easter.c

// easter.c - calculate date of easter
//
// based on https://youtu.be/u7UNEDYN2xY
// "How to Calculate the Date of the Easter Weekend \
//     - Simple Formula - Step by Step Tutorial"
//
// see also:
// http://pubs.opengroup.org/onlinepubs/7908799/xsh/time.h.html
// https://www.timeanddate.com/calendar/determining-easter-date.html
//
// previous: https://pastebin.com/e0QDUfJW
//           https://boards.4channel.org/g/thread/70603712#p70608743
// coding style is still inconsistant...

#include <stdio.h>
#include <stdlib.h>
#include <time.h>

int main(int argc, char *argv[]) {
    int x, year;

    if(argc-1 == 1) {
        x = sscanf(argv[1], "%d", &year);
        if(x == 1) {
            if(year < 32) {
                puts("easter is under construction...");
                exit(0);
            }

            // unix epoch:        Jan 1st, 1970
            // MS-DOS epoch:      Jan 1st, 1980
            // Common Lisp epoch: Jan 1st, 1900 etc...

            if(year < 1970) {
                puts("unix time routines don't work before 1970");
                exit(1)
            }
        }
    }

    if(argc-1 != 1 || x <= 0) {
        printf("usage: easter <year>\n");
        exit(1);
    }

    int metonic = 1 + (year % 19); // lookup table [1..20]

    typedef struct { int month; int day; } month_day;
    enum
    {
        Jan=0, Feb, Mar, Apr, May, Jun,
        Jul,   Aug, Sep, Oct, Nov, Dec
    };

    month_day table[] = {
        // entry [0] is dummy, end wrap-around
        { Mar, 27 }, { Apr, 14 }, { Apr,  3 }, { Mar, 23 },
        { Apr, 11 }, { Mar, 31 }, { Apr, 18 }, { Apr,  8 },
        { Mar, 28 }, { Apr, 16 }, { Apr,  5 }, { Mar, 25 },
        { Apr, 13 }, { Apr,  2 }, { Mar, 22 }, { Apr, 10 },
        { Mar, 30 }, { Apr, 17 }, { Apr,  7 }, { Mar, 27 }
    };

    int month = table[metonic].month;
    int day   = table[metonic].day;

    struct tm date;
    date.tm_year = year-1900;
    date.tm_mon  = month;
    date.tm_mday = day;

    /* round-trip conversion to fill in day of week */
    time_t t = mktime(&date); localtime_r(&t, &date);

    /* can DST or leap year/seconds cause negative weekday? */
    int dow = (date.tm_wday + 7) % 7; // Sun == 0

    date.tm_mday += (7-dow);

    /* round-trip to fix day of week (and possibly month) */
    t = mktime(&date); localtime_r(&t, &date);

    char str[128];
    strftime(str, sizeof(str)-1, "%a, %b %d, %Y", &date);
    printf("%s\n", str);
    return 0;
}