Doesn't WSPR also use the CRC in messages? So it would be a combination of
collision + valid CRC.The 50/50 point for 32768 values is 214.Why does WSPR
remember the hash value?
We do see bogus matches in FT8 modes and such -- not real often but every once
in while a callsign hash will match a random decode....same 15 bