From ddcc52b5541ed662ecb8399a364a3eb5b6234a32 Mon Sep 17 00:00:00 2001
From: Jon Schlueter <jschlueter@redhat.com>
Date: Wed, 7 Oct 2015 21:29:30 -0400
Subject: AIS: Add better handling of non byte aligned data blobs

I ran into a problem with the type 8 unstructured data. When the number
of bits available is not dividable by 8, the last byte may contain
random bits. I would expect the bits beyond the valid range to be 0.

An example of this can be found in the following message:
!AIVDM,3,1,4,A,85PH6TAKfDOkp95`nCRt5w<:qFUiaihFhBc7s4AHGsQ,0*40
!AIVDM,3,2,4,A,DcMJM18k6<=m7rwVm3b5RRWEskwJWej8uP<0:W5K6PA,0*61
!AIVDM,3,3,4,A,gPs<dwulp,4*14

The current version decodes this to:

{"class":"AIS","device":"stdin","type":8,"repeat":0,"mmsi":369493649,"scaled":true,"dac":366,"fid":57,"data":"510:47f3e09168d938bc17f30ae56971a71c16c12ac7ec44585fb854add69d048cc630dd47ebf9b50ea1628a757bcff6a7b7223d80c00a9c56c6811be0eccb3ff74e"}

The 510 bits of data are 63 full bytes and then 6 remaining bits. The
last byte has value: 4e, which is 0100 1110. I would expect 4c, 0100 1100.

To fix this, the following patch could be applied. Please note that the
problem might also be present in other message types, but I have not
encountered any problems in other message types in the 1 million
messages I have in my test dataset. Message types that could be impacted
are: 6, 17, 25 and 26, as they all use the same memcpy to get
unstructured data.

Identified by: Stefan Roels <sroels-gpsd-dev@42solutions.nl>

Added new sample entry to sample.aivdm that exercises this case
Had to regenerate large ais test data log
---
 test/sample.aivdm.chk | 1 +
 1 file changed, 1 insertion(+)

(limited to 'test/sample.aivdm.chk')

diff --git a/test/sample.aivdm.chk b/test/sample.aivdm.chk
index 0b514723..bee2d762 100644
--- a/test/sample.aivdm.chk
+++ b/test/sample.aivdm.chk
@@ -85,3 +85,4 @@
 8|1|002300059|1|11|1325788|3622678|20T18:28Z|12|14|92|84|586|90|1023|228|2|255|511|3|255|511|255|511|31|255|511|31|255|63|511|255|63|511|15|1023|7|511|3
 8|0|002766080|1|11|1382800|3532000|20T18:30Z|0|0|0|0|0|0|0|0|0|0|0|0|255|0|0|0|0|0|0|0|0|3|0|0|0|0|0|1|0|0|0
 6|0|002053507|2|244670500|0|200|3|16:3200
+8|0|366999607|366|57|510:7ffdef068fbded3c0a1943dfb997b4a10fadec21e6e0a6608f0f44b686a8747ff71e595dcb5ec5b4e05221d886233d2e4916f6eb6fe8ecd38d69792816f3a2d0
-- 
cgit v1.2.1