fn do_partial_sum(start: i64, to_do: i64) -> i64 {
    let mut result = 0;
    for i in start..(start + to_do) {
        if i % 2 == 0 {
            result += i;
        }
    }
    result
}

const MAX: i64 = 2000000000;

fn main() {
    let thread_count = 4;
    let mut result;
    if thread_count == 0 {
        result = do_partial_sum(0, MAX);
    } else {
        let each = {
            let c = thread_count as i64;
            (MAX + c - 1) / c
        };
        let mut threads = Vec::new();
        result = 0;
        let mut start = 0;
        while start < MAX {
            let result_ptr = &mut result;
            threads.push(std::thread::spawn(move || {
                let incr = if each + start > MAX { MAX - start } else { each };
                *result_ptr += do_partial_sum(start, incr);
            }));
            start += each;
        }
        for t in threads {
            t.join().unwrap();
        }
    }

    println!("result: {}", result);
}