diff --git a/data_structure/test/assosiative_array.test.cpp b/data_structure/test/assosiative_array.test.cpp
index d9b105bb..fcdc245a 100644
--- a/data_structure/test/assosiative_array.test.cpp
+++ b/data_structure/test/assosiative_array.test.cpp
@@ -1,24 +1,7 @@
 #define PROBLEM "https://judge.yosupo.jp/problem/associative_array"
+#include "../../random/custom_hash.hpp"
 #include <iostream>
-#include <unordered_map>
 
-#include <chrono>
-struct custom_hash {
-    // <https://codeforces.com/blog/entry/62393>
-    static uint64_t splitmix64(uint64_t x) {
-        // http://xorshift.di.unimi.it/splitmix64.c
-        x += 0x9e3779b97f4a7c15;
-        x = (x ^ (x >> 30)) * 0xbf58476d1ce4e5b9;
-        x = (x ^ (x >> 27)) * 0x94d049bb133111eb;
-        return x ^ (x >> 31);
-    }
-
-    size_t operator()(uint64_t x) const {
-        static const uint64_t FIXED_RANDOM =
-            std::chrono::steady_clock::now().time_since_epoch().count();
-        return splitmix64(x + FIXED_RANDOM);
-    }
-};
 #include <ext/pb_ds/assoc_container.hpp>
 using namespace __gnu_pbds;
 
diff --git a/random/custom_hash.hpp b/random/custom_hash.hpp
index 7079ba86..865619ac 100644
--- a/random/custom_hash.hpp
+++ b/random/custom_hash.hpp
@@ -1,6 +1,7 @@
 #pragma once
 #include <chrono>
 #include <cstdlib>
+#include <functional>
 
 struct custom_hash {
     // https://codeforces.com/blog/entry/62393
@@ -19,10 +20,19 @@ struct custom_hash {
     }
 };
 
-// Usage
-#include <unordered_map>
-std::unordered_map<int, int, custom_hash> robust_unordered_map;
+// Template of std::hash for arbitrary structs
+// template <> struct std::hash<T> {
+//     std::size_t operator()(const T &x) const noexcept {
+//         static custom_hash h;
+//         return h(/* */);
+//     }
+// };
 
-#include <ext/pb_ds/assoc_container.hpp>
-using namespace __gnu_pbds;
-gp_hash_table<int, null_type, custom_hash> robust_hash_table; // fast unordered_set / unordered_map
+// robust unordered_map
+// #include <unordered_map>
+// std::unordered_map<int, int, custom_hash> robust_unordered_map;
+
+// fast unordered_set / unordered_map
+// #include <ext/pb_ds/assoc_container.hpp>
+// using namespace __gnu_pbds;
+// gp_hash_table<int, null_type, custom_hash> fast_hash_table;
diff --git a/random/custom_hash.md b/random/custom_hash.md
new file mode 100644
index 00000000..d4032e72
--- /dev/null
+++ b/random/custom_hash.md
@@ -0,0 +1,11 @@
+---
+title: Custom hash functions （各種データ構造のためのハッシュ関数）
+documentation_of: ./custom_hash.hpp
+---
+
+Codeforces 等で `std::unordered_set<>` や `std::unordered_map<>` を使用した場合のハッシュ衝突攻撃を防止するハッシュ関数．また，任意の構造体をこれらのキーとして与えるための `std::hash` の特殊化のテンプレートを含む．
+
+## Link
+
+- [Blowing up unordered_map, and how to stop getting hacked on it - Codeforces](https://codeforces.com/blog/entry/62393)
+- [<unordered_set> [🟢C++20 対応]｜競プロのための標準 C++](https://zenn.dev/reputeless/books/standard-cpp-for-competitive-programming/viewer/unordered_set)