contrib/ntp/libntp/caljulian.c

   1 /*
   2  * caljulian - determine the Julian date from an NTP time.
   3  */
   4 #include <sys/types.h>
   5
   6 #include "ntp_types.h"
   7 #include "ntp_calendar.h"
   8 #include "ntp_stdlib.h"
   9
  10 /*
  11  * calmonthtab - days-in-the-month table
  12  */
  13 static u_short calmonthtab[11] = {
  14         JAN,
  15         FEB,
  16         MAR,
  17         APR,
  18         MAY,
  19         JUN,
  20         JUL,
  21         AUG,
  22         SEP,
  23         OCT,
  24         NOV
  25 };
  26
  27 void
  28 caljulian(
  29         u_long                          ntptime,
  30         register struct calendar        *jt
  31         )
  32 {
  33         u_long ntp_day;
  34         u_long minutes;
  35         /*
  36          * Absolute, zero-adjusted Christian era day, starting from the
  37          * mythical day 12/1/1 BC
  38          */
  39         u_long acez_day;
  40
  41         u_long d400;                             /* Days into a Gregorian cycle */
  42         u_long d100;                             /* Days into a normal century */
  43         u_long d4;                                       /* Days into a 4-year cycle */
  44         u_long n400;                             /* # of Gregorian cycles */
  45         u_long n100;                             /* # of normal centuries */
  46         u_long n4;                                       /* # of 4-year cycles */
  47         u_long n1;                                       /* # of years into a leap year */
  48                                                  /*   cycle */
  49
  50         /*
  51          * Do the easy stuff first: take care of hh:mm:ss, ignoring leap
  52          * seconds
  53          */
  54         jt->second = (u_char)(ntptime % SECSPERMIN);
  55         minutes    = ntptime / SECSPERMIN;
  56         jt->minute = (u_char)(minutes % MINSPERHR);
  57         jt->hour   = (u_char)((minutes / MINSPERHR) % HRSPERDAY);
  58
  59         /*
  60          * Find the day past 1900/01/01 00:00 UTC
  61          */
  62         ntp_day = ntptime / SECSPERDAY;
  63         acez_day = DAY_NTP_STARTS + ntp_day - 1;
  64         n400     = acez_day/GREGORIAN_CYCLE_DAYS;
  65         d400     = acez_day%GREGORIAN_CYCLE_DAYS;
  66         n100     = d400 / GREGORIAN_NORMAL_CENTURY_DAYS;
  67         d100     = d400 % GREGORIAN_NORMAL_CENTURY_DAYS;
  68         n4               = d100 / GREGORIAN_NORMAL_LEAP_CYCLE_DAYS;
  69         d4               = d100 % GREGORIAN_NORMAL_LEAP_CYCLE_DAYS;
  70         n1               = d4 / DAYSPERYEAR;
  71
  72         /*
  73          * Calculate the year and year-of-day
  74          */
  75         jt->yearday = (u_short)(1 + d4%DAYSPERYEAR);
  76         jt->year        = (u_short)(400*n400 + 100*n100 + n4*4 + n1);
  77
  78         if (n100 == 4 || n1 == 4)
  79         {
  80         /*
  81          * If the cycle year ever comes out to 4, it must be December 31st
  82          * of a leap year.
  83          */
  84         jt->month        = 12;
  85         jt->monthday = 31;
  86         jt->yearday  = 366;
  87         }
  88         else
  89         {
  90         /*
  91          * Else, search forwards through the months to get the right month
  92          * and date.
  93          */
  94         int monthday;
  95
  96         jt->year++;
  97         monthday = jt->yearday;
  98
  99         for (jt->month=0;jt->month<11; jt->month++)
 100         {
 101                 int t;
 102
 103                 t = monthday - calmonthtab[jt->month];
 104                 if (jt->month == 1 && is_leapyear(jt->year))
 105                 t--;
 106
 107                 if (t > 0)
 108                 monthday = t;
 109                 else
 110                 break;
 111         }
 112         jt->month++;
 113         jt->monthday = monthday;
 114         }
 115 }