#include <stdio.h>

struct date
{
    int month;
    int day;
    int year;
};

int calc_date_number(struct date a_date)
{
    long f;
    long g;
    long N;

    if (a_date.month <= 2) {
        f = a_date.year - 1;
        g = a_date.month + 13;
    }
    else
    {
        f = a_date.year;
        g = a_date.month + 1;
    }
    printf("f = %ld\n", f);
    printf("g = %ld\n", g);
    printf("day = %d\n", a_date.day);

    N = (1461 * f / 4) + (153 * g / 5) + a_date.day;
    printf("calc_date_number: %ld\n", N);

    return N;
}

int calc_day_number(long int number)
{

    return (number - 621049) % 7;
}

const char* display_day_of_week(int day_of_week)
{
    const char* days[7] = {"Sunday", "Monday", "Tuesday", "Wednesday", "Thursday", "Friday", "Saturday"};

    return days[day_of_week];
}

int main()
{
    struct date a_date;
    long days;
    long day_number;
    const char* dayofweek;
    int daysPerMonth[12] = {31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31};

    printf("Welcome to the Date to Day-of-Week program.\n");
    printf("\nThe program will give the day of the week for any date from 1/1/1900\n\n");

    while (1)
    {
        printf("Enter date (mm/dd/yyyy): ");
        scanf("%d/%d/%d", &a_date.month, &a_date.day, &a_date.year);


        if (a_date.month < 1 || a_date.month > 12)
        {
            printf("Invalid month, please re-enter date.\n");
            continue;
        }
        else if (a_date.day < 1 || a_date.day > 31)
        {
            printf("Invalid day, please re-enter date.\n");
            continue;
        }
        else if (a_date.year < 1900) {
            printf("Invalid year, please re-enter date. Year must be greater than 1900\n");
            continue;
        }

        days = calc_date_number(a_date);
        day_number = calc_day_number(days);
        dayofweek = display_day_of_week(day_number);

        printf("%d/%d/%d falls on a %s.\n", a_date.month, a_date.day, a_date.year, dayofweek);
        break;
    }

    return 0;
}