iterator is reached.
  • Definition Classes
    • Iterator
  • Note
    • Reuse: After calling this method, one should discard the iterator it was called on. Using it is undefined and subject to change.

(defined at scala.collection.Iterator)

def map[B](f: (A) ⇒ B): Iterator[B]

Creates a new iterator that maps all produced values of this iterator to new values using a transformation function.

  • f
    • the transformation function
  • returns
    • a new iterator which transforms every value produced by this iterator by applying the function f to it.
  • Definition Classes
    • Iterator
  • Note
    • Reuse: 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.

(defined at scala.collection.Iterator)

def padTo[A1 >: A](len: Int, elem: A1): Iterator[A1]

[use case]

Appends an element value to this iterator until a given target length is reached.

  • len
    • the target length
  • elem
    • the padding value
  • returns
    • 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 .
  • Definition Classes
    • Iterator

(defined at scala.collection.Iterator)

def partition(p: (A) ⇒ Boolean): (Iterator[A], Iterator[A])

Partitions this iterator in two iterators according to a predicate.

  • p
    • the predicate on which to partition
  • returns
    • 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.
  • Definition Classes
    • Iterator
  • Note
    • Reuse: 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.

(defined at scala.collection.Iterator)

def patch[B >: A](from: Int, patchElems: Iterator[B], replaced: Int): Iterator[B]

Returns this iterator with patched values. Patching at negative indices is the same as patching starting at 0. Patching at indices at or larger than the length of the original iterator appends the patch to the end. If more values are replaced than actually exist, the excess is ignored.

  • from
    • The start index from which to patch
  • patchElems
    • The iterator of patch values
  • replaced
    • The number of values in the original iterator that are replaced by the patch.
  • Definition Classes
    • Iterator
  • Note
    • Reuse: 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.

(defined at scala.collection.Iterator)

def sameElements(that: Iterator[_]): Boolean

Tests if another iterator produces the same values as this one.

Note: will not terminate for infinite iterators.

  • that
    • the other iterator
  • returns
    • true , if both iterators produce the same elements in the same order, false otherwise.
  • Definition Classes
    • Iterator
  • Note
    • Reuse: 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.

(defined at scala.collection.Iterator)

def scanLeft[B](z: B)(op: (B, A) ⇒ B): Iterator[B]

Produces a collection containing cumulative results of applying the operator going left to right.

Note: will not terminate for infinite iterators.

Note: might return different results for different runs, unless the underlying collection type is ordered.

  • B
    • the type of the elements in the resulting collection
  • z
    • the initial value
  • op
    • the binary operator applied to the intermediate result and the element
  • returns
    • iterator with intermediate results
  • Definition Classes
    • Iterator
  • Note
    • Reuse: 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.

(defined at scala.collection.Iterator)

def scanRight[B](z: B)(op: (A, B) ⇒ B): Iterator[B]

Produces a collection containing cumulative results of applying the operator going right to left. The head of the collection is the last cumulative result.

Note: will not terminate for infinite iterators.

Note: might return different results for different runs, unless the underlying collection type is ordered.

  • B
    • the type of the elements in the resulting collection
  • z
    • the initial value
  • op
    • the binary operator applied to the intermediate result and the element
  • returns
    • iterator with intermediate results
  • Definition Classes
    • Iterator
  • Note
    • Reuse: 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.

Example:

Iterator(1, 2, 3, 4).scanRight(0)(_ + _).toList == List(10, 9, 7, 4, 0)

(defined at scala.collection.Iterator)

def slice(from: Int, until: Int): Iterator[A]

Creates an iterator returning an interval of the values produced by this iterator.

  • from
    • the index of the first element in this iterator which forms part of the slice. If negative, the slice starts at zero.
  • until
    • the index of the first element following the slice. If negative, the slice is empty.
  • returns
    • an iterator which advances this iterator past the first from elements using drop , and then takes until - from elements, using take .
  • Definition Classes
    • Iterator
  • Note
    • Reuse: 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.

(defined at scala.collection.Iterator)

def sliceIterator(from: Int, until: Int): Iterator[A]

Creates an optionally bounded slice, unbounded if until is negative.

  • Attributes
    • protected
  • Definition Classes
    • Iterator

(defined at scala.collection.Iterator)

def sliding[B >: A](size: Int, step: Int = 1): GroupedIterator[B]

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
  • Definition Classes
    • Iterator
  • Note
    • Reuse: 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.

(defined at scala.collection.Iterator)

def span(p: (A) ⇒ Boolean): (Iterator[A], Iterator[A])

Splits this Iterator into a prefix/suffix pair according to a predicate.

