Keeping a variable alive across multiple function calls in rust

Question

I am trying to memoize a recursive collatz sequence function in rust, however I need the hashmap of memoized values to keep its contents across separate function calls. Is there an elegant way to do this in rust, or do I have to declare the hashmap in main and pass it to the function each time? I believe the hashmap is being redeclared as an empty map each time I call the function. Here is my code:

fn collatz(n: int) -> int {
    let mut map = HashMap::<int, int>::new();
    if map.contains_key(&n) {return *map.get(&n);}
    if n == 1 { return 0; }
    map.insert(n, 
        match n % 2 {
            0 => { 1 + collatz(n/2) }
            _ => { 1 + collatz(n*3+1) }
        }
    );
    return *map.get(&n);
}

On a side note, why do I need to add all of the &'s and *'s when I am inserting and pulling items out of the HashMap? I just did it because the compiler was complaining and adding them fixed it but I'm not sure why. Can't I just pass by value? Thanks.

Answer 1

You can use thread_local for thread-local statics.

thread_local! (static COLLATZ_MEM: HashMap<i32, i32> = HashMap::new());
fn collatz(n: i32) -> i32 {
    COLLATZ_MEM.with (|collatz_mem| {
        0  // Your code here.
    })
}

P.S. There's also an excellent lazy-static macro which can be used for the truly global static caches. Here's an example.

Answer 2

There are no "static" locals in Rust the way there are in C, no. Maybe make an object, put the hash in it, and make collatz a method of it.

You can't pass by value because that does either a copy (which might be expensive for complex keys) or a move (which would make you unable to use the key again). In this case your keys are just ints, but the API is meant to work for arbitrary types.