jens-maus/yam

/* Return the difference between two timestamps.  */

/*
** This file is in the public domain, so clarified as of
** 1996-06-05 by Arthur David Olson.
*/

/*LINTLIBRARY*/

#include "private.h"    /* for time_t and TYPE_SIGNED */

/* Return -X as a double.  Using this avoids casting to 'double'.  */
static double
dminus(double x)
{
  return -x;
}

double
difftime(time_t time1, time_t time0)
{
    /*
    ** If double is large enough, simply convert and subtract
    ** (assuming that the larger type has more precision).
    */
    if (sizeof (time_t) < sizeof (double)) {
      double t1 = time1, t0 = time0;
      return t1 - t0;
    }

    /*
    ** The difference of two unsigned values can't overflow
    ** if the minuend is greater than or equal to the subtrahend.
    */
    if (!TYPE_SIGNED(time_t))
      return time0 <= time1 ? time1 - time0 : dminus(time0 - time1);

    /* Use uintmax_t if wide enough.  */
    if (sizeof (time_t) <= sizeof (uintmax_t)) {
      uintmax_t t1 = time1, t0 = time0;
      return time0 <= time1 ? t1 - t0 : dminus(t0 - t1);
    }

    /*
    ** Handle cases where both time1 and time0 have the same sign
    ** (meaning that their difference cannot overflow).
    */
    if ((time1 < 0) == (time0 < 0))
      return time1 - time0;

    /*
    ** The values have opposite signs and uintmax_t is too narrow.
    ** This suffers from double rounding; attempt to lessen that
    ** by using long double temporaries.
    */
    {
      long double t1 = time1, t0 = time0;
      return t1 - t0;
    }
}