  • p
    • the test predicate
  • returns
    • a pair of Iterators consisting of the longest prefix of this whose elements all satisfy p , and the rest of the Iterator.
  • Definition Classes
    • Iterator
  • Note
    • Reuse: 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.

(defined at scala.collection.Iterator)

def take(n: Int): Iterator[A]

Selects first n values of this iterator.

  • n
    • the number of values to take
  • returns
    • an iterator producing only the first n values of this iterator, or else the whole iterator, if it produces fewer than n values.
  • Definition Classes
    • Iterator
  • Note
    • Reuse: 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.

(defined at scala.collection.Iterator)

def takeWhile(p: (A) ⇒ Boolean): Iterator[A]

Takes longest prefix of values produced by this iterator that satisfy a predicate.

  • p
    • The predicate used to test elements.
  • returns
    • An iterator returning the values produced by this iterator, until this iterator produces a value that does not satisfy the predicate p .
  • Definition Classes
    • Iterator
  • Note
    • Reuse: 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.

(defined at scala.collection.Iterator)

def toStream: immutable.Stream[A]

Converts this traversable or iterator to a stream.

  • returns
    • a stream containing all elements of this traversable or iterator.
  • Definition Classes
    • Iterator → GenTraversableOnce

(defined at scala.collection.Iterator)

def withFilter(p: (A) ⇒ Boolean): Iterator[A]

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.

  • p
    • the predicate used to test values.
  • returns
    • an iterator which produces those values of this iterator which satisfy the predicate p .
  • Definition Classes
    • Iterator
  • Note
    • Reuse: 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.

(defined at scala.collection.Iterator)

def zipAll[B, A1 >: A, B1 >: B](that: Iterator[B], thisElem: A1, thatElem: B1): Iterator[(A1, B1)]

[use case]

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.

  • that
    • iterator that may have a different length as the self iterator.
  • thisElem
    • element thisElem is used to fill up the resulting iterator if the self iterator is shorter than that
  • thatElem
    • element thatElem is used to fill up the resulting iterator if that is shorter than the self iterator
  • returns
    • 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.
  • Definition Classes
    • Iterator

(defined at scala.collection.Iterator)

def zip[B](that: Iterator[B]): Iterator[(A, B)]

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.

  • that
    • The iterator providing the second half of each result pair
  • returns
    • 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 .
  • Definition Classes
    • Iterator
  • Note
    • Reuse: 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.

(defined at scala.collection.Iterator)

Concrete Value Members From scala.collection.TraversableOnce

def /:[B](z: B)(op: (B, A) ⇒ B): B

Applies a binary operator to a start value and all elements of this traversable or iterator, going left to right.

Note: /: is alternate syntax for foldLeft ; z /: xs is the same as xs foldLeft z .

Examples:

Note that the folding function used to compute b is equivalent to that used to compute c.

scala> val a = List(1,2,3,4)
a: List[Int] = List(1, 2, 3, 4)

scala> val b = (5 /: a)(_+_)
b: Int = 15

scala> val c = (5 /: a)((x,y) => x + y)
c: Int = 15

Note: will not terminate for infinite-sized collections.

Note: might return different results for different runs, unless the underlying collection type is ordered or the operator is associative and commutative.

  • B
    • the result type of the binary operator.
  • z
    • the start value.
  • op
    • the binary operator.
  • returns
    • the result of inserting op between consecutive elements of this traversable or iterator, going left to right with the start value z on the left:
    op(...op(op(z, x_1), x_2), ..., x_n)
    
where `x1, ..., xn` are the elements of this traversable or iterator.
  • Definition Classes
    • TraversableOnce → GenTraversableOnce

(defined at scala.collection.TraversableOnce)

def :\[B](z: B)(op: (A, B) ⇒ B): B

Applies a binary operator to all elements of this traversable or iterator and a start value, going right to left.

Note: :\ is alternate syntax for foldRight ; xs :\ z is the same as xs foldRight z .

Note: will not terminate for infinite-sized collections.

Note: might return different results for different runs, unless the underlying collection type is ordered or the operator is associative and commutative.

