Easter - calculate Easter Sunday (Gregorian calender)

easter.c

/**
 *  Easter - calculate Easter Sunday (Gregorian calender)
 *  Copyright (C) 2011 Matous J. Fialka, <http://mjf.cz/>
 *  Released under the terms of The MIT License
 *
 *  $ cc -pedantic -ansi -pedantic-errors -Wall -Werror easter.c -o easter
 *  $ su -c 'cp easter /usr/local/bin'
 */

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

const char *s[12] = {
	"January", "February", "March", "April", "May", "June",
	"July", "August", "September", "October", "November", "December"
};

int
main(int argc, char *argv[])
{
	int y, g, c, h, i, j, l, m, d;

	if(argc < 2) {
		fprintf(stderr, "Usage: easter [year ...]\n");
		return 1;
	}
	while(*++argv) {
		y = atoi(*argv);
		g = y % 19;
		c = y / 100;
		h = (c - c / 4 - (8 * c + 13) / 25 + 19 * g + 15) % 30;
		i = h - (h / 28) * (1 - (29 / (h + 1)) * ((21 - g) / 11));
		j = (y + y / 4 + i + 2 - c + c / 4) % 7;
		l = i - j;
		m = 3 + (l + 40) / 44;
		d = l + 28 - 31 * (m / 4);
		printf("Sunday, %s %d, %d\n", s[m - 1], d, y);
	}
	return 0;
}

Example usage may be as follows:

$ easter 2010 2011 2012
Sunday, April 4, 2010
Sunday, April 24, 2011
Sunday, April 8, 2012

Enjoy!