How to store state within a module between function calls?

user_10232 0
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I have the following functions in a module:

pub fn square(s: u32) -> u64 {
    if s < 1 || s > 64 {
        panic!("Square must be between 1 and 64")
    }

    total_for_square(s) - total_for_square(s - 1)
}

fn total_for_square(s: u32) -> u64 {
    if s == 64 {
        return u64::max_value();
    }

    2u64.pow(s) - 1
}

pub fn total() -> u64 {
    u64::max_value()
}

This works fine when calling individual functions directly. However, I want to optimize it and cache values to total_for_square to speed up future look ups (storing in a HashMap). How should I approach where to store the HashMap so it's available between calls? I know I could refactor to put all of this in a struct, but in this case, I cannot change the API.

In other, higher level languages I have used, I would just have a variable in the same scope as the functions. However, it's not clear if that is possible in Rust on the module level.

rust
2 Answers

In other, higher level languages I have used, I would just have a variable in the same scope as the functions.

You can use something similar in Rust but it's syntactically more complicated: you need to create a global for your cache using lazy_static or once_cell for instance.

The cache will need to be thread-safe though, so either a regular map sitting behind a Mutex or RwLock, or some sort of concurrent map.

Although given you only have 64 inputs, you could just precompute the entire thing and return precomputed values directly.

user_81821180

The cached crate comes in handy:

use cached::proc_macro::cached;

#[cached]
fn total_for_square(s: u32) -> u64 {
    if s == 64 {
        return u64::MAX;
    }

    2u64.pow(s) - 1
}

Indeed, you only need to write two lines, and the crate will take care of everything. Internally, the cached values are stored in a hash map.

(Note that u64::max_value() has been superseded by u64::MAX)

Side note: in this specific case, the simplest solution is probably to modify square so that it returns s * s.

user_97165970
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