diff --git a/src/coprocessor/memoset/multiset.rs b/src/coprocessor/memoset/multiset.rs
index 34abb1d3a4..253cf2c5b0 100644
--- a/src/coprocessor/memoset/multiset.rs
+++ b/src/coprocessor/memoset/multiset.rs
@@ -3,17 +3,57 @@ use std::default::Default;
 use std::hash::Hash;
 
 #[derive(PartialEq, Eq, Debug, Default, Clone)]
-pub(crate) struct MultiSet<T: Hash + Eq>(pub(crate) HashMap<T, usize>);
+pub(crate) struct MultiSet<T: Hash + Eq> {
+    map: HashMap<T, usize>,
+    cardinality: usize,
+}
 
 impl<T: Hash + Eq> MultiSet<T> {
     pub(crate) fn new() -> Self {
-        Self(Default::default())
+        Self {
+            map: Default::default(),
+            cardinality: 0,
+        }
     }
     pub(crate) fn add(&mut self, element: T) {
-        *self.0.entry(element).or_insert(0) += 1;
+        *self.map.entry(element).or_insert(0) += 1;
+        self.cardinality += 1;
     }
 
     pub(crate) fn get(&self, element: &T) -> Option<usize> {
-        self.0.get(element).copied()
+        self.map.get(element).copied()
+    }
+
+    #[allow(dead_code)]
+    pub(crate) fn cardinality(&self) -> usize {
+        self.cardinality
+    }
+
+    #[allow(dead_code)]
+    pub(crate) fn len(&self) -> usize {
+        self.map.len()
+    }
+}
+
+#[cfg(test)]
+mod test {
+    use super::*;
+
+    #[test]
+    fn test_multiset() {
+        let mut m = MultiSet::<usize>::new();
+        let mut c = 0;
+        let n = 5;
+
+        for i in 1..n {
+            for _ in 0..i {
+                m.add(i);
+            }
+            c += i;
+            assert_eq!(i, m.len());
+            assert_eq!(c, m.cardinality());
+            assert_eq!(Some(i), m.get(&i));
+            assert_eq!(None, m.get(&(i + n)));
+        }
     }
 }