use std::collections::HashMap;
use std::hash::Hash;

pub struct Cacher<T, VIn, VOut>
    where T: Fn(&VIn) -> VOut,
          VIn: Eq + Hash
{
    calculation: T,
    values: HashMap<VIn, VOut>
}

impl<T, VIn, VOut> Cacher<T, VIn, VOut>
    where T: Fn(&VIn) -> VOut,
          VIn: Eq + Hash
{
    pub fn new(calculation: T) -> Cacher<T, VIn, VOut> {
        Cacher {
            calculation,
            values: HashMap::new(),
        }
    }

    pub fn value(&mut self, arg: VIn) -> &VOut {
        if self.values.contains_key(&arg) {
            self.values.get(&arg).unwrap()
        } else {
            let result = (self.calculation)(&arg);
            let entry = self.values.entry(arg).or_insert_with(|| result);
            &*entry
        }
    }
}

fn main() {
    let mut cacher = Cacher::new(|a| *a + 1);
    println!("{}", cacher.value(3));
    println!("{}", cacher.value(4));
    println!("{}", cacher.value(3));
}