contrib/tzcode/stdtime/difftime.c

   1 /*
   2 ** This file is in the public domain, so clarified as of
   3 ** 1996-06-05 by Arthur David Olson.
   4 */
   5
   6 #include <sys/cdefs.h>
   7 #ifndef lint
   8 #ifndef NOID
   9 static char     elsieid[] __unused = "@(#)difftime.c    8.1";
  10 #endif /* !defined NOID */
  11 #endif /* !defined lint */
  12 __FBSDID("$FreeBSD$");
  13
  14 /*LINTLIBRARY*/
  15
  16 #include "namespace.h"
  17 #include "private.h"    /* for time_t, TYPE_INTEGRAL, and TYPE_SIGNED */
  18 #include "un-namespace.h"
  19
  20 double
  21 difftime(const time_t time1, const time_t time0)
  22 {
  23         /*
  24         ** If (sizeof (double) > sizeof (time_t)) simply convert and subtract
  25         ** (assuming that the larger type has more precision).
  26         ** This is the common real-world case circa 2004.
  27         */
  28         if (sizeof (double) > sizeof (time_t))
  29                 return (double) time1 - (double) time0;
  30         if (!TYPE_INTEGRAL(time_t)) {
  31                 /*
  32                 ** time_t is floating.
  33                 */
  34                 return time1 - time0;
  35         }
  36         if (!TYPE_SIGNED(time_t)) {
  37                 /*
  38                 ** time_t is integral and unsigned.
  39                 ** The difference of two unsigned values can't overflow
  40                 ** if the minuend is greater than or equal to the subtrahend.
  41                 */
  42                 if (time1 >= time0)
  43                         return time1 - time0;
  44                 else    return -((double) (time0 - time1));
  45         }
  46         /*
  47         ** time_t is integral and signed.
  48         ** Handle cases where both time1 and time0 have the same sign
  49         ** (meaning that their difference cannot overflow).
  50         */
  51         if ((time1 < 0) == (time0 < 0))
  52                 return time1 - time0;
  53         /*
  54         ** time1 and time0 have opposite signs.
  55         ** Punt if unsigned long is too narrow.
  56         */
  57         if (sizeof (unsigned long) < sizeof (time_t))
  58                 return (double) time1 - (double) time0;
  59         /*
  60         ** Stay calm...decent optimizers will eliminate the complexity below.
  61         */
  62         if (time1 >= 0 /* && time0 < 0 */)
  63                 return (unsigned long) time1 +
  64                         (unsigned long) (-(time0 + 1)) + 1;
  65         return -(double) ((unsigned long) time0 +
  66                 (unsigned long) (-(time1 + 1)) + 1);
  67 }