Implement sub.

src/lib.rs

@@ -246,6 +246,27 @@ impl<'a> ops::BitOr<&'a Self> for BitSet {
     }
 }
 
+impl<'a> ops::Sub<&'a BitSet> for &'a BitSet {
+    type Output = BitSet;
+
+    #[inline]
+    fn sub(self, rhs: &'a BitSet) -> BitSet {
+        let mut set = self.clone();
+
+        set.nbits = 0;
+
+        for (key, value) in &rhs.blocks {
+            let block = set.blocks.entry(*key).or_insert(0);
+
+            *block = !value & *block;
+
+            set.nbits += block.count_ones() as usize;
+        }
+
+        set
+    }
+}
+
 pub struct Iter<'a> {
     iter: std::collections::hash_map::Iter<'a, u64, u64>,
     block: u64,
@@ -548,6 +569,22 @@ mod tests {
         assert_eq!(observed, 0xFFFF_FFFF);
     }
 
+    #[test]
+    fn test_sub() {
+        let a: BitSet = vec![1, 2, 3].into_iter().collect();
+        let b: BitSet = vec![3, 4, 5].into_iter().collect();
+
+        let set = &a - &b;
+
+        let mut i = 0;
+        let expected = [1, 2];
+        for x in &set {
+            assert!(expected.contains(&x));
+            i += 1;
+        }
+        assert_eq!(i, expected.len());
+    }
+
     #[test]
     fn test_from_iter() {
         let xs = [1, 2, 3, 4, 5, 6, 7, 8, 9];