easter/easter.c

#include <stdlib.h>
#include <stdio.h>
#include <stdbool.h>
#include <ctype.h>
#include <math.h>
#include <errno.h>
#include <limits.h>

#define METONIC_PERIOD       19

void usage() {
    fprintf(stderr, "Usage: easter <YYYY>\n");
}

bool is_year(char *arg) {
    int c = *arg == '-' ? 1 : 0; // Skip leading negative sign
    while(*(arg + c) != '\0') {
        if(!isdigit(*(arg + c))) {
            return false;
        }
        ++c;
    }
    return true;
}

bool strtoy(char *y_str, long *y_num) {
    errno = 0;
    *y_num = strtol(y_str, NULL, 10);
    if((errno == ERANGE && (*y_num == LONG_MAX || *y_num == LONG_MIN))
        || (errno != 0 && *y_num == 0)) {
        return false;
    }
    return true;
}

// Ref: https://en.wikipedia.org/wiki/Computus#Algorithms
void easter(const long year) {
    long a = year % METONIC_PERIOD;
    long b = year/100;
    long c = year%100;
    long d = b/4;
    long e = b%4;
    long f = (b + 8)/25;
    long g = (b - f + 1)/3;
    long h = (19*a + b - d - g + 15) % 30;
    long i = c/4;
    long k = c%4;
    long l = (32 + 2*e + 2*i - h - k) % 7;
    long m = (a + 11*h + 22*l)/451;

    long date = h + l - 7*m + 114;
    long month = date/31;
    long day = date % 31 + 1;
    
    printf("%04ld-%02ld-%02ld\n", year, month, day);
}

int main(int argc, char **argv) {
    if(argc != 2) {
        usage();
        exit(1);
    }
    if(!is_year(argv[1])) {
        usage();
        exit(1);
    }

    long Y;
    if(!strtoy(argv[1], &Y)) {
        usage();
        exit(1);
    }
    
    easter(Y);
    return 0;
}