/*

Calculates the date of Easter Sunday or Good Friday
according to the Gregorian Calendar

Michael Sanders 2024
https://busybox.neocities.org/notes/is-easter-or-goodfriday.txt

Further reading...
https://www.algorithm-archive.org/contents/computus/computus.html
https://en.wikipedia.org/wiki/Date_of_Easter
https://en.wikipedia.org/wiki/Gregorian_calendar

*/

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

int isLeapYear(int year) {
    return (year % 4 == 0 && (year % 100 != 0 || year % 400 == 0));
}

int isEasterOrGoodFriday(const struct tm *now, int checkGoodFriday) {

    int Y = now->tm_year + 1900; // correcting the year
    int a = Y % 19;
    int b = Y / 100;
    int c = Y % 100;
    int d = b / 4;
    int e = b % 4;
    int f = (b + 8) / 25;
    int g = (b - f + 1) / 3;
    int h = (19 * a + b - d - g + 15) % 30;
    int i = c / 4;
    int k = c % 4;
    int L = (32 + 2 * e + 2 * i - h - k) % 7;
    int m = (a + 11 * h + 22 * L) / 451;
    int month = (h + L - 7 * m + 114) / 31;
    int day = ((h + L - 7 * m + 114) % 31) + 1;

    if (checkGoodFriday) {
        day -= 2;
        if (day <= 0) {
            month -= 1;
            if (month <= 0) {
                month = 12;
                Y -= 1;
            }
            day += (month == 2) ? (28 + isLeapYear(Y)) : 31;
        }
    }

    return (now->tm_mday == day && now->tm_mon + 1 == month);
}

int main() {

    // set a test date - change as needed
    struct tm testDate;

    // good friday 2024
    testDate.tm_year = 2024 - 1900;
    testDate.tm_mon  = 2;
    testDate.tm_mday = 29;

/*
    // easter 2024
    testDate.tm_year = 2024 - 1900;
    testDate.tm_mon  = 2;
    testDate.tm_mday = 31;
*/

    char Date[30] = {0};
    strftime(Date, sizeof(Date), "%A, %B %d, %Y", &testDate);
    printf("%s\n", Date);

    if (isEasterOrGoodFriday(&testDate, 0)) printf("The date is Easter Sunday.\n");
    else if (isEasterOrGoodFriday(&testDate, 1)) printf("The date is Good Friday.\n");
    else printf("The date is neither Easter Sunday nor Good Friday.\n");

    return 0;
}

// eof