birthday.c -- for lecture

birthday.c

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

// Returns 1 if the given date exists, 0 if not.
// Ref: Leap year https://ja.wikipedia.org/wiki/%E9%96%8F%E5%B9%B4
int is_correct_day(int y, int m, int d) {
    if (m == 2) {
        if (d > 29) { // d = 30,31 is incorrect
            return 0;
        } else if (d == 29) {
            if (y % 400 == 0 || (y % 4 == 0 && y % 100 != 0)) {
                return 1;
            } else {
                return 0;
           }
        }
    } else if (m == 4 || m == 6 || m == 9 || m == 11) {
        if (d > 30) { // d = 31 is incorrect
            return 0;
        }
    }
    return 1;
}

// Calculate the day of the week.
// Ref: Zeller's congruence https://ja.wikipedia.org/wiki/%E3%83%84%E3%82%A7%E3%83%A9%E3%83%BC%E3%81%AE%E5%85%AC%E5%BC%8F
int Zeller(int y, int m, int d) {
    if (m == 1 || m == 2) {
        m += 12;
        y--;
    }

    int Y = y % 100;
    int C = y / 100;
    int Gamma = 5 * C + C / 4;

    int h = (d + (26 * (m + 1)) / 10 + Y + Y / 4 + Gamma) % 7;

    return h;
}

int main(void) {
    int y, m, d;

    printf("Input your birthday\n");
    printf("-------------------\n");

    printf("Year  (yyyy): ");
    scanf("%d", &y);
    if (y < 1600 || y > 2020) {
        printf("Error: Year must be between 1600 and 2020.\n");
        exit(1);
    }

    printf("Month (mm)  : ");
    scanf("%d", &m);
    if (m < 1 || m > 12) {
        printf("Error: Month must be between 1 and 12.\n");
        exit(1);
    }

    printf("Day   (dd)  : ");
    scanf("%d", &d);
    if (d < 1 || d > 31) {
        printf("Error: Day must be between 1 and 31.\n");
        exit(1);
    }

    printf("\n");
    if (is_correct_day(y, m, d) == 0) {
        printf("Error: %04d/%02d/%02d does not exist.\n", y, m, d);
        exit(1);
    }

    int h = Zeller(y, m, d);

    char days[7][10] = {
        "Saturday", "Sunday" , "Monday", "Tuesday",
        "Wednesday", "Thursday", "Friday"
    };
    printf("%04d/%02d/%02d is %s.\n", y, m, d, days[h]);

    return 0;
}