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difftime work

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SCCS-file: difftime.c
SCCS-SID: 7.10
This commit is contained in:
Arthur David Olson
2004-10-21 10:29:28 -04:00
committed by Paul Eggert
parent 3626176d0e
commit e0ba4abff7
1 changed files with 33 additions and 61 deletions
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94
difftime.c
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@ -11,73 +11,45 @@ static char elsieid[] = "%W%";
/*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 */
#include "sys/types.h" /* for time_t */
#include "private.h" /* for TYPE_INTEGRAL and TYPE_SIGNED */
double
difftime(time1, time0)
const time_t time1;
const time_t time0;
{
time_t delta;
time_t hibit;
if (!TYPE_INTEGRAL(time_t)) {
/*
** time_t is floating.
** Do the math in whichever of time_t or double is wider.
*/
if (sizeof (time_t) >= sizeof (double))
return time1 - time0;
else return (double) time1 - (double) time0;
} else if (!TYPE_SIGNED(time_t)) {
/*
** time_t is integral and unsigned.
** The difference of two time_t's won't overflow if
** the minuend is greater than or equal to the subtrahend.
*/
if (time1 >= time0)
return time1 - time0;
else return -((double) (time0 - time1));
} else {
/*
** time_t is integral and signed.
** As elsewhere in the time zone package,
** use modular arithmetic to avoid overflow.
** We could check to see if double is sufficiently wider
** than time_t to let us simply return
** (double) time1 - (double) time0
*/
register time_t lead;
register time_t trail;
{
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;
lead = time1 / 2 - time0 / 2;
trail = time1 % 2 - time0 % 2;
return 2 * ((double) lead) + trail;
}
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;
}