Clear unused bits in Not() (#38)

bitlist64.go

@@ -308,6 +308,7 @@ func (b *Bitlist64) NoAllocNot(ret *Bitlist64) {
 	for idx, word := range b.data {
 		ret.data[idx] = ^word
 	}
+	ret.clearUnusedBits()
 }
 
 // BitIndices returns list of bit indexes of bitlist where value is set to true.
@@ -367,3 +368,11 @@ func (b *Bitlist64) Clone() *Bitlist64 {
 func numWordsRequired(n uint64) int {
 	return int((n + (wordSize - 1)) >> wordSizeLog2)
 }
+
+// clearUnusedBits zeroes unused bits in the last word.
+func (b *Bitlist64) clearUnusedBits() {
+	// Unless bitlist is divisible by a word evenly, we need to zero unused bits in the last word.
+	if !(b.size%wordSize == 0) {
+		b.data[len(b.data)-1] &= allBitsSet >> (wordSize - b.size%wordSize)
+	}
+}

bitlist64_test.go

@@ -1195,6 +1195,16 @@ func TestBitlist64_Not(t *testing.T) {
 			a:    NewBitlist64From([]uint64{}), // zero-length bitlist
 			want: NewBitlist64From([]uint64{}),
 		},
+		{
+			// Last word bits are unused, single word.
+			a:    NewBitlist64(3),                  // 0b*****000
+			want: NewBitlist64From([]uint64{0x07}), // 0b00000111
+		},
+		{
+			// Last word bits are unused, multiple words.
+			a:    NewBitlist64(131),                                        // 0x00..00, 0x00..00, 0b*****000
+			want: NewBitlist64From([]uint64{allBitsSet, allBitsSet, 0x07}), // 0xFF..FF, 0xFF..FF, 0b00000111
+		},
 		{
 			a:    NewBitlist64From([]uint64{0x01}),               // 0b00000001
 			want: NewBitlist64From([]uint64{0xFFFFFFFFFFFFFFFE}), // 0b11111110
@@ -1252,7 +1262,7 @@ func TestBitlist64_Not(t *testing.T) {
 	t.Run("Not()", func(t *testing.T) {
 		for _, tt := range tests {
 			if !reflect.DeepEqual(tt.a.Not().data, tt.want.data) {
-				t.Errorf("(%+v).Not() = %x, wanted %x", tt.a, tt.a.Not().data, tt.want)
+				t.Errorf("(%+v).Not() = %x, wanted %x", tt.a, tt.a.Not().data, tt.want.data)
 			}
 		}
 	})