src/core/lib/promise/if.h

// 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_CORE_LIB_PROMISE_IF_H
#define GRPC_CORE_LIB_PROMISE_IF_H

#include <grpc/support/port_platform.h>

#include <type_traits>

#include "absl/status/statusor.h"
#include "absl/types/variant.h"

#include "src/core/lib/promise/detail/promise_factory.h"
#include "src/core/lib/promise/detail/promise_like.h"
#include "src/core/lib/promise/poll.h"

namespace grpc_core {

namespace promise_detail {

template <typename CallPoll, typename T, typename F>
typename CallPoll::PollResult ChooseIf(CallPoll call_poll, bool result,
                                       T* if_true, F* if_false) {
  if (result) {
    auto promise = if_true->Once();
    return call_poll(promise);
  } else {
    auto promise = if_false->Once();
    return call_poll(promise);
  }
}

template <typename CallPoll, typename T, typename F>
typename CallPoll::PollResult ChooseIf(CallPoll call_poll,
                                       absl::StatusOr<bool> result, T* if_true,
                                       F* if_false) {
  if (!result.ok()) {
    return typename CallPoll::PollResult(result.status());
  } else if (*result) {
    auto promise = if_true->Once();
    return call_poll(promise);
  } else {
    auto promise = if_false->Once();
    return call_poll(promise);
  }
}

template <typename C, typename T, typename F>
class If {
 private:
  using TrueFactory = promise_detail::PromiseFactory<void, T>;
  using FalseFactory = promise_detail::PromiseFactory<void, F>;
  using ConditionPromise = PromiseLike<C>;
  using TruePromise = typename TrueFactory::Promise;
  using FalsePromise = typename FalseFactory::Promise;
  using Result =
      typename PollTraits<decltype(std::declval<TruePromise>()())>::Type;

 public:
  If(C condition, T if_true, F if_false)
      : state_(Evaluating{ConditionPromise(std::move(condition)),
                          TrueFactory(std::move(if_true)),
                          FalseFactory(std::move(if_false))}) {}

  Poll<Result> operator()() {
    return absl::visit(CallPoll<false>{this}, state_);
  }

 private:
  struct Evaluating {
    ConditionPromise condition;
    TrueFactory if_true;
    FalseFactory if_false;
  };
  using State = absl::variant<Evaluating, TruePromise, FalsePromise>;
  State state_;

  template <bool kSetState>
  struct CallPoll {
    using PollResult = Poll<Result>;

    If* const self;

    PollResult operator()(Evaluating& evaluating) const {
      static_assert(
          !kSetState,
          "shouldn't need to set state coming through the initial branch");
      auto r = evaluating.condition();
      if (auto* p = absl::get_if<kPollReadyIdx>(&r)) {
        return ChooseIf(CallPoll<true>{self}, std::move(*p),
                        &evaluating.if_true, &evaluating.if_false);
      }
      return Pending();
    }

    template <class Promise>
    PollResult operator()(Promise& promise) const {
      auto r = promise();
      if (kSetState && absl::holds_alternative<Pending>(r)) {
        self->state_.template emplace<Promise>(std::move(promise));
      }
      return r;
    }
  };
};

}  // namespace promise_detail

// If promise combinator.
// Takes 3 promise factories, and evaluates the first.
// If it returns failure, returns failure for the entire combinator.
// If it returns true, evaluates the second promise.
// If it returns false, evaluates the third promise.
template <typename C, typename T, typename F>
promise_detail::If<C, T, F> If(C condition, T if_true, F if_false) {
  return promise_detail::If<C, T, F>(std::move(condition), std::move(if_true),
                                     std::move(if_false));
}

}  // namespace grpc_core

#endif  // GRPC_CORE_LIB_PROMISE_IF_H