Some tweaks to slice to make it run faster, no ...

Some tweaks to slice to make it run faster, no review.

2 files changed, 12 insertions, 2 deletions

diff --git a/src/library/scala/collection/TraversableLike.scala b/src/library/scala/collection/TraversableLike.scala
index 2d3a4c229c..fe1d5d7d77 100644
--- a/src/library/scala/collection/TraversableLike.scala
+++ b/src/library/scala/collection/TraversableLike.scala
@@ -434,10 +434,14 @@ trait TraversableLike[+A, +Repr] extends HasNewBuilder[A, Repr]
   def take(n: Int): Repr = slice(0, n)
 
   def drop(n: Int): Repr =
-    if (n <= 0) newBuilder ++= thisCollection result
+    if (n <= 0) {
+      val b = newBuilder
+      b.sizeHint(this)
+      b ++= thisCollection result
+    }
     else sliceWithKnownDelta(n, Int.MaxValue, -n)
 
-  def slice(from: Int, until: Int): Repr = sliceWithKnownBound(from max 0, until)
+  def slice(from: Int, until: Int): Repr = sliceWithKnownBound(math.max(from, 0), until)
 
   // Precondition: from >= 0, until > 0, builder already configured for building.
   private[this] def sliceInternal(from: Int, until: Int, b: Builder[A, Repr]): Repr = {
diff --git a/src/library/scala/collection/immutable/List.scala b/src/library/scala/collection/immutable/List.scala
index c14e24ab47..d8c8268a2e 100644
--- a/src/library/scala/collection/immutable/List.scala
+++ b/src/library/scala/collection/immutable/List.scala
@@ -171,6 +171,12 @@ sealed abstract class List[+A] extends LinearSeq[A]
     these
   }
 
+  override def slice(from: Int, until: Int): List[A] = {
+    val lo = math.max(from, 0)
+    if (until <= lo || isEmpty) Nil
+    else this drop lo take (until - lo)
+  }
+
   override def takeRight(n: Int): List[A] = {
     @tailrec
     def loop(lead: List[A], lag: List[A]): List[A] = lead match {