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use std::collections::HashSet;

#[derive(PartialEq, Clone, Debug)]
enum E {
    L(i64, Box<E>),
    A(Box<E>, Box<E>),
    I(i64),
}

fn sub(term: E, var: i64, rep: &E, offset: i64, bound: &mut HashSet<i64>) -> E {
    match term {
        E::L(var, body) => {
            bound.insert(var);
            let x = E::L(var + offset, Box::new(sub(*body, var, rep, offset, bound)));
            bound.remove(&var);
            x
        }
        E::A(func, arg) => E::A(
            Box::new(sub(*func, var, rep, offset, bound)),
            Box::new(sub(*arg, var, rep, offset, bound)),
        ),
        x => {
            if x == E::I(var) {
                rep.clone()
            } else {
                x
            }
        }
    }
}

fn red(term: E) -> E {
    match term {
        E::L(var, body) => E::L(var, Box::new(red(*body))),
        E::A(func, arg) => {
            let rfunc = red(*func);
            match rfunc {
                E::L(var, body) => red(sub(
                    *body,
                    var,
                    &arg,
                    rand::random_range(0..1 << 32),
                    &mut HashSet::<i64>::new(),
                )),
                x => x,
            }
        }
        x => x,
    }
}

fn main() {
    // Prints nothing?
    println!(
        "{:?}",
        red(E::A(
            Box::new(E::L(1, Box::new(E::L(2, Box::new(E::I(1)))))),
            Box::new(E::I(2))
        ))
    );
}