Examples:

Note that the folding function used to compute b is equivalent to that used to compute c.

scala> val a = List(1,2,3,4)
a: List[Int] = List(1, 2, 3, 4)

scala> val b = (a :\ 5)(_+_)
b: Int = 15

scala> val c = (a :\ 5)((x,y) => x + y)
c: Int = 15
  • B
    • the result type of the binary operator.
  • z
    • the start value
  • op
    • the binary operator
  • returns
    • the result of inserting op between consecutive elements of this traversable or iterator, going right to left with the start value z on the right:
    op(x_1, op(x_2, ... op(x_n, z)...))
    
where `x1, ..., xn` are the elements of this traversable or iterator.
  • Definition Classes
    • TraversableOnce → GenTraversableOnce

(defined at scala.collection.TraversableOnce)

def addString(b: StringBuilder): StringBuilder

Appends all elements of this traversable or iterator to a string builder. The written text consists of the string representations (w.r.t. the method toString ) of all elements of this traversable or iterator without any separator string.

Example:

scala> val a = List(1,2,3,4)
a: List[Int] = List(1, 2, 3, 4)

scala> val b = new StringBuilder()
b: StringBuilder =

scala> val h = a.addString(b)
h: StringBuilder = 1234
  • b
    • the string builder to which elements are appended.
  • returns
    • the string builder b to which elements were appended.
  • Definition Classes
    • TraversableOnce

(defined at scala.collection.TraversableOnce)

def addString(b: StringBuilder, sep: String): StringBuilder

Appends all elements of this traversable or iterator to a string builder using a separator string. The written text consists of the string representations (w.r.t. the method toString ) of all elements of this traversable or iterator, separated by the string sep .

Example:

scala> val a = List(1,2,3,4)
a: List[Int] = List(1, 2, 3, 4)

scala> val b = new StringBuilder()
b: StringBuilder =

scala> a.addString(b, ", ")
res0: StringBuilder = 1, 2, 3, 4
  • b
    • the string builder to which elements are appended.
  • sep
    • the separator string.
  • returns
    • the string builder b to which elements were appended.
  • Definition Classes
    • TraversableOnce

(defined at scala.collection.TraversableOnce)

def addString(b: StringBuilder, start: String, sep: String, end: String): StringBuilder

Appends all elements of this traversable or iterator to a string builder using start, end, and separator strings. The written text begins with the string start and ends with the string end . Inside, the string representations (w.r.t. the method toString ) of all elements of this traversable or iterator are separated by the string sep .

Example:

scala> val a = List(1,2,3,4)
a: List[Int] = List(1, 2, 3, 4)

scala> val b = new StringBuilder()
b: StringBuilder =

scala> a.addString(b , "List(" , ", " , ")")
res5: StringBuilder = List(1, 2, 3, 4)
  • b
    • the string builder to which elements are appended.
  • start
    • the starting string.
  • sep
    • the separator string.
  • end
    • the ending string.
  • returns
    • the string builder b to which elements were appended.
  • Definition Classes
    • TraversableOnce

(defined at scala.collection.TraversableOnce)

def aggregate[B](z: ⇒ B)(seqop: (B, A) ⇒ B, combop: (B, B) ⇒ B): B

Aggregates the results of applying an operator to subsequent elements.

This is a more general form of fold and reduce . It is similar to foldLeft in that it doesn’t require the result to be a supertype of the element type. In addition, it allows parallel collections to be processed in chunks, and then combines the intermediate results.

aggregate splits the traversable or iterator into partitions and processes each partition by sequentially applying seqop , starting with z (like foldLeft ). Those intermediate results are then combined by using combop (like fold ). The implementation of this operation may operate on an arbitrary number of collection partitions (even 1), so combop may be invoked an arbitrary number of times (even 0).

As an example, consider summing up the integer values of a list of chars. The initial value for the sum is 0. First, seqop transforms each input character to an Int and adds it to the sum (of the partition). Then, combop just needs to sum up the intermediate results of the partitions:

List('a', 'b', 'c').aggregate(0)({ (sum, ch) => sum + ch.toInt }, { (p1, p2) => p1 + p2 })
  • B
    • the type of accumulated results
  • z
    • the initial value for the accumulated result of the partition - this will typically be the neutral element for the seqop operator (e.g. Nil for list concatenation or 0 for summation) and may be evaluated more than once
  • seqop
    • an operator used to accumulate results within a partition
  • combop
    • an associative operator used to combine results from different partitions
  • Definition Classes
    • TraversableOnce → GenTraversableOnce

(defined at scala.collection.TraversableOnce)

def collectFirst[B](pf: PartialFunction[A, B]): Option[B]

Finds the first element of the traversable or iterator for which the given partial function is defined, and applies the partial function to it.

Note: may not terminate for infinite-sized collections.

Note: might return different results for different runs, unless the underlying collection type is ordered.

