fn main() {
    let fib_instance = FibItr::default();
    println!("{}",
        fib_instance
            .filter(is_even)
            .take_while(|x| *x < 4000000)
            .sum::<usize>()
        );
}

fn is_even(x: &usize) -> bool {
    x % 2 == 0
}

struct FibItr {
    index: usize,
    prev: usize
}

impl Iterator for FibItr {
    type Item = usize;
    fn next ( &mut self ) -> Option<usize> {
        let result = self.index + self.prev;
        self.prev = self.index;
        self.index = result;
        return Some(result);
    }
}

impl Default for FibItr {
    fn default() -> Self {
        FibItr {
            index: 1,
            prev: 1
        }
    }
}