contrib/tzcode/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 /*LINTLIBRARY*/
   7
   8 #include "private.h"    /* for time_t and TYPE_SIGNED */
   9
  10 /* Return -X as a double.  Using this avoids casting to 'double'.  */
  11 static double
  12 dminus(double x)
  13 {
  14   return -x;
  15 }
  16
  17 double ATTRIBUTE_CONST
  18 difftime(time_t time1, time_t time0)
  19 {
  20         /*
  21         ** If double is large enough, simply convert and subtract
  22         ** (assuming that the larger type has more precision).
  23         */
  24         if (sizeof (time_t) < sizeof (double)) {
  25           double t1 = time1, t0 = time0;
  26           return t1 - t0;
  27         }
  28
  29         /*
  30         ** The difference of two unsigned values can't overflow
  31         ** if the minuend is greater than or equal to the subtrahend.
  32         */
  33         if (!TYPE_SIGNED(time_t))
  34           return time0 <= time1 ? time1 - time0 : dminus(time0 - time1);
  35
  36         /* Use uintmax_t if wide enough.  */
  37         if (sizeof (time_t) <= sizeof (uintmax_t)) {
  38           uintmax_t t1 = time1, t0 = time0;
  39           return time0 <= time1 ? t1 - t0 : dminus(t0 - t1);
  40         }
  41
  42         /*
  43         ** Handle cases where both time1 and time0 have the same sign
  44         ** (meaning that their difference cannot overflow).
  45         */
  46         if ((time1 < 0) == (time0 < 0))
  47           return time1 - time0;
  48
  49         /*
  50         ** The values have opposite signs and uintmax_t is too narrow.
  51         ** This suffers from double rounding; attempt to lessen that
  52         ** by using long double temporaries.
  53         */
  54         {
  55           long double t1 = time1, t0 = time0;
  56           return t1 - t0;
  57         }
  58 }