From 9ce07ff617dd53d6351ff6fc4fe9e11fdc409bcf Mon Sep 17 00:00:00 2001
From: Michele Caini <michele.caini@gmail.com>
Date: Sun, 26 Mar 2023 12:46:27 +0200
Subject: [PATCH] type_traits: value_list_diff[_t]

---
 src/entt/core/type_traits.hpp  | 22 ++++++++++++++++++++++
 test/entt/core/type_traits.cpp |  6 ++++++
 2 files changed, 28 insertions(+)

diff --git a/src/entt/core/type_traits.hpp b/src/entt/core/type_traits.hpp
index 0dfdfdff0..45df48471 100644
--- a/src/entt/core/type_traits.hpp
+++ b/src/entt/core/type_traits.hpp
@@ -558,6 +558,28 @@ struct value_list_contains<value_list<Value...>, Other>
 template<typename List, auto Value>
 inline constexpr bool value_list_contains_v = value_list_contains<List, Value>::value;
 
+/*! @brief Primary template isn't defined on purpose. */
+template<typename...>
+struct value_list_diff;
+
+/**
+ * @brief Computes the difference between two value lists.
+ * @tparam Value Values provided by the first value list.
+ * @tparam Other Values provided by the second value list.
+ */
+template<auto... Value, auto... Other>
+struct value_list_diff<value_list<Value...>, value_list<Other...>> {
+    /*! @brief A value list that is the difference between the two value lists. */
+    using type = value_list_cat_t<std::conditional_t<value_list_contains_v<value_list<Other...>, Value>, value_list<>, value_list<Value>>...>;
+};
+
+/**
+ * @brief Helper type.
+ * @tparam List Value lists between which to compute the difference.
+ */
+template<typename... List>
+using value_list_diff_t = typename value_list_diff<List...>::type;
+
 /*! @brief Same as std::is_invocable, but with tuples. */
 template<typename, typename>
 struct is_applicable: std::false_type {};
diff --git a/test/entt/core/type_traits.cpp b/test/entt/core/type_traits.cpp
index 935cc7440..a6f2e4647 100644
--- a/test/entt/core/type_traits.cpp
+++ b/test/entt/core/type_traits.cpp
@@ -138,6 +138,12 @@ TEST(ValueList, Functionalities) {
     static_assert(entt::value_list_index_v<0, value> == 0u);
     static_assert(entt::value_list_index_v<2, value> == 1u);
     static_assert(entt::value_list_index_v<1, other> == 0u);
+
+    static_assert(std::is_same_v<entt::value_list_diff_t<entt::value_list<0, 1, 2>, entt::value_list<3, 4>>, entt::value_list<0, 1, 2>>);
+    static_assert(std::is_same_v<entt::value_list_diff_t<entt::value_list<0, 1, 2>, entt::value_list<0, 1, 2>>, entt::value_list<>>);
+    static_assert(std::is_same_v<entt::value_list_diff_t<entt::value_list<0, 1, 2>, entt::value_list<0, 1>>, entt::value_list<2>>);
+    static_assert(std::is_same_v<entt::value_list_diff_t<entt::value_list<0, 1, 2>, entt::value_list<1, 2>>, entt::value_list<0>>);
+    static_assert(std::is_same_v<entt::value_list_diff_t<entt::value_list<0, 1, 2>, entt::value_list<1>>, entt::value_list<0, 2>>);
 }
 
 TEST(IsApplicable, Functionalities) {