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.
scala · May 9, 2016 · 11def59 · 11def59
1 parent 4c4c5e6
commit 11def59
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Showing 6 changed files with 54 additions and 51 deletions.
diff --git 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
@@ -123,15 +124,17 @@ extends AbstractMap[A, B]
    */
   def - (key: A): ListMap[A, B] = this
 
+  private[ListMap] def unchecked_+[B1 >: B](kv: (A, B1)): ListMap[A, B1] = new Node(kv._1, kv._2)
+
   /** 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 }
+        if (hasNext) { val res = (self.key, self.value); self = self.next; res }
+        else Iterator.empty.next()
     }.toList.reverseIterator
 
   protected def key: A = throw new NoSuchElementException("empty map")
@@ -210,14 +213,9 @@ 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
@@ -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
@@ -104,7 +78,7 @@ 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] =
@@ -119,13 +93,9 @@ sealed class ListSet[A] extends AbstractSet[A]
     var that: ListSet[A] = self
     def hasNext = that.nonEmpty
     def next: A =
-      if (hasNext) {
-        val res = that.head
-        that = that.tail
-        res
-      }
+      if (hasNext) { val res = that.head; that = that.tail; res }
       else Iterator.empty.next()
-  }
+  }.toList.reverseIterator
 
   /**
    *  @throws java.util.NoSuchElementException
@@ -174,9 +144,12 @@ 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.head) (cur.tail /: acc) { case (t, h) => new t.Node(h.head) }
+      else removeInternal(k, cur.tail, cur :: acc)
 
     override def tail: ListSet[A] = self
   }

diff --git 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
@@ -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/junit/scala/collection/immutable/ListMapTest.scala b/test/junit/scala/collection/immutable/ListMapTest.scala
@@ -0,0 +1,16 @@
+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 hasCorrectBuilder(): Unit = {
+    val m = ListMap[String, String]("a" -> "1", "b" -> "2", "c" -> "3", "b" -> "2.2", "d" -> "4")
+    assertEquals(List("a" -> "1", "c" -> "3", "b" -> "2.2", "d" -> "4"), m.toList)
+  }
+}
diff --git a/test/junit/scala/collection/immutable/ListSetTest.scala b/test/junit/scala/collection/immutable/ListSetTest.scala
@@ -0,0 +1,16 @@
+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 hasTailRecursiveDelete(): Unit = {
+    val s = ListSet[Int](1 to 50000: _*)
+    try s - 25000 catch { case e: StackOverflowError => fail("A stack overflow occurred") }
+  }
+}