src/Computer/DeltaTime.hpp

   1 /*
   2 Copyright_License {
   3
   4   XCSoar Glide Computer - http://www.xcsoar.org/
   5   Copyright (C) 2000-2013 The XCSoar Project
   6   A detailed list of copyright holders can be found in the file "AUTHORS".
   7
   8   This program is free software; you can redistribute it and/or
   9   modify it under the terms of the GNU General Public License
  10   as published by the Free Software Foundation; either version 2
  11   of the License, or (at your option) any later version.
  12
  13   This program is distributed in the hope that it will be useful,
  14   but WITHOUT ANY WARRANTY; without even the implied warranty of
  15   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
  16   GNU General Public License for more details.
  17
  18   You should have received a copy of the GNU General Public License
  19   along with this program; if not, write to the Free Software
  20   Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA  02111-1307, USA.
  21 }
  22 */
  23
  24 #ifndef XCSOAR_DELTA_TIME_HPP
  25 #define XCSOAR_DELTA_TIME_HPP
  26
  27 #include "Math/fixed.hpp"
  28
  29 /**
  30  * Calculates the time difference between two events, and detects time
  31  * warps.
  32  */
  33 class DeltaTime {
  34   /**
  35    * The time stamp of the previous call.  A negative value means
  36    * "unavailable".
  37    */
  38   fixed last_time;
  39
  40 public:
  41   void Reset() {
  42     last_time = fixed(-1);
  43   }
  44
  45   gcc_pure
  46   bool IsDefined() const {
  47     return !negative(last_time);
  48   }
  49
  50   /**
  51    * Update the "last" time stamp, and return the difference.  Returns
  52    * -1 on time warp.
  53    *
  54    * @param current_time the current time stamp or -1 if not known
  55    * @param min_delta returns zero and does not update the "last" time
  56    * stamp if the difference is smaller than this value
  57    * @param warp_tolerance if the time warp is smaller than this
  58    * value, then zero is returned instead of -1
  59    * @return the (non-negative) time stamp difference since the last
  60    * call, or 0 if difference is too small, or -1 on time warp
  61    */
  62   fixed Update(fixed current_time, fixed min_delta, fixed warp_tolerance) {
  63     assert(!negative(current_time));
  64     assert(!negative(min_delta));
  65     assert(!negative(warp_tolerance));
  66
  67     if (!IsDefined()) {
  68       /* first call */
  69       last_time = current_time;
  70       return fixed(0);
  71     }
  72
  73     if (current_time < last_time) {
  74       /* time warp */
  75
  76       const fixed delta = last_time - current_time;
  77       last_time = current_time;
  78       return delta < warp_tolerance ? fixed(0) : fixed(-1);
  79     }
  80
  81     const fixed delta = current_time - last_time;
  82     if (delta < min_delta)
  83       /* difference too small, don't update "last" time stamp to let
  84          small differences add up eventually */
  85       return fixed(0);
  86
  87     last_time = current_time;
  88
  89     if (delta > fixed(4 * 3600))
  90       /* after several hours without a signal, we can assume there was
  91          a time warp */
  92       return fixed(-1);
  93
  94     return delta;
  95   }
  96 };
  97
  98 #endif