Calculate the target difficulty at a particular difficulty interval (index).
        Return (bits, target) on success

def get_target(cls, path, index, chain=None):
        """
        Calculate the target difficulty at a particular difficulty interval (index).
        Return (bits, target) on success
        """
        if chain is None:
            chain = []  # Do not use mutables as default values!

        max_target = 0x00000000FFFF0000000000000000000000000000000000000000000000000000
        if index == 0:
            return 0x1d00ffff, max_target

        first = SPVClient.read_header( path, (index-1)*BLOCK_DIFFICULTY_CHUNK_SIZE)
        last = SPVClient.read_header( path, index*BLOCK_DIFFICULTY_CHUNK_SIZE - 1, allow_none=True)
        if last is None:
            for h in chain:
                if h.get('block_height') == index*BLOCK_DIFFICULTY_CHUNK_SIZE - 1:
                    last = h

        nActualTimespan = last.get('timestamp') - first.get('timestamp')
        nTargetTimespan = BLOCK_DIFFICULTY_INTERVAL
        nActualTimespan = max(nActualTimespan, nTargetTimespan/4)
        nActualTimespan = min(nActualTimespan, nTargetTimespan*4)

        bits = last.get('bits')
        # convert to bignum
        MM = 256*256*256
        a = bits%MM
        if a < 0x8000:
            a *= 256
        target = (a) * pow(2, 8 * (bits/MM - 3))

        # new target
        new_target = min( max_target, (target * nActualTimespan)/nTargetTimespan )

        # convert it to bits
        c = ("%064X"%new_target)[2:]
        i = 31
        while c[0:2]=="00":
            c = c[2:]
            i -= 1

        c = int('0x'+c[0:6],16)
        if c >= 0x800000:
            c /= 256
            i += 1

        new_bits = c + MM * i
        return new_bits, new_target