Fix Shrink not clearing unused bits

when new length requires multiple words
bits-and-blooms · Nov 18, 2022 · fd65337 · fd65337
1 parent 6752638
commit fd65337
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Showing 2 changed files with 12 additions and 2 deletions.
diff --git a/bitset.go b/bitset.go
@@ -272,8 +272,9 @@ func (b *BitSet) Shrink(lastbitindex uint) *BitSet {
 	copy(shrunk, b.set[:idx])
 	b.set = shrunk
 	b.length = length
-	if length < 64 {
-		b.set[idx-1] &= allBits >> uint64(64-wordsIndex(length))
+	lastWordUsedBits := length % 64
+	if lastWordUsedBits != 0 {
+		b.set[idx-1] &= allBits >> uint64(64-wordsIndex(lastWordUsedBits))
 	}
 	return b
 }

diff --git a/bitset_test.go b/bitset_test.go
@@ -707,6 +707,15 @@ func TestShrink(t *testing.T) {
 		t.Error("24 should be set")
 		return
 	}
+
+	b = New(110)
+	b.Set(80)
+	b.Shrink(70)
+	for _, word := range b.set {
+		if (word != 0) {
+			t.Error("word should be 0", word)
+		}
+	}
 }
 
 func TestInsertAtWithSet(t *testing.T) {