diff options
Diffstat (limited to 'src/library')
-rw-r--r-- | src/library/scala/collection/immutable/Stream.scala | 32 |
1 files changed, 26 insertions, 6 deletions
diff --git a/src/library/scala/collection/immutable/Stream.scala b/src/library/scala/collection/immutable/Stream.scala index 78c4d76eda..5566806c55 100644 --- a/src/library/scala/collection/immutable/Stream.scala +++ b/src/library/scala/collection/immutable/Stream.scala @@ -181,6 +181,7 @@ import scala.language.implicitConversions * @define coll stream * @define orderDependent * @define orderDependentFold + * @define willTerminateInf Note: lazily evaluated; will terminate for infinite-sized collections. */ abstract class Stream[+A] extends AbstractSeq[A] with LinearSeq[A] @@ -286,9 +287,8 @@ self => len } - /** It's an imperfect world, but at least we can bottle up the - * imperfection in a capsule. - */ + // It's an imperfect world, but at least we can bottle up the + // imperfection in a capsule. @inline private def asThat[That](x: AnyRef): That = x.asInstanceOf[That] @inline private def asStream[B](x: AnyRef): Stream[B] = x.asInstanceOf[Stream[B]] @inline private def isStreamBuilder[B, That](bf: CanBuildFrom[Stream[A], B, That]) = @@ -725,10 +725,15 @@ self => * // produces: "5, 6, 7, 8, 9" * }}} */ - override def take(n: Int): Stream[A] = + override def take(n: Int): Stream[A] = ( + // Note that the n == 1 condition appears redundant but is not. + // It prevents "tail" from being referenced (and its head being evaluated) + // when obtaining the last element of the result. Such are the challenges + // of working with a lazy-but-not-really sequence. if (n <= 0 || isEmpty) Stream.empty else if (n == 1) cons(head, Stream.empty) else cons(head, tail take n-1) + ) @tailrec final override def drop(n: Int): Stream[A] = if (n <= 0 || isEmpty) this @@ -784,8 +789,23 @@ self => these } - // there's nothing we can do about dropRight, so we just keep the definition - // in LinearSeq + /** + * @inheritdoc + * $willTerminateInf + */ + override def dropRight(n: Int): Stream[A] = { + // We make dropRight work for possibly infinite streams by carrying + // a buffer of the dropped size. As long as the buffer is full and the + // rest is non-empty, we can feed elements off the buffer head. When + // the rest becomes empty, the full buffer is the dropped elements. + def advance(stub0: List[A], stub1: List[A], rest: Stream[A]): Stream[A] = { + if (rest.isEmpty) Stream.empty + else if (stub0.isEmpty) advance(stub1.reverse, Nil, rest) + else cons(stub0.head, advance(stub0.tail, rest.head :: stub1, rest.tail)) + } + if (n <= 0) this + else advance((this take n).toList, Nil, this drop n) + } /** Returns the longest prefix of this `Stream` whose elements satisfy the * predicate `p`. |