use std::time::Instant;

fn sunday_the_first_in_year(start_day : usize, leap_year : bool) -> usize {
    let mut current_day = start_day;
    let mut sundays = 0;
    for i in 1..12 {
        if current_day == 0 {
            sundays += 1;
        }

        let modulo = match i {
            1 | 3 | 5 | 7 | 8 | 10 | 12 => {
                31 % 7
            },
            4 | 6 | 9 | 11 => {
                30 % 7
            },
            2 => {
                if leap_year {
                    29 % 7
                }else{
                    28 % 7
                }
            }
            _ => unreachable!()
        };
        current_day = (current_day + modulo) % 7;
    }

    if current_day == 0 {
        sundays += 1;
    }

    sundays
}

pub fn counting_sundays() -> usize {
    let mut days : [usize;14] = [0;14];

    for i in 0..7 {
        days[i] = sunday_the_first_in_year(i,false);
        days[i+7] = sunday_the_first_in_year(i,true);
    }

    let mut sundays = 0;
    let mut start_day = 2;
    for year in 1901..2001 {
        let is_leap_year = year % 4 == 0 && (year % 100 != 0 || year % 400 == 0);

        let index = if is_leap_year {
            start_day + 7
        }else {
            start_day
        };

        sundays += days[index];

        if is_leap_year {
            start_day = (start_day + 2) % 7;
        }else{
            start_day = (start_day + 1) % 7;
        }
    }

    sundays
}

pub fn main() {
    let now = Instant::now();
    let solution = counting_sundays();
    println!("{} in {:?}", solution, now.elapsed());
}