Improve performance and behavior of ListMap and ListSet

Makes the immutable `ListMap` and `ListSet` collections more alike one another, both in their semantics and in their performance.

In terms of semantics, makes the `ListSet` iterator return the elements in their insertion order, as `ListMap` already does. While, as mentioned in SI-8985, `ListMap` and `ListSet` doesn't seem to make any guarantees in terms of iteration order, I believe users expect `ListSet` and `ListMap` to behave in the same way, particularly when they are implemented in the exact same way.

In terms of performance, `ListSet` has a custom builder that avoids creation in O(N^2) time. However, this significantly reduces its performance in the creation of small sets, as its requires the instantiation and usage of an auxilliary HashSet. As `ListMap` and `ListSet` are only suitable for small sizes do to their performance characteristics, the builder is removed, the default `SetBuilder` being used instead.

9 files changed, 144 insertions, 98 deletions

diff --git a/src/library/scala/collection/immutable/ListMap.scala b/src/library/scala/collection/immutable/ListMap.scala
index e1bcc0711c..9af05183dd 100644
--- a/src/library/scala/collection/immutable/ListMap.scala
+++ b/src/library/scala/collection/immutable/ListMap.scala
@@ -113,7 +113,8 @@ extends AbstractMap[A, B]
    *  @param xs     the traversable object.
    */
   override def ++[B1 >: B](xs: GenTraversableOnce[(A, B1)]): ListMap[A, B1] =
-    ((repr: ListMap[A, B1]) /: xs.seq) (_ + _)
+    if (xs.isEmpty) this
+    else ((repr: ListMap[A, B1]) /: xs) (_ + _)
 
   /** This creates a new mapping without the given `key`.
    *  If the map does not contain a mapping for the given key, the
@@ -125,14 +126,18 @@ extends AbstractMap[A, B]
 
   /** Returns an iterator over key-value pairs.
    */
-  def iterator: Iterator[(A,B)] =
-    new AbstractIterator[(A,B)] {
-      var self: ListMap[A,B] = ListMap.this
-      def hasNext = !self.isEmpty
-      def next(): (A,B) =
-        if (!hasNext) throw new NoSuchElementException("next on empty iterator")
-        else { val res = (self.key, self.value); self = self.next; res }
-    }.toList.reverseIterator
+  def iterator: Iterator[(A, B)] = {
+    def reverseList = {
+      var curr: ListMap[A, B] = this
+      var res: List[(A, B)] = Nil
+      while (!curr.isEmpty) {
+        res = (curr.key, curr.value) :: res
+        curr = curr.next
+      }
+      res
+    }
+    reverseList.iterator
+  }
 
   protected def key: A = throw new NoSuchElementException("empty map")
   protected def value: B = throw new NoSuchElementException("empty map")
@@ -210,14 +215,9 @@ extends AbstractMap[A, B]
     override def - (k: A): ListMap[A, B1] = remove0(k, this, Nil)
 
     @tailrec private def remove0(k: A, cur: ListMap[A, B1], acc: List[ListMap[A, B1]]): ListMap[A, B1] =
-      if (cur.isEmpty)
-        acc.last
-      else if (k == cur.key)
-        (cur.next /: acc) {
-          case (t, h) => val tt = t; new tt.Node(h.key, h.value) // SI-7459
-        }
-      else
-        remove0(k, cur.next, cur::acc)
+      if (cur.isEmpty) acc.last
+      else if (k == cur.key) (cur.next /: acc) { case (t, h) => new t.Node(h.key, h.value) }
+      else remove0(k, cur.next, cur::acc)
 
     override protected def next: ListMap[A, B1] = ListMap.this
 
diff --git a/src/library/scala/collection/immutable/ListSet.scala b/src/library/scala/collection/immutable/ListSet.scala
index d20e7bc6d2..7803e055ed 100644
--- a/src/library/scala/collection/immutable/ListSet.scala
+++ b/src/library/scala/collection/immutable/ListSet.scala
@@ -12,7 +12,6 @@ package immutable
 
 import generic._
 import scala.annotation.tailrec
-import mutable.{Builder, ReusableBuilder}
 
 /** $factoryInfo
  *  @define Coll immutable.ListSet
@@ -23,33 +22,8 @@ object ListSet extends ImmutableSetFactory[ListSet] {
   /** setCanBuildFromInfo */
   implicit def canBuildFrom[A]: CanBuildFrom[Coll, A, ListSet[A]] = setCanBuildFrom[A]
 