  • pf
    • the partial function
  • returns
    • an option value containing pf applied to the first value for which it is defined, or None if none exists.
  • Definition Classes
    • TraversableOnce

Example:

Seq("a", 1, 5L).collectFirst({ case x: Int => x*10 }) = Some(10)

(defined at scala.collection.TraversableOnce)

def copyToArray[B >: A](xs: Array[B]): Unit

[use case]

Copies the elements of this traversable or iterator to an array. Fills the given array xs with values of this traversable or iterator. Copying will stop once either the end of the current traversable or iterator is reached, or the end of the target array is reached.

Note: will not terminate for infinite iterators.

  • xs
    • the array to fill.
  • Definition Classes
    • TraversableOnce → GenTraversableOnce

(defined at scala.collection.TraversableOnce)

def copyToArray[B >: A](xs: Array[B], start: Int): Unit

[use case]

Copies the elements of this traversable or iterator to an array. Fills the given array xs with values of this traversable or iterator, beginning at index start . Copying will stop once either the end of the current traversable or iterator is reached, or the end of the target array is reached.

Note: will not terminate for infinite iterators.

  • xs
    • the array to fill.
  • start
    • the starting index.
  • Definition Classes
    • TraversableOnce → GenTraversableOnce

(defined at scala.collection.TraversableOnce)

def copyToBuffer[B >: A](dest: Buffer[B]): Unit

Copies all elements of this traversable or iterator to a buffer.

Note: will not terminate for infinite-sized collections.

  • dest
    • The buffer to which elements are copied.
  • Definition Classes
    • TraversableOnce

(defined at scala.collection.TraversableOnce)

def count(p: (A) ⇒ Boolean): Int

Counts the number of elements in the traversable or iterator which satisfy a predicate.

  • p
    • the predicate used to test elements.
  • returns
    • the number of elements satisfying the predicate p .
  • Definition Classes
    • TraversableOnce → GenTraversableOnce

(defined at scala.collection.TraversableOnce)

def foldLeft[B](z: B)(op: (B, A) ⇒ B): B

Applies a binary operator to a start value and all elements of this traversable or iterator, going left to right.

Note: will not terminate for infinite-sized collections.

Note: might return different results for different runs, unless the underlying collection type is ordered or the operator is associative and commutative.

  • B
    • the result type of the binary operator.
  • z
    • the start value.
  • op
    • the binary operator.
  • returns
    • the result of inserting op between consecutive elements of this traversable or iterator, going left to right with the start value z on the left:
    op(...op(z, x_1), x_2, ..., x_n)
    
where `x1, ..., xn` are the elements of this traversable or iterator.
Returns `z` if this traversable or iterator is empty.
  • Definition Classes
    • TraversableOnce → GenTraversableOnce

(defined at scala.collection.TraversableOnce)

def foldRight[B](z: B)(op: (A, B) ⇒ B): B

Applies a binary operator to all elements of this traversable or iterator and a start value, going right to left.

Note: will not terminate for infinite-sized collections.

Note: might return different results for different runs, unless the underlying collection type is ordered or the operator is associative and commutative.

  • B
    • the result type of the binary operator.
  • z
    • the start value.
  • op
    • the binary operator.
  • returns
    • the result of inserting op between consecutive elements of this traversable or iterator, going right to left with the start value z on the right:
    op(x_1, op(x_2, ... op(x_n, z)...))
    
where `x1, ..., xn` are the elements of this traversable or iterator.
Returns `z` if this traversable or iterator is empty.
  • Definition Classes
    • TraversableOnce → GenTraversableOnce

(defined at scala.collection.TraversableOnce)

def fold[A1 >: A](z: A1)(op: (A1, A1) ⇒ A1): A1

Folds the elements of this traversable or iterator using the specified associative binary operator.

The order in which operations are performed on elements is unspecified and may be nondeterministic.

Note: will not terminate for infinite-sized collections.

  • A1
    • a type parameter for the binary operator, a supertype of A .
  • z
    • a neutral element for the fold operation; may be added to the result an arbitrary number of times, and must not change the result (e.g., Nil for list concatenation, 0 for addition, or 1 for multiplication).
  • op
    • a binary operator that must be associative.
  • returns
    • the result of applying the fold operator op between all the elements and z , or z if this traversable or iterator is empty.
  • Definition Classes
    • TraversableOnce → GenTraversableOnce

(defined at scala.collection.TraversableOnce)

def maxBy[B](f: (A) ⇒ B)(implicit cmp: Ordering[B]): A

[use case]

Finds the first element which yields the largest value measured by function f.

