def __init__(self, raw: bytes):
# Set all values to None initially
[setattr(self, name, None) for name in self.__slots__]
# Store raw data
self.raw = raw
# An AIS NMEA message consists of seven, comma separated parts
values = raw.split(b",")
# Only encapsulated messages are currently supported
if values[0][0] != 0x21:
return
if len(values) != 7:
raise InvalidNMEAMessageException("A NMEA message needs to have exactly 7 comma separated entries.")
# Unpack NMEA message parts
(
head,
count,
index,
seq_id,
channel,
data,
checksum
) = values
# The talker is identified by the next 2 characters
self.talker = head[1:3].decode('ascii')
# The type of message is then identified by the next 3 characters
self.msg_type = head[3:].decode('ascii')
# Store other important parts
self.count = int(count)
self.index = int(index)
self.seq_id = seq_id
self.channel = channel
self.data = data
self.checksum = int(checksum[2:], 16)
# Verify if the checksum is correct
if not self.is_valid:
raise InvalidChecksumException(
f"Invalid Checksum. Expected {self.checksum}, got {compute_checksum(self.data)}.")
# Finally decode bytes into bits
self.bit_array = decode_into_bit_array(self.data)
self.ais_id = get_int(self.bit_array, 0, 6)
def validate_message(msg: bytes) -> None:
"""
Validates a given message.
It checks if the messages complies with the AIS standard.
It is based on:
1. https://en.wikipedia.org/wiki/Automatic_identification_system
2. https://en.wikipedia.org/wiki/NMEA_0183
If not errors are found, nothing is returned.
Otherwise an InvalidNMEAMessageException is raised.
"""
values = msg.split(b",")
# A message has exactly 7 comma separated values
if len(values) != 7:
raise InvalidNMEAMessageException(
"A NMEA message needs to have exactly 7 comma separated entries.")
# The only allowed blank value may be the message ID
if not values[0]:
raise InvalidNMEAMessageException("The NMEA message type is empty!")
if not values[1]:
raise InvalidNMEAMessageException("Number of sentences is empty!")
if not values[2]:
raise InvalidNMEAMessageException("Sentence number is empty!")
if not values[4]:
raise InvalidNMEAMessageException("The AIS channel (A or B) is empty.")
if not values[5]:
raise InvalidNMEAMessageException(
"The NMEA message body (payload) is empty.")
if not values[6]:
raise InvalidNMEAMessageException(
"NMEA checksum (NMEA 0183 Standard CRC16) is empty.")
try:
sentence_num = int(values[1])
if sentence_num > 9:
raise InvalidNMEAMessageException(
"Number of sentences exceeds limit of 9 total sentences.")
except ValueError:
raise InvalidNMEAMessageException(
"Invalid sentence number. No Number.")
if values[2]:
try:
sentence_num = int(values[2])
if sentence_num > 9:
raise InvalidNMEAMessageException(
" Sentence number exceeds limit of 9 total sentences.")
except ValueError:
raise InvalidNMEAMessageException(
"Invalid Sentence number. No Number.")
if values[3]:
try:
sentence_num = int(values[3])
if sentence_num > 9:
raise InvalidNMEAMessageException(
"Number of sequential message ID exceeds limit of 9 total sentences."
)
except ValueError:
raise InvalidNMEAMessageException(
"Invalid sequential message ID. No Number.")
# It should not have more than 82 chars (including starting $ or ! and <LF>)
if len(msg) > 82:
raise InvalidNMEAMessageException(
f"{msg.decode('utf-8')} has more than 82 characters.")
# Only encapsulated messages are currently supported
if values[0][0] != 0x21:
# https://en.wikipedia.org/wiki/Automatic_identification_system
raise InvalidNMEAMessageException(
"'NMEAMessage' only supports !AIVDM/!AIVDO encapsulated messages. "
f"These start with an '!', but got '{chr(values[0][0])}'")