source: buchla-68k/orig/DATE/CTIME.C@ a6f5b95

/*
   =============================================================================
        ctime.c -- time and date support ala' Unix
        Version 3 -- 1989-01-13 -- D.N. Lynx Crowe
   =============================================================================
*/

#define HAS_ENV         0               /* define non-zero if getenv() exists */

/*
 * This routine converts time as follows.
 * The epoch is 0000 Jan 1 1970 GMT.
 * The argument time is in seconds since then.
 * The localtime(t) entry returns a pointer to an array containing:
 *
 *  seconds (0-59)
 *  minutes (0-59)
 *  hours (0-23)
 *  day of month (1-31)
 *  month (0-11)
 *  year-1900
 *  weekday (0-6, Sun is 0)
 *  day of the year
 *  daylight savings flag
 *
 * The routine corrects for daylight savings time and will work in any
 * time zone provided that "timezone" is adjusted to the difference between
 * Greenwich and local standard time (measured in seconds).
 *
 * In places like Michigan "daylight" must be initialized to 0 to prevent
 * the conversion to daylight savings time.
 *
 * There is a table which accounts for the peculiarities undergone by
 * daylight savings time in 1974-1975.
 *
 * This routine does not work for Saudi Arabia,  which runs on Solar time.
 *
 * asctime(tvec)
 *
 *      where tvec is produced by localtime()
 *
 *      Returns a ptr to a character string that has the ascii time in the form:
 *
 *              Thu Jan 01 00:00:00 1970n0\\
 *              01234567890123456789012345
 *              0         1         2
 *
 * ctime(t)
 *
 *      Just calls localtime(), then asctime()
 *
 * tzset()
 *
 *      Looks for an environment variable named TZ.
 *
 *      It should be in the form "ESTn" or "ESTnEDT",
 *      where "n" represents a string of digits with an optional
 *      negative sign (for locations east of Greenwich, England).
 *
 *      If the variable is present, it will set the external
 *      variables "timezone", "daylight", and "tzname"
 *      appropriately. It is called by localtime, and
 *      may also be called explicitly by the user.
 */

/* 
 */

#define dysize(A) (((A)%4)? 365: 366)

#define void    int

#include "time.h"

long    timezone = 8*60*60;
int     daylight = 0;
char    *tzname[] = {"PST", "PDT"};

struct tm *gmtime(), *localtime();
char    *ctime(), *asctime();
void    tzset();

static char cbuf[26];
static int dmsize[12] = { 31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31 };

/*
 * The following table is used for 1974 and 1975 and
 * gives the day number of the first day after the Sunday of the
 * change.
 */

static struct {

        int     daylb;
        int     dayle;

} daytab[] = {

        5,      333,    /* 1974: Jan 6 - last Sun. in Nov */
        58,     303,    /* 1975: Last Sun. in Feb - last Sun in Oct */
};

/* 
 */

/*
   =============================================================================
        ctime(t)
   =============================================================================
*/

char *
ctime(t)
long    *t;
{
        return(asctime(localtime(t)));
}

/* 
 */

/*
   =============================================================================
        localtime(tim)
   =============================================================================
*/

struct tm *
localtime(tim)
long    *tim;
{
        register int dayno, daylbegin, daylend;
        register struct tm *ct;
        long copyt;

        tzset();
        copyt = *tim - timezone;
        ct = gmtime(&copyt);
        dayno = ct->tm_yday;
        daylbegin = 119;        /* last Sun in Apr */
        daylend = 303;          /* Last Sun in Oct */

        if(ct->tm_year == 74 || ct->tm_year == 75) {

                daylbegin = daytab[ct->tm_year-74].daylb;
                daylend = daytab[ct->tm_year-74].dayle;
        }

        daylbegin = sunday(ct, daylbegin);
        daylend = sunday(ct, daylend);

        if(daylight &&
            (dayno>daylbegin || (dayno==daylbegin && ct->tm_hour>=2)) &&
            (dayno<daylend || (dayno==daylend && ct->tm_hour<1))) {

                copyt += 1*60*60;
                ct = gmtime(&copyt);
                ct->tm_isdst++;
        }

        return(ct);
}

/* 
 */

/*
   =============================================================================
        sunday(t,d)