  • B
    • The result type of the function f.
  • f
    • The measuring function.
  • returns
    • the first element of this traversable or iterator with the largest value measured by function f.
  • Definition Classes
    • TraversableOnce → GenTraversableOnce

(defined at scala.collection.TraversableOnce)

def minBy[B](f: (A) ⇒ B)(implicit cmp: Ordering[B]): A

[use case]

Finds the first element which yields the smallest value measured by function f.

  • B
    • The result type of the function f.
  • f
    • The measuring function.
  • returns
    • the first element of this traversable or iterator with the smallest value measured by function f.
  • Definition Classes
    • TraversableOnce → GenTraversableOnce

(defined at scala.collection.TraversableOnce)

def mkString(sep: String): String

Displays all elements of this traversable or iterator in a string using a separator string.

  • sep
    • the separator string.
  • returns
    • a string representation of this traversable or iterator. In the resulting string the string representations (w.r.t. the method toString ) of all elements of this traversable or iterator are separated by the string sep .
  • Definition Classes
    • TraversableOnce → GenTraversableOnce

Example:

List(1, 2, 3).mkString("|") = "1|2|3"

(defined at scala.collection.TraversableOnce)

def mkString(start: String, sep: String, end: String): String

Displays all elements of this traversable or iterator in a string using start, end, and separator strings.

  • start
    • the starting string.
  • sep
    • the separator string.
  • end
    • the ending string.
  • returns
    • a string representation of this traversable or iterator. The resulting string begins with the string start and ends with the string end . Inside, the string representations (w.r.t. the method toString ) of all elements of this traversable or iterator are separated by the string sep .
  • Definition Classes
    • TraversableOnce → GenTraversableOnce

Example:

List(1, 2, 3).mkString("(", "; ", ")") = "(1; 2; 3)"

(defined at scala.collection.TraversableOnce)

def reduceLeftOption[B >: A](op: (B, A) ⇒ B): Option[B]

Optionally applies a binary operator to all elements of this traversable or iterator, going left to right.

Note: will not terminate for infinite-sized collections.

Note: might return different results for different runs, unless the underlying collection type is ordered or the operator is associative and commutative.

  • B
    • the result type of the binary operator.
  • op
    • the binary operator.
  • returns
    • an option value containing the result of reduceLeft(op) if this traversable or iterator is nonempty, None otherwise.
  • Definition Classes
    • TraversableOnce → GenTraversableOnce

(defined at scala.collection.TraversableOnce)

def reduceLeft[B >: A](op: (B, A) ⇒ B): B

Applies a binary operator to all elements of this traversable or iterator, going left to right.

Note: will not terminate for infinite-sized collections.

Note: might return different results for different runs, unless the underlying collection type is ordered or the operator is associative and commutative.

  • B
    • the result type of the binary operator.
  • op
    • the binary operator.
  • returns
    • the result of inserting op between consecutive elements of this traversable or iterator, going left to right:
    op( op( ... op(x_1, x_2) ..., x_{n-1}), x_n)
    
where `x1, ..., xn` are the elements of this traversable or iterator.
  • Definition Classes
    • TraversableOnce
  • Exceptions thrown
    • UnsupportedOperationException if this traversable or iterator is empty.

(defined at scala.collection.TraversableOnce)

def reduceOption[A1 >: A](op: (A1, A1) ⇒ A1): Option[A1]

Reduces the elements of this traversable or iterator, if any, using the specified associative binary operator.

The order in which operations are performed on elements is unspecified and may be nondeterministic.

  • A1
    • A type parameter for the binary operator, a supertype of A .
  • op
    • A binary operator that must be associative.
  • returns
    • An option value containing result of applying reduce operator op between all the elements if the collection is nonempty, and None otherwise.
  • Definition Classes
    • TraversableOnce → GenTraversableOnce

(defined at scala.collection.TraversableOnce)

def reduceRightOption[B >: A](op: (A, B) ⇒ B): Option[B]

Optionally applies a binary operator to all elements of this traversable or iterator, going right to left.

Note: will not terminate for infinite-sized collections.

Note: might return different results for different runs, unless the underlying collection type is ordered or the operator is associative and commutative.

  • B
    • the result type of the binary operator.
  • op
    • the binary operator.
  • returns
    • an option value containing the result of reduceRight(op) if this traversable or iterator is nonempty, None otherwise.
  • Definition Classes
    • TraversableOnce → GenTraversableOnce

(defined at scala.collection.TraversableOnce)

def reduceRight[B >: A](op: (A, B) ⇒ B): B

Applies a binary operator to all elements of this traversable or iterator, going right to left.

