Improved handling of empty BitSet

Author: linasm

src/main/scala/scala/collection/decorators/BitSetDecorator.scala

@@ -15,7 +15,8 @@ class BitSetDecorator[+C <: BitSet with BitSetOps[C]](protected val bs: C) {
     */
   def <<(shiftBy: Int): C = {
 
-    val shiftedBits = if (shiftBy > 0) shiftLeft(shiftBy)
+    val shiftedBits = if (bs.nwords == 0 || bs.nwords == 1 && bs.word(0) == 0) Array.emptyLongArray
+    else if (shiftBy > 0) shiftLeft(shiftBy)
     else if (shiftBy == 0) bs.toBitMask
     else shiftRight(-shiftBy)
 
@@ -30,7 +31,8 @@ class BitSetDecorator[+C <: BitSet with BitSetOps[C]](protected val bs: C) {
     */
   def >>(shiftBy: Int): C = {
 
-    val shiftedBits = if (shiftBy > 0) shiftRight(shiftBy)
+    val shiftedBits = if (bs.nwords == 0 || bs.nwords == 1 && bs.word(0) == 0) Array.emptyLongArray
+    else if (shiftBy > 0) shiftRight(shiftBy)
     else if (shiftBy == 0) bs.toBitMask
     else shiftLeft(-shiftBy)
 

src/test/scala/scala/collection/decorators/BitSetDecoratorTest.scala

@@ -6,13 +6,13 @@ import scala.collection.BitSet
 
 class BitSetDecoratorTest {
 
-  import Assert.assertEquals
+  import Assert.{assertEquals, assertSame}
   import BitSet.empty
 
   @Test
   def shiftEmptyLeft(): Unit = {
     for (shiftBy <- 0 to 128) {
-      assertEquals(empty, empty << shiftBy)
+      assertSame(empty, empty << shiftBy)
     }
   }
 
@@ -39,15 +39,15 @@ class BitSetDecoratorTest {
   @Test
   def shiftEmptyRight(): Unit = {
     for (shiftBy <- 0 to 128) {
-      assertEquals(empty, empty >> shiftBy)
+      assertSame(empty, empty >> shiftBy)
     }
   }
 
   @Test
   def shiftLowestBitRight(): Unit = {
     assertEquals(BitSet(0), BitSet(0) >> 0)
     for (shiftBy <- 1 to 128) {
-      assertEquals(empty, BitSet(0) >> shiftBy)
+      assertSame(empty, BitSet(0) >> shiftBy)
     }
   }