Custom implementations of drop/take/slice.

This mainly helps performance when comparing keys is expensive.

src/library/scala/collection/immutable/RedBlackTree.scala

@@ -56,6 +56,10 @@ object RedBlackTree {
   def to[A: Ordering, B](tree: Tree[A, B], to: A): Tree[A, B] = blacken(doTo(tree, to))
   def until[A: Ordering, B](tree: Tree[A, B], key: A): Tree[A, B] = blacken(doUntil(tree, key))
 
+  def drop[A: Ordering, B](tree: Tree[A, B], n: Int): Tree[A, B] = blacken(doDrop(tree, n))
+  def take[A: Ordering, B](tree: Tree[A, B], n: Int): Tree[A, B] = blacken(doTake(tree, n))
+  def slice[A: Ordering, B](tree: Tree[A, B], from: Int, until: Int): Tree[A, B] = blacken(doSlice(tree, from, until))
+
   def smallest[A, B](tree: Tree[A, B]): Tree[A, B] = {
     if (tree eq null) throw new NoSuchElementException("empty map")
     var result = tree
@@ -86,7 +90,7 @@ object RedBlackTree {
 
   @tailrec
   def nth[A, B](tree: Tree[A, B], n: Int): Tree[A, B] = {
-    val count = RedBlackTree.count(tree.left)
+    val count = this.count(tree.left)
     if (n < count) nth(tree.left, n)
     else if (n > count) nth(tree.right, n - count - 1)
     else tree
@@ -243,6 +247,39 @@ object RedBlackTree {
     else rebalance(tree, newLeft, newRight)
   }
 
+  private[this] def doDrop[A: Ordering, B](tree: Tree[A, B], n: Int): Tree[A, B] = {
+    if (n <= 0) return tree
+    if (n >= this.count(tree)) return null
+    val count = this.count(tree.left)
+    if (n > count) return doDrop(tree.right, n - count - 1)
+    val newLeft = doDrop(tree.left, n)
+    if (newLeft eq tree.left) tree
+    else if (newLeft eq null) upd(tree.right, tree.key, tree.value)
+    else rebalance(tree, newLeft, tree.right)
+  }
+  private[this] def doTake[A: Ordering, B](tree: Tree[A, B], n: Int): Tree[A, B] = {
+    if (n <= 0) return null
+    if (n >= this.count(tree)) return tree
+    val count = this.count(tree.left)
+    if (n <= count) return doTake(tree.left, n)
+    val newRight = doTake(tree.right, n - count - 1)
+    if (newRight eq tree.right) tree
+    else if (newRight eq null) upd(tree.left, tree.key, tree.value)
+    else rebalance(tree, tree.left, newRight)
+  }
+  private[this] def doSlice[A: Ordering, B](tree: Tree[A, B], from: Int, until: Int): Tree[A, B] = {
+    if (tree eq null) return null
+    val count = this.count(tree.left)
+    if (from > count) return doSlice(tree.right, from - count - 1, until - count - 1)
+    if (until <= count) return doSlice(tree.left, from, until)
+    val newLeft = doDrop(tree.left, from)
+    val newRight = doTake(tree.right, until - count - 1)
+    if ((newLeft eq tree.left) && (newRight eq tree.right)) tree
+    else if (newLeft eq null) upd(newRight, tree.key, tree.value)
+    else if (newRight eq null) upd(newLeft, tree.key, tree.value)
+    else rebalance(tree, newLeft, newRight)
+  }
+
   // The zipper returned might have been traversed left-most (always the left child)
   // or right-most (always the right child). Left trees are traversed right-most,
   // and right trees are traversed leftmost.

src/library/scala/collection/immutable/TreeMap.scala

@@ -89,20 +89,20 @@ class TreeMap[A, +B] private (tree: RB.Tree[A, B])(implicit val ordering: Orderi
   override def drop(n: Int) = {
     if (n <= 0) this
     else if (n >= size) empty
-    else from(RB.nth(tree, n).key)
+    else new TreeMap(RB.drop(tree, n))
   }
 
   override def take(n: Int) = {
     if (n <= 0) empty
     else if (n >= size) this
-    else until(RB.nth(tree, n).key)
+    else new TreeMap(RB.take(tree, n))
   }
 
   override def slice(from: Int, until: Int) = {
     if (until <= from) empty
     else if (from <= 0) take(until)
     else if (until >= size) drop(from)
-    else range(RB.nth(tree, from).key, RB.nth(tree, until).key)
+    else new TreeMap(RB.slice(tree, from, until))
   }
 
