AK: Add a Tuple implementation

Please don't use this outside of metaprogramming needs, *please*.

AK/Tuple.h

@@ -0,0 +1,218 @@
+/*
+ * Copyright (c) 2021, Ali Mohammad Pur <mpfard@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include <AK/StdLibExtras.h>
+#include <AK/TypeList.h>
+
+namespace AK::Detail {
+
+template<typename... Ts>
+struct Tuple {
+};
+
+template<typename T>
+struct Tuple<T> {
+    Tuple(T&& value) requires(!IsSame<T&&, const T&>)
+        : value(forward<T>(value))
+    {
+    }
+
+    Tuple(const T& value)
+        : value(value)
+    {
+    }
+
+    template<typename U>
+    U& get()
+    {
+        static_assert(IsSame<T, U>, "Invalid tuple access");
+        return value;
+    }
+
+    template<typename U>
+    const U& get() const
+    {
+        return const_cast<Tuple<T>&>(*this).get<U>();
+    }
+
+    template<typename U, unsigned index>
+    U& get_with_index()
+    {
+        static_assert(IsSame<T, U> && index == 0, "Invalid tuple access");
+        return value;
+    }
+
+    template<typename U, unsigned index>
+    const U& get_with_index() const
+    {
+        return const_cast<Tuple<T>&>(*this).get_with_index<U, index>();
+    }
+
+private:
+    T value;
+};
+
+template<typename T, typename... TRest>
+struct Tuple<T, TRest...> : Tuple<TRest...> {
+    Tuple(T&& first, TRest&&... rest)
+        : Tuple<TRest...>(forward<TRest>(rest)...)
+        , value(forward<T>(first))
+    {
+    }
+
+    Tuple(const T& first, const TRest&... rest)
+        : Tuple<TRest...>(rest...)
+        , value(first)
+    {
+    }
+
+    template<typename U>
+    U& get()
+    {
+        if constexpr (IsSame<T, U>)
+            return value;
+        else
+            return Tuple<TRest...>::template get<U>();
+    }
+
+    template<typename U>
+    const U& get() const
+    {
+        return const_cast<Tuple<T, TRest...>&>(*this).get<U>();
+    }
+
+    template<typename U, unsigned index>
+    U& get_with_index()
+    {
+        if constexpr (IsSame<T, U> && index == 0)
+            return value;
+        else
+            return Tuple<TRest...>::template get_with_index<U, index - 1>();
+    }
+
+    template<typename U, unsigned index>
+    const U& get_with_index() const
+    {
+        return const_cast<Tuple<T, TRest...>&>(*this).get_with_index<U, index>();
+    }
+
+private:
+    T value;
+};
+
+}
+
+namespace AK {
+
+template<typename... Ts>
+struct Tuple : Detail::Tuple<Ts...> {
+    using Types = TypeList<Ts...>;
+    using Detail::Tuple<Ts...>::Tuple;
+    using Indices = MakeIndexSequence<sizeof...(Ts)>;
+
+    Tuple(Tuple&& other)
+        : Tuple(move(other), Indices())
+    {
+    }
+
+    Tuple(const Tuple& other)
+        : Tuple(other, Indices())
+    {
+    }
+
+    Tuple& operator=(Tuple&& other)
+    {
+        set(move(other), Indices());
+        return *this;
+    }
+
+    Tuple& operator=(const Tuple& other)
+    {
+        set(other, Indices());
+        return *this;
+    }
+
+    template<typename T>
+    auto& get()
+    {
+        return Detail::Tuple<Ts...>::template get<T>();
+    }
+
+    template<unsigned index>
+    auto& get()
+    {
+        return Detail::Tuple<Ts...>::template get_with_index<typename Types::template Type<index>, index>();
+    }
+
+    template<typename T>
+    auto& get() const
+    {
+        return Detail::Tuple<Ts...>::template get<T>();
+    }
+
+    template<unsigned index>
+    auto& get() const
+    {
+        return Detail::Tuple<Ts...>::template get_with_index<typename Types::template Type<index>, index>();
+    }
+
+    template<typename F>
+    auto apply_as_args(F&& f)
+    {
+        return apply_as_args(forward<F>(f), Indices());
+    }
+
+    template<typename F>
+    auto apply_as_args(F&& f) const
+    {
+        return apply_as_args(forward<F>(f), Indices());
+    }
+
+    static constexpr auto size() { return sizeof...(Ts); }
+
+private:
+    template<unsigned... Is>
+    Tuple(Tuple&& other, IndexSequence<Is...>)
+        : Detail::Tuple<Ts...>(move(other.get<Is>())...)
+    {
+    }
+
+    template<unsigned... Is>
+    Tuple(const Tuple& other, IndexSequence<Is...>)
+        : Detail::Tuple<Ts...>(other.get<Is>()...)
+    {
+    }
+
+    template<unsigned... Is>
+    void set(Tuple&& other, IndexSequence<Is...>)
+    {
+        ((get<Is>() = move(other.get<Is>())), ...);
+    }
+
+    template<unsigned... Is>
+    void set(const Tuple& other, IndexSequence<Is...>)
+    {
+        ((get<Is>() = other.get<Is>()), ...);
+    }
+
+    template<typename F, unsigned... Is>
+    auto apply_as_args(F&& f, IndexSequence<Is...>)
+    {
+        return forward<F>(f)(get<Is>()...);
+    }
+
+    template<typename F, unsigned... Is>
+    auto apply_as_args(F&& f, IndexSequence<Is...>) const
+    {
+        return forward<F>(f)(get<Is>()...);
+    }
+};
+
+}
+
+using AK::Tuple;