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

#define MAX_STRING_LENGTH 50
#define MAX_WEEKDAYS 7

char week_days[MAX_WEEKDAYS][MAX_STRING_LENGTH] = {"Sunday", "Monday", "Tuesday", "Wednesday", "Thursday", "Friday", "Saturday"};

int main(void){
    int year;
    printf("Enter year between 0 and 2019:\n> ");
    do{
        scanf("%d", &year);
        if(year < 0 || year > 2019){
            printf("Wrong input! Try again.\n> ");
        }
    } while(year < 0 || year > 2019);

    /* number of days in given year */
    bool leap_year;
    if (year % 400 == 0){
        leap_year = true;
    } else if (year % 100 == 0){
        leap_year = false;
    } else if (year % 4 == 0){
        leap_year = true;
    } else{
        leap_year = false;
    }

    /* 1.1.#### */
    int d = 1;
    int m = 1;
    int y = year;

    /* 0 -> Sunday, 1 -> Monday, 2 -> Tuesday, 3 -> Wednesday, 4 -> Thursday, 5 -> Friday, 6 -> Sunday */
    int first_of_january = (d += m < 3 ? y-- : y - 2, 23 * m / 9 + d + 4 + y / 4 - y / 100 + y / 400) % 7;

    printf("\nMost days in year %d: ", year);
    /* non-leap year = 365 days -> most days: day on the 1st of January */
    if(!leap_year){
        printf("%s\n", week_days[first_of_january]);
    /* leap year = 366 days -> most days: day on the 1st of January and the day after */
    } else{
        printf("%s and %s\n", week_days[first_of_january], week_days[(first_of_january + 1) % 7]);
    }

    return 0;
}