use std::collections::HashMap;

struct Cacher<T, I, O> where I: std::hash::Hash, I: std::cmp::Eq, T: Fn(&I) -> O {
    func: T,
    map: HashMap<I, O>,
}

impl<T, I, O> Cacher<T, I, O> where I: std::hash::Hash, I: std::cmp::Eq, T: Fn(&I) -> O {
    fn new(calc: T) -> Cacher<T, I, O> {
        Cacher{
            func: calc,
            map: HashMap::new()}
    }

    fn value(&mut self, v: I) -> O {
        let c = self.map.get(&v);
        match c {
            Some(a) => *a,
            None => {
//                println!("Calculating for {}", v);
                let r = (self.func)(&v);
                self.map.insert(v, r);
                r
            }
        }
    }
}

fn main() {

    let mut v = Cacher::new(|x| x + 1);

    println!("{}", &mut v.value(1));
    println!("{}", &mut v.value(1));
    println!("{}", &mut v.value(2));
}