text: // Copyright (c) 2017-2018, The InSPa Project
  //
  // All rights reserved.
  //
  // Redistribution and use in source and binary forms, with or without modification, are
  // permitted provided that the following conditions are met:
  //
  // 1. Redistributions of source code must retain the above copyright notice, this list of
  //    conditions and the following disclaimer.
  //
  // 2. Redistributions in binary form must reproduce the above copyright notice, this list
  //    of conditions and the following disclaimer in the documentation and/or other
  //    materials provided with the distribution.
  //
  // 3. Neither the name of the copyright holder nor the names of its contributors may be
  //    used to endorse or promote products derived from this software without specific
  //    prior written permission.
  //
  // THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS" AND ANY
  // EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF
  // MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL
  // THE COPYRIGHT HOLDER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
  // SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO,
  // PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
  // INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT,
  // STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF
  // THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
  #pragma once
  
  #include <boost/thread/condition_variable.hpp>
  #include <boost/thread/mutex.hpp>
  #include <boost/thread/thread.hpp>
  #include <cstddef>
  #include <functional>
  #include <utility>
  #include <vector>
  #include <stdexcept>
  
  namespace tools
  {
  //! A global thread pool
  class threadpool
  {
  public:
    static threadpool& getInstance() {
      static threadpool instance;
      return instance;
    }
    static threadpool *getNewForUnitTests(unsigned max_threads = 0) {
      return new threadpool(max_threads);
    }
  
    // The waiter lets the caller know when all of its
    // tasks are completed.
    class waiter {
      boost::mutex mt;
      boost::condition_variable cv;
      int num;
      public:
      void inc();
      void dec();
      void wait(threadpool *tpool);  //! Wait for a set of tasks to finish.
      waiter() : num(0){}
      ~waiter();
    };
  
    // Submit a task to the pool. The waiter pointer may be
    // NULL if the caller doesn't care to wait for the
    // task to finish.
    void submit(waiter *waiter, std::function<void()> f, bool leaf = false);
  
    unsigned int get_max_concurrency() const;
  
    ~threadpool();
  
    private:
      threadpool(unsigned int max_threads = 0);
      typedef struct entry {
        waiter *wo;
        std::function<void()> f;
        bool leaf;
      } entry;
      std::deque<entry> queue;
      boost::condition_variable has_work;
      boost::mutex mutex;
      std::vector<boost::thread> threads;
      unsigned int active;
      unsigned int max;
      bool running;
      void run(bool flush = false);
  };
  
  }
  