-  override def newBuilder[A]: Builder[A, ListSet[A]] = new ListSetBuilder[A]
-
   private object EmptyListSet extends ListSet[Any] { }
   private[collection] def emptyInstance: ListSet[Any] = EmptyListSet
-
-  /** A custom builder because forgetfully adding elements one at
-   *  a time to a list backed set puts the "squared" in N^2.  There is a
-   *  temporary space cost, but it's improbable a list backed set could
-   *  become large enough for this to matter given its pricy element lookup.
-   *
-   *  This builder is reusable.
-   */
-  class ListSetBuilder[Elem](initial: ListSet[Elem]) extends ReusableBuilder[Elem, ListSet[Elem]] {
-    def this() = this(empty[Elem])
-    protected val elems = (new mutable.ListBuffer[Elem] ++= initial).reverse
-    protected val seen  = new mutable.HashSet[Elem] ++= initial
-
-    def +=(x: Elem): this.type = {
-      if (!seen(x)) {
-        elems += x
-        seen += x
-      }
-      this
-    }
-    def clear() = { elems.clear() ; seen.clear() }
-    def result() = elems.foldLeft(empty[Elem])(_ unchecked_+ _)
-  }
 }
 
 /** This class implements immutable sets using a list-based data
@@ -104,9 +78,8 @@ sealed class ListSet[A] extends AbstractSet[A]
    */
   override def ++(xs: GenTraversableOnce[A]): ListSet[A] =
     if (xs.isEmpty) this
-    else (new ListSet.ListSetBuilder(this) ++= xs.seq).result()
+    else (repr /: xs) (_ + _)
 
-  private[ListSet] def unchecked_+(e: A): ListSet[A] = new Node(e)
   private[ListSet] def unchecked_outer: ListSet[A] =
     throw new NoSuchElementException("Empty ListSet has no outer pointer")
 
@@ -115,33 +88,34 @@ sealed class ListSet[A] extends AbstractSet[A]
    *  @throws java.util.NoSuchElementException
    *  @return the new iterator
    */
-  def iterator: Iterator[A] = new AbstractIterator[A] {
-    var that: ListSet[A] = self
-    def hasNext = that.nonEmpty
-    def next: A =
-      if (hasNext) {
-        val res = that.head
-        that = that.tail
-        res
+  def iterator: Iterator[A] = {
+    def reverseList = {
+      var curr: ListSet[A] = self
+      var res: List[A] = Nil
+      while (!curr.isEmpty) {
+        res = curr.elem :: res
+        curr = curr.next
       }
-      else Iterator.empty.next()
+      res
+    }
+    reverseList.iterator
   }
 
   /**
    *  @throws java.util.NoSuchElementException
    */
-  override def head: A = throw new NoSuchElementException("Set has no elements")
+  protected def elem: A = throw new NoSuchElementException("elem of empty set")
 
   /**
    *  @throws java.util.NoSuchElementException
    */
-  override def tail: ListSet[A] = throw new NoSuchElementException("Next of an empty set")
+  protected def next: ListSet[A] = throw new NoSuchElementException("Next of an empty set")
 
   override def stringPrefix = "ListSet"
 
   /** Represents an entry in the `ListSet`.
    */
-  protected class Node(override val head: A) extends ListSet[A] with Serializable {
+  protected class Node(override val elem: A) extends ListSet[A] with Serializable {
     override private[ListSet] def unchecked_outer = self
 
     /** Returns the number of elements in this set.
@@ -166,7 +140,7 @@ sealed class ListSet[A] extends AbstractSet[A]
      */
     override def contains(e: A) = containsInternal(this, e)
     @tailrec private def containsInternal(n: ListSet[A], e: A): Boolean =
-      !n.isEmpty && (n.head == e || containsInternal(n.unchecked_outer, e))
+      !n.isEmpty && (n.elem == e || containsInternal(n.unchecked_outer, e))
 
     /** This method creates a new set with an additional element.
      */
@@ -174,11 +148,14 @@ sealed class ListSet[A] extends AbstractSet[A]
 
     /** `-` can be used to remove a single element from a set.
      */
-    override def -(e: A): ListSet[A] = if (e == head) self else {
-      val tail = self - e; new tail.Node(head)
-    }
+    override def -(e: A): ListSet[A] = removeInternal(e, this, Nil)
+
+    @tailrec private def removeInternal(k: A, cur: ListSet[A], acc: List[ListSet[A]]): ListSet[A] =
+      if (cur.isEmpty) acc.last
+      else if (k == cur.elem) (cur.next /: acc) { case (t, h) => new t.Node(h.elem) }
+      else removeInternal(k, cur.next, cur :: acc)
 
