/*

*/

#include <sys/cdefs.h>

#ifndef lint

#ifndef NOID

static char	elsieid[] __unused = "@(#)difftime.c	7.9";

#endif /* !defined NOID */

#endif /* !defined lint */

__FBSDID("$FreeBSD: src/lib/libc/stdtime/difftime.c,v 1.8 2004/06/14 10:31:52 stefanf Exp $");

/*LINTLIBRARY*/

#include "namespace.h"

#include "private.h"

#include "un-namespace.h"

/*

** Algorithm courtesy Paul Eggert (eggert@twinsun.com).

*/

#ifdef HAVE_LONG_DOUBLE

#define long_double	long double

#endif /* defined HAVE_LONG_DOUBLE */

#ifndef HAVE_LONG_DOUBLE

#define long_double	double

#endif /* !defined HAVE_LONG_DOUBLE */

EXPORT_C

double

difftime(time1, time0)

const time_t	time1;

const time_t	time0;

{

	time_t	delta;

	time_t	hibit;

	{

		time_t		tt;

		double		d;

		long_double	ld;

		if (sizeof tt < sizeof d)

			return (double) time1 - (double) time0;

		if (sizeof tt < sizeof ld)

			return (long_double) time1 - (long_double) time0;

	}

	if (time1 < time0)

		return -difftime(time0, time1);

	/*

	** As much as possible, avoid loss of precision

	** by computing the difference before converting to double.

	*/

	delta = time1 - time0;

	if (delta >= 0)

		return delta;

	/*

	** Repair delta overflow.

	*/

	hibit = (~ (time_t) 0) << (TYPE_BIT(time_t) - 1);

	/*

	** The following expression rounds twice, which means

	** the result may not be the closest to the true answer.

	** For example, suppose time_t is 64-bit signed int,

	** long_double is IEEE 754 double with default rounding,

	** time1 = 9223372036854775807 and time0 = -1536.

	** Then the true difference is 9223372036854777343,

	** which rounds to 9223372036854777856

	** with a total error of 513.

	** But delta overflows to -9223372036854774273,

	** which rounds to -9223372036854774784, and correcting

	** this by subtracting 2 * (long_double) hibit

	** (i.e. by adding 2**64 = 18446744073709551616)

	** yields 9223372036854776832, which

	** rounds to 9223372036854775808

	** with a total error of 1535 instead.

	** This problem occurs only with very large differences.

	** It's too painful to fix this portably.

	** We are not alone in this problem;

	** some C compilers round twice when converting

	** large unsigned types to small floating types,

	** so if time_t is unsigned the "return delta" above

	** has the same double-rounding problem with those compilers.

	*/

	return delta - 2 * (long_double) hibit;

}