[aprssig] Aprx v2 beta, now with Viscous Digipeater

[Scott Miller]

> I did some studies over various "rapid hash algorithms", how well
> they distributed their results  -  of what I had been recommended
> to use by other people, worst ones did result in about 25% of
> value space getting any hits at all, and even smaller fraction
> getting most hits.  As a young man then, that did tech me not to
> trust hearsay, and want to do validation with _my_ data.

The T2 has a fixed number of slots, and the first available one (or 
oldest if the list is full) is taken.  I'm using CRC16.  Assuming 
perfect hashing, the chance of two 16-bit hashes in a set of 15 (the 
default T2 dupe list size) colliding is about 0.16%.

I considered that when I coded it, and decided that was an acceptable 
rate for falsely dropping packets as dupes.

> There are very few "good" hash algorthms, and they tend to be called
> "Cryptographic Hash XYZ".   But your first task on storage constrained

A cryptographic hash has the additional property of being very hard to 
deliberately generate a collision for.  You don't need that when 
someone's not deliberately trying to screw up your hashes.

Scott
N1VG