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  #include "qelapsedtimer.h"
  
  QT_BEGIN_NAMESPACE
  
  /*!
      \class QElapsedTimer
      \inmodule QtCore
      \brief The QElapsedTimer class provides a fast way to calculate elapsed times.
      \since 4.7
  
      \reentrant
      \ingroup tools
  
      The QElapsedTimer class is usually used to quickly calculate how much
      time has elapsed between two events. Its API is similar to that of QTime,
      so code that was using that can be ported quickly to the new class.
  
      However, unlike QTime, QElapsedTimer tries to use monotonic clocks if
      possible. This means it's not possible to convert QElapsedTimer objects
      to a human-readable time.
  
      The typical use-case for the class is to determine how much time was
      spent in a slow operation. The simplest example of such a case is for
      debugging purposes, as in the following example:
  
      \snippet qelapsedtimer/main.cpp 0
  
      In this example, the timer is started by a call to start() and the
      elapsed timer is calculated by the elapsed() function.
  
      The time elapsed can also be used to recalculate the time available for
      another operation, after the first one is complete. This is useful when
      the execution must complete within a certain time period, but several
      steps are needed. The \tt{waitFor}-type functions in QIODevice and its
      subclasses are good examples of such need. In that case, the code could
      be as follows:
  
      \snippet qelapsedtimer/main.cpp 1
  
      Another use-case is to execute a certain operation for a specific
      timeslice. For this, QElapsedTimer provides the hasExpired() convenience
      function, which can be used to determine if a certain number of
      milliseconds has already elapsed:
  
      \snippet qelapsedtimer/main.cpp 2
  
      \section1 Reference Clocks
  
      QElapsedTimer will use the platform's monotonic reference clock in all
      platforms that support it (see QElapsedTimer::isMonotonic()). This has
      the added benefit that QElapsedTimer is immune to time adjustments, such
      as the user correcting the time. Also unlike QTime, QElapsedTimer is
      immune to changes in the timezone settings, such as daylight-saving
      periods.
  
      On the other hand, this means QElapsedTimer values can only be compared
      with other values that use the same reference. This is especially true if
      the time since the reference is extracted from the QElapsedTimer object
      (QElapsedTimer::msecsSinceReference()) and serialised. These values
      should never be exchanged across the network or saved to disk, since
      there's no telling whether the computer node receiving the data is the
      same as the one originating it or if it has rebooted since.
  
      It is, however, possible to exchange the value with other processes
      running on the same machine, provided that they also use the same
      reference clock. QElapsedTimer will always use the same clock, so it's
      safe to compare with the value coming from another process in the same
      machine. If comparing to values produced by other APIs, you should check
      that the clock used is the same as QElapsedTimer (see
      QElapsedTimer::clockType()).
  
      \section2 32-bit overflows
  
      Some of the clocks used by QElapsedTimer have a limited range and may
      overflow after hitting the upper limit (usually 32-bit). QElapsedTimer
      deals with this overflow issue and presents a consistent timing. However,
      when extracting the time since reference from QElapsedTimer, two
      different processes in the same machine may have different understanding
      of how much time has actually elapsed.
  
      The information on which clocks types may overflow and how to remedy that
      issue is documented along with the clock types.
  
      \sa QTime, QTimer
  */
  
  /*!
      \enum QElapsedTimer::ClockType
  
      This enum contains the different clock types that QElapsedTimer may use.
  
      QElapsedTimer will always use the same clock type in a particular
      machine, so this value will not change during the lifetime of a program.
      It is provided so that QElapsedTimer can be used with other non-Qt
      implementations, to guarantee that the same reference clock is being
      used.
  
      \value SystemTime         The human-readable system time. This clock is not monotonic.
      \value MonotonicClock     The system's monotonic clock, usually found in Unix systems. This clock is monotonic and does not overflow.
      \value TickCounter        The system's tick counter, used on Windows systems. This clock may overflow.
      \value MachAbsoluteTime   The Mach kernel's absolute time (\macos and iOS). This clock is monotonic and does not overflow.
      \value PerformanceCounter The high-resolution performance counter provided by Windows. This clock is monotonic and does not overflow.
  
