Bench sub.

Improve performance for sub.

src/lib.rs

@@ -254,17 +254,15 @@ impl<'a> ops::Sub<&'a BitSet> for &'a BitSet {
         let mut set = self.clone();
 
         for (key, block) in &mut set.blocks {
-            if let Some(value) = rhs.blocks.get(&key) {
-                // let n = block.count_ones();
+            if let Some(value) = rhs.blocks.get(key) {
+                let n = block.count_ones() as i32;
 
                 *block &= !value;
 
-                // set.nbits += (block.count_ones() - n) as usize;
+                set.nbits = (set.nbits as i32 + (block.count_ones() as i32 - n)) as usize;
             }
         }
 
-        set.nbits = set.blocks.values().map(|block| block.count_ones()).sum::<u32>() as usize;
-
         set
     }
 }
@@ -587,7 +585,9 @@ mod tests {
         assert_eq!(i, expected.len());
         assert_eq!(set.len(), expected.len());
 
-        let a: BitSet = vec![1, 2, 3, 4, 5, 100, 101, 1001, 1002].into_iter().collect();
+        let a: BitSet = vec![1, 2, 3, 4, 5, 100, 101, 1001, 1002]
+            .into_iter()
+            .collect();
         let b: BitSet = vec![1, 2, 3].into_iter().collect();
 
         let set = &a - &b;