#include <stdio.h>

/**
 * returns number of days in a month. 
 * Month is identified by an integer -
 * 0 = Jan and 11 = December 
 * For February, adjust for leap year outside the function.
 **/
char get_month_days(short month)
{
    if (month == 1) /* February has 28 days. Adjust leap year in the loop */
        return 28;
    else if (month <= 6) /* odd months till July have 30 days - Jan = 0 (even)*/
    {
        if (month & 0x01)
            return 30;
        else
            return 31;
    } else if (month >= 7) /* odd months after July have 31 days*/
    {
        if (month & 0x01)
            return 31;
        else
            return 30;
    }
    /* should never reach here! */
    perror("Should never have reached this point!\n");
    return -1;
}

/**
 * return 1 if input year is a leap year
 * otherwise, return 0
 **/
char is_leap_year(short year)
{
    if ((year % 400 == 0) || ((year % 4 == 0) && (year % 100 != 0)))
        return 1;
    
    return 0;
}

#ifdef DEBUG
const char* day_string(int day)
{
    switch(day)
    {
        case 0:
            return "Sunday";
        case 1:
            return "Monday";
        case 2:
            return "Tuesday";
        case 3:
            return "Wednesday";
        case 4:
            return "Thursday";
        case 5:
            return "Friday";
        case 6:
            return "Saturday";
        default:
            return "Shouldnt see this!";
    }
}
#endif


int main(int argc, char **argv)
{
    int count_sundays = 0;
    const short start_year = 1901;
    const short end_year = 2000;

    /**
     * Let us identify days i.e., Sunday thru Saturday with integers - 0 thru 6 respectively
     *  Jan 1 1901 was a Tuesday
     **/
    char start_day = 2;

    for (int year = start_year; year <= end_year; year++)
    {
        char is_leap = is_leap_year(year);
        for (char month = 0; month < 12; month ++)
        {
            /**
             * These two for-loops count the start of day for the next month. Hence, 
             * we have to skip the last December count */
            if (year == end_year && month == 11)
                continue;

            int days = get_month_days(month);
    
            if (is_leap && month == 1)  /* for a leap year february, add a day */
                days ++;
            
            #ifdef DEBUG
            if (year == end_year)
            {
                printf("Year: %d\t Month: %d\t Days: %d\t First of day: %s\n", year, month, days, day_string(start_day));
            }
            #endif

            /** Main Algorithm:
             * every week has 7 days hence, the start of next day would be modulo 7
             * add to this, the current start date and ensure the result is still
             * modulo 7!
             **/
            start_day = ((days % 7) + start_day) % 7;

            /* If start-day is a Sunday, increment counter */
            if (start_day == 0)
                count_sundays ++;
        }
    }

    printf("Total number of Sundays that happened on the 1st of a month in the last century: %d\n", count_sundays);

    return 0;
}