SI-7102 Specialize isEmpty for bitsets

Currently bitsets use default isEmpty implementation inherited from
Set, which tests for "size == 0".

Calculating the size of a word in a bitmap requires summing through
all bits set, whereas testing for emptyness needs only one comparison
with zero.

This commit overrides the default implementation with the specialized
one looking for a non-zero word in this bitmap.

1 files changed, 13 insertions, 0 deletions

diff --git a/test/files/run/bitsets.scala b/test/files/run/bitsets.scala
index 0ea43fcb95..d55f9e4e83 100644
--- a/test/files/run/bitsets.scala
+++ b/test/files/run/bitsets.scala
@@ -37,6 +37,19 @@ object TestMutable {
   Console.println("mi1 = " + ms1.toImmutable)
   Console.println("mi2 = " + ms2.toImmutable)
   Console.println
+
+  val N = 257
+  val gen = 3
+  val bs = BitSet((1 until N): _*)
+  (1 until N).foldLeft(gen) {
+    case (acc, i) =>
+      assert(bs.size == N-i, s"Bad size for $bs, expected ${N-i} actual ${bs.size}")
+      assert(!bs.isEmpty, s"Unexpected isEmpty for $bs")
+      bs -= acc
+      acc*gen % N
+  }
+  assert(bs.size == 0, s"Expected size == 0 for $bs")
+  assert(bs.isEmpty, s"Expected isEmpty for $bs")
 }
 
 object TestMutable2 {