Note: will not terminate for infinite-sized collections.

Note: might return different results for different runs, unless the underlying collection type is ordered or the operator is associative and commutative.

  • B
    • the result type of the binary operator.
  • op
    • the binary operator.
  • returns
    • the result of inserting op between consecutive elements of this traversable or iterator, going right to left:
    op(x_1, op(x_2, ..., op(x_{n-1}, x_n)...))
    
where `x1, ..., xn` are the elements of this traversable or iterator.
  • Definition Classes
    • TraversableOnce → GenTraversableOnce
  • Exceptions thrown
    • UnsupportedOperationException if this traversable or iterator is empty.

(defined at scala.collection.TraversableOnce)

def reduce[A1 >: A](op: (A1, A1) ⇒ A1): A1

Reduces the elements of this traversable or iterator using the specified associative binary operator.

The order in which operations are performed on elements is unspecified and may be nondeterministic.

  • A1
    • A type parameter for the binary operator, a supertype of A .
  • op
    • A binary operator that must be associative.
  • returns
    • The result of applying reduce operator op between all the elements if the traversable or iterator is nonempty.
  • Definition Classes
    • TraversableOnce → GenTraversableOnce
  • Exceptions thrown
    • UnsupportedOperationException if this traversable or iterator is empty.

(defined at scala.collection.TraversableOnce)

def toBuffer[B >: A]: Buffer[B]

Uses the contents of this traversable or iterator to create a new mutable buffer.

Note: will not terminate for infinite-sized collections.

  • returns
    • a buffer containing all elements of this traversable or iterator.
  • Definition Classes
    • TraversableOnce → GenTraversableOnce

(defined at scala.collection.TraversableOnce)

def toIndexedSeq: immutable.IndexedSeq[A]

Converts this traversable or iterator to an indexed sequence.

Note: will not terminate for infinite-sized collections.

  • returns
    • an indexed sequence containing all elements of this traversable or iterator.
  • Definition Classes
    • TraversableOnce → GenTraversableOnce

(defined at scala.collection.TraversableOnce)

def toMap[T, U](implicit ev: <:<[A, (T, U)]): immutable.Map[T, U]

[use case]

Converts this traversable or iterator to a map. This method is unavailable unless the elements are members of Tuple2, each ((T, U)) becoming a key-value pair in the map. Duplicate keys will be overwritten by later keys: if this is an unordered collection, which key is in the resulting map is undefined.

Note: will not terminate for infinite iterators.

  • returns
    • a map of type immutable.Map[T, U] containing all key/value pairs of type (T, U) of this traversable or iterator.
  • Definition Classes
    • TraversableOnce → GenTraversableOnce

(defined at scala.collection.TraversableOnce)

def toSet[B >: A]: immutable.Set[B]

Converts this traversable or iterator to a set.

Note: will not terminate for infinite-sized collections.

  • returns
    • a set containing all elements of this traversable or iterator.
  • Definition Classes
    • TraversableOnce → GenTraversableOnce (defined at scala.collection.TraversableOnce)

Full Source:

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

package scala
package collection

import mutable.ArrayBuffer
import scala.annotation.{tailrec, 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 {

  /** 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
    private[this] var currentHasNextChecked = false
    // Advance current to the next non-empty iterator
    // current is set to null when all iterators are exhausted
    @tailrec
    private[this] def advance(): Boolean = {
      if (queue.isEmpty) {
        current = null
        false
      }
      else {
        current = queue.head()
        queue = queue.tail
        if (current.hasNext) {
          currentHasNextChecked = true
          true
        } else advance()
      }
    }
    def hasNext =
      if (currentHasNextChecked) true
      else if (current eq null) false
      else if (current.hasNext) {
        currentHasNextChecked = true
        true
      } else advance()
    def next()  =
      if (hasNext) {
        currentHasNextChecked = false
        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] var state = 0 // 0: lhs not checked, 1: lhs has next, 2: switched to rhs
    private[this] lazy val rhs: Iterator[A] = that.toIterator
    def hasNext = state match {
      case 0 =>
        if (lhs.hasNext) {
          state = 1
          true
        } else {
          state = 2
          rhs.hasNext
        }
      case 1 => true
      case _ => rhs.hasNext
    }
    def next() = state match {
      case 0 =>
        if (lhs.hasNext) lhs.next()
        else {
          state = 2
          rhs.next()
        }
      case 1 =>
        state = 0
        lhs.next()
      case _ =>
        rhs.next()
    }

