text: // Copyright 2017 The Abseil Authors.
  //
  // 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.
  //
  // This header file contains C++11 versions of standard <utility> header
  // abstractions available within C++14 and C++17, and are designed to be drop-in
  // replacement for code compliant with C++14 and C++17.
  //
  // The following abstractions are defined:
  //
  //   * integer_sequence<T, Ints...>  == std::integer_sequence<T, Ints...>
  //   * index_sequence<Ints...>       == std::index_sequence<Ints...>
  //   * make_integer_sequence<T, N>   == std::make_integer_sequence<T, N>
  //   * make_index_sequence<N>        == std::make_index_sequence<N>
  //   * index_sequence_for<Ts...>     == std::index_sequence_for<Ts...>
  //   * apply<Functor, Tuple>         == std::apply<Functor, Tuple>
  //   * exchange<T>                   == std::exchange<T>
  //
  // This header file also provides the tag types `in_place_t`, `in_place_type_t`,
  // and `in_place_index_t`, as well as the constant `in_place`, and
  // `constexpr` `std::move()` and `std::forward()` implementations in C++11.
  //
  // References:
  //
  //  http://en.cppreference.com/w/cpp/utility/integer_sequence
  //  http://en.cppreference.com/w/cpp/utility/apply
  //  http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2013/n3658.html
  //
  
  #ifndef ABSL_UTILITY_UTILITY_H_
  #define ABSL_UTILITY_UTILITY_H_
  
  #include <cstddef>
  #include <cstdlib>
  #include <tuple>
  #include <utility>
  
  #include "absl/base/config.h"
  #include "absl/base/internal/inline_variable.h"
  #include "absl/base/internal/invoke.h"
  #include "absl/meta/type_traits.h"
  
  namespace absl {
  inline namespace lts_2018_12_18 {
  
  // integer_sequence
  //
  // Class template representing a compile-time integer sequence. An instantiation
  // of `integer_sequence<T, Ints...>` has a sequence of integers encoded in its
  // type through its template arguments (which is a common need when
  // working with C++11 variadic templates). `absl::integer_sequence` is designed
  // to be a drop-in replacement for C++14's `std::integer_sequence`.
  //
  // Example:
  //
  //   template< class T, T... Ints >
  //   void user_function(integer_sequence<T, Ints...>);
  //
  //   int main()
  //   {
  //     // user_function's `T` will be deduced to `int` and `Ints...`
  //     // will be deduced to `0, 1, 2, 3, 4`.
  //     user_function(make_integer_sequence<int, 5>());
  //   }
  template <typename T, T... Ints>
  struct integer_sequence {
    using value_type = T;
    static constexpr size_t size() noexcept { return sizeof...(Ints); }
  };
  
  // index_sequence
  //
  // A helper template for an `integer_sequence` of `size_t`,
  // `absl::index_sequence` is designed to be a drop-in replacement for C++14's
  // `std::index_sequence`.
  template <size_t... Ints>
  using index_sequence = integer_sequence<size_t, Ints...>;
  
  namespace utility_internal {
  
  template <typename Seq, size_t SeqSize, size_t Rem>
  struct Extend;
  
  // Note that SeqSize == sizeof...(Ints). It's passed explicitly for efficiency.
  template <typename T, T... Ints, size_t SeqSize>
  struct Extend<integer_sequence<T, Ints...>, SeqSize, 0> {
    using type = integer_sequence<T, Ints..., (Ints + SeqSize)...>;
  };
  
  template <typename T, T... Ints, size_t SeqSize>
  struct Extend<integer_sequence<T, Ints...>, SeqSize, 1> {
    using type = integer_sequence<T, Ints..., (Ints + SeqSize)..., 2 * SeqSize>;
  };
  
  // Recursion helper for 'make_integer_sequence<T, N>'.
  // 'Gen<T, N>::type' is an alias for 'integer_sequence<T, 0, 1, ... N-1>'.
  template <typename T, size_t N>
  struct Gen {
    using type =
        typename Extend<typename Gen<T, N / 2>::type, N / 2, N % 2>::type;
  };
  
  template <typename T>
  struct Gen<T, 0> {
    using type = integer_sequence<T>;
  };
  
  }  // namespace utility_internal
  
  // Compile-time sequences of integers
  
  // make_integer_sequence
  //
  // This template alias is equivalent to
  // `integer_sequence<int, 0, 1, ..., N-1>`, and is designed to be a drop-in
  // replacement for C++14's `std::make_integer_sequence`.
  template <typename T, T N>
  using make_integer_sequence = typename utility_internal::Gen<T, N>::type;
  
  // make_index_sequence
  //
  // This template alias is equivalent to `index_sequence<0, 1, ..., N-1>`,
  // and is designed to be a drop-in replacement for C++14's
  // `std::make_index_sequence`.
  template <size_t N>
  using make_index_sequence = make_integer_sequence<size_t, N>;
  
  // index_sequence_for
  //
  // Converts a typename pack into an index sequence of the same length, and
  // is designed to be a drop-in replacement for C++14's
  // `std::index_sequence_for()`
  template <typename... Ts>
  using index_sequence_for = make_index_sequence<sizeof...(Ts)>;
  
  // Tag types
  
  #ifdef ABSL_HAVE_STD_OPTIONAL
  
  using std::in_place_t;
  using std::in_place;
  
