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110 lines
3.0 KiB
110 lines
3.0 KiB
/*
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* Copyright 2020 The Android Open Source Project
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*
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* Licensed under the Apache License, Version 2.0 (the "License");
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* you may not use this file except in compliance with the License.
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* You may obtain a copy of the License at
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*
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* http://www.apache.org/licenses/LICENSE-2.0
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*
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* Unless required by applicable law or agreed to in writing, software
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* distributed under the License is distributed on an "AS IS" BASIS,
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* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
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* See the License for the specific language governing permissions and
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* limitations under the License.
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*/
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#pragma once
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#include <future>
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#include <type_traits>
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#include <utility>
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namespace android::ftl {
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// Creates a future that defers a function call until its result is queried.
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//
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// auto future = ftl::defer([](int x) { return x + 1; }, 99);
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// assert(future.get() == 100);
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//
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template <typename F, typename... Args>
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inline auto defer(F&& f, Args&&... args) {
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return std::async(std::launch::deferred, std::forward<F>(f), std::forward<Args>(args)...);
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}
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// Creates a future that wraps a value.
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//
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// auto future = ftl::yield(42);
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// assert(future.get() == 42);
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//
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// auto ptr = std::make_unique<char>('!');
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// auto future = ftl::yield(std::move(ptr));
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// assert(*future.get() == '!');
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//
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template <typename T>
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inline std::future<T> yield(T&& v) {
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return defer([](T&& v) { return std::forward<T>(v); }, std::forward<T>(v));
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}
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namespace details {
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template <typename T>
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struct future_result {
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using type = T;
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};
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template <typename T>
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struct future_result<std::future<T>> {
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using type = T;
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};
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template <typename T>
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using future_result_t = typename future_result<T>::type;
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// Attaches a continuation to a future. The continuation is a function that maps T to either R or
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// std::future<R>. In the former case, the chain wraps the result in a future as if by ftl::yield.
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//
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// auto future = ftl::yield(123);
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// std::future<char> futures[] = {ftl::yield('a'), ftl::yield('b')};
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//
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// std::future<char> chain =
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// ftl::chain(std::move(future))
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// .then([](int x) { return static_cast<std::size_t>(x % 2); })
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// .then([&futures](std::size_t i) { return std::move(futures[i]); });
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//
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// assert(chain.get() == 'b');
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//
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template <typename T>
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struct Chain {
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// Implicit conversion.
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Chain(std::future<T>&& f) : future(std::move(f)) {}
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operator std::future<T>&&() && { return std::move(future); }
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T get() && { return future.get(); }
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template <typename F, typename R = std::invoke_result_t<F, T>>
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auto then(F&& op) && -> Chain<future_result_t<R>> {
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return defer(
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[](auto&& f, F&& op) {
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R r = op(f.get());
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if constexpr (std::is_same_v<R, future_result_t<R>>) {
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return r;
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} else {
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return r.get();
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}
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},
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std::move(future), std::forward<F>(op));
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}
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std::future<T> future;
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};
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} // namespace details
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template <typename T>
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inline auto chain(std::future<T>&& f) -> details::Chain<T> {
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return std::move(f);
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}
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} // namespace android::ftl
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