include/ftl/initializer_list.h

/*
 * Copyright 2020 The Android Open Source Project
 *
 * 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 <tuple>
#include <utility>

namespace android::ftl {

// Compile-time counterpart of std::initializer_list<T> that stores per-element constructor
// arguments with heterogeneous types. For a container with elements of type T, given Sizes
// (S0, S1, ..., SN), N elements are initialized: the first element is initialized with the
// first S0 arguments, the second element is initialized with the next S1 arguments, and so
// on. The list of Types (T0, ..., TM) is flattened, so M is equal to the sum of the Sizes.
//
// An InitializerList is created using ftl::init::list, and is consumed by constructors of
// containers. The function call operator is overloaded such that arguments are accumulated
// in a tuple with each successive call. For instance, the following calls initialize three
// strings using different constructors, i.e. string literal, default, and count/character:
//
//   ... = ftl::init::list<std::string>("abc")()(3u, '?');
//
// The following syntax is a shorthand for key-value pairs, where the first argument is the
// key, and the rest construct the value. The types of the key and value are deduced if the
// first pair contains exactly two arguments:
//
//   ... = ftl::init::map<int, std::string>(-1, "abc")(-2)(-3, 3u, '?');
//
//   ... = ftl::init::map(0, 'a')(1, 'b')(2, 'c');
//
// WARNING: The InitializerList returned by an ftl::init::list expression must be consumed
// immediately, since temporary arguments are destroyed after the full expression. Storing
// an InitializerList results in dangling references.
//
template <typename T, typename Sizes = std::index_sequence<>, typename... Types>
struct InitializerList;

template <typename T, std::size_t... Sizes, typename... Types>
struct InitializerList<T, std::index_sequence<Sizes...>, Types...> {
  // Creates a superset InitializerList by appending the number of arguments to Sizes, and
  // expanding Types with forwarding references for each argument.
  template <typename... Args>
  [[nodiscard]] constexpr auto operator()(Args&&... args) && -> InitializerList<
      T, std::index_sequence<Sizes..., sizeof...(Args)>, Types..., Args&&...> {
    return {std::tuple_cat(std::move(tuple), std::forward_as_tuple(std::forward<Args>(args)...))};
  }

  // The temporary InitializerList returned by operator() is bound to an rvalue reference in
  // container constructors, which extends the lifetime of any temporary arguments that this
  // tuple refers to until the completion of the full expression containing the construction.
  std::tuple<Types...> tuple;
};

template <typename K, typename V>
struct KeyValue {};

// Shorthand for key-value pairs that assigns the first argument to the key, and the rest to the
// value. The specialization is on KeyValue rather than std::pair, so that ftl::init::list works
// with the latter.
template <typename K, typename V, std::size_t... Sizes, typename... Types>
struct InitializerList<KeyValue<K, V>, std::index_sequence<Sizes...>, Types...> {
  // Accumulate the three arguments to std::pair's piecewise constructor.
  template <typename... Args>
  [[nodiscard]] constexpr auto operator()(K&& k, Args&&... args) && -> InitializerList<
      KeyValue<K, V>, std::index_sequence<Sizes..., 3>, Types..., std::piecewise_construct_t,
      std::tuple<K&&>, std::tuple<Args&&...>> {
    return {std::tuple_cat(
        std::move(tuple),
        std::forward_as_tuple(std::piecewise_construct, std::forward_as_tuple(std::forward<K>(k)),
                              std::forward_as_tuple(std::forward<Args>(args)...)))};
  }

  std::tuple<Types...> tuple;
};

namespace init {

template <typename T, typename... Args>
[[nodiscard]] constexpr auto list(Args&&... args) {
  return InitializerList<T>{}(std::forward<Args>(args)...);
}

template <typename K, typename V, typename... Args>
[[nodiscard]] constexpr auto map(Args&&... args) {
  return list<KeyValue<K, V>>(std::forward<Args>(args)...);
}

template <typename K, typename V>
[[nodiscard]] constexpr auto map(K&& k, V&& v) {
  return list<KeyValue<K, V>>(std::forward<K>(k), std::forward<V>(v));
}

}  // namespace init
}  // namespace android::ftl