/*      $NetBSD: difftime.c,v 1.24 2024/01/20 14:52:49 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>
#if defined(LIBC_SCCS) && !defined(lint)
#if 0
static char     elsieid[] = "@(#)difftime.c     8.1";
#else
__RCSID("$NetBSD: difftime.c,v 1.24 2024/01/20 14:52:49 christos Exp $");
#endif
#endif /* LIBC_SCCS and not lint */

/*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).
        */
        /*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))
                /*NOTREACHED*/
                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;
        }
}