source: Sophya/trunk/SophyaExt/XephemAstroLib/mjd.c@ 1516

/* functions to manipulate the modified-julian-date used throughout xephem. */

#include <stdio.h>
#include <math.h>

#include "P_.h"
#include "astro.h"

/* given a date in months, mn, days, dy, years, yr,
 * return the modified Julian date (number of days elapsed since 1900 jan 0.5),
 * *mjd.
 */
void
cal_mjd (mn, dy, yr, mjd)
int mn, yr;
double dy;
double *mjd;
{
        static double last_mjd, last_dy;
        static int last_mn, last_yr;
        int b, d, m, y;
        long c;

        if (mn == last_mn && yr == last_yr && dy == last_dy) {
            *mjd = last_mjd;
            return;
        }

        m = mn;
        y = (yr < 0) ? yr + 1 : yr;
        if (mn < 3) {
            m += 12;
            y -= 1;
        }

        if (yr < 1582 || (yr == 1582 && (mn < 10 || (mn == 10 && dy < 15))))
            b = 0;
        else {
            int a;
            a = y/100;
            b = 2 - a + a/4;
        }

        if (y < 0)
            c = (long)((365.25*y) - 0.75) - 694025L;
        else
            c = (long)(365.25*y) - 694025L;

        d = (int)(30.6001*(m+1));

        *mjd = b + c + d + dy - 0.5;

        last_mn = mn;
        last_dy = dy;
        last_yr = yr;
        last_mjd = *mjd;
}

/* given the modified Julian date (number of days elapsed since 1900 jan 0.5,),
 * mjd, return the calendar date in months, *mn, days, *dy, and years, *yr.
 */
void
mjd_cal (mjd, mn, dy, yr)
double mjd;
int *mn, *yr;
double *dy;
{
        static double last_mjd, last_dy;
        static int last_mn, last_yr;
        double d, f;
        double i, a, b, ce, g;

        /* we get called with 0 quite a bit from unused epoch fields.
         * 0 is noon the last day of 1899.
         */
        if (mjd == 0.0) {
            *mn = 12;
            *dy = 31.5;
            *yr = 1899;
            return;
        }

        if (mjd == last_mjd) {
            *mn = last_mn;
            *yr = last_yr;
            *dy = last_dy;
            return;
        }

        d = mjd + 0.5;
        i = floor(d);
        f = d-i;
        if (f == 1) {
            f = 0;
            i += 1;
        }

        if (i > -115860.0) {
            a = floor((i/36524.25)+.99835726)+14;
            i += 1 + a - floor(a/4.0);
        }

        b = floor((i/365.25)+.802601);
        ce = i - floor((365.25*b)+.750001)+416;
        g = floor(ce/30.6001);
        *mn = (int)(g - 1);
        *dy = ce - floor(30.6001*g)+f;
        *yr = (int)(b + 1899);

        if (g > 13.5)
            *mn = (int)(g - 13);
        if (*mn < 2.5)
            *yr = (int)(b + 1900);
        if (*yr < 1)
            *yr -= 1;

        last_mn = *mn;
        last_dy = *dy;
        last_yr = *yr;
        last_mjd = mjd;
}

/* given an mjd, set *dow to 0..6 according to which day of the week it falls
 * on (0=sunday).
 * return 0 if ok else -1 if can't figure it out.
 */
int
mjd_dow (mjd, dow)
double mjd;
int *dow;
{
        /* cal_mjd() uses Gregorian dates on or after Oct 15, 1582.
         * (Pope Gregory XIII dropped 10 days, Oct 5..14, and improved the leap-
         * year algorithm). however, Great Britian and the colonies did not
         * adopt it until Sept 14, 1752 (they dropped 11 days, Sept 3-13,
         * due to additional accumulated error). leap years before 1752 thus
         * can not easily be accounted for from the cal_mjd() number...
         */
        if (mjd < -53798.5) {
            /* pre sept 14, 1752 too hard to correct |:-S */
            return (-1);
        }
        *dow = ((long)floor(mjd-.5) + 1) % 7;/* 1/1/1900 (mjd 0.5) is a Monday*/
        if (*dow < 0)
            *dow += 7;
        return (0);
}

/* given a year, return whether it is a leap year */
int
isleapyear (y)
int y;
{
        return ((y%4==0 && y%100!=0) || y%400==0);
}

/* given a mjd, return the the number of days in the month.  */
void
mjd_dpm (mjd, ndays)
double mjd;
int *ndays;
{
        static short dpm[] = {31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31};
        int m, y;
        double d;

        mjd_cal (mjd, &m, &d, &y);
        *ndays = (m==2 && isleapyear(y)) ? 29 : dpm[m-1];
}

/* given a mjd, return the year and number of days since 00:00 Jan 1 */
void
mjd_dayno (mjd, yr, dy)
double mjd;
int *yr;
double *dy;
{
        double yrd;
        int yri;
        int dpy;

        mjd_year (mjd, &yrd);
        *yr = yri = (int)yrd;
        dpy = isleapyear(yri) ? 366 : 365;
        *dy = dpy*(yrd-yri);
}

/* given a mjd, return the year as a double. */
void
mjd_year (mjd, yr)
double mjd;
double *yr;
{
        static double last_mjd, last_yr;
        int m, y;
        double d;
        double e0, e1;  /* mjd of start of this year, start of next year */

        if (mjd == last_mjd) {
            *yr = last_yr;
            return;
        }

        mjd_cal (mjd, &m, &d, &y);
        if (y == -1) y = -2;
        cal_mjd (1, 1.0, y, &e0);
        cal_mjd (1, 1.0, y+1, &e1);
        *yr = y + (mjd - e0)/(e1 - e0);

        last_mjd = mjd;
        last_yr = *yr;
}

/* given a decimal year, return mjd */
void
year_mjd (y, mjd)
double y;
double *mjd;
{
        double e0, e1;  /* mjd of start of this year, start of next year */
        int yf = (int)floor (y);
        if (yf == -1) yf = -2;

        cal_mjd (1, 1.0, yf, &e0);
        cal_mjd (1, 1.0, yf+1, &e1);
        *mjd = e0 + (y - yf)*(e1-e0);
}

/* round a time in days, *t, to the nearest second, IN PLACE. */
void
rnd_second (t)
double *t;
{
        *t = floor(*t*SPD+0.5)/SPD;
}
        
/* given an mjd, truncate it to the beginning of the whole day */
double
mjd_day(jd)
double jd;
{
        return (floor(jd-0.5)+0.5);
}

/* given an mjd, return the number of hours past midnight of the whole day */
double
mjd_hr(jd)
double jd;
{
        return ((jd-mjd_day(jd))*24.0);
}

/* insure 0 <= *v < r.
 */
void
range (v, r)
double *v, r;
{
        *v -= r*floor(*v/r);
}

/* For RCS Only -- Do Not Edit */
static char *rcsid[2] = {(char *)rcsid, "@(#) $RCSfile: mjd.c,v $ $Date: 2001-04-10 14:40:46 $ $Revision: 1.1.1.1 $ $Name: not supported by cvs2svn $"};