diff --git a/misc/set_intersection.cc b/misc/set_intersection.cc
new file mode 100644
index 0000000..cdeacb0
--- /dev/null
+++ b/misc/set_intersection.cc
@@ -0,0 +1,15 @@
+// Set intersection supporting std::unordered_set.
+template <typename Set, typename Key = typename Set::value_type>
+static inline Set set_intersection(const Set& lhs, const Set& rhs) {
+    if (lhs.size() <= rhs.size()) {
+        Set iset;
+        for (const Key& key : lhs) {
+            if (rhs.count(key) > 0) {
+                iset.insert(key);
+            }
+        }
+        return std::move(iset);
+    } else {
+        return set_intersection(rhs, lhs);
+    }
+}
\ No newline at end of file