import java.io.UnsupportedEncodingException;

import java.security.MessageDigest;

import java.security.NoSuchAlgorithmException;

import java.util.logging.Level;

import java.util.logging.Logger;

//a block stores its own hash, previous nodes hash,

//timestamp of creation, data, and a arbitary number used for cryptoraphy

public class Block

{

private static Logger logger = Logger.getLogger(Block.class.getName());

private String hash;

private String previoushash;

private String data;

private long timestamp;

private int nonce;

public Block(String data, String previoushash, long timestamp)

{

this.data = data;

this.previoushash = previoushash;

this.timestamp = timestamp;

this.hash = calculateBlockHash();

}

public String mineBlock(int prefix)

{

String prefixString = new String(new char[prefix]).replace('\0', '0');

while (!hash.substring(0, prefix)

.equals(prefixString)) {

nonce++;

hash = calculateBlockHash();

}

return hash;

}

public String calculateBlockHash()

{

String dataToHash = previoushash

+ Long.toString(timestamp)

+ Integer.toString(nonce)

+ data;

MessageDigest digest = null;

byte[] bytes = null;

try{

digest = MessageDigest.getInstance("SHA-256");

bytes = digest.digest(dataToHash.getBytes("UTF_8"));

}

catch(NoSuchAlgorithmException | UnsupportedEncodingException ex)

{

logger.log(Level.SEVERE, ex.getMessage());

}

StringBuffer buffer = new StringBuffer();

for (byte b : bytes)

{

buffer.append(String.format("%02x",b));

}

return buffer.toString();

}

}

/*

public class Main

{

public static void main(String[] args) {

System.out.println("hello world");

}

}

*/