      \section2 SystemTime
  
      The system time clock is purely the real time, expressed in milliseconds
      since Jan 1, 1970 at 0:00 UTC. It's equivalent to the value returned by
      the C and POSIX \tt{time} function, with the milliseconds added. This
      clock type is currently only used on Unix systems that do not support
      monotonic clocks (see below).
  
      This is the only non-monotonic clock that QElapsedTimer may use.
  
      \section2 MonotonicClock
  
      This is the system's monotonic clock, expressed in milliseconds since an
      arbitrary point in the past. This clock type is used on Unix systems
      which support POSIX monotonic clocks (\tt{_POSIX_MONOTONIC_CLOCK}).
  
      This clock does not overflow.
  
      \section2 TickCounter
  
      The tick counter clock type is based on the system's or the processor's
      tick counter, multiplied by the duration of a tick. This clock type is
      used on Windows platforms. If the high-precision performance
      counter is available on Windows, the \tt{PerformanceCounter} clock type
      is used instead.
  
      The TickCounter clock type is the only clock type that may overflow.
      Windows Vista and Windows Server 2008 support the extended 64-bit tick
      counter, which allows avoiding the overflow.
  
      On Windows systems, the clock overflows after 2^32 milliseconds, which
      corresponds to roughly 49.7 days. This means two processes' reckoning of
      the time since the reference may be different by multiples of 2^32
      milliseconds. When comparing such values, it's recommended that the high
      32 bits of the millisecond count be masked off.
  
      \section2 MachAbsoluteTime
  
      This clock type is based on the absolute time presented by Mach kernels,
      such as that found on \macos. This clock type is presented separately
      from MonotonicClock since \macos and iOS are also Unix systems and may support
      a POSIX monotonic clock with values differing from the Mach absolute
      time.
  
      This clock is monotonic and does not overflow.
  
      \section2 PerformanceCounter
  
      This clock uses the Windows functions \tt{QueryPerformanceCounter} and
      \tt{QueryPerformanceFrequency} to access the system's high-precision
      performance counter. Since this counter may not be available on all
      systems, QElapsedTimer will fall back to the \tt{TickCounter} clock
      automatically, if this clock cannot be used.
  
      This clock is monotonic and does not overflow.
  
      \sa clockType(), isMonotonic()
  */
  
  /*!
      \fn QElapsedTimer::QElapsedTimer()
      \since 5.4
  
      Constructs an invalid QElapsedTimer. A timer becomes valid once it has been
      started.
  
      \sa isValid(), start()
  */
  
  
  /*!
      \fn bool QElapsedTimer::operator ==(const QElapsedTimer &other) const
  
      Returns \c true if this object and \a other contain the same time.
  */
  
  /*!
      \fn bool QElapsedTimer::operator !=(const QElapsedTimer &other) const
  
      Returns \c true if this object and \a other contain different times.
  */
  
  static const qint64 invalidData = Q_INT64_C(0x8000000000000000);
  
  /*!
      Marks this QElapsedTimer object as invalid.
  
      An invalid object can be checked with isValid(). Calculations of timer
      elapsed since invalid data are undefined and will likely produce bizarre
      results.
  
      \sa isValid(), start(), restart()
  */
  void QElapsedTimer::invalidate() Q_DECL_NOTHROW
  {
       t1 = t2 = invalidData;
  }
  
  /*!
      Returns \c false if the timer has never been started or invalidated by a
      call to invalidate().
  
      \sa invalidate(), start(), restart()
  */
  bool QElapsedTimer::isValid() const Q_DECL_NOTHROW
  {
      return t1 != invalidData && t2 != invalidData;
  }
  
  /*!
      Returns \c true if this QElapsedTimer has already expired by \a timeout
      milliseconds (that is, more than \a timeout milliseconds have elapsed).
      The value of \a timeout can be -1 to indicate that this timer does not
      expire, in which case this function will always return false.
  
      \sa elapsed()
  */
  bool QElapsedTimer::hasExpired(qint64 timeout) const Q_DECL_NOTHROW
  {
      // if timeout is -1, quint64(timeout) is LLINT_MAX, so this will be
      // considered as never expired
      return quint64(elapsed()) > quint64(timeout);
  }
  
  QT_END_NAMESPACE
  