variant.h

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/* *****************************************************************************
 * kblib is a general utility library for C++14 and C++17, intended to provide
 * performant high-level abstractions and more expressive ways to do simple
 * things.
 *
 * Copyright (c) 2021 killerbee
 *
 * This program is free software: you can redistribute it and/or modify
 * it under the terms of the GNU General Public License as published by
 * the Free Software Foundation, either version 3 of the License, or
 * (at your option) any later version.
 *
 * This program is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 * GNU General Public License for more details.
 *
 * You should have received a copy of the GNU General Public License
 * along with this program.  If not, see <https://www.gnu.org/licenses/>.
 * ****************************************************************************/
 
#ifndef KBLIB_VARIANT_H
#define KBLIB_VARIANT_H
 
#include "convert.h"
#include "logic.h"
#include "tdecl.h"
 
#include <cstddef>
#include <new>
 
#if KBLIB_USE_CXX17
#  include <variant>
#endif
 
namespace KBLIB_NS {
 
#if KBLIB_USE_CXX17
 
template <typename T, typename = void>
constexpr inline bool is_variant_like_v = false;
template <typename T>
constexpr inline bool is_variant_like_v<T, void_t<std::variant_size<T>>> = true;
 
template <typename T>
struct is_variant_like : std::bool_constant<is_variant_like_v<T>> {};
 
template <typename To, typename... Ts>
KBLIB_NODISCARD [[deprecated("Use new lexical_coerce instead.")]] auto coerce(
    const std::variant<Ts...>& v) -> To {
   return std::visit([](const auto& t) -> To { return lexical_cast<To>(t); },
                     v);
}
 
template <typename To, typename... Ts>
KBLIB_NODISCARD auto static_coerce(const std::variant<Ts...>& v) -> To {
   return std::visit([](const auto& t) -> To { return static_cast<To>(t); }, v);
}
 
template <typename To, typename... Ts>
KBLIB_NODISCARD auto lexical_coerce(const std::variant<Ts...>& v) -> To {
   return std::visit([](const auto& t) { return lexical_cast<To>(t); }, v);
}
 
template <typename... Ts>
struct visitor : Ts... {
   using Ts::operator()...;
};
template <typename... Ts>
visitor(Ts...) -> visitor<Ts...>;
 
namespace detail {
 
   template <typename T>
   struct tuple_type {
      using type = std::tuple<>;
   };
   template <typename... Ts>
   struct tuple_type<std::variant<Ts...>> {
      using type = std::tuple<Ts...>;
   };
   template <typename T>
   using tuple_type_t = typename tuple_type<T>::type;
 
   template <typename Variant, typename F, std::size_t... Is>
   constexpr auto indexed_visitor_impl(std::index_sequence<Is...>) -> auto {
      return std::array{+[](Variant&& variant, F&& f) -> decltype(auto) {
         return std::forward<F>(f)(
             kblib::constant<std::size_t, Is>{},
             std::get<Is>(std::forward<Variant>(variant)));
      }...};
   }
 
} // namespace detail
 
inline namespace literals {
 
   template <char... Cs>
   KBLIB_NODISCARD constexpr auto operator""_vi() {
      constexpr char arr[] = {Cs...};
      return std::in_place_index_t<parse_integer<std::size_t>(arr)>{};
   }
 
} // namespace literals
 
template <typename Variant, typename... Fs>
constexpr auto visit_indexed(Variant&& variant, Fs&&... fs) -> decltype(auto) {
   if (variant.valueless_by_exception()) {
      throw std::bad_variant_access();
   }
   using visitor_t = decltype(visitor{std::forward<Fs>(fs)...});
   return detail::indexed_visitor_impl<Variant, visitor_t>(
       std::make_index_sequence<
           std::variant_size_v<std::decay_t<Variant>>>())[variant.index()](
       std::forward<Variant>(variant), visitor{std::forward<Fs>(fs)...});
}
 
template <typename To, typename From>
KBLIB_NODISCARD constexpr auto variant_cast(From&& v) -> To {
   static_assert(contains_types_v<detail::tuple_type_t<std::decay_t<To>>,
                                  detail::tuple_type_t<std::decay_t<From>>>,
                 "To must include all types in From");
 
   return visit_indexed(std::forward<From>(v), [](auto constant, auto&& x) {
      return To(std::in_place_type<
                    std::variant_alternative_t<constant, std::decay_t<From>>>,
                std::forward<decltype(x)>(x));
   });
 
