/* 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 "private.h"

/* 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;
		double 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;
		uintmax_t 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;
		long double t0 = time0;

		return t1 - t0;
	}
}
