Java: Generate Bitcoin & Ethereum Target from Difficulty

Java: Generate Bitcoin & Ethereum Target from Difficulty

  October 19, 2016    Nikos Siatras

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The other day I was making a small CPU miner for Ethereum using Java. I decided to implement the stratum protocol and I came to the following... difficulty.

Stratum does not send the target directly to the miner as the Getwork protocol does. Stratum sends a double data type that holds the difficulty and the miner have to generate the target from that.

Stratum example message of mining difficulty:

{ 
  "id": null, 
  "method": "mining.set_difficulty", 
  "params": [
    0.5
  ]
}\n

The Java Solution!

As I read online the conversion between difficulty and target is done the same way as with Bitcoin but it is not very clear how can someone do that so... I decided to post my code and maybe make the life of others a bit easier.

The code is written in simple Java and does not need any external libraries.

    public static void main(String[] args)
    {
        String target = GetTargetFromDifficulty(1);
        System.out.println("Target for difficulty 1 is " + target);
    }

    private static String GetTargetFromDifficulty(double difficulty)
    {
        // Note: difficulty of 1 is transformed to target being in HEX:
        // 00000000ffff0000000000000000000000000000000000000000000000000000

        long m;
        int k;
        byte[] target = new byte[8 * 4];
        for (k = 6; k > 0 && difficulty > 1.0; k--)
        {
            difficulty /= 4294967296.0;
        }
        m = (long) (4294901760.0 / difficulty);
        if (m == 0 && k == 6)
        {
            Arrays.fill(target, (byte) 0xff);
        }
        else
        {
            Arrays.fill(target, (byte) 0);
            for (int i = 0; i < 8; i++)
            {
                target[k * 4 + i] = (byte) ((m >> (i * 8)) & 0xff);
            }
        }

        String tmp = toHexString(target);
        String reverse = new StringBuffer(tmp).reverse().toString();
        return reverse;
    }

    private static String toHexString(byte[] b)
    {
        StringBuilder sb = new StringBuilder(80);
        for (int i = 0; i < b.length; i++)
        {
            sb.append(Integer.toString((b[i] & 0xff) + 0x100, 16).substring(1));
        }
        return sb.toString();
    }