   // return std::visit([](auto&& x) { return std::forward<decltype(x)>(x); },
   //                   std::forward<From>(v));
}
 
template <typename V, typename F, typename... Fs>
KBLIB_NODISCARD constexpr auto visit(V&& v, F&& f, Fs&&... fs)
    -> decltype(auto) {
   return std::visit(visitor{std::forward<F>(f), std::forward<Fs>(fs)...},
                     std::forward<V>(v));
}
 
namespace detail {
 
   template <typename F, typename... Ts>
   constexpr bool invocable_with_all_v
       = (ignore_t<std::invoke_result_t<F, Ts>, std::true_type>::value and ...);
 
   template <typename Callable, typename Variant>
   constexpr bool v_invocable_with_all_v = false;
 
   template <typename F, typename... Ts>
   constexpr bool v_invocable_with_all_v<
       F, std::variant<Ts...>> = invocable_with_all_v<F, Ts...>;
 
   template <typename F, typename... Ts>
   constexpr bool v_invocable_with_all_v<
       F, const std::variant<Ts...>> = invocable_with_all_v<F, const Ts...>;
 
   template <typename F, typename... Ts>
   constexpr bool v_invocable_with_all_v<
       F, std::variant<Ts...>&> = invocable_with_all_v<F, Ts&...>;
 
   template <typename F, typename... Ts>
   constexpr bool v_invocable_with_all_v<
       F, const std::variant<Ts...>&> = invocable_with_all_v<F, const Ts&...>;
 
   template <typename F, typename... Ts>
   constexpr bool v_invocable_with_all_v<
       F, std::variant<Ts...>&&> = invocable_with_all_v<F, Ts&&...>;
 
   template <typename F, typename... Ts>
   constexpr bool v_invocable_with_all_v<
       F, const std::variant<Ts...>&&> = invocable_with_all_v<F, const Ts&&...>;
 
   template <typename V, typename F, std::size_t I, std::size_t... Is>
   [[gnu::always_inline]] constexpr decltype(auto) visit_impl(
       V&& v, F&& f, std::index_sequence<I, Is...>) {
      static_assert(I < std::variant_size_v<std::decay_t<V>>);
      if (auto* p = std::get_if<I>(&v)) {
         return std::forward<F>(f)(
             static_cast<decltype(std::get<I>(std::forward<V>(v)))>(*p));
      } else if constexpr (sizeof...(Is) > 0) {
         return visit_impl(std::forward<V>(v), std::forward<F>(f),
                           std::index_sequence<Is...>{});
      } else {
         throw std::bad_variant_access();
      }
   }
 
   template <typename V, typename F, std::size_t I, std::size_t... Is>
   [[gnu::always_inline]] constexpr void visit_nop_impl(
       V&& v, F&& f, std::index_sequence<I, Is...>) {
      static_assert(I < std::variant_size_v<std::decay_t<V>>);
      if (auto* p = std::get_if<I>(&v)) {
         std::forward<F>(f)(
             static_cast<decltype(std::get<I>(std::forward<V>(v)))>(*p));
         return;
      } else if constexpr (sizeof...(Is) > 0) {
         return visit_nop_impl(std::forward<V>(v), std::forward<F>(f),
                               std::index_sequence<Is...>{});
      } else {
         // valueless_by_exception case
         return;
      }
   }
 
} // namespace detail
 
template <typename V, typename F, typename... Fs>
[[gnu::always_inline]] constexpr auto visit2(V&& v, F&& f, Fs&&... fs)
    -> decltype(auto) {
   auto visitor_obj = visitor{std::forward<F>(f), std::forward<Fs>(fs)...};
   static_assert(detail::v_invocable_with_all_v<decltype(visitor_obj), V&&>,
                 "Some variant types not accepted by any visitors.");
   return detail::visit_impl(
       std::forward<V>(v), std::move(visitor_obj),
       std::make_index_sequence<std::variant_size_v<std::decay_t<V>>>{});
}
 
template <typename V, typename F, typename... Fs>
[[gnu::always_inline]] constexpr auto visit2_nop(V&& v, F&& f, Fs&&... fs)
    -> void {
   auto visitor_obj = visitor{std::forward<F>(f), std::forward<Fs>(fs)...};
   static_assert(detail::v_invocable_with_all_v<decltype(visitor_obj), V&&>,
                 "Some variant types not accepted by any visitors.");
   return detail::visit_nop_impl(
       std::forward<V>(v), visitor_obj,
       std::make_index_sequence<std::variant_size_v<std::decay_t<V>>>{});
}
 
template <typename V>
KBLIB_NODISCARD constexpr auto visit(V& v) -> auto {
   return [&v](auto... fs) -> decltype(auto) {
      return kblib::visit(v, std::forward<decltype(fs)>(fs)...);
   };
}
 
#endif // KBLIB_USE_CXX17
 
} // namespace KBLIB_NS
 
#endif // KBLIB_VARIANT_H