Calculates E using Mathematica. Use 'SHOW_PROGRESS=X' to show progress every X iteration, Set LOOPS=N to loop N times or exclude to run forever. CTRL+C will end early and report result

ecalculator.rs

use std::time::Instant;
use rand::prelude::*;
use std::f64::consts;
use console::Term;
use std::sync::atomic::{AtomicBool, Ordering};
use std::sync::Arc;

fn main() {
    //Handler to terminate on CTRL+C
    let running = Arc::new(AtomicBool::new(true));
    let r = running.clone();
    ctrlc::set_handler(move || {
        r.store(false, Ordering::SeqCst);
        println!();
    }).expect("Error setting Ctrl-C handler");

    let term = Term::stdout();
    
    //Parses env 'SHOW_PROGRESS', value must be an int and sets how frequent to log progress
    //If has a value, should log progress.
    let log_interval: Option<u64> = match std::env::var("SHOW_PROGRESS").ok() {
        Some(value) => Some(value.parse::<u64>().expect("not a valid integer")),
        None => None
    };
    
    //Gets the maximum loops to run or none for infinity
    let max_loops: Option<u64> = match std::env::var("LOOPS") {
        Ok(val) => Some(val.parse::<u64>().expect("not a valid integer")),
        _ => None
    };

    match max_loops {
        Some(max_loops) => {
            println!("Running {} iterations. Press CTRL+C to exit at any time.", max_loops);
            run_limited(term, running, log_interval, max_loops);
        },
        None => {
            println!("No maximum loops specified, running forever. Press CTRL+C to exit at any time.");
            run_unlimited(term, running, log_interval);
        }
    }
}

//Runs upto X times
fn run_limited(term: Term, running: Arc<AtomicBool>, log_interval: Option<u64>, max_loops: u64) {
    let now = Instant::now();
    
    let mut total_count: u64 = 0;
    let mut count: u64;
	let mut sum: f64;
    
    //Print header only if showing progress
    if log_interval.is_some() {
        println!("Iteration\tValue       \t% Err\tCurrent %\tTime Elapsed");
    }

    for i in 0..max_loops {
        count = 0;
        sum = 0.0;
        //Run until sum hits 1.0
        while sum <= 1.0 {
            count += 1;
            sum += rand::thread_rng().gen::<f64>();
        }

        total_count += count;

        //Prints progress:
        if log_interval.is_some() && i % log_interval.unwrap() == 0 {
            let average = total_count as f64 / i as f64;
            let pererror = (average - consts::E).abs() / consts::E * 100.0;
            let cur_percent = i as f64 / max_loops as f64 * 100.0;
            term.write_line(&format!(" {:9.10}\t{:.10}\t{:5.2}%\t{:9.2}%\t{:?}", i, average, pererror, cur_percent, now.elapsed())).ok();
            term.move_cursor_up(1).ok();
        }

        //Checks if CTRL+C was called, end loop here
        if !running.load(Ordering::SeqCst) {
            break;
        }
    }

    let average = total_count as f64 / max_loops as f64;
    let pererror = (average - consts::E).abs() / consts::E * 100.0;
    println!("---------------------------");
    println!("Iterations: {}", max_loops);
    println!("Final Average: {}", average);
    println!("Final % Error: {:.4}%", pererror);
    println!("Took: {:?}", now.elapsed());
}

fn run_unlimited(term: Term, running: Arc<AtomicBool>, log_interval: Option<u64>) {
    let now = Instant::now();
    let mut total_count: u64 = 0;
    let mut count: u64;
	let mut sum: f64;
    let mut i: u64 = 0;

    if log_interval.is_some() {
        println!("Iteration\tValue       \t% Err\tTime Elapsed");
    }

    loop {
        count = 0;
        sum = 0.0;
        //Run until sum hits 1.0
        while sum <= 1.0 {
            count += 1;
            sum += rand::thread_rng().gen::<f64>();
        }

        total_count += count;

        //Prints progress:
        if log_interval.is_some() && i % log_interval.unwrap() == 0 {
            let average = total_count as f64 / i as f64;
            let pererror = (average - consts::E).abs() / consts::E * 100.0;
            term.write_line(&format!(" {:9.10}\t{:.10}\t{:5.2}%\t{:?}", i, average, pererror, now.elapsed())).ok();
            term.move_cursor_up(1).ok();
        }

        //Checks if CTRL+C was called, end loop here
        if !running.load(Ordering::SeqCst) {
            break;
        }

        i += 1;
    }

    let average = total_count as f64 / i as f64;
    let pererror = (average - consts::E).abs() / consts::E * 100.0;
    println!("---------------------------");
    println!("Iterations: {}", i);
    println!("Final Average: {}", average);
    println!("Final % Error: {:.4}%", pererror);
    println!("Took: {:?}", now.elapsed());
}