Merge pull request #10651 from som-snytt/followup/array-builder-followup

Size hint with delta is not negative

src/library/scala/collection/mutable/Builder.scala

@@ -35,10 +35,7 @@ trait Builder[-A, +To] extends Growable[A] { self =>
    *
    *  Some builder classes will optimize their representation based on the hint.
    *  However, builder implementations are required to work correctly even if the hint is
-   *  wrong, i.e., if a different number of elements is added.
-   *
-   *  The semantics of supplying a hint out of range, such as a size that is negative
-   *  or unreasonably large, is not specified but is implementation-dependent.
+   *  wrong, e.g., a different number of elements is added, or the hint is out of range.
    *
    *  The default implementation simply ignores the hint.
    *
@@ -54,9 +51,13 @@ trait Builder[-A, +To] extends Growable[A] { self =>
    *
    *  {{{
    *    if (coll.knownSize != -1)
-   *      sizeHint(coll.knownSize + delta)
+   *      if (coll.knownSize + delta <= 0) sizeHint(0)
+   *      else sizeHint(coll.knownSize + delta)
    *  }}}
    *
+   *  If the delta is negative and the result size is known to be negative,
+   *  then the size hint is issued at zero.
+   *
    *  Some builder classes will optimize their representation based on the hint.
    *  However, builder implementations are required to work correctly even if the hint is
    *  wrong, i.e., if a different number of elements is added.
@@ -67,7 +68,7 @@ trait Builder[-A, +To] extends Growable[A] { self =>
   final def sizeHint(coll: scala.collection.IterableOnce[_], delta: Int = 0): Unit =
     coll.knownSize match {
       case -1 =>
-      case sz => sizeHint(sz + delta)
+      case sz => sizeHint(0 max sz + delta)
     }
 
   /** Gives a hint how many elements are expected to be added

test/junit/scala/collection/mutable/BuilderTest.scala

@@ -0,0 +1,25 @@
+
+package scala.collection.mutable
+
+import org.junit.Test
+import org.junit.Assert.assertEquals
+
+class BuilderTest {
+  class TestBuilder extends Builder[String, String] {
+    val stuff = ListBuffer.empty[String]
+    def clear() = stuff.clear()
+    def result() = stuff.mkString("hello, world: ", ",", "")
+    def addOne(s: String) = { stuff.addOne(s); this }
+    var expectedSize = 16
+    override def sizeHint(size: Int) = expectedSize = size
+  }
+
+  @Test def `sizeHint is capped`: Unit = {
+    val b = new TestBuilder
+    val prototype = Nil
+    b.sizeHint(prototype, delta = -1)
+    assertEquals(0, b.expectedSize)
+    b.addOne("tested!")
+    assertEquals("hello, world: tested!", b.result())
+  }
+}