NetBSD/lib/libntp/caljulian.c

/*	$NetBSD: caljulian.c,v 1.3 1998/03/06 18:17:14 christos Exp $	*/

/*
 * caljulian - determine the Julian date from an NTP time.
 */
#include <sys/types.h>

#include "ntp_types.h"
#include "ntp_calendar.h"
#include "ntp_stdlib.h"

/*
 * calmonthtab - days-in-the-month table
 */
static u_short calmonthtab[11] = {
    JAN,
    FEB,
    MAR,
    APR,
    MAY,
    JUN,
    JUL,
    AUG,
    SEP,
    OCT,
    NOV
};

void
caljulian(ntptime, jt)
    u_long                    ntptime;
    register struct calendar *jt;
{
    u_long ntp_day;
    u_long minutes;
    /*
     * Absolute, zero-adjusted Christian era day, starting from the
     * mythical day 12/1/1 BC
     */
    u_long acez_day;

    u_long d400;			     /* Days into a Gregorian cycle */
    u_long d100;			     /* Days into a normal century */
    u_long d4;				     /* Days into a 4-year cycle */
    u_long n400;			     /* # of Gregorian cycles */
    u_long n100;			     /* # of normal centuries */
    u_long n4;				     /* # of 4-year cycles */
    u_long n1;				     /* # of years into a leap year */
					     /*   cycle */

    /*
     * Do the easy stuff first: take care of hh:mm:ss, ignoring leap
     * seconds
     */
    jt->second = ntptime % SECSPERMIN;
    minutes    = ntptime / SECSPERMIN;
    jt->minute = minutes % MINSPERHR;
    jt->hour   = (minutes / MINSPERHR) % HRSPERDAY;
    
    /*
     * Find the day past 1900/01/01 00:00 UTC
     */
    ntp_day = ntptime / SECSPERDAY;
    acez_day = DAY_NTP_STARTS + ntp_day - 1;
    n400     = acez_day/GREGORIAN_CYCLE_DAYS;
    d400     = acez_day%GREGORIAN_CYCLE_DAYS;
    n100     = d400 / GREGORIAN_NORMAL_CENTURY_DAYS;
    d100     = d400 % GREGORIAN_NORMAL_CENTURY_DAYS;
    n4       = d100 / GREGORIAN_NORMAL_LEAP_CYCLE_DAYS;
    d4       = d100 % GREGORIAN_NORMAL_LEAP_CYCLE_DAYS;
    n1       = d4 / DAYSPERYEAR;

    /*
     * Calculate the year and year-of-day
     */
    jt->yearday = 1 + d4%DAYSPERYEAR;
    jt->year    = 400*n400 + 100*n100 + n4*4 + n1;

    if (n100 == 4 || n1 == 4)
    {
	/*
	 * If the cycle year ever comes out to 4, it must be December 31st
	 * of a leap year.
	 */
	jt->month    = 12;
	jt->monthday = 31;
	jt->yearday  = 366;
    }
    else
    {
	/*
	 * Else, search forwards through the months to get the right month
	 * and date.
	 */
	int monthday;

	jt->year++;
	monthday = jt->yearday;

	for (jt->month=0;jt->month<11; jt->month++)
	{
	    int t;
	
	    t = monthday - calmonthtab[jt->month];
	    if (jt->month == 1 && is_leapyear(jt->year))
		t--;

	    if (t > 0)
		monthday = t;
	    else
		break;
	}
	jt->month++;
	jt->monthday = monthday;
    }
}