include/ftl/details/future.h

/*
 * Copyright 2022 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

namespace android::ftl {

template <typename, template <typename> class>
class Future;

namespace details {

template <typename T>
struct future_result {
  using type = T;
};

template <typename T>
struct future_result<std::future<T>> {
  using type = T;
};

template <typename T>
struct future_result<std::shared_future<T>> {
  using type = T;
};

template <typename T, template <typename> class FutureImpl>
struct future_result<Future<T, FutureImpl>> {
  using type = T;
};

template <typename T>
using future_result_t = typename future_result<T>::type;

struct ValueTag {};

template <typename, typename T, template <typename> class>
class BaseFuture;

template <typename Self, typename T>
class BaseFuture<Self, T, std::future> {
  using Impl = std::future<T>;

 public:
  Future<T, std::shared_future> share() {
    if (T* value = std::get_if<T>(&self())) {
      return {ValueTag{}, std::move(*value)};
    }

    return std::get<Impl>(self()).share();
  }

 protected:
  T get() {
    if (T* value = std::get_if<T>(&self())) {
      return std::move(*value);
    }

    return std::get<Impl>(self()).get();
  }

  template <class Rep, class Period>
  std::future_status wait_for(const std::chrono::duration<Rep, Period>& timeout_duration) const {
    if (std::holds_alternative<T>(self())) {
      return std::future_status::ready;
    }

    return std::get<Impl>(self()).wait_for(timeout_duration);
  }

 private:
  auto& self() { return static_cast<Self&>(*this).future_; }
  const auto& self() const { return static_cast<const Self&>(*this).future_; }
};

template <typename Self, typename T>
class BaseFuture<Self, T, std::shared_future> {
  using Impl = std::shared_future<T>;

 protected:
  const T& get() const {
    if (const T* value = std::get_if<T>(&self())) {
      return *value;
    }

    return std::get<Impl>(self()).get();
  }

  template <class Rep, class Period>
  std::future_status wait_for(const std::chrono::duration<Rep, Period>& timeout_duration) const {
    if (std::holds_alternative<T>(self())) {
      return std::future_status::ready;
    }

    return std::get<Impl>(self()).wait_for(timeout_duration);
  }

 private:
  const auto& self() const { return static_cast<const Self&>(*this).future_; }
};

}  // namespace details
}  // namespace android::ftl