In the past, I’ve implemented Euler’s totient function for shits and giggles. It’s not a hard exercise by any means, but it can be if you haven’t had education in math at a level to understand the formula.

$$\varphi(n) =n \prod_{p\mid n} \left(1-\frac{1}{p}\right)$$

Alright, I’ve revised my rust totient calculator to be easier to use, and more flexible. It’ll accept a “start” and “end” calculation value. That way you can start right away crunching some huge primes. I started with checking command line arguments, I also convert them safely to u128, and then off she goes. It’s really fast.
  1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51  use std::env; fn gcd(mut m: u128, mut n: u128) -> u128 { while m != 0 { let old_m = m; m = n % m; n = old_m; } n } fn totient(mut a: u128) -> u128 { let mut y = 0; let o = a; while a != 0 { if gcd(o, a) == 1 { y += 1; } a -= 1; } y } fn main() { let args: Vec = env::args().collect(); let x: Vec = <[String]>::iter(&args).filter_map(|c| { c.parse::().ok() } ).collect(); println!("Arguments: {}", &x.len()); match &x.len() { 1 => { println!("Not enough arguments. Expecting two.") }, 2 => { for i in x[0]..x[1] { let o = totient(i); println!("{:?}, {:?}", i, o); } }, 3 => { println!("Too many. Only need two.") }, _ => { println!("Only two arguments. Please.") } } } 
Alright, a few things to note, since the code is completely uncommented. First, you’ll notice I’m using .filter_map. That’s because Rust is fucking wild. It’s so “safe” that it’ll take a shit converting from “string” to u128 if the string has no numbers. That means pulling command line arguments is going to be more difficult with .map. By using .filter_map for an iterator we’re telling it “We don’t give a shit about what has no numbers in it”. God, I wish this stuff were better documented or more obvious.
 25 26 27 28 29 30   let args: Vec = env::args().collect(); let x: Vec = <[String]>::iter(&args).filter_map(|c| { c.parse::().ok() } ).collect();