/*
 * 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.
 *
 * Copyright 2018 Danny Robson <danny@nerdcruft.net>
 */

#ifndef CRUFT_UTIL_JOB_EVENT_HPP
#define CRUFT_UTIL_JOB_EVENT_HPP

#include <atomic>

namespace util::job {
    /// a reusable synchronisation object that allows threads to wait until
    /// notify is called.
    ///
    /// there is no internal state so it is easy to create races between wait
    /// and notify calls. this makes the class mostly suitable for recurring
    /// events.
    ///
    /// the user should ensure no callers are waiting at destruction time
    /// otherwise they may remain blocked indefinitely.
    ///
    /// the address of the object is important so it must _never_ be
    /// relocated in any manner if any caller may be waiting. it may be safe
    /// to do so if there are no callers waiting (but the relevant functions
    /// are deleted for safety anyway).
    class event {
    public:
        event ();

        event (const event&) = delete;
        event (event&&) = delete;
        event& operator= (const event&) = delete;
        event& operator= (event&&) = delete;

        /// block until notified
        void wait (void);

        /// wake all threads that are waiting
        int notify (void);

        /// wait `count' threads that are waiting
        int notify (int count);

    private:
        alignas (4) std::atomic<int> value;
    };
}

#endif