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[aprssig] !x! (was: IS-to-RF proposal (rev b))

Steve Dimse steve at dimse.com
Tue Jan 3 15:20:47 UTC 2012

On Jan 3, 2012, at 8:48 AM, Heikki Hannikainen wrote:

> For positions I probably thought it could be a bad idea to allowed it to appear anywhere - it might be valid encoded thing in some APRS data extension, or could appear in someone's comment text accidentally.
> !x! can probably appear in some valid compressed packets given the right coordinates.
This is theoretically possible in the base 91 compression format, if anyone wants to do the reverse math and see what !x! corresponds to in the most significant positions that might shed some light on the situation, not all base 91 possibilities correspond to physical locations, these might be lats > 90 or lons > 180. 

I can only say that no one has ever complained that findU was filtering positions, and that the last three months or so of log files show no cases where the filtering was not obviously intended by the originator.

findU's implementation of this has been simple from day one. The first test an incoming APRS IS line faces in the parser is a search for !x! or x-no-ar anywhere in the line, if it is found then the line is printed to the log file (so I can answer questions like these about statistics and what is filtered) and then the line is thrown out from further processing. 

findU does these full string searches very easily because it is written in perl which was designed to handle text strings efficiently. In perl it is far more efficient to do this than to inspect the content of the packet and look only in certain places in certain packet types for !x!.

Steve K4HG

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