index_sequence.h

/*
 * Copyright 2017 Google 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.
 */

// This is a trimed version of absl's utility.h, since dimsum doesn't have to
// depend on absl. Specifically, it contains all index_sequence related
// utilities.
#ifndef DIMSUM_INDEX_SEQUENCE_H_
#define DIMSUM_INDEX_SEQUENCE_H_

#include <cstddef>

namespace dimsum {

// 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 detail {

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 detail

// 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 detail::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)>;

}  // namespace dimsum

#endif  // DIMSUM_INDEX_SEQUENCE_H_