Simple Proof-of-Work

simple-pow.java

import org.apache.commons.codec.binary.Hex;
import org.apache.commons.codec.digest.DigestUtils;
import org.apache.commons.lang3.StringUtils;

import java.nio.charset.StandardCharsets;
import java.security.NoSuchAlgorithmException;
import java.time.Instant;

class PoW {
    private int bitsNeeded = 0;
    private int nonce = 0;
    private long timestamp;
    private String hash;

    public PoW(byte[] msg, int numZeroes) throws NoSuchAlgorithmException {
        bitsNeeded = numZeroes;
        timestamp = Instant.now().toEpochMilli();

        work(msg);

        System.out.println("Work factor " + nonce);
    }

    private void work(byte[] msg) throws NoSuchAlgorithmException {
        String prefix = StringUtils.repeat("0", bitsNeeded);

        String hexMsg = Hex.encodeHexString(msg);
        hash = DigestUtils.sha256Hex(reMessage(hexMsg));

        while(!hash.startsWith(prefix)) {
            timestamp = Instant.now().toEpochMilli();
            nonce += 1;

            hash = DigestUtils.sha256Hex(reMessage(hexMsg));
        }
    }

    private String reMessage(String msg) {
        StringBuilder sb = new StringBuilder(String.valueOf(timestamp));
        sb.append(msg);
        sb.append(nonce);

        return sb.toString();
    }

    public static void main(String[] args) throws NoSuchAlgorithmException {
        PoW pow1 = new PoW("0000000Hello worlds this is kitty mew mew!".getBytes(StandardCharsets.UTF_8), 4);
        PoW pow2 = new PoW("RAWR me hangry value is soul".getBytes(StandardCharsets.UTF_8), 3);
    }
}