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/* Copyright (C) 1991-2018 Free Software Foundation, Inc.
This file is part of the GNU C Library.
The GNU C Library is free software; you can redistribute it and/or
modify it under the terms of the GNU Lesser General Public
License as published by the Free Software Foundation; either
version 2.1 of the License, or (at your option) any later version.
The GNU C Library is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
Lesser General Public License for more details.
You should have received a copy of the GNU Lesser General Public
License along with the GNU C Library; if not, see
<http://www.gnu.org/licenses/>. */
/* Written by Paul Eggert <eggert@cs.ucla.edu>. */
#include <time.h>
#include <limits.h>
#include <float.h>
#include <stdint.h>
#define TYPE_BITS(type) (sizeof (type) * CHAR_BIT)
#define TYPE_FLOATING(type) ((type) 0.5 == 0.5)
#define TYPE_SIGNED(type) ((type) -1 < 0)
/* Return the difference between TIME1 and TIME0, where TIME0 <= TIME1.
time_t is known to be an integer type. */
static double
subtract (time_t time1, time_t time0)
{
if (! TYPE_SIGNED (time_t))
return time1 - time0;
else
{
/* Optimize the common special cases where time_t
can be converted to uintmax_t without losing information. */
uintmax_t dt = (uintmax_t) time1 - (uintmax_t) time0;
double delta = dt;
if (UINTMAX_MAX / 2 < INTMAX_MAX)
{
/* This is a rare host where uintmax_t has padding bits, and possibly
information was lost when converting time_t to uintmax_t.
Check for overflow by comparing dt/2 to (time1/2 - time0/2).
Overflow occurred if they differ by more than a small slop.
Thanks to Clive D.W. Feather for detailed technical advice about
hosts with padding bits.
In the following code the "h" prefix means half. By range
analysis, we have:
-0.5 <= ht1 - 0.5*time1 <= 0.5
-0.5 <= ht0 - 0.5*time0 <= 0.5
-1.0 <= dht - 0.5*(time1 - time0) <= 1.0
If overflow has not occurred, we also have:
-0.5 <= hdt - 0.5*(time1 - time0) <= 0
-1.0 <= dht - hdt <= 1.5
and since dht - hdt is an integer, we also have:
-1 <= dht - hdt <= 1
or equivalently:
0 <= dht - hdt + 1 <= 2
In the above analysis, all the operators have their exact
mathematical semantics, not C semantics. However, dht - hdt +
1 is unsigned in C, so it need not be compared to zero. */
uintmax_t hdt = dt / 2;
time_t ht1 = time1 / 2;
time_t ht0 = time0 / 2;
time_t dht = ht1 - ht0;
if (2 < dht - hdt + 1)
{
/* Repair delta overflow.
The following expression contains a second rounding,
so the result may not be the closest to the true answer.
This problem occurs only with very large differences.
It's too painful to fix this portably. */
delta = dt + 2.0L * (UINTMAX_MAX - UINTMAX_MAX / 2);
}
}
return delta;
}
}
/* Return the difference between TIME1 and TIME0. */
double
__difftime (time_t time1, time_t time0)
{
/* Convert to double and then subtract if no double-rounding error could
result. */
if (TYPE_BITS (time_t) <= DBL_MANT_DIG
|| (TYPE_FLOATING (time_t) && sizeof (time_t) < sizeof (long double)))
return (double) time1 - (double) time0;
/* Likewise for long double. */
if (TYPE_BITS (time_t) <= LDBL_MANT_DIG || TYPE_FLOATING (time_t))
return (long double) time1 - (long double) time0;
/* Subtract the smaller integer from the larger, convert the difference to
double, and then negate if needed. */
return time1 < time0 ? - subtract (time0, time1) : subtract (time1, time0);
}
strong_alias (__difftime, difftime)
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