Rust Closures Cache

main-with-naive-impl.rs

struct Cacher<T>
where
    T: Fn(u32) -> u32,
{
    calculation: T,
    value: HashMap<u32, u32>,
}

impl<T> Cacher<T>
where
    T: Fn(u32) -> u32,
{
    fn new(calculation: T) -> Cacher<T> {
        Cacher {
            calculation,
            value: HashMap::new(),
        }
    }

    fn value(&mut self, arg: u32) -> u32 {
        // FIXME: Getting a clippy warning here, recommending
        // `self.value.entry().or_insert` instead.
        if self.value.contains_key(&arg) {
            *self.value.get(&arg).unwrap()
        } else {
            self.value.insert(arg, (self.calculation)(arg));
            *self.value.get(&arg).unwrap()
        }
    }
}

fn generate_workout(intensity: u32, random_number: u32) {
    let mut expensive_resultexpensive_result = Cacher::new(|num| {
        println!("calculating ...");
        thread::sleep(Duration::from_secs(2));
        num
    });

    if intensity < 25 {
        println!(
            "Today, do {} pushups!",
            expensive_resultexpensive_result.value(intensity)
        );
        println!(
            "Next, do {} situps!",
            expensive_resultexpensive_result.value(intensity)
        );
    } else if random_number == 3 {
        println!("Take a break today! Remember to stay hydrated!");
    } else {
        println!(
            "Today, run for {} minutes!",
            expensive_resultexpensive_result.value(intensity)
        );
    }
}

fn main() {
    let simulated_user_specified_value = 10;
    let simulated_random_number = 7;

    generate_workout(simulated_user_specified_value, simulated_random_number);
}

main.rs

struct Cacher<T>
where
    T: Fn(u32) -> u32,
{
    calculation: T,
    value: HashMap<u32, u32>,
}

impl<T> Cacher<T>
where
    T: Fn(u32) -> u32,
{
    fn new(calculation: T) -> Cacher<T> {
        Cacher {
            calculation,
            value: HashMap::new(),
        }
    }

    fn value(&mut self, arg: u32) -> u32 {
        let f = || (self.calculation)(arg);
        // FIXME
        // [rustc E0502] [E] cannot borrow `self.value` as mutable because it is also borrowed as
        // immutable
        //
        // mutable borrow occurs here
        *self.value.entry(arg).or_insert_with(f)
    }
}

fn generate_workout(intensity: u32, random_number: u32) {
    let mut expensive_resultexpensive_result = Cacher::new(|num| {
        println!("calculating ...");
        thread::sleep(Duration::from_secs(2));
        num
    });

    if intensity < 25 {
        println!(
            "Today, do {} pushups!",
            expensive_resultexpensive_result.value(intensity)
        );
        println!(
            "Next, do {} situps!",
            expensive_resultexpensive_result.value(intensity)
        );
    } else if random_number == 3 {
        println!("Take a break today! Remember to stay hydrated!");
    } else {
        println!(
            "Today, run for {} minutes!",
            expensive_resultexpensive_result.value(intensity)
        );
    }
}

fn main() {
    let simulated_user_specified_value = 10;
    let simulated_random_number = 7;

    generate_workout(simulated_user_specified_value, simulated_random_number);
}