        The argument is a 0-origin day number.
        The value is the day number of the first
        Sunday on or after the day.
   =============================================================================
 */

static int
sunday(t, d)
register struct tm *t;
register int d;
{
        if(d >= 58)
                d += dysize(t->tm_year) - 365;

        return(d - (d - t->tm_yday + t->tm_wday + 700) % 7);
}

/* 
 */

/*
   =============================================================================
        gmtime(tim)
   =============================================================================
*/

struct tm *
gmtime(tim)
long    *tim;
{
        register int d0, d1;
        long hms, day;
        static struct tm xtime;

        /*
         * break initial number into days
         */

        hms = *tim % 86400L;
        day = *tim / 86400L;

        if(hms < 0) {
                hms += 86400L;
                day -= 1;
        }

        /*
         * generate hours:minutes:seconds
         */

        xtime.tm_sec = hms % 60;
        d1 = hms / 60;
        xtime.tm_min = d1 % 60;
        d1 /= 60;
        xtime.tm_hour = d1;

        /*
         * day is the day number.
         * generate day of the week.
         * The addend is 4 mod 7 (1/1/1970 was Thursday)
         */

        xtime.tm_wday = (day + 7340036L) % 7;

        /*
         * year number
         */

        if(day >= 0)
                for(d1=70; day >= dysize(d1); d1++)
                        day -= dysize(d1);
        else
                for(d1=70; day < 0; d1--)
                        day += dysize(d1-1);

        xtime.tm_year = d1;
        xtime.tm_yday = d0 = day;

        /*
         * generate month
         */

        if(dysize(d1) == 366)
                dmsize[1] = 29;

        for(d1=0; d0 >= dmsize[d1]; d1++)
                d0 -= dmsize[d1];

        dmsize[1] = 28;
        xtime.tm_mday = d0+1;
        xtime.tm_mon = d1;
        xtime.tm_isdst = 0;
        return(&xtime);
}

/* 
 */

/*
   =============================================================================
        asctime(t)
   =============================================================================
*/

char *
asctime(t)
struct tm *t;
{
        register char *cp, *ncp;
        register int *tp;
        char    *ct_numb();

        cp = cbuf;

        for(ncp = "Day Mon 00 00:00:00 1900\n"; *cp++ = *ncp++; );

        ncp = &"SunMonTueWedThuFriSat"[3*t->tm_wday];
        cp = cbuf;
        *cp++ = *ncp++;
        *cp++ = *ncp++;
        *cp++ = *ncp++;
        cp++;
        tp = &t->tm_mon;
        ncp = &"JanFebMarAprMayJunJulAugSepOctNovDec"[(*tp)*3];
        *cp++ = *ncp++;
        *cp++ = *ncp++;
        *cp++ = *ncp++;
        cp = ct_numb(cp, *--tp);
        cp = ct_numb(cp, *--tp+100);
        cp = ct_numb(cp, *--tp+100);
        cp = ct_numb(cp, *--tp+100);

        if(t->tm_year >= 100) {
                cp[1] = '2';
                cp[2] = '0';
        }

        cp += 2;
        cp = ct_numb(cp, t->tm_year+100);
        return(cbuf);
}

/* 
 */

/*
   =============================================================================
        ct_numb(cp,n)
   =============================================================================
*/

static char *
ct_numb(cp, n)
register char *cp;
int     n;
{
        cp++;

        if(n >= 10)
                *cp++ = (n/10)%10 + '0';
        else
                *cp++ = ' ';

        *cp++ = n%10 + '0';
        return(cp);
}

/* 
 */

/*
   =============================================================================
        tzset()
   =============================================================================
*/

void
tzset()
{
#if     HAS_ENV
        register char *p, *q;
        register int n;
        int sign;
        char *getenv();

        if((p = getenv ("TZ")) && *p) {

                n = 3;
                q = tzname[0];

                do {

                        *q++ = *p? *p++: ' ';

                } while(--n);

                if(sign = *p == '-')
                        p++;

                n = 0;

                while(*p >= '0' && *p <= '9')
                        n = (n * 10) + *p++ - '0';

                if(sign)
                        n = -n;

                timezone = ((long)(n * 60)) * 60;

                if(daylight = *p != '\0') {

                        q = tzname[1];
                        n = 3;

                        do {

                                *q++ = *p? *p++: ' ';

                        } while(--n);
                }
        }
#endif
}