/*	$NetBSD: difftime.c,v 1.22 2019/04/04 19:25:38 christos Exp $	*/

/* Return the difference between two timestamps.  */

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

#include <sys/cdefs.h>
#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).
  */
  /*CONSTCOND*/
  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((double)(time0 - time1));

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