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// Copyright 2021 gRPC 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.
#ifndef GRPC_SRC_CORE_LIB_PROMISE_RACE_H
#define GRPC_SRC_CORE_LIB_PROMISE_RACE_H
#include <grpc/support/port_platform.h>
#include <type_traits>
#include <utility>
namespace grpc_core {
namespace promise_detail {
// Implementation type for Race combinator.
template <typename... Promises>
class Race;
template <typename Promise, typename... Promises>
class Race<Promise, Promises...> {
public:
using Result = decltype(std::declval<Promise>()());
explicit Race(Promise promise, Promises... promises)
: promise_(std::move(promise)), next_(std::move(promises)...) {}
Result operator()() {
// Check our own promise.
auto r = promise_();
if (r.pending()) {
// Check the rest of them.
return next_();
}
// Return the first ready result.
return std::move(r.value());
}
private:
// The Promise checked by this instance.
Promise promise_;
// We recursively expand to check the rest of the instances.
Race<Promises...> next_;
};
template <typename Promise>
class Race<Promise> {
public:
using Result = decltype(std::declval<Promise>()());
explicit Race(Promise promise) : promise_(std::move(promise)) {}
Result operator()() { return promise_(); }
private:
Promise promise_;
};
} // namespace promise_detail
/// Run all the promises, return the first result that's available.
/// If two results are simultaneously available, bias towards the first result
/// listed.
template <typename... Promises>
promise_detail::Race<Promises...> Race(Promises... promises) {
return promise_detail::Race<Promises...>(std::move(promises)...);
}
} // namespace grpc_core
#endif // GRPC_SRC_CORE_LIB_PROMISE_RACE_H
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