diff --git a/a8/commonrank.h b/a8/commonrank.h
new file mode 100644
index 0000000..67e671b
--- /dev/null
+++ b/a8/commonrank.h
@@ -0,0 +1,99 @@
+#ifndef A8_COMMONRANK_H
+#define A8_COMONRANK_H
+
+#include <algorithm>
+
+namespace a8
+{
+
+    template <class T>
+    class CommonRank
+    {
+    private:
+        std::vector<T*> list_;
+        std::set<T*> set_;
+        std::function<bool (const T*, const T*)> cmp_func_;
+        size_t max_num_ = 0;
+
+    public:
+
+        CommonRank() {}
+        CommonRank(std::function<bool (const T*, const T*)> cmp_func, size_t max_num)
+        {
+            cmp_func_ = cmp_func;
+            max_num_ = max_num;
+        }
+
+        size_t Size()
+        {
+            return list_.size();
+        }
+
+        const std::vector<T*>& GetList()
+        {
+            return list_;
+        }
+
+        void Sort()
+        {
+            std::sort(list_.begin(), list_.end(), cmp_func_);
+        }
+
+        void Update(T* val)
+        {
+            if (Size() < max_num_) {
+                if (!Exists(val)) {
+                    list_.push_back(val);
+                    set_.insert(val);
+                    Sort();
+                }
+            } else {
+                if (cmp_func_(val, list_[list_.size() - 1])) {
+                    if (!Exists(val)) {
+                        set_.erase(list_[list_.size() - 1]);
+                        list_[list_.size() - 1] = val;
+                        set_.insert(val);
+                        Sort();
+                    }
+                }
+            }
+        }
+
+        bool Exists(T* val)
+        {
+            return set_.find(val) != set_.end();
+        }
+
+    };
+
+    template <typename T>
+    a8::CommonRank<T>& ForceCreateCommonRankList(std::map<long long, a8::CommonRank<T>>& rank_list,
+                                                 long long key,
+                                                 std::function<bool (const T*, const T*)> cmp_func,
+                                                 size_t max_num)
+    {
+        auto itr = rank_list.find(key);
+        if (itr != rank_list.end()) {
+            return itr->second;
+        }
+        rank_list[key] = a8::CommonRank<T>(cmp_func, max_num);
+        return rank_list[key];
+    }
+
+    template <typename T>
+    static a8::CommonRank<T>& ForceCreateCommonRankList(std::map<std::string, a8::CommonRank<T>>& rank_list,
+                                                        const std::string& key,
+                                                        std::function<bool (const T*, const T*)> cmp_func,
+                                                        size_t max_num)
+    {
+        auto itr = rank_list.find(key);
+        if (itr != rank_list.end()) {
+            return itr->second;
+        }
+        rank_list[key] = a8::CommonRank<T>(cmp_func, max_num);
+        return rank_list[key];
+    }
+
+}
+
+#endif