Merge pull request #50 from linasm/skip-zero-words-on-shift-left

BitSet shift left - skip zero words

Author: julienrf

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

@@ -8,7 +8,7 @@ class BitSetDecorator[+C <: BitSet with BitSetOps[C]](protected val bs: C) {
   import BitSetOps._
 
   /**
-    * Bitwise left shift of this BitSet by given the shift distance.
+    * Bitwise left shift of this BitSet by the given shift distance.
     * The shift distance may be negative, in which case this method performs a right shift.
     * @param shiftBy shift distance, in bits
     * @return a new BitSet whose value is a bitwise shift left of this BitSet by given shift distance (`shiftBy`)
@@ -44,8 +44,13 @@ class BitSetDecorator[+C <: BitSet with BitSetOps[C]](protected val bs: C) {
     val bitOffset = shiftBy & WordMask
     val wordOffset = shiftBy >>> LogWL
 
+    var significantWordCount = bs.nwords
+    while (significantWordCount > 0 && bs.word(significantWordCount - 1) == 0) {
+      significantWordCount -= 1
+    }
+
     if (bitOffset == 0) {
-      val newSize = bs.nwords + wordOffset
+      val newSize = significantWordCount + wordOffset
       require(newSize <= MaxSize)
       val newBits = Array.ofDim[Long](newSize)
       var i = wordOffset
@@ -56,14 +61,14 @@ class BitSetDecorator[+C <: BitSet with BitSetOps[C]](protected val bs: C) {
       newBits
     } else {
       val revBitOffset = WordLength - bitOffset
-      val extraBits = bs.word(bs.nwords - 1) >>> revBitOffset
+      val extraBits = bs.word(significantWordCount - 1) >>> revBitOffset
       val extraWordCount = if (extraBits == 0) 0 else 1
-      val newSize = bs.nwords + wordOffset + extraWordCount
+      val newSize = significantWordCount + wordOffset + extraWordCount
       require(newSize <= MaxSize)
       val newBits = Array.ofDim[Long](newSize)
       var previous = 0L
       var i = 0
-      while (i < bs.nwords) {
+      while (i < significantWordCount) {
         val current = bs.word(i)
         newBits(i + wordOffset) = (previous >>> revBitOffset) | (current << bitOffset)
         previous = current

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

@@ -36,6 +36,13 @@ class BitSetDecoratorTest {
     }
   }
 
+  @Test
+  def skipZeroWordsOnShiftLeft(): Unit = {
+    val result = BitSet(5 * 64 - 1) << 64
+    assertEquals(BitSet(6 * 64 - 1), result)
+    assertEquals(6, result.nwords)
+  }
+
   @Test
   def shiftEmptyRight(): Unit = {
     for (shiftBy <- 0 to 128) {