  #else  // ABSL_HAVE_STD_OPTIONAL
  
  // in_place_t
  //
  // Tag type used to specify in-place construction, such as with
  // `absl::optional`, designed to be a drop-in replacement for C++17's
  // `std::in_place_t`.
  struct in_place_t {};
  
  ABSL_INTERNAL_INLINE_CONSTEXPR(in_place_t, in_place, {});
  
  #endif  // ABSL_HAVE_STD_OPTIONAL
  
  #if defined(ABSL_HAVE_STD_ANY) || defined(ABSL_HAVE_STD_VARIANT)
  using std::in_place_type_t;
  #else
  
  // in_place_type_t
  //
  // Tag type used for in-place construction when the type to construct needs to
  // be specified, such as with `absl::any`, designed to be a drop-in replacement
  // for C++17's `std::in_place_type_t`.
  template <typename T>
  struct in_place_type_t {};
  #endif  // ABSL_HAVE_STD_ANY || ABSL_HAVE_STD_VARIANT
  
  #ifdef ABSL_HAVE_STD_VARIANT
  using std::in_place_index_t;
  #else
  
  // in_place_index_t
  //
  // Tag type used for in-place construction when the type to construct needs to
  // be specified, such as with `absl::any`, designed to be a drop-in replacement
  // for C++17's `std::in_place_index_t`.
  template <size_t I>
  struct in_place_index_t {};
  #endif  // ABSL_HAVE_STD_VARIANT
  
  // Constexpr move and forward
  
  // move()
  //
  // A constexpr version of `std::move()`, designed to be a drop-in replacement
  // for C++14's `std::move()`.
  template <typename T>
  constexpr absl::remove_reference_t<T>&& move(T&& t) noexcept {
    return static_cast<absl::remove_reference_t<T>&&>(t);
  }
  
  // forward()
  //
  // A constexpr version of `std::forward()`, designed to be a drop-in replacement
  // for C++14's `std::forward()`.
  template <typename T>
  constexpr T&& forward(
      absl::remove_reference_t<T>& t) noexcept {  // NOLINT(runtime/references)
    return static_cast<T&&>(t);
  }
  
  namespace utility_internal {
  // Helper method for expanding tuple into a called method.
  template <typename Functor, typename Tuple, std::size_t... Indexes>
  auto apply_helper(Functor&& functor, Tuple&& t, index_sequence<Indexes...>)
      -> decltype(absl::base_internal::Invoke(
          absl::forward<Functor>(functor),
          std::get<Indexes>(absl::forward<Tuple>(t))...)) {
    return absl::base_internal::Invoke(
        absl::forward<Functor>(functor),
        std::get<Indexes>(absl::forward<Tuple>(t))...);
  }
  
  }  // namespace utility_internal
  
  // apply
  //
  // Invokes a Callable using elements of a tuple as its arguments.
  // Each element of the tuple corresponds to an argument of the call (in order).
  // Both the Callable argument and the tuple argument are perfect-forwarded.
  // For member-function Callables, the first tuple element acts as the `this`
  // pointer. `absl::apply` is designed to be a drop-in replacement for C++17's
  // `std::apply`. Unlike C++17's `std::apply`, this is not currently `constexpr`.
  //
  // Example:
  //
  //   class Foo{void Bar(int);};
  //   void user_function(int, string);
  //   void user_function(std::unique_ptr<Foo>);
  //
  //   int main()
  //   {
  //       std::tuple<int, string> tuple1(42, "bar");
  //       // Invokes the user function overload on int, string.
  //       absl::apply(&user_function, tuple1);
  //
  //       auto foo = absl::make_unique<Foo>();
  //       std::tuple<Foo*, int> tuple2(foo.get(), 42);
  //       // Invokes the method Bar on foo with one argument 42.
  //       absl::apply(&Foo::Bar, foo.get(), 42);
  //
  //       std::tuple<std::unique_ptr<Foo>> tuple3(absl::make_unique<Foo>());
  //       // Invokes the user function that takes ownership of the unique
  //       // pointer.
  //       absl::apply(&user_function, std::move(tuple));
  //   }
  template <typename Functor, typename Tuple>
  auto apply(Functor&& functor, Tuple&& t)
      -> decltype(utility_internal::apply_helper(
          absl::forward<Functor>(functor), absl::forward<Tuple>(t),
          absl::make_index_sequence<std::tuple_size<
              typename std::remove_reference<Tuple>::type>::value>{})) {
    return utility_internal::apply_helper(
        absl::forward<Functor>(functor), absl::forward<Tuple>(t),
        absl::make_index_sequence<std::tuple_size<
            typename std::remove_reference<Tuple>::type>::value>{});
  }
  
  // exchange
  //
  // Replaces the value of `obj` with `new_value` and returns the old value of
  // `obj`.  `absl::exchange` is designed to be a drop-in replacement for C++14's
  // `std::exchange`.
  //
  // Example:
  //
  //   Foo& operator=(Foo&& other) {
  //     ptr1_ = absl::exchange(other.ptr1_, nullptr);
  //     int1_ = absl::exchange(other.int1_, -1);
  //     return *this;
  //   }
  template <typename T, typename U = T>
  T exchange(T& obj, U&& new_value) {
    T old_value = absl::move(obj);
    obj = absl::forward<U>(new_value);
    return old_value;
  }
  
  }  // inline namespace lts_2018_12_18
  }  // namespace absl
  
  #endif  // ABSL_UTILITY_UTILITY_H_
  