// Copyright (c) 2014 GeometryFactory Sarl (France)
// All rights reserved.
//
// This file is part of CGAL (www.cgal.org); you can redistribute it and/or
// modify it under the terms of the GNU Lesser General Public License as
// published by the Free Software Foundation; either version 3 of the License,
// or (at your option) any later version.
//
// Licensees holding a valid commercial license may use this file in
// accordance with the commercial license agreement provided with the software.
//
// This file is provided AS IS with NO WARRANTY OF ANY KIND, INCLUDING THE
// WARRANTY OF DESIGN, MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE.
//
// $URL$
// $Id$
// SPDX-License-Identifier: LGPL-3.0+
//
// Author(s)     : Jane Tournois

#ifndef CGAL_TIME_STAMPER_H
#define CGAL_TIME_STAMPER_H

#include <CGAL/Has_timestamp.h>
#include <CGAL/atomic.h>

namespace CGAL {

template <typename T>
struct Time_stamper
{
  Time_stamper()
   : time_stamp_() {}

  Time_stamper(const Time_stamper& ts)
    : time_stamp_()
  {
    time_stamp_ = std::size_t(ts.time_stamp_);
  }

  Time_stamper& operator=(const Time_stamper& ts)
  {
    time_stamp_ = std::size_t(ts.time_stamp_);
    return *this;
  }

  static void initialize_time_stamp(T* pt) {
    pt->set_time_stamp(std::size_t(-1));
  }

  void set_time_stamp(T* pt) {
    if(pt->time_stamp() == std::size_t(-1)) {
      const std::size_t new_ts = time_stamp_++;
      pt->set_time_stamp(new_ts);
    }
    else {
      // else: the time stamp is re-used

      // Enforces that the time stamp is greater than the current value.
      // That is used when a TDS_3 is copied: in that case, the
      // time stamps are copied from the old element to the new one,
      // but the time stamper does not know.
#ifdef CGAL_NO_ATOMIC
      time_stamp_ = (std::max)(time_stamp_, pt->time_stamp() + 1);
#else
      // How to atomically update a maximum value?
      //   https://stackoverflow.com/a/16190791/1728537
      const std::size_t new_value = pt->time_stamp() + 1;
      std::size_t prev_value = time_stamp_;
      while(prev_value < new_value &&
            !time_stamp_.compare_exchange_weak(prev_value, new_value))
        ;
#endif // atomic
    }
  }

  static std::size_t time_stamp(const T* pt)
  {
    if(pt == NULL){
      return std::size_t(-1);
    }
    return pt->time_stamp();
  }

  static std::size_t hash_value(const T* p) {
    if(NULL == p)
      return std::size_t(-1);
    else
      return p->time_stamp();
  }

  static bool less(const T* p_t1, const T* p_t2) {
    if(p_t1 == NULL)      return (p_t2 != NULL);
    else if(p_t2 == NULL) return false;
    else {
      CGAL_assertion((p_t1 == p_t2) == (time_stamp(p_t1) == time_stamp(p_t2)));
      return time_stamp(p_t1) < time_stamp(p_t2);
    }
  }

  void reset() {
    time_stamp_ = 0;
  }
private:
#ifdef CGAL_NO_ATOMIC
  std::size_t time_stamp_;
#else
  CGAL::cpp11::atomic<std::size_t> time_stamp_;
#endif
}; // end class template Time_stamper<T>

template <typename T>
struct No_time_stamp
{
public:
  void set_time_stamp(T*)  {}
  static bool less(const T* p_t1,const T* p_t2) {
    return p_t1 < p_t2;
  }

  static void initialize_time_stamp(T*) {
  }

  static std::size_t time_stamp(const T*)
  {
    return 0;
  }

  static std::size_t hash_value(const T* p) {
    return reinterpret_cast<std::size_t>(p)/sizeof(T);
  }

  void reset()                {}
}; // end class template No_time_stamp<T>

// That class template is an auxiliary class.  It has a
// specialization for the case where `T::Has_timestamp` does not exists.
// The non-specialized template, when `T::Has_timestamp` exists, derives
// from `Time_stamper<T>` or `No_time_stamp<T>` depending on the
// value of the Boolean constant `T::Has_timestamp`.
// The declaration of that class template requires `T` to be a complete type.
template <class T, bool has_timestamp = internal::Has_timestamp<T>::value>
struct Get_time_stamper{
  typedef Time_stamper<T> type;
};

// Specialization when `T::Has_timestamp` does not exist, derives from
// `TimeStamper_`, or from `No_time_stamp<T>`.
template <class T>
struct Get_time_stamper<T,false>{
  typedef No_time_stamp<T> type;
};

// Implementation of the timestamp policy. It is very important that the
// declaration of that class template does not require `T` to be a complete
// type.  That way, the declaration of a pointer of type `Time_stamper_impl<T, Ts>
// in `Compact_container` for example is possible with an incomplete type.
template <class T>
struct Time_stamper_impl : public Get_time_stamper<T>::type {};

} //end of namespace CGAL

#endif // CGAL_TIME_STAMPER_H
