Fix NSet.SetBits in GetDifference

nset.go

@@ -257,7 +257,13 @@ func (n *NSet[T]) GetDifference(otherSet *NSet[T]) *NSet[T] {
 		}
 
 		if b1.StorageUnitCount > b2.StorageUnitCount {
+
 			copy(newB.Data[b2.StorageUnitCount:], b1.Data[b2.StorageUnitCount:])
+			for j := uint32(b2.StorageUnitCount); j < newB.StorageUnitCount; j++ {
+
+				storage := newB.Data[j]
+				outSet.SetBits += uint64(bits.OnesCount64(uint64(storage)))
+			}
 		}
 	}
 

nset_test.go

@@ -126,7 +126,7 @@ func TestNSet(t *testing.T) {
 
 	nDiff1.AddMany(1, 2, 3, 4, 5, math.MaxUint32)
 	nDiff3 = nDiff1.GetDifference(nDiff2)
-	AllTrue(t, nDiff3.SetBits == 2, nDiff3.ContainsAll(2, 5, math.MaxUint32), !nDiff3.ContainsAny(1, 3, 4))
+	AllTrue(t, nDiff3.SetBits == 3, nDiff3.ContainsAll(2, 5, math.MaxUint32), !nDiff3.ContainsAny(1, 3, 4))
 }
 
 func TestNSetFullRange(t *testing.T) {