////////////////////////////////////////////////////////////////////////////////
/// DISCLAIMER
///
/// Copyright 2014-2016 ArangoDB GmbH, Cologne, Germany
/// Copyright 2004-2014 triAGENS GmbH, Cologne, Germany
///
/// 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 holder is ArangoDB GmbH, Cologne, Germany
///
/// @author Jan Steemann
////////////////////////////////////////////////////////////////////////////////

#ifndef ARANGODB_PREGEL_THREAD_POOL_H
#define ARANGODB_PREGEL_THREAD_POOL_H 1

#include "Basics/Common.h"
#include "Basics/ConditionLocker.h"
#include "Basics/ConditionVariable.h"
#include "Basics/Thread.h"

#include <deque>

namespace arangodb {
namespace pregel {

class WorkerThread;
class ThreadPool {
 public:
  ThreadPool(ThreadPool const&) = delete;
  ThreadPool& operator=(ThreadPool const&) = delete;

  ThreadPool(size_t, std::string const&);

  ~ThreadPool();

 public:
  //////////////////////////////////////////////////////////////////////////////
  /// @brief return the number of threads in the pool
  //////////////////////////////////////////////////////////////////////////////

  size_t numThreads() const { return _threads.size(); }

  //////////////////////////////////////////////////////////////////////////////
  /// @brief return the name of the pool
  //////////////////////////////////////////////////////////////////////////////

  char const* name() const { return _name.c_str(); }

  //////////////////////////////////////////////////////////////////////////////
  /// @brief dequeue a task
  //////////////////////////////////////////////////////////////////////////////

  bool dequeue(std::function<void()>&);

  //////////////////////////////////////////////////////////////////////////////
  /// @brief enqueue a task
  //////////////////////////////////////////////////////////////////////////////

  template <typename T>
  void enqueue(T task) {
    {
      CONDITION_LOCKER(guard, _condition);
      _tasks.emplace_back(std::function<void()>(task));
    }

    _condition.signal();
  }

 private:
  arangodb::basics::ConditionVariable _condition;

  std::vector<WorkerThread*> _threads;

  std::deque<std::function<void()>> _tasks;

  std::string _name;

  std::atomic<bool> _stopping;
};

class WorkerThread : public arangodb::Thread {
 public:
  WorkerThread(WorkerThread const&) = delete;
  WorkerThread operator=(WorkerThread const&) = delete;

  WorkerThread(ThreadPool* pool)
      : Thread(pool->name()), _pool(pool), _status(0) {}

  ~WorkerThread() {
    waitForDone();
    shutdown();
  }

  //////////////////////////////////////////////////////////////////////////////
  /// @brief stops the worker thread
  //////////////////////////////////////////////////////////////////////////////

  void waitForDone() {
    int expected = 0;
    _status.compare_exchange_strong(expected, 1, std::memory_order_relaxed);

    while (_status != 2) {
      usleep(5000);
    }
  }

 protected:
  void run() {
    while (_status == 0) {
      if (isStopping()) {
        break;
      }

      std::function<void()> task;

      if (!_pool->dequeue(task)) {
        break;
      }

      task();
    }

    _status = 2;
  }

 private:
  ThreadPool* _pool;

  std::atomic<int> _status;
};

}  // namespace arangodb::basics
}  // namespace arangodb

#endif