use std::collections::HashMap;

struct Cacher<T, U, F> {
    values: HashMap<T, U>,
    calculation: F,
}

impl<T: std::cmp::Eq + std::hash::Hash, U, F> Cacher<T, U, F> {
    fn new(calculation: impl Fn(T) -> U) -> Cacher<T, U, F> {
        Cacher { values: HashMap::new(), calculation: calculation }
    }

    fn value(&mut self, arg: T) -> U {
        match self.values.get(&arg) {
            Some(&cached) => cached,
            None => {
                let value = (self.calculation)(arg);
                self.values.insert(arg, value);
                value
            }
        }
    }
}

fn main() {
    let mut calculator = Cacher::new(|n| n + 1);
    println!("value: {}", calculator.value(5));
    println!("value: {}", calculator.value(200));
}