#include <stdio.h>

/* Structs */
/* ------- */

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

/* This function takes a date and returns a number representing the date entered */
/* ----------------------------------------------------------------------------- */

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

    N = (1461 * f / 4) + (153 * g / 5) + a_date.day;

    return N;
}

/* This function takes the number calculated in calc_date_number and returns a number */
/* representing the day of the week (Sunday- Saturday                                 */
/* ---------------------------------------------------------------------------------- */

int calc_day_number(long int number)
{

    return (number - 621049) % 7;
}

/* This function takes the numeric representation and returns the English representation */
/* of the day of the week                                                                */
/* ------------------------------------------------------------------------------------- */

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

    return days[day_of_week];
}

/* Begin function main */
/* ------------------- */

int main()
{
    /* Declare variables */
    /* ----------------- */

    struct date a_date;
    long int days;
    long int day_number;
    const char* dayofweek;
    long 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");

    /* Checks to see if year is a leap year. */
    /* ------------------------------------- */

    if((a_date.year%4==0 && a_date.year%100!=0) || (a_date.year%4==0 && a_date.year%400==0))
    daysPerMonth[1] = 29;

    while (1)
    {
        /* Prompt user for date in format mm/dd/yyyy entered. */
        /* -------------------------------------------------- */

        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);

        /* Display the results. */
        /* -------------------- */

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

    } /* End loop */

    /* Output ending message */
    /* --------------------- */

    printf("Thank you for using the Date to Day-of-Week program.\n");

    getchar();

    return 0;

} /* End main */