'''

# tag::example1[]

>>> from io import BytesIO

>>> from network import SimpleNode, GetHeadersMessage, HeadersMessage

>>> from block import Block, GENESIS_BLOCK, LOWEST_BITS

>>> from helper import calculate_new_bits

>>> previous = Block.parse(BytesIO(GENESIS_BLOCK))

>>> first_epoch_timestamp = previous.timestamp

>>> expected_bits = LOWEST_BITS

>>> count = 1

>>> node = SimpleNode('mainnet.programmingbitcoin.com', testnet=False)

>>> node.handshake()

>>> for _ in range(19):

... getheaders = GetHeadersMessage(start_block=previous.hash())

... node.send(getheaders)

... headers = node.wait_for(HeadersMessage)

... for header in headers.blocks:

... if not header.check_pow(): # <1>

... raise RuntimeError('bad PoW at block {}'.format(count))

... if header.prev_block != previous.hash(): # <2>

... raise RuntimeError('discontinuous block at {}'.format(count))

... if count % 2016 == 0:

... time_diff = previous.timestamp - first_epoch_timestamp

... expected_bits = calculate_new_bits(previous.bits, time_diff)

... print(expected_bits.hex())

... first_epoch_timestamp = header.timestamp

... if header.bits != expected_bits: # <3>

... raise RuntimeError('bad bits at block {}'.format(count))

... previous = header

... count += 1

ffff001d

ffff001d

ffff001d

ffff001d

ffff001d

ffff001d

ffff001d

ffff001d

ffff001d

ffff001d

ffff001d

ffff001d

ffff001d

ffff001d

ffff001d

6ad8001d

28c4001d

71be001d

# end::example1[]

'''