// ----------------------------------------------------------------------
// File: Timekeeper.hh
// Author: Georgios Bitzes - CERN
// ----------------------------------------------------------------------

/************************************************************************
 * quarkdb - a redis-like highly available key-value store              *
 * Copyright (C) 2018 CERN/Switzerland                                  *
 *                                                                      *
 * This program is free software: you can redistribute it and/or modify *
 * it under the terms of the GNU General Public License as published by *
 * the Free Software Foundation, either version 3 of the License, or    *
 * (at your option) any later version.                                  *
 *                                                                      *
 * This program is distributed in the hope that it will be useful,      *
 * but WITHOUT ANY WARRANTY; without even the implied warranty of       *
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the        *
 * GNU General Public License for more details.                         *
 *                                                                      *
 * You should have received a copy of the GNU General Public License    *
 * along with this program.  If not, see <http://www.gnu.org/licenses/>.*
 ************************************************************************/

#ifndef QUARKDB_TIME_KEEPER_H
#define QUARKDB_TIME_KEEPER_H

#include <chrono>
#include <shared_mutex>

namespace quarkdb {

using ClockValue = uint64_t;

class Timekeeper {
public:
  //----------------------------------------------------------------------------
  // Construct Timekeeper with the given initial ClockValue. Time starts
  // rolling forward as soon as the object is constructed.
  //----------------------------------------------------------------------------
  Timekeeper(ClockValue startup);

  //----------------------------------------------------------------------------
  // Reset a Timekeeper object completely, disregarding its previous state.
  // You probably want to use synchronize() to update the clock value!
  //----------------------------------------------------------------------------
  void reset(ClockValue startup);

  //----------------------------------------------------------------------------
  // The static clock has been updated to the given value. The static clock
  // should _never_  go back in time, that indicates serious corruption - an
  // assertion in synchronize() enforces this.
  //
  // However, the dynamic clock (as given by getCurrentTime) might go back
  // if the following happens:
  // - synchronize(0)
  // - sleep(10 ms)
  // - getCurrentTime() -> 10
  // - synchronize(5)
  // - getCurrentTime() -> 5
  //
  // The static clock only went forward in time, but the dynamic clock was
  // set back, and that's okay in the context we're using this.
  //----------------------------------------------------------------------------
  void synchronize(ClockValue newval);

  //----------------------------------------------------------------------------
  // Get the current dynamic time in milliseconds.
  //----------------------------------------------------------------------------
  ClockValue getDynamicTime() const;

private:
  mutable std::shared_mutex mtx;
  ClockValue staticClock;
  std::chrono::steady_clock::time_point anchorPoint;

  //----------------------------------------------------------------------------
  // Get time elapsed since last anchor point
  //----------------------------------------------------------------------------
  std::chrono::milliseconds getTimeSinceAnchor() const;
};

}

#endif