struct Counter {
    count: u32,
    max: u32,
}

impl Counter {
    fn new(max: u32) -> Counter {
        Counter { count: 0, max }
    }
}

impl Iterator for Counter {
    type Item = u32;

    fn next(&mut self) -> Option<u32> {
        if self.count < self.max {
            self.count += 1;
            Some(self.count)
        } else {
            None
        }
    }
}

fn main() {
    let sum: u32 = Counter::new(5).sum();
    println!("sum: {}", sum);

    let pairs: Vec<(u32, u32)> = Counter::new(3)
        .zip(Counter::new(3).skip(1))
        .collect();
    println!("{:?}", pairs);
}