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difftime.c revision 1.1 
      1 #ifndef lint
      2 #ifndef NOID
      3 static char	elsieid[] = "@(#)difftime.c	7.5";
      4 #endif /* !defined NOID */
      5 #endif /* !defined lint */
      6 
      7 /*LINTLIBRARY*/
      8 
      9 #include "private.h"
     10 
     11 /*
     12 ** Algorithm courtesy Paul Eggert (eggert (at) twinsun.com).
     13 */
     14 
     15 #ifdef HAVE_LONG_DOUBLE
     16 #define long_double	long double
     17 #endif /* defined HAVE_LONG_DOUBLE */
     18 #ifndef HAVE_LONG_DOUBLE
     19 #define long_double	double
     20 #endif /* !defined HAVE_LONG_DOUBLE */
     21 
     22 double
     23 difftime(time1, time0)
     24 const time_t	time1;
     25 const time_t	time0;
     26 {
     27 	time_t	delta;
     28 	time_t	hibit;
     29 
     30 	if (sizeof(time_t) < sizeof(double))
     31 		return (double) time1 - (double) time0;
     32 	if (sizeof(time_t) < sizeof(long_double))
     33 		return (long_double) time1 - (long_double) time0;
     34 	if (time1 < time0)
     35 		return -difftime(time0, time1);
     36 	/*
     37 	** As much as possible, avoid loss of precision
     38 	** by computing the difference before converting to double.
     39 	*/
     40 	delta = time1 - time0;
     41 	if (delta >= 0)
     42 		return delta;
     43 	/*
     44 	** Repair delta overflow.
     45 	*/
     46 	hibit = 1;
     47 	while ((hibit <<= 1) > 0)
     48 		continue;
     49 	/*
     50 	** The following expression rounds twice, which means
     51 	** the result may not be the closest to the true answer.
     52 	** For example, suppose time_t is 64-bit signed int,
     53 	** long_double is IEEE 754 double with default rounding,
     54 	** time1 = 9223372036854775807 and time0 = -1536.
     55 	** Then the true difference is 9223372036854777343,
     56 	** which rounds to 9223372036854777856
     57 	** with a total error of 513.
     58 	** But delta overflows to -9223372036854774273,
     59 	** which rounds to -9223372036854774784, and correcting
     60 	** this by subtracting 2 * (long_double) hibit
     61 	** (i.e. by adding 2**64 = 18446744073709551616)
     62 	** yields 9223372036854776832, which
     63 	** rounds to 9223372036854775808
     64 	** with a total error of 1535 instead.
     65 	** This problem occurs only with very large differences.
     66 	** It's too painful to fix this portably.
     67 	** We are not alone in this problem;
     68 	** many C compilers round twice when converting
     69 	** large unsigned types to small floating types,
     70 	** so if time_t is unsigned the "return delta" above
     71 	** has the same double-rounding problem.
     72 	*/
     73 	return delta - 2 * (long_double) hibit;
     74 }
     75