fn entropy(s: &[u8]) -> f32 {
    let mut entropy: f32 = 0.0;
    let mut histogram = [0; 256];

    for i in 0..s.len() {
        histogram.get_mut(s[i] as usize).map(|v| *v += 1);
    }
    for i in 0..256 {
        if histogram[i] > 0 {
            let ratio = (histogram[i] as f32 / s.len() as f32) as f32;
            entropy -= (ratio * ratio.log2()) as f32;
        }
    }
    entropy
}

fn main() {
    let arg = std::env::args().nth(1).expect("Need a string.");
    println!("Entropy of {} is {}.", arg, entropy(&arg.bytes().collect::<Vec<_>>()));
}