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difftime.c revision 1.2 
      1 /*	$NetBSD: difftime.c,v 1.2 1995/03/09 23:41:15 jtc Exp $	*/
      2 
      3 #ifndef lint
      4 #ifndef NOID
      5 static char	elsieid[] = "@(#)difftime.c	7.5";
      6 #endif /* !defined NOID */
      7 #endif /* !defined lint */
      8 
      9 /*LINTLIBRARY*/
     10 
     11 #include "private.h"
     12 
     13 /*
     14 ** Algorithm courtesy Paul Eggert (eggert (at) twinsun.com).
     15 */
     16 
     17 #ifdef HAVE_LONG_DOUBLE
     18 #define long_double	long double
     19 #endif /* defined HAVE_LONG_DOUBLE */
     20 #ifndef HAVE_LONG_DOUBLE
     21 #define long_double	double
     22 #endif /* !defined HAVE_LONG_DOUBLE */
     23 
     24 double
     25 difftime(time1, time0)
     26 const time_t	time1;
     27 const time_t	time0;
     28 {
     29 	time_t	delta;
     30 	time_t	hibit;
     31 
     32 	if (sizeof(time_t) < sizeof(double))
     33 		return (double) time1 - (double) time0;
     34 	if (sizeof(time_t) < sizeof(long_double))
     35 		return (long_double) time1 - (long_double) time0;
     36 	if (time1 < time0)
     37 		return -difftime(time0, time1);
     38 	/*
     39 	** As much as possible, avoid loss of precision
     40 	** by computing the difference before converting to double.
     41 	*/
     42 	delta = time1 - time0;
     43 	if (delta >= 0)
     44 		return delta;
     45 	/*
     46 	** Repair delta overflow.
     47 	*/
     48 	hibit = 1;
     49 	while ((hibit <<= 1) > 0)
     50 		continue;
     51 	/*
     52 	** The following expression rounds twice, which means
     53 	** the result may not be the closest to the true answer.
     54 	** For example, suppose time_t is 64-bit signed int,
     55 	** long_double is IEEE 754 double with default rounding,
     56 	** time1 = 9223372036854775807 and time0 = -1536.
     57 	** Then the true difference is 9223372036854777343,
     58 	** which rounds to 9223372036854777856
     59 	** with a total error of 513.
     60 	** But delta overflows to -9223372036854774273,
     61 	** which rounds to -9223372036854774784, and correcting
     62 	** this by subtracting 2 * (long_double) hibit
     63 	** (i.e. by adding 2**64 = 18446744073709551616)
     64 	** yields 9223372036854776832, which
     65 	** rounds to 9223372036854775808
     66 	** with a total error of 1535 instead.
     67 	** This problem occurs only with very large differences.
     68 	** It's too painful to fix this portably.
     69 	** We are not alone in this problem;
     70 	** many C compilers round twice when converting
     71 	** large unsigned types to small floating types,
     72 	** so if time_t is unsigned the "return delta" above
     73 	** has the same double-rounding problem.
     74 	*/
     75 	return delta - 2 * (long_double) hibit;
     76 }
     77