Cacher.rs

// Solution from:
// https://users.rust-lang.org/t/30856/5
use std::collections::{HashMap, hash_map::Entry};
use std::cmp::Eq;
use std::hash::Hash;
use std::thread::sleep;
use std::time::Duration;

struct Cacher<T, U, V>
where T: Fn(&U) -> V, U: Eq + Hash {
    func: T,
    cache: HashMap<U, V>,
}

impl<T, U, V> Cacher<T, U, V>
where T: Fn(&U) -> V, U: Eq + Hash {
    fn new(func: T) -> Cacher<T, U, V> {
        Cacher {
            func,
            cache: HashMap::new()
        }
    }

    fn get(&mut self, value: U) -> &V {
        match self.cache.entry(value) {
            Entry::Occupied(entry) => entry.into_mut(),
            Entry::Vacant(entry) => {
                let v = (self.func)(entry.key());
                entry.insert(v)
            }
        }
    }
}

fn main() {
    let mut cache = Cacher::new(|x| {
        sleep(Duration::from_secs(2));
        x + 1
    });

    println!("Start!");
    println!("First: {}", cache.get(1));
    println!("Second: {}", cache.get(1));
}