/*-*- mode:c;indent-tabs-mode:t;c-basic-offset:8;tab-width:8;coding:utf-8   -*-│
│ vi: set noet ft=c ts=8 sw=8 fenc=utf-8                                   :vi │
╚─────────────────────────────────────────────────────────────────────────────*/
#include "libc/calls/weirdtypes.h"
#include "libc/macros.h"
#include "libc/time.h"

/* Return the difference between two timestamps.  */

/* 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, 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, 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, t0 = time0;
		return t1 - t0;
	}
}