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

  /** Creates a delegating iterator capped by a limit count. Negative limit means unbounded.
   *  Lazily skip to start on first evaluation.  Avoids daisy-chained iterators due to slicing.
   */
  private[scala] final class SliceIterator[A](val underlying: Iterator[A], start: Int, limit: Int) extends AbstractIterator[A] {
    private var remaining = limit
    private var dropping  = start
    @inline private def unbounded = remaining < 0
    private def skip(): Unit =
      while (dropping > 0) {
        if (underlying.hasNext) {
          underlying.next()
          dropping -= 1
        } else
          dropping = 0
      }
    def hasNext = { skip(); remaining != 0 && underlying.hasNext }
    def next()  = {
      skip()
      if (remaining > 0) {
        remaining -= 1
        underlying.next()
      }
      else if (unbounded) underlying.next()
      else empty.next()
    }
    override protected def sliceIterator(from: Int, until: Int): Iterator[A] = {
      val lo = from max 0
      def adjustedBound =
        if (unbounded) -1
        else 0 max (remaining - lo)
      val rest =
        if (until < 0) adjustedBound          // respect current bound, if any
        else if (until <= lo) 0               // empty
        else if (unbounded) until - lo        // now finite
        else adjustedBound min (until - lo)   // keep lesser bound
      if (rest == 0) empty
      else {
        dropping += lo
        remaining = rest
        this
      }
    }
  }
}

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 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] = sliceIterator(0, n max 0)

  /** 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] = {
    var j = 0
    while (j < n && hasNext) {
      next()
      j += 1
    }
    this
  }

  /** 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.
   *                If negative, the slice starts at zero.
   *  @param until  the index of the first element following the slice. If negative, the slice is empty.
   *  @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] = sliceIterator(from, until max 0)

  /** Creates an optionally bounded slice, unbounded if `until` is negative. */
  protected def sliceIterator(from: Int, until: Int): Iterator[A] = {
    val lo = from max 0
    val rest =
      if (until < 0) -1            // unbounded
      else if (until <= lo) 0      // empty
      else until - lo              // finite

    if (rest == 0) empty
    else new Iterator.SliceIterator(this, lo, rest)
  }

  /** 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
    private def nextCur() { cur = f(self.next()).toIterator }
    def hasNext: Boolean = {
      // Equivalent to cur.hasNext || self.hasNext && { nextCur(); hasNext }
      // but slightly shorter bytecode (better JVM inlining!)
      while (!cur.hasNext) {
        if (!self.hasNext) return false
        nextCur()
      }
      true
    }
    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] {
    // TODO 2.12 - Make a full-fledged FilterImpl that will reverse sense of p
    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] = new AbstractIterator[B] {
    // Manually buffer to avoid extra layer of wrapping with buffered
    private[this] var hd: A = _

    // Little state machine to keep track of where we are
    // Seek = 0; Found = 1; Empty = -1
    // Not in vals because scalac won't make them static (@inline def only works with -optimize)
    // BE REALLY CAREFUL TO KEEP COMMENTS AND NUMBERS IN SYNC!
    private[this] var status = 0/*Seek*/

    def hasNext = {
      while (status == 0/*Seek*/) {
        if (self.hasNext) {
          hd = self.next()
          if (pf.isDefinedAt(hd)) status = 1/*Found*/
        }
        else status = -1/*Empty*/
      }
      status == 1/*Found*/
    }
    def next() = if (hasNext) { status = 0/*Seek*/; pf(hd) } else Iterator.empty.next()
  }

  /** Produces a collection containing cumulative 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 cumulative results of applying the operator going right to left.
   *  The head of the collection is the last cumulative 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]) = {
    /*
     * 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] {
      var lookahead: mutable.Queue[A] = null
      var hd: A = _
      /* Status is kept with magic numbers
       *   1 means next element is in hd and we're still reading into this iterator
       *   0 means we're still reading but haven't found a next element
       *   -1 means we are done reading into the iterator, so we must rely on lookahead
       *   -2 means we are done but have saved hd for the other iterator to use as its first element
       */
      var status = 0
      private def store(a: A) {
        if (lookahead == null) lookahead = new mutable.Queue[A]
        lookahead += a
      }
      def hasNext = {
        if (status < 0) (lookahead ne null) && lookahead.nonEmpty
        else if (status > 0) true
        else {
          if (self.hasNext) {
            hd = self.next()
            status = if (p(hd)) 1 else -2
          }
          else status = -1
          status > 0
        }
      }
      def next() = {
        if (hasNext) {
          if (status == 1) { status = 0; hd }
          else lookahead.dequeue()
        }
        else empty.next()
      }
      def finish(): Boolean = {
        if (status == -1) false
        else if (status == -2) {
          status = -1
          true
        }
        else {
          if (status == 1) store(hd)
          while (self.hasNext) {
            val a = self.next()
            if (p(a)) store(a)
            else {
              hd = a
              status = -1
              return true
            }
          }
          false
        }
      }
    }

