From b1369582e105f7c07309727cf1cf211dbc2773a6 Mon Sep 17 00:00:00 2001
From: Michele Caini <michele.caini@gmail.com>
Date: Wed, 2 Dec 2020 15:49:05 +0100
Subject: [PATCH] type_traits: added value_list

---
 src/entt/core/type_traits.hpp  | 68 ++++++++++++++++++++++++++++++++++
 test/entt/core/type_traits.cpp | 13 +++++++
 2 files changed, 81 insertions(+)

diff --git a/src/entt/core/type_traits.hpp b/src/entt/core/type_traits.hpp
index de7970be8..b1e5deee8 100644
--- a/src/entt/core/type_traits.hpp
+++ b/src/entt/core/type_traits.hpp
@@ -229,6 +229,74 @@ template<class List, typename Type>
 inline constexpr auto type_list_contains_v = type_list_contains<List, Type>::value;
 
 
+/**
+ * @brief A class to use to push around lists of constant values, nothing more.
+ * @tparam Value Values provided by the value list.
+ */
+template<auto... Value>
+struct value_list {
+    /*! @brief Value list type. */
+    using type = value_list;
+    /*! @brief Compile-time number of elements in the value list. */
+    static constexpr auto size = sizeof...(Value);
+};
+
+
+/**
+ * @brief Concatenates multiple value lists.
+ * @tparam Value Values provided by the first value list.
+ * @tparam Other Values provided by the second value list.
+ * @return A value list composed by the values of both the value lists.
+ */
+template<auto... Value, auto... Other>
+constexpr value_list<Value..., Other...> operator+(value_list<Value...>, value_list<Other...>) { return {}; }
+
+
+/*! @brief Primary template isn't defined on purpose. */
+template<typename...>
+struct value_list_cat;
+
+
+/*! @brief Concatenates multiple value lists. */
+template<>
+struct value_list_cat<> {
+    /*! @brief A value list composed by the values of all the value lists. */
+    using type = value_list<>;
+};
+
+
+/**
+ * @brief Concatenates multiple value lists.
+ * @tparam Value Values provided by the first value list.
+ * @tparam Other Values provided by the second value list.
+ * @tparam List Other value lists, if any.
+ */
+template<auto... Value, auto... Other, typename... List>
+struct value_list_cat<value_list<Value...>, value_list<Other...>, List...> {
+    /*! @brief A value list composed by the values of all the value lists. */
+    using type = typename value_list_cat<value_list<Value..., Other...>, List...>::type;
+};
+
+
+/**
+ * @brief Concatenates multiple value lists.
+ * @tparam Value Values provided by the value list.
+ */
+template<auto... Value>
+struct value_list_cat<value_list<Value...>> {
+    /*! @brief A value list composed by the values of all the value lists. */
+    using type = value_list<Value...>;
+};
+
+
+/**
+ * @brief Helper type.
+ * @tparam List Value lists to concatenate.
+ */
+template<typename... List>
+using value_list_cat_t = typename value_list_cat<List...>::type;
+
+
 /**
  * @brief Provides the member constant `value` to true if a given type is
  * equality comparable, false otherwise.
diff --git a/test/entt/core/type_traits.cpp b/test/entt/core/type_traits.cpp
index 7afd49ea7..c3c514e1a 100644
--- a/test/entt/core/type_traits.cpp
+++ b/test/entt/core/type_traits.cpp
@@ -56,6 +56,19 @@ TEST(TypeTraits, TypeList) {
     static_assert(!entt::type_list_contains_v<type, double>);
 }
 
+TEST(TypeTraits, ValueList) {
+    using value = entt::value_list<0, 2>;
+    using other = entt::value_list<1>;
+
+    static_assert(value::size == 2u);
+    static_assert(other::size == 1u);
+
+    static_assert(std::is_same_v<decltype(value{} + other{}), entt::value_list<0, 2, 1>>);
+    static_assert(std::is_same_v<entt::value_list_cat_t<value, other, value, other>, entt::value_list<0, 2, 1, 0, 2, 1>>);
+    static_assert(std::is_same_v<entt::value_list_cat_t<value, other>, entt::value_list<0, 2, 1>>);
+    static_assert(std::is_same_v<entt::value_list_cat_t<value, value>, entt::value_list<0, 2, 0, 2>>);
+}
+
 TEST(TypeTraits, IsEqualityComparable) {
     static_assert(entt::is_equality_comparable_v<int>);
     static_assert(!entt::is_equality_comparable_v<void>);