memo function

memo_func.rs

use std::hash::Hash;
use std::{collections::HashMap, thread, time::Duration};

fn factorial(n: i32) -> i32 {
    if n <= 1 {
        return 1;
    }
    thread::sleep(Duration::from_millis(100));
    factorial(n - 1).wrapping_mul(n)
}

fn memo<F, X, R>(f: F) -> impl FnMut(X) -> R
where
    F: Fn(X) -> R,
    X: Hash + Eq + Copy,
    R: Copy,
{
    let mut cache: HashMap<X, R> = HashMap::new();
    move |x| {
        if let Some(&ans) = cache.get(&x) {
            return ans;
        }
        let ans = f(x);
        cache.insert(x, ans);
        ans
    }
}

fn main() {
    println!("{}", factorial(5));
    println!("{}", factorial(5));

    let mut mf = memo(factorial);
    println!("{}", mf(5));
    println!("{}", mf(5));
    println!("{}", mf(5));
    println!("{}", mf(5));

    let double = |x| {
        thread::sleep(Duration::from_millis(1_000));
        x*2
    };
    let mut memoized_double = memo(double);
    println!("{}", memoized_double(22));
    println!("{}", memoized_double(22));
    println!("{}", memoized_double(22));
    println!("{}", memoized_double(22));
}