fn proof_of_work(data: String, difficulty: i16) {
    let mut nonce: i64 = 0;
    let mut proof: bool = false;
    let mut hash: String = String::from("");
    let mut difficulty_str: String = String::from("");

    for _ in 0..difficulty {
        difficulty_str = String::from(difficulty_str + "0");
    }

    while proof == false {
        hash = hash_block(&data, nonce);
        nonce = nonce + 1;
        if hash.find(&difficulty_str) != None {
            proof = true;
        } else {
            println!("Nonce increased to {}", nonce);
        }
    }

    println!("found proof of work at nonce {} with hash {}", nonce, hash);
}