Utility.h

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/*
 * Copyright 2016-present Facebook, Inc.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *   http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

#pragma once

#include <cstdint>
#include <limits>
#include <type_traits>
#include <utility>

#include <folly/CPortability.h>
#include <folly/Traits.h>

namespace folly {

template <typename T>
constexpr typename std::decay<T>::type copy(T&& value) noexcept(
    noexcept(typename std::decay<T>::type(std::forward<T>(value)))) {
  return std::forward<T>(value);
}

#if __cpp_lib_as_const || _LIBCPP_STD_VER > 14 || _MSC_VER

/* using override */ using std::as_const;

#else

template <class T>
constexpr T const& as_const(T& t) noexcept {
  return t;
}

template <class T>
void as_const(T const&&) = delete;

#endif

//  mimic: forward_like, p0847r0
template <typename Src, typename Dst>
constexpr like_t<Src, Dst>&& forward_like(Dst&& dst) noexcept {
  return static_cast<like_t<Src, Dst>&&>(std::forward<Dst>(dst));
}

#if __cpp_lib_exchange_function || _LIBCPP_STD_VER > 11 || _MSC_VER

/* using override */ using std::exchange;

#else

//  mimic: std::exchange, C++14
//  from: http://en.cppreference.com/w/cpp/utility/exchange, CC-BY-SA
template <class T, class U = T>
T exchange(T& obj, U&& new_value) {
  T old_value = std::move(obj);
  obj = std::forward<U>(new_value);
  return old_value;
}

#endif

namespace utility_detail {
template <typename...>
struct make_seq_cat;
template <
    template <typename T, T...> class S,
    typename T,
    T... Ta,
    T... Tb,
    T... Tc>
struct make_seq_cat<S<T, Ta...>, S<T, Tb...>, S<T, Tc...>> {
  using type =
      S<T,
        Ta...,
        (sizeof...(Ta) + Tb)...,
        (sizeof...(Ta) + sizeof...(Tb) + Tc)...>;
};

// Not parameterizing by `template <typename T, T...> class, typename` because
// clang precisely v4.0 fails to compile that. Note that clang v3.9 and v5.0
// handle that code correctly.
//
// For this to work, `S0` is required to be `Sequence<T>` and `S1` is required
// to be `Sequence<T, 0>`.

template <std::size_t Size>
struct make_seq {
  template <typename S0, typename S1>
  using apply = typename make_seq_cat<
      typename make_seq<Size / 2>::template apply<S0, S1>,
      typename make_seq<Size / 2>::template apply<S0, S1>,
      typename make_seq<Size % 2>::template apply<S0, S1>>::type;
};
template <>
struct make_seq<1> {
  template <typename S0, typename S1>
  using apply = S1;
};
template <>
struct make_seq<0> {
  template <typename S0, typename S1>
  using apply = S0;
};
} // namespace utility_detail

#if __cpp_lib_integer_sequence || _MSC_VER

/* using override */ using std::index_sequence;
/* using override */ using std::integer_sequence;

#else

// TODO: Remove after upgrading to C++14 baseline

template <class T, T... Ints>
struct integer_sequence {
  using value_type = T;

  static constexpr std::size_t size() noexcept {
    return sizeof...(Ints);
  }
};

template <std::size_t... Ints>
using index_sequence = integer_sequence<std::size_t, Ints...>;

#endif

#if FOLLY_HAS_BUILTIN(__make_integer_seq) || _MSC_FULL_VER >= 190023918

template <typename T, std::size_t Size>
using make_integer_sequence = __make_integer_seq<integer_sequence, T, Size>;

#else

template <typename T, std::size_t Size>
using make_integer_sequence = typename utility_detail::make_seq<
    Size>::template apply<integer_sequence<T>, integer_sequence<T, 0>>;

#endif

template <std::size_t Size>
using make_index_sequence = make_integer_sequence<std::size_t, Size>;
template <class... T>
using index_sequence_for = make_index_sequence<sizeof...(T)>;

struct in_place_tag {};
template <class>
struct in_place_type_tag {};
template <std::size_t>
struct in_place_index_tag {};

using in_place_t = in_place_tag (&)(in_place_tag);
template <class T>
using in_place_type_t = in_place_type_tag<T> (&)(in_place_type_tag<T>);
template <std::size_t I>
using in_place_index_t = in_place_index_tag<I> (&)(in_place_index_tag<I>);

inline in_place_tag in_place(in_place_tag = {}) {
  return {};
}
template <class T>
inline in_place_type_tag<T> in_place_type(in_place_type_tag<T> = {}) {
  return {};
}
template <std::size_t I>
inline in_place_index_tag<I> in_place_index(in_place_index_tag<I> = {}) {
  return {};
}

struct initlist_construct_t {};
constexpr initlist_construct_t initlist_construct{};

struct presorted_t {};
constexpr presorted_t presorted{};

struct unsorted_t {};
constexpr unsorted_t unsorted{};

template <typename T>
struct transparent : T {
  using is_transparent = void;
  using T::T;
};

struct Identity {
  template <class T>
  constexpr T&& operator()(T&& x) const noexcept {
    return static_cast<T&&>(x);
  }
};

namespace moveonly_ { // Protection from unintended ADL.

class MoveOnly {
 protected:
  constexpr MoveOnly() = default;
  ~MoveOnly() = default;

  MoveOnly(MoveOnly&&) = default;
  MoveOnly& operator=(MoveOnly&&) = default;
  MoveOnly(const MoveOnly&) = delete;
  MoveOnly& operator=(const MoveOnly&) = delete;
};

} // namespace moveonly_

using MoveOnly = moveonly_::MoveOnly;

template <typename T>
constexpr auto to_signed(T const& t) -> typename std::make_signed<T>::type {
  using S = typename std::make_signed<T>::type;
  // note: static_cast<S>(t) would be more straightforward, but it would also be
  // implementation-defined behavior and that is typically to be avoided; the
  // following code optimized into the same thing, though
  return std::numeric_limits<S>::max() < t ? -static_cast<S>(~t) + S{-1}
                                           : static_cast<S>(t);
}

template <typename T>
constexpr auto to_unsigned(T const& t) -> typename std::make_unsigned<T>::type {
  using U = typename std::make_unsigned<T>::type;
  return static_cast<U>(t);
}

} // namespace folly