    val leading = new Leading

    val trailing = new AbstractIterator[A] {
      private[this] var myLeading = leading
      /* Status flags meanings:
       *   -1 not yet accesssed
       *   0 single element waiting in leading
       *   1 defer to self
       */
      private[this] var status = -1
      def hasNext = {
        if (status > 0) self.hasNext
        else {
          if (status == 0) true
          else if (myLeading.finish()) {
            status = 0
            true
          }
          else {
            status = 1
            myLeading = null
            self.hasNext
          }
        }
      }
      def next() = {
        if (hasNext) {
          if (status > 0) self.next()
          else {
            status = 1
            val ans = myLeading.hd
            myLeading = null
            ans
          }
        }
        else Iterator.empty.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] = new AbstractIterator[A] {
    // Magic value: -1 = hasn't dropped, 0 = found first, 1 = defer to parent iterator
    private[this] var status = -1
    // Local buffering to avoid double-wrap with .buffered
    private[this] var fst: A = _
    def hasNext: Boolean =
      if (status == 1) self.hasNext
      else if (status == 0) true
      else {
        while (self.hasNext) {
          val a = self.next()
          if (!p(a)) {
            fst = a
            status = 0
            return true
          }
        }
        status = 1
        false
      }
    def next() =
      if (hasNext) {
        if (status == 1) self.next()
        else {
          status = 1
          fst
        }
      }
      else Iterator.empty.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)    // Note--this seems faster than manual inlining!

  /** 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] = {
    while (hasNext) {
      val a = next()
      if (p(a)) return Some(a)
    }
    None
  }

  /** 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 = indexWhere(p, 0)

  /** Returns the index of the first produced value satisfying a predicate, or -1, after or at
   *  some start index.
   *  $mayNotTerminateInf
   *
   *  @param p the predicate to test values
   *  @param from the start index
   *  @return the index `>= from` 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, from: Int): Int = {
    var i = 0
    while (i < from && hasNext) {
      next()
      i += 1
    }

    while (hasNext) {
      if (p(next())) return i
      i += 1
    }
    -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 = indexOf(elem, 0)

  /** Returns the index of the first occurrence of the specified object in this iterable object
   *  after or at some start index.
   *  $mayNotTerminateInf
   *
   *  @param elem element to search for.
   *  @param from the start index
   *  @return the index `>= from` 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, from: Int): Int = {
    var i = 0
    while (i < from && hasNext) {
      next()
      i += 1
    }

    while (hasNext) {
      if (next() == elem) return i
      i += 1
    }
    -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.
     *  NB: self.take.toSeq is slice.toStream, lazily built on self,
     *  so a subsequent self.hasNext would not test self after the
     *  group was consumed.
     */
    private def takeDestructively(size: Int): Seq[A] = {
      val buf = new ArrayBuffer[A]
      var i = 0
      // The order of terms in the following condition is important
      // here as self.hasNext could be blocking
      while (i < size && self.hasNext) {
        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)
        // was: extra checks so we don't calculate length unless there's reason
        // but since we took the group eagerly, just use the fast length
        val shortBy = count - res.length
        if (shortBy > 0 && pad.isDefined) res ++ padding(shortBy) 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.
   *  Patching at negative indices is the same as patching starting at 0.
   *  Patching at indices at or larger than the length of the original iterator appends the patch to the end.
   *  If more values are replaced than actually exist, the excess is ignored.
   *
   *  @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 = (if (from > 0) from else 0)  // Counts down, switch to patch on 0, -1 means use patch first
    def hasNext: Boolean = {
      if (i == 0) {
        origElems = origElems drop replaced
        i = -1
      }
      origElems.hasNext || patchElems.hasNext
    }
    def next(): B = {
      if (i == 0) {
        origElems = origElems drop replaced
        i = -1
      }
      if (i < 0) {
        if (patchElems.hasNext) patchElems.next()
        else origElems.next()
      }
      else {
        if (origElems.hasNext) {
          i -= 1
          origElems.next()
        }
        else {
          i = -1
          patchElems.next()
        }
      }
    }
  }

  /** 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 = {
    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]