gistfile1.txt

extern crate rayon;
use rayon::prelude::*;

use std::time::Instant;

fn collatz(mut x: u64) -> usize {
    let mut steps = 0;
    while x != 1 {
        if x % 2 == 0 {
            x /= 2;
        } else {
            x = 3*x + 1;
        }
        steps += 1;
    }
    return steps;
}

fn imperative(upper_limit: u64) -> (u64, usize) {
    let mut champion: u64 = 0;
    let mut max_steps: usize = 0;

    for i in 1 .. upper_limit {
        let steps = collatz(i);
        if steps > max_steps {
            champion = i;
            max_steps = steps;
        }
    }
    return (champion, max_steps);
}

fn serial(upper_limit: u64) -> (u64, usize) {
    (1 .. upper_limit)
        .into_iter()
        .map(|i| (i, collatz(i)))
        .max_by(|a, b| (a.1).cmp(&b.1))
        .unwrap()
}

fn parallel(upper_limit: u64) -> (u64, usize) {
    (1 .. upper_limit)
        .into_par_iter()
        .map(|i| (i, collatz(i)))
        .max_by(|a, b| (a.1).cmp(&b.1))
        .unwrap()
}

fn main() {
    let top = Instant::now();
    let result = imperative(1_000_000);
    let elapsed = top.elapsed();
    println!("Imperative: {:?}: {}.{:09}s", result, elapsed.as_secs(), elapsed.subsec_nanos());

    let top = Instant::now();
    let result = serial(1_000_000);
    let elapsed = top.elapsed();
    println!("Serial    : {:?}: {}.{:09}s", result, elapsed.as_secs(), elapsed.subsec_nanos());

    let top = Instant::now();
    let result = parallel(1_000_000);
    let elapsed = top.elapsed();
    println!("Parallel  : {:?}: {}.{:09}s", result, elapsed.as_secs(), elapsed.subsec_nanos());
}