external/bsd/ntp/dist/libntp/calyearstart.c

   1 /*      $NetBSD$        */
   2
   3 /*
   4  * calyearstart - determine the NTP time at midnight of January 1 in
   5  *                the year of the given date.
   6  */
   7 #include <sys/types.h>
   8
   9 #include "ntp_types.h"
  10 #include "ntp_calendar.h"
  11 #include "ntp_stdlib.h"
  12 #include "ntp_assert.h"
  13
  14 /*
  15  * Juergen Perlinger, 2008-11-12
  16  * Use the result of 'caljulian' to get the delta from the time stamp to the
  17  * beginning of the year. Do not make a second trip through 'caltontp' after
  18  * fixing the date, apart for invariant tests.
  19  */
  20 u_long
  21 calyearstart(u_long ntp_time)
  22 {
  23         struct calendar jt;
  24         ntp_u_int32_t   delta;
  25
  26         caljulian(ntp_time,&jt);
  27
  28         /*
  29         * Now we have days since yearstart (unity-based) and the time in that
  30         * day. Simply merge these together to seconds and subtract that from
  31         * input time. That's faster than going through the calendar stuff
  32         * again...
  33         */
  34         delta = (ntp_u_int32_t)jt.yearday * SECSPERDAY
  35               + (ntp_u_int32_t)jt.hour    * MINSPERHR * SECSPERMIN
  36               + (ntp_u_int32_t)jt.minute  * SECSPERMIN
  37               + (ntp_u_int32_t)jt.second
  38               - SECSPERDAY;     /* yearday is unity-based... */
  39
  40 #   if ISC_CHECK_INVARIANT
  41         /*
  42          * check that this computes properly: do a roundtrip! That's the only
  43          * sensible test here, but it's a rather expensive invariant...
  44          */
  45         jt.yearday  = 0;
  46         jt.month    = 1;
  47         jt.monthday = 1;
  48         jt.hour     = 0;
  49         jt.minute   = 0;
  50         jt.second   = 0;
  51         NTP_INVARIANT((ntp_u_int32_t)(caltontp(&jt) + delta) == (ntp_u_int32_t)ntp_time);
  52 #   endif
  53
  54         /* The NTP time stamps (l_fp) count seconds unsigned mod 2**32, so we
  55          * have to calculate this in the proper way!
  56          */
  57         return (ntp_u_int32_t)(ntp_time - delta);
  58 }