print 'Decode aivdm encoded ais files.'

print 'Usage: %s [options] <filename.aivdm> ...'

print

print 'Options:'

def __init__(self):

spec_dir = os.path.join(os.path.split(__file__)[0], 'spec')

self._fake_6_bit_text_field = Field('6 bit ascii', length=0,

format=Field.TEXT)

try:

self._aivdm = load_specs([os.path.join(spec_dir, 'aivdm.xml')])

self._ais = load_specs(glob(os.path.join(spec_dir, 'ais*.xml')))

except LoadError, ex:

sys.exit(str(ex))

def _filter_6_bit_ascii(self, items):

"""Convert 6-bit ascii decode items into a single text field.

This doesn't change the decoding, but hacks the decoded output

so it displays the data in a nicer fashion.

items -- An iterable list of decoded entries.

return -- A iterable list of decoded entries, 6-bit ascii items

collapsed into a single field."""

iterable = iter(items)

for is_starting, name, entry, data, value in iterable:

if is_starting and entry.name == '6 bit ascii':

# We found a 6-bit ascii string. Get the data and values for

# the child entries, and collapse them into a single result.

yield True, name, self._fake_6_bit_text_field, None, None

text = []

d = Data()

for is_starting, name, entry, data, value in iterable:

if entry.name == '6 bit ascii':

break

elif value is not None and name=='character':

d += data

text += chr(value)

yield False, name, self._fake_6_bit_text_field, d, ''.join(text)

else:

yield is_starting, name, entry, data, value

def _get_crc(self, data):

result = 0

data = data.copy()

# Skip the leading '!'

data.pop(8)

while True:

char = data.pop(8)

if int(char) == ord('*'):

return result

result ^= int(char)

def decode(self, input_file):

# Encode aivdm data back into binary

fragments = []

payload = self._aivdm[1][0]

data = Data(input_file)

while data:

try:

calculated_crc = self._get_crc(data)

packet = decode(self._aivdm[0], data)

if packet.checksum != calculated_crc:

sys.stderr.write('Crc mismatch; expected %02X, but '

'calculated %02X. Skipping packet.\n' %

(packet.checksum, calculated_crc))

continue

bits = encode(payload, list(ord(c.character) for c in packet.payload))

bits = bits.pop(len(bits) - packet.num_fill_bits)

if len(fragments) == packet.fragment_number - 1:

fragments.append(bits)

if len(fragments) == packet.fragment_count:

self._decode_ais(reduce(operator.add, fragments))

fragments = []

else:

sys.stderr.write('Expected fragment number %i, got %i (of %i)\n' % (

len(fragments) + 1, packet.fragment_number, packet.fragment_count))

fragments = []

except DecodeError, ex:

filename, line_number, column_number = self._aivdm[2][ex.entry]

sys.stderr.write('%s[%i] - %s\n' % (filename, line_number, ex))

fragments = []

# Read to the next newline

while data and data.pop(8).text('ascii') != '\n':

pass

def _decode_ais(self, data):

# Decode the ais binary

try:

to_file(self._filter_6_bit_ascii(self._ais[0].decode(data)), sys.stdout)

except DecodeError, ex:

filename, line_number, column_number = self._ais[2][ex.entry]

try:

opts, args = getopt.getopt(args[1:], 'h', [])

except getopt.GetoptError, ex:

sys.exit(ex)

for opt, value in opts:

if opt == '-h':

usage(args[0])

return 0

else:

raise NotImplementedError(opt)

if not args:

sys.exit("No files to decode. Run '%s -h' for more details.", args[0])

decoder = Decoder()

for filename in args: