type CZS = Vec<Option<u32>>;
struct Collatz { czs: CZS }

impl Collatz {
    pub fn new() -> Collatz {
        let cl0 = Some(0); // for indexing
        let cl1 = Some(1);
        Collatz { czs: vec![cl0, cl1] }
    }
    pub fn compute(&mut self, n: usize) -> u32 {
        match self.ensure_get(n) {
            Some(cl) => cl,
            None => {
                //println!("compute {}", n);
                let cl_prev = if n % 2 == 0 { 
                    self.compute(n / 2)
                } else {
                    self.compute(3 * n + 1)
                };
                self.czs[n] = Some(cl_prev + 1);
                cl_prev + 1
            }
        }
    }
    pub fn ensure_get(&mut self, i: usize) -> Option<u32> {
        while self.czs.len() <= i { self.czs.push(None); }
        self.czs[i]
    }
}

fn main() {
    let mut collatz = Collatz::new();

    for k in 0..100000usize {
        let clk = collatz.compute(k);
        println!("{}: {}", k, clk);
    }
}