77 lines
2.1 KiB
C
77 lines
2.1 KiB
C
/* $NetBSD: difftime.c,v 1.2 1995/03/09 23:41:15 jtc Exp $ */
|
|
|
|
#ifndef lint
|
|
#ifndef NOID
|
|
static char elsieid[] = "@(#)difftime.c 7.5";
|
|
#endif /* !defined NOID */
|
|
#endif /* !defined lint */
|
|
|
|
/*LINTLIBRARY*/
|
|
|
|
#include "private.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 */
|
|
|
|
double
|
|
difftime(time1, time0)
|
|
const time_t time1;
|
|
const time_t time0;
|
|
{
|
|
time_t delta;
|
|
time_t hibit;
|
|
|
|
if (sizeof(time_t) < sizeof(double))
|
|
return (double) time1 - (double) time0;
|
|
if (sizeof(time_t) < sizeof(long_double))
|
|
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 = 1;
|
|
while ((hibit <<= 1) > 0)
|
|
continue;
|
|
/*
|
|
** 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;
|
|
** many 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.
|
|
*/
|
|
return delta - 2 * (long_double) hibit;
|
|
}
|