-    override def tail: ListSet[A] = self
+    override protected def next: ListSet[A] = self
   }
 
   override def toSet[B >: A]: Set[B] = this.asInstanceOf[ListSet[B]]
diff --git a/test/files/jvm/serialization-new.check b/test/files/jvm/serialization-new.check
index cb26446f40..91248320d4 100644
--- a/test/files/jvm/serialization-new.check
+++ b/test/files/jvm/serialization-new.check
@@ -89,8 +89,8 @@ x = Map(buffers -> 20, layers -> 2, title -> 3)
 y = Map(buffers -> 20, layers -> 2, title -> 3)
 x equals y: true, y equals x: true
 
-x = ListSet(5, 3)
-y = ListSet(5, 3)
+x = ListSet(3, 5)
+y = ListSet(3, 5)
 x equals y: true, y equals x: true
 
 x = Queue(a, b, c)
diff --git a/test/files/jvm/serialization.check b/test/files/jvm/serialization.check
index cb26446f40..91248320d4 100644
--- a/test/files/jvm/serialization.check
+++ b/test/files/jvm/serialization.check
@@ -89,8 +89,8 @@ x = Map(buffers -> 20, layers -> 2, title -> 3)
 y = Map(buffers -> 20, layers -> 2, title -> 3)
 x equals y: true, y equals x: true
 
-x = ListSet(5, 3)
-y = ListSet(5, 3)
+x = ListSet(3, 5)
+y = ListSet(3, 5)
 x equals y: true, y equals x: true
 
 x = Queue(a, b, c)
diff --git a/test/files/run/t3822.scala b/test/files/run/t3822.scala
deleted file mode 100644
index c35804035e..0000000000
--- a/test/files/run/t3822.scala
+++ /dev/null
@@ -1,19 +0,0 @@
-import scala.collection.{ mutable, immutable, generic }
-import immutable.ListSet
-
-object Test {
-  def main(args: Array[String]): Unit = {
-    val xs = ListSet(-100000 to 100001: _*)
-
-    assert(xs.size == 200002)
-    assert(xs.sum == 100001)
-
-    val ys = ListSet[Int]()
-    val ys1 = (1 to 12).grouped(3).foldLeft(ys)(_ ++ _)
-    val ys2 = (1 to 12).foldLeft(ys)(_ + _)
-
-    assert(ys1 == ys2)
-  }
-}
-
-
diff --git a/test/files/run/t6198.scala b/test/files/run/t6198.scala
index 5aa8f1c1cf..65dbaf8160 100644
--- a/test/files/run/t6198.scala
+++ b/test/files/run/t6198.scala
@@ -1,13 +1,6 @@
 import scala.collection.immutable._
 
 object Test extends App {
-  // test that ListSet.tail does not use a builder
-  // we can't test for O(1) behavior, so the best we can do is to
-  // check that ls.tail always returns the same instance
-  val ls = ListSet.empty[Int] + 1 + 2
-
-  if(ls.tail ne ls.tail)
-    println("ListSet.tail should not use a builder!")
 
