// Licensed to the Apache Software Foundation (ASF) under one

// or more contributor license agreements.  See the NOTICE file

// distributed with this work for additional information

// regarding copyright ownership.  The ASF licenses this file

// to you 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.

// bthread - An M:N threading library to make applications more concurrent.

// Date: 2017/07/27 23:07:06

#ifndef BTHREAD_PARKING_LOT_H

#define BTHREAD_PARKING_LOT_H

#include <gflags/gflags.h>

#include "butil/atomicops.h"

#include "bthread/sys_futex.h"

namespace bthread {

DECLARE_bool(parking_lot_no_signal_when_no_waiter);

// Park idle workers.

class BAIDU_CACHELINE_ALIGNMENT ParkingLot {

public:

    class State {

    public:

        State(): val(0) {}

        bool stopped() const { return val & 1; }

    private:

    friend class ParkingLot;

        State(int val) : val(val) {}

        int val;

    };

    ParkingLot()

        : _pending_signal(0), _waiter_num(0)

        , _no_signal_when_no_waiter(FLAGS_parking_lot_no_signal_when_no_waiter) {}

    // Wake up at most `num_task' workers.

    // Returns #workers woken up.

    int signal(int num_task) {

        _pending_signal.fetch_add((num_task << 1), butil::memory_order_release);

        if (_no_signal_when_no_waiter && _waiter_num.load(butil::memory_order_relaxed) == 0) {

            return 0;

        }

        return futex_wake_private(&_pending_signal, num_task);

    }

    // Get a state for later wait().

    State get_state() {

        return _pending_signal.load(butil::memory_order_acquire);

    }

    // Wait for tasks.

    // If the `expected_state' does not match, wait() may finish directly.

    void wait(const State& expected_state) {

        if (get_state().val != expected_state.val) {

            // Fast path, no need to futex_wait.

            return;

        }

        if (_no_signal_when_no_waiter) {

            _waiter_num.fetch_add(1, butil::memory_order_relaxed);

        }

        futex_wait_private(&_pending_signal, expected_state.val, NULL);

        if (_no_signal_when_no_waiter) {

            _waiter_num.fetch_sub(1, butil::memory_order_relaxed);

        }

    }

    // Wakeup suspended wait() and make them unwaitable ever. 

    void stop() {

        _pending_signal.fetch_or(1);

        futex_wake_private(&_pending_signal, 10000);

    }

private:

    // higher 31 bits for signalling, LSB for stopping.

    butil::atomic<int> _pending_signal;

    butil::atomic<int> _waiter_num;

    // Whether to signal when there is no waiter.

    // In busy worker scenarios, signal overhead

    // can be reduced.

    bool _no_signal_when_no_waiter;

};

}  // namespace bthread

#endif  // BTHREAD_PARKING_LOT_H