[gpsd-dev] Overlay issue in pseudoais.c

[Eric S. Raymond]

This is mainly for Reinhard Arlt, but copied to the dev list for the
record.

Revision 4 of ITU-R 1371-4 introduced an incompatible change in the
handling of AIS type 24 vendor ID fields. Before this, the bit span
from 48 to 64 was treated as a single string of 7 packet sixbit
characters.  In revision 4 it became three sixbit characters followed
by numeric 4- and 20-bit model-code and serial number numeric fields.

I'm dealing with this by coninuing to decode the string as 7
characters rather than 3, also reporting the (overlapping) model
and serial values.  Human eyeballs will have to decide when the
trailing 4 bytes of string (and thus, oppositely, the numeric fields)
are valid.

The code in pseudoais.c needs to be adjusted, but I'm not sure how
this should be done as I don't know what the use case for it is.
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                                        >>esr>>