def get_target(self, index):
        # compute target from chunk x, used in chunk x+1
        if constants.net.TESTNET:
            return 0
        if index == -1:
            return MAX_TARGET
        if index < len(self.checkpoints):
            h, t = self.checkpoints[index]
            return t
        # new target
        first = self.read_header(index * 2016)
        last = self.read_header(index * 2016 + 2015)
        bits = last.get('bits')
        target = self.bits_to_target(bits)
        nActualTimespan = last.get('timestamp') - first.get('timestamp')
        nTargetTimespan = 14 * 24 * 60 * 60
        nActualTimespan = max(nActualTimespan, nTargetTimespan // 4)
        nActualTimespan = min(nActualTimespan, nTargetTimespan * 4)
        new_target = min(MAX_TARGET, (target * nActualTimespan) // nTargetTimespan)
        return new_target

def verify_header(self, header, prev_hash, target):
        _hash = hash_header(header)
        if prev_hash != header.get('prev_block_hash'):
            raise Exception("prev hash mismatch: %s vs %s" % (prev_hash, header.get('prev_block_hash')))
        if constants.net.TESTNET:
            return
        bits = self.target_to_bits(target)
        if bits != header.get('bits'):
            raise Exception("bits mismatch: %s vs %s" % (bits, header.get('bits')))
        if int('0x' + _hash, 16) > target:
            raise Exception("insufficient proof of work: %s vs target %s" % (int('0x' + _hash, 16), target))