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75 lines
2.1 KiB
C
75 lines
2.1 KiB
C
#ifndef lint
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#ifndef NOID
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static char elsieid[] = "@(#)difftime.c 7.4";
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#endif /* !defined NOID */
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#endif /* !defined lint */
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/*LINTLIBRARY*/
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#include "private.h"
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/*
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** Algorithm courtesy Paul Eggert (eggert@twinsun.com).
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*/
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#ifdef HAVE_LONG_DOUBLE
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#define long_double long double
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#endif /* defined HAVE_LONG_DOUBLE */
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#ifndef HAVE_LONG_DOUBLE
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#define long_double double
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#endif /* !defined HAVE_LONG_DOUBLE */
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double
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difftime(time1, time0)
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const time_t time1;
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const time_t time0;
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{
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time_t delta;
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time_t 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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/*
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** As much as possible, avoid loss of precision
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** by computing the difference before converting to double.
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*/
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delta = time1 - time0;
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if (delta >= 0)
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return delta;
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/*
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** Repair delta overflow.
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*/
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hibit = 1;
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while ((hibit <<= 1) > 0)
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continue;
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/*
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** The following expression rounds twice, which means
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** the result may not be the closest to the true answer.
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** For example, suppose time_t is 64-bit signed int,
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** long_double is IEEE 754 double with default rounding,
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** time1 = 9223372036854775807 and time0 = -1536.
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** Then the true difference is 9223372036854777343,
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** which rounds to 9223372036854777856
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** with a total error of 513.
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** But delta overflows to -9223372036854774273,
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** which rounds to -9223372036854774784, and correcting
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** this by subtracting 2 * (long_double) hibit
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** (i.e. by adding 2**64 = 18446744073709551616)
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** yields 9223372036854776832, which
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** rounds to 9223372036854775808
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** with a total error of 1535 instead.
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** This problem occurs only with very large differences.
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** It's too painful to fix this portably.
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** We are not alone in this problem;
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** many C compilers round twice when converting
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** large unsigned types to small floating types,
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** so if time_t is unsigned the "return delta" above
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** has the same double-rounding problem.
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*/
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return delta - 2 * (long_double) hibit;
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}
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