problem_02.rs

struct Fibonacci {
    current: u64,
    next: u64,
}

impl Iterator for Fibonacci {
    type Item = u64;

    fn next(&mut self) -> Option<u64> {
        let t = self.current + self.next;
        self.current = self.next;
        self.next = t;
        Some(self.current)
    }
}

fn fibonacci() -> Fibonacci {
    Fibonacci {
        current: 1,
        next: 1,
    }
}

fn main() {
    let mut sum: u64 = 0;
    for i in fibonacci().take_while(|&x| x < 4000000).filter(
        |&x| x % 2 == 0,
    )
    {
        sum += i;
    }
    println!("sum: {}", sum);
}