future.hpp

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/*
 
Copyright 2025 Matthew Tolman
 
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 MTCORE_MTTHREAD_FUTURE_HPP
#define MTCORE_MTTHREAD_FUTURE_HPP
 
#include "mtcore_thread/arc.hpp"
#include "mtcore_thread/core.hpp"
#include <condition_variable>
#include <functional>
#include <mutex>
 
namespace mtcore::thread {
  template<typename T, typename E = void>
  struct FutureState;

  template<typename T, typename E = void>
  struct Future {
    Arc<FutureState<T, E>> state;

    Result<Future, ArcError> acquire(Allocator &alloc) {
      auto res = state.acquire(alloc);
      if (res.is_error()) {
        return res.error();
      }
      return Future{.state = res.value()};
    }

    [[nodiscard]] T wait() { return state->wait(); }

    void deinit(Allocator &alloc) { state.deinit(alloc); }
  };

  template<typename T>
  struct FutureState<T, void> {
    std::mutex mux             = {};
    std::condition_variable cv = {};
    Optional<T> value          = nullopt;

    void set_val(const T &v) {
      auto l = std::unique_lock{mux};
      ensure(value.empty(), "CANNOT SET FUTURE VALUE MULTIPLE TIMES! CONSIDER USING A CANNEL OR INBOX!");
      value = v;
      cv.notify_all();
    }

    T wait() {
      auto l = std::unique_lock{mux};
      while (value.empty()) {
        cv.wait(l);
      }
      return *value;
    }
 
    struct make_res {
      Future<T> future;
      Arc<FutureState> state;
    };

    static Result<make_res, AllocationError> make(Allocator &alloc) {
      Arc<FutureState> state;
      if (auto err = state.init(alloc); err.is_error()) {
        return err.error();
      }
      auto futureArc = state.acquire(alloc);
      if (futureArc.is_error()) {
        state.deinit(alloc);
        return error(AllocationError::ALLOCATION_FAILED);
      }
      return make_res{.future = Future<T>{.state = futureArc.value()}, .state = state};
    }
  };

  template<typename T, typename E>
  struct FutureState {
    std::mutex mux             = {};
    std::condition_variable cv = {};
    Optional<T> value          = nullopt;
    Arc<E> extra               = {};
 
    void set_val(const T &v) {
      auto l = std::unique_lock{mux};
      ensure(value.empty(), "CANNOT SET FUTURE VALUE MULTIPLE TIMES! CONSIDER USING A "
                            "CANNEL OR INBOX!");
      value = v;
      cv.notify_all();
    }

    T wait() {
      auto l = std::unique_lock{mux};
      while (value.empty()) {
        cv.wait(l);
      }
      return *value;
    }
 
    void deinit(Allocator &alloc) {
      if (extra) {
        extra.deinit(alloc);
      }
    }
 
    struct make_res {
      Future<T, E> future;
      Arc<FutureState> state;
    };

    static Result<make_res, AllocationError>
    make(Allocator &alloc,
         std::function<Result<Arc<E>, AllocationError>(Allocator &alloc, Arc<FutureState> &)> getExtra) {
      Arc<FutureState> state;
      if (auto err = state.init(alloc); err.is_error()) {
        return err.error();
      }
      Result<Arc<E>, AllocationError> extra = getExtra(alloc, state);
      if (extra.is_error()) {
        state.deinit(alloc);
        return extra.error();
      }
 
      state->extra   = extra.value();
      auto futureArc = state.acquire(alloc);
      if (futureArc.is_error()) {
        state.deinit(alloc);
        return error(AllocationError::ALLOCATION_FAILED);
      }
      return make_res{.future = Future<T, E>{.state = futureArc.value()}, .state = state};
    }
  };
 
}  // namespace mtcore::thread
 
#endif  // MTCORE_MTTHREAD_FUTURE_HPP