Rust: concurrent filter example. Keeps empty elements and original order. Using thread pool.

filter.rs

// Add to your Cargo.toml:
// [dependencies]
// rayon = "1.1"

use rayon::prelude::*;
use rayon::ThreadPoolBuilder;
use std::thread;
use std::time::{Duration, Instant};

fn main() {
    let input = vec!["A", "B", "C", "D", "E", "F", "G", "H", "I", "J", "", ""];
    let expected = vec!["B", "D", "F", "H", "J", ""];

    /// Returns true if item should stay in the collection given to filter
    ///
    /// # Arguments
    ///
    /// * `item` - A tuple with current index and element of the collection
    fn keep_fn(item: &(usize, &&str)) -> bool {
        thread::sleep(Duration::from_secs(3)); // Sleep for 3 seconds to simulate heavy task.
        item.0 % 2 != 0 // Keep every even element (remove every odd).
    }

    let max_workers = 6;
    let pool = ThreadPoolBuilder::new().num_threads(max_workers).build().unwrap();

    let start = Instant::now();
    let output: Vec<&str> = pool.scope(|_| {
        input
            .par_iter()
            .enumerate()
            .filter(keep_fn)
            .map(|(_, e)| *e)
            .collect()
    });
    let duration = start.elapsed();

    println!("Filtering took {:?}", duration);
    if output != expected {
        println!("Unexpected output {:?}", output);
    }
}