   // class that always causes hash collisions
   case class Collision(value:Int) { override def hashCode = 0 }
diff --git a/test/files/run/t7445.scala b/test/files/run/t7445.scala
deleted file mode 100644
index e4ffeb8e1a..0000000000
--- a/test/files/run/t7445.scala
+++ /dev/null
@@ -1,6 +0,0 @@
-import scala.collection.immutable.ListMap
-
-object Test extends App {
-	val a = ListMap(1 -> 1, 2 -> 2, 3 -> 3, 4 -> 4, 5 -> 5);
-	require(a.tail == ListMap(2 -> 2, 3 -> 3, 4 -> 4, 5 -> 5));
-}
diff --git a/test/junit/scala/collection/immutable/ListMapTest.scala b/test/junit/scala/collection/immutable/ListMapTest.scala
new file mode 100644
index 0000000000..320a976755
--- /dev/null
+++ b/test/junit/scala/collection/immutable/ListMapTest.scala
@@ -0,0 +1,48 @@
+package scala.collection.immutable
+
+import org.junit.Assert._
+import org.junit.Test
+import org.junit.runner.RunWith
+import org.junit.runners.JUnit4
+
+@RunWith(classOf[JUnit4])
+class ListMapTest {
+
+  @Test
+  def t7445(): Unit = {
+    val m = ListMap(1 -> 1, 2 -> 2, 3 -> 3, 4 -> 4, 5 -> 5)
+    assertEquals(ListMap(2 -> 2, 3 -> 3, 4 -> 4, 5 -> 5), m.tail)
+  }
+
+  @Test
+  def hasCorrectBuilder(): Unit = {
+    val m = ListMap("a" -> "1", "b" -> "2", "c" -> "3", "b" -> "2.2", "d" -> "4")
+    assertEquals(List("a" -> "1", "c" -> "3", "b" -> "2.2", "d" -> "4"), m.toList)
+  }
+
+  @Test
+  def hasCorrectHeadTailLastInit(): Unit = {
+    val m = ListMap(1 -> 1, 2 -> 2, 3 -> 3)
+    assertEquals(1 -> 1, m.head)
+    assertEquals(ListMap(2 -> 2, 3 -> 3), m.tail)
+    assertEquals(3 -> 3, m.last)
+    assertEquals(ListMap(1 -> 1, 2 -> 2), m.init)
+  }
+
+  @Test
+  def hasCorrectAddRemove(): Unit = {
+    val m = ListMap(1 -> 1, 2 -> 2, 3 -> 3)
+    assertEquals(ListMap(1 -> 1, 2 -> 2, 3 -> 3, 4 -> 4), m + (4 -> 4))
+    assertEquals(ListMap(1 -> 1, 3 -> 3, 2 -> 4), m + (2 -> 4))
+    assertEquals(ListMap(1 -> 1, 2 -> 2, 3 -> 3), m + (2 -> 2))
+    assertEquals(ListMap(2 -> 2, 3 -> 3), m - 1)
+    assertEquals(ListMap(1 -> 1, 3 -> 3), m - 2)
+    assertEquals(ListMap(1 -> 1, 2 -> 2, 3 -> 3), m - 4)
+  }
+
+  @Test
+  def hasCorrectIterator(): Unit = {
+    val m = ListMap(1 -> 1, 2 -> 2, 3 -> 3, 5 -> 5, 4 -> 4)
+    assertEquals(List(1 -> 1, 2 -> 2, 3 -> 3, 5 -> 5, 4 -> 4), m.iterator.toList)
+  }
+}
diff --git a/test/junit/scala/collection/immutable/ListSetTest.scala b/test/junit/scala/collection/immutable/ListSetTest.scala
new file mode 100644
index 0000000000..395da88c75
--- /dev/null
+++ b/test/junit/scala/collection/immutable/ListSetTest.scala
@@ -0,0 +1,53 @@
+package scala.collection.immutable
+
+import org.junit.Assert._
+import org.junit.Test
+import org.junit.runner.RunWith
+import org.junit.runners.JUnit4
+
+@RunWith(classOf[JUnit4])
+class ListSetTest {
+
+  @Test
+  def t7445(): Unit = {
+    val s = ListSet(1, 2, 3, 4, 5)
+    assertEquals(ListSet(2, 3, 4, 5), s.tail)
+  }
+
+  @Test
+  def hasCorrectBuilder(): Unit = {
+    val m = ListSet("a", "b", "c", "b", "d")
+    assertEquals(List("a", "b", "c", "d"), m.toList)
+  }
+
+  @Test
+  def hasTailRecursiveDelete(): Unit = {
+    val s = ListSet(1 to 50000: _*)
+    try s - 25000 catch { case e: StackOverflowError => fail("A stack overflow occurred") }
+  }
+
+  @Test
+  def hasCorrectHeadTailLastInit(): Unit = {
+    val m = ListSet(1, 2, 3)
+    assertEquals(1, m.head)
+    assertEquals(ListSet(2, 3), m.tail)
+    assertEquals(3, m.last)
+    assertEquals(ListSet(1, 2), m.init)
+  }
+
+  @Test
+  def hasCorrectAddRemove(): Unit = {
+    val m = ListSet(1, 2, 3)
+    assertEquals(ListSet(1, 2, 3, 4), m + 4)
+    assertEquals(ListSet(1, 2, 3), m + 2)
+    assertEquals(ListSet(2, 3), m - 1)
+    assertEquals(ListSet(1, 3), m - 2)
+    assertEquals(ListSet(1, 2, 3), m - 4)
+  }
+
+  @Test
+  def hasCorrectIterator(): Unit = {
+    val s = ListSet(1, 2, 3, 5, 4)
+    assertEquals(List(1, 2, 3, 5, 4), s.iterator.toList)
+  }
+}