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difftime.c revision 1.4 
      1 /*	$NetBSD: difftime.c,v 1.4 1996/09/10 22:04:29 jtc Exp $	*/
      2 
      3 /*
      4 ** This file is in the public domain, so clarified as of
      5 ** June 5, 1996 by Arthur David Olson (arthur_david_olson (at) nih.gov).
      6 */
      7 
      8 #ifndef lint
      9 #ifndef NOID
     10 static char	elsieid[] = "@(#)difftime.c	7.7";
     11 #endif /* !defined NOID */
     12 #endif /* !defined lint */
     13 
     14 /*LINTLIBRARY*/
     15 
     16 #include "private.h"
     17 
     18 /*
     19 ** Algorithm courtesy Paul Eggert (eggert (at) twinsun.com).
     20 */
     21 
     22 #ifdef HAVE_LONG_DOUBLE
     23 #define long_double	long double
     24 #endif /* defined HAVE_LONG_DOUBLE */
     25 #ifndef HAVE_LONG_DOUBLE
     26 #define long_double	double
     27 #endif /* !defined HAVE_LONG_DOUBLE */
     28 
     29 double
     30 difftime(time1, time0)
     31 const time_t	time1;
     32 const time_t	time0;
     33 {
     34 	time_t	delta;
     35 	time_t	hibit;
     36 
     37 	if (sizeof(time_t) < sizeof(double))
     38 		return (double) time1 - (double) time0;
     39 	if (sizeof(time_t) < sizeof(long_double))
     40 		return (long_double) time1 - (long_double) time0;
     41 	if (time1 < time0)
     42 		return -difftime(time0, time1);
     43 	/*
     44 	** As much as possible, avoid loss of precision
     45 	** by computing the difference before converting to double.
     46 	*/
     47 	delta = time1 - time0;
     48 	if (delta >= 0)
     49 		return delta;
     50 	/*
     51 	** Repair delta overflow.
     52 	*/
     53 	hibit = (~ (time_t) 0) << (TYPE_BIT(time_t) - 1);
     54 	/*
     55 	** The following expression rounds twice, which means
     56 	** the result may not be the closest to the true answer.
     57 	** For example, suppose time_t is 64-bit signed int,
     58 	** long_double is IEEE 754 double with default rounding,
     59 	** time1 = 9223372036854775807 and time0 = -1536.
     60 	** Then the true difference is 9223372036854777343,
     61 	** which rounds to 9223372036854777856
     62 	** with a total error of 513.
     63 	** But delta overflows to -9223372036854774273,
     64 	** which rounds to -9223372036854774784, and correcting
     65 	** this by subtracting 2 * (long_double) hibit
     66 	** (i.e. by adding 2**64 = 18446744073709551616)
     67 	** yields 9223372036854776832, which
     68 	** rounds to 9223372036854775808
     69 	** with a total error of 1535 instead.
     70 	** This problem occurs only with very large differences.
     71 	** It's too painful to fix this portably.
     72 	** We are not alone in this problem;
     73 	** some C compilers round twice when converting
     74 	** large unsigned types to small floating types,
     75 	** so if time_t is unsigned the "return delta" above
     76 	** has the same double-rounding problem with those compilers.
     77 	*/
     78 	return delta - 2 * (long_double) hibit;
     79 }
     80