factory_cache.rs

trait Resource: Read {}

trait Factory {
    type R: Resource;

    fn get(&self, index: u8) -> Result<Self::R, ()>;
}


#[derive(Clone)]
struct CacheEntry<R: Resource> {
    value: Arc<Mutex<R>>,
}

struct Cache<F: Factory> {
    factory: F,
    map: RefCell<BTreeMap<u8, CacheEntry<F::R>>>,
}

impl<R: Resource> Resource for CacheEntry<R> {}

impl<R: Resource> Read for CacheEntry<R> {
    fn read(&mut self, buf: &mut [u8]) -> std::io::Result<usize> {
        self.value.lock().unwrap().read(buf)
    }
}

impl<F: Factory> Factory for Cache<F> {
    type R = CacheEntry<F::R>;

    fn get(&self, index: u8) -> Result<Self::R, ()> {
        let mut map = self.map.borrow_mut();

        match map.entry(index) {
            Entry::Occupied(e) => Ok(CacheEntry { value: e.get().value.clone() }),
            Entry::Vacant(v) => {
                let resource = self.factory.get(index)?;
                let entry = CacheEntry { value: Arc::new(Mutex::new(resource)) };

                Ok(CacheEntry { value: v.insert(entry).value.clone() })
            }
        }
    }
}