diff options
| -rwxr-xr-x | devtools/ais.py | 19 |
1 files changed, 17 insertions, 2 deletions
diff --git a/devtools/ais.py b/devtools/ais.py index a0c1874f..c75013a3 100755 --- a/devtools/ais.py +++ b/devtools/ais.py @@ -849,13 +849,20 @@ def parse_ais_messages(source, scaled=False, skiperr=False, verbose=0): payload = '' raw = '' values = {} + crc_ok = False while True: line = source.readline() if not line: return raw += line + # Compute CRC-16 checksum + packet = line.strip()[1:-3] # Strip leading !, trailing * and CRC + csum = 0 + for c in packet: + csum ^= ord(c) + csum = "%02X" % csum # Ignore comments - if line.startswith("#"): + if not line.startswith("!"): continue # Assemble fragments from single- and multi-line payloads fields = line.split(",") @@ -863,9 +870,17 @@ def parse_ais_messages(source, scaled=False, skiperr=False, verbose=0): fragment = fields[2] if fragment == '1': payload = '' + crc_ok = True payload += fields[5] pad = int(fields[6].split('*')[0]) - if fragment < expect: + crc = fields[6].split('*')[1].strip() + if csum != crc: + if skiperr: + sys.stderr.write("Bad checksum %s, expecting %s: %s\n" % (csum, `crc`, line.strip())) + crc_ok = False + else: + raise AISUnpackingException("checksum", crc) + if fragment < expect or not crc_ok: continue # Render assembled payload to packed bytes bits = BitVector() |