el_diablo/src/level_ladder.rs

pub struct LevelLadder {
    curr: usize,
    next: usize,
}

impl Iterator for LevelLadder {
    type Item = usize;

    fn next(&mut self) -> Option<Self::Item> {
        let current = self.curr;

        self.curr = self.next;
        self.next += current;

        // Since there's no endpoint to a Fibonacci sequence, the `Iterator`
        // will never return `None`, and `Some` is always returned.
        Some(current * 30)
    }
}

// Returns a Fibonacci sequence generator
pub fn get_level_ladder() -> LevelLadder {
    LevelLadder { curr: 1, next: 2 }
}

#[test]
fn test_level_ladder() {
    let mut iter = get_level_ladder();
    assert_eq!(iter.next(), Some(30));
}