commands/zoneinfo/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 #ifndef lint
   7 #ifndef NOID
   8 static char     elsieid[] = "@(#)difftime.c     7.19";
   9 #endif /* !defined NOID */
  10 #endif /* !defined lint */
  11
  12 /*LINTLIBRARY*/
  13
  14 #include "private.h"    /* for time_t, TYPE_INTEGRAL, and TYPE_SIGNED */
  15
  16 double
  17 difftime(time1, time0)
  18 const time_t    time1;
  19 const time_t    time0;
  20 {
  21         /*
  22         ** If (sizeof (double) > sizeof (time_t)) simply convert and subtract
  23         ** (assuming that the larger type has more precision).
  24         ** This is the common real-world case circa 2004.
  25         */
  26         if (sizeof (double) > sizeof (time_t))
  27                 return (double) time1 - (double) time0;
  28         if (!TYPE_INTEGRAL(time_t)) {
  29                 /*
  30                 ** time_t is floating.
  31                 */
  32                 return time1 - time0;
  33         }
  34         if (!TYPE_SIGNED(time_t)) {
  35                 /*
  36                 ** time_t is integral and unsigned.
  37                 ** The difference of two unsigned values can't overflow
  38                 ** if the minuend is greater than or equal to the subtrahend.
  39                 */
  40                 if (time1 >= time0)
  41                         return time1 - time0;
  42                 else    return -((double) (time0 - time1));
  43         }
  44         /*
  45         ** time_t is integral and signed.
  46         ** Handle cases where both time1 and time0 have the same sign
  47         ** (meaning that their difference cannot overflow).
  48         */
  49         if ((time1 < 0) == (time0 < 0))
  50                 return time1 - time0;
  51         /*
  52         ** time1 and time0 have opposite signs.
  53         ** Punt if unsigned long is too narrow.
  54         */
  55         if (sizeof (unsigned long) < sizeof (time_t))
  56                 return (double) time1 - (double) time0;
  57         /*
  58         ** Stay calm...decent optimizers will eliminate the complexity below.
  59         */
  60         if (time1 >= 0 /* && time0 < 0 */)
  61                 return (unsigned long) time1 +
  62                         (unsigned long) (-(time0 + 1)) + 1;
  63         return -(double) ((unsigned long) time0 +
  64                 (unsigned long) (-(time1 + 1)) + 1);
  65 }