   override def dropRight(n: Int) = take(size - n)

src/library/scala/collection/immutable/TreeSet.scala

@@ -66,20 +66,20 @@ class TreeSet[A] private (tree: RB.Tree[A, Unit])(implicit val ordering: Orderin
   override def drop(n: Int) = {
     if (n <= 0) this
     else if (n >= size) empty
-    else from(RB.nth(tree, n).key)
+    else newSet(RB.drop(tree, n))
   }
 
   override def take(n: Int) = {
     if (n <= 0) empty
     else if (n >= size) this
-    else until(RB.nth(tree, n).key)
+    else newSet(RB.take(tree, n))
   }
 
   override def slice(from: Int, until: Int) = {
     if (until <= from) empty
     else if (from <= 0) take(until)
     else if (until >= size) drop(from)
-    else range(RB.nth(tree, from).key, RB.nth(tree, until).key)
+    else newSet(RB.slice(tree, from, until))
   }
 
   override def dropRight(n: Int) = take(size - n)

test/files/scalacheck/treemap.scala

@@ -7,11 +7,12 @@ import util._
 import Buildable._
 
 object Test extends Properties("TreeMap") {
-  implicit def arbTreeMap[A : Arbitrary : Ordering, B : Arbitrary]: Arbitrary[TreeMap[A, B]] =
-    Arbitrary(for {
+  def genTreeMap[A: Arbitrary: Ordering, B: Arbitrary]: Gen[TreeMap[A, B]] =
+    for {
       keys <- listOf(arbitrary[A])
       values <- listOfN(keys.size, arbitrary[B])
-    } yield TreeMap(keys zip values: _*))
+    } yield TreeMap(keys zip values: _*)
+  implicit def arbTreeMap[A : Arbitrary : Ordering, B : Arbitrary] = Arbitrary(genTreeMap[A, B])
 
   property("foreach/iterator consistency") = forAll { (subject: TreeMap[Int, String]) =>
     val it = subject.iterator
@@ -96,6 +97,17 @@ object Test extends Properties("TreeMap") {
     prefix == subject.take(n) && suffix == subject.drop(n)
   }
 
+  def genSliceParms = for {
+    tree <- genTreeMap[Int, String]
+    from <- choose(0, tree.size)
+    until <- choose(from, tree.size)
+  } yield (tree, from, until)
+
+  property("slice") = forAll(genSliceParms) { case (subject, from, until) =>
+    val slice = subject.slice(from, until)
+    slice.size == until - from && subject.toSeq == subject.take(from).toSeq ++ slice ++ subject.drop(until)
+  }
+
   property("takeWhile") = forAll { (subject: TreeMap[Int, String]) =>
     val result = subject.takeWhile(_._1 < 0)
     result.forall(_._1 < 0) && result == subject.take(result.size)

test/files/scalacheck/treeset.scala

@@ -6,8 +6,11 @@ import Arbitrary._
 import util._
 
 object Test extends Properties("TreeSet") {
-  implicit def arbTreeSet[A : Arbitrary : Ordering]: Arbitrary[TreeSet[A]] =
-    Arbitrary(listOf(arbitrary[A]) map (elements => TreeSet(elements: _*)))
+  def genTreeSet[A: Arbitrary: Ordering]: Gen[TreeSet[A]] =
+    for {
+      elements <- listOf(arbitrary[A])
+    } yield TreeSet(elements: _*)
+  implicit def arbTreeSet[A : Arbitrary : Ordering]: Arbitrary[TreeSet[A]] = Arbitrary(genTreeSet)
 
   property("foreach/iterator consistency") = forAll { (subject: TreeSet[Int]) =>
     val it = subject.iterator
@@ -92,6 +95,17 @@ object Test extends Properties("TreeSet") {
     prefix == subject.take(n) && suffix == subject.drop(n)
   }
 
+  def genSliceParms = for {
+    tree <- genTreeSet[Int]
+    from <- choose(0, tree.size)
+    until <- choose(from, tree.size)
+  } yield (tree, from, until)
+
+  property("slice") = forAll(genSliceParms) { case (subject, from, until) =>
+    val slice = subject.slice(from, until)
+    slice.size == until - from && subject.toSeq == subject.take(from).toSeq ++ slice ++ subject.drop(until)
+  }
+
   property("takeWhile") = forAll { (subject: TreeSet[Int]) =>
     val result = subject.takeWhile(_ < 0)
     result.forall(_ < 0) && result == subject.take(result.size)