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(time1, time0)
  22 const time_t    time1;
  23 const time_t    time0;
  24 {
  25         /*
  26         ** If (sizeof (double) > sizeof (time_t)) simply convert and subtract
  27         ** (assuming that the larger type has more precision).
  28         ** This is the common real-world case circa 2004.
  29         */
  30         if (sizeof (double) > sizeof (time_t))
  31                 return (double) time1 - (double) time0;
  32         if (!TYPE_INTEGRAL(time_t)) {
  33                 /*
  34                 ** time_t is floating.
  35                 */
  36                 return time1 - time0;
  37         }
  38         if (!TYPE_SIGNED(time_t)) {
  39                 /*
  40                 ** time_t is integral and unsigned.
  41                 ** The difference of two unsigned values can't overflow
  42                 ** if the minuend is greater than or equal to the subtrahend.
  43                 */
  44                 if (time1 >= time0)
  45                         return time1 - time0;
  46                 else    return -((double) (time0 - time1));
  47         }
  48         /*
  49         ** time_t is integral and signed.
  50         ** Handle cases where both time1 and time0 have the same sign
  51         ** (meaning that their difference cannot overflow).
  52         */
  53         if ((time1 < 0) == (time0 < 0))
  54                 return time1 - time0;
  55         /*
  56         ** time1 and time0 have opposite signs.
  57         ** Punt if unsigned long is too narrow.
  58         */
  59         if (sizeof (unsigned long) < sizeof (time_t))
  60                 return (double) time1 - (double) time0;
  61         /*
  62         ** Stay calm...decent optimizers will eliminate the complexity below.
  63         */
  64         if (time1 >= 0 /* && time0 < 0 */)
  65                 return (unsigned long) time1 +
  66                         (unsigned long) (-(time0 + 1)) + 1;
  67         return -(double) ((unsigned long) time0 +
  68                 (unsigned long) (-(time1 + 1)) + 1);
  69 }