rustlings/exercises/24_async/async1.rs

// Alice is an elementary school teacher who needs to calculate the mean test
// score for three classes she teaches. Instead of calculating them one after
// the other, she decides to ask her friends Bob and Catherine for help. Working
// together, they can finish the job much faster.
//
// Let's simulate this using asynchronous programming. Each person is
// represented as an asynchronous task, which can be executed concurrently.

const SCORES_CLASS_A: &str = "exercises/24_async/scores_class_a.txt";
const SCORES_CLASS_B: &str = "exercises/24_async/scores_class_b.txt";
const SCORES_CLASS_C: &str = "exercises/24_async/scores_class_c.txt";

// Async tasks need to be executed by a "runtime", which is not provided by
// Rust's standard library. Here, we use the mainstream runtime `tokio`.
// The macro `tokio::main` wraps the entire main function in a runtime.
#[tokio::main]
async fn main() {
    let mean_score_a = tokio::spawn(calculate_mean_score(SCORES_CLASS_A));
    let mean_score_b = tokio::spawn(calculate_mean_score(SCORES_CLASS_B));
    let mean_score_c = tokio::spawn(calculate_mean_score(SCORES_CLASS_C));

    // TODO: Await the spawned tasks to check their results.
    assert_eq!(mean_score_a, 84); // alice
    assert_eq!(mean_score_b, 89); // bob
    assert_eq!(mean_score_c, 76); // catherine
}

// TODO: Fix the compiler errors by making the spawned function async.
fn calculate_mean_score(scores_file: &str) -> usize {
    // Read the file asynchronously
    let file = tokio::fs::read_to_string(scores_file).await.unwrap();

    // Initialize the sum and the number of scores
    let mut sum = 0;
    let mut n = 0;
    for line in file.lines() {
        // Parse every line as a score
        let score = line.parse::<usize>().unwrap();
        sum += score;
        n += 1;
    }

    sum / n
}