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67 lines
2.6 KiB
C
67 lines
2.6 KiB
C
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/* Copyright (C) 1991, 1994 Free Software Foundation, Inc.
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This file is part of the GNU C Library.
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The GNU C Library is free software; you can redistribute it and/or
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modify it under the terms of the GNU Library General Public License as
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published by the Free Software Foundation; either version 2 of the
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License, or (at your option) any later version.
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The GNU C Library is distributed in the hope that it will be useful,
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but WITHOUT ANY WARRANTY; without even the implied warranty of
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MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
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Library General Public License for more details.
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You should have received a copy of the GNU Library General Public
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License along with the GNU C Library; see the file COPYING.LIB. If
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not, write to the Free Software Foundation, Inc., 675 Mass Ave,
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Cambridge, MA 02139, USA. */
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#include <ansidecl.h>
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#include <time.h>
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/* Return the difference between TIME1 and TIME0. */
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double
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DEFUN(difftime, (time1, time0), time_t time1 AND time_t time0)
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{
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/* Algorithm courtesy Paul Eggert (eggert@twinsun.com). */
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time_t delta, hibit;
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if (sizeof (time_t) < sizeof (double))
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return (double) time1 - (double) time0;
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if (sizeof (time_t) < sizeof (LONG_DOUBLE))
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return (LONG_DOUBLE) time1 - (LONG_DOUBLE) time0;
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if (time1 < time0)
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return - difftime (time0, time1);
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/* As much as possible, avoid loss of precision by computing the
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difference before converting to double. */
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delta = time1 - time0;
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if (delta >= 0)
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return delta;
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/* Repair delta overflow. */
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hibit = 1;
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while ((hibit <<= 1) > 0)
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continue;
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/* The following expression rounds twice, which means the result may not
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be the closest to the true answer. For example, suppose time_t is
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64-bit signed int, long_double is IEEE 754 double with default
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rounding, time1 = 9223372036854775807 and time0 = -1536. Then the
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true difference is 9223372036854777343, which rounds to
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9223372036854777856 with a total error of 513. But delta overflows to
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-9223372036854774273, which rounds to -9223372036854774784, and
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correcting this by subtracting 2 * (long_double) hibit (i.e. by adding
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2**64 = 18446744073709551616) yields 9223372036854776832, which rounds
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to 9223372036854775808 with a total error of 1535 instead. This
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problem occurs only with very large differences. It's too painful to
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fix this portably. We are not alone in this problem; many C compilers
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round twice when converting large unsigned types to small floating
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types, so if time_t is unsigned the "return delta" above has the same
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double-rounding problem. */
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return delta - 2 * (LONG_DOUBLE) hibit;
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}
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