xref: /src/lib/libc/time/difftime.c

/*	$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;
	}
}