trait Stepper {
    fn inc(&mut self) -> ();
    fn get(&self) -> u32;
}

// Big Stepper
struct BigStepper {
    value: u32,
}

impl BigStepper {
    fn new() -> BigStepper { BigStepper{ value: 1u32 } }
}

impl Stepper for BigStepper {
    fn inc(&mut self) {
        self.value = self.value*3 + 1;
    }

    fn get(&self) -> u32 {
        self.value
    }
}
// END Big Stepper

// Little Stepper
struct LittleStepper {
    value: u32
}

impl LittleStepper {
    fn new() -> LittleStepper { LittleStepper{ value: 0u32 } }
}

impl Stepper for LittleStepper {
    fn inc(&mut self) {
        self.value = self.value / 2;
    }
    fn get(&self) -> u32 {
        self.value
    }
}
// END Little Stepper

fn main() {
    let mut big_stepper: Box<Stepper> = Box::new(BigStepper::new());
    let mut little_stepper: Box<Stepper> = Box::new(LittleStepper::new());
    let mut stepper: &mut Box<Stepper> = &mut big_stepper;
    while stepper.get() != 1 {
        stepper.inc();
        println!("{}", stepper.get());
        stepper = match stepper.get() % 2 {
            0 => &mut little_stepper,
            1 => &mut big_stepper,
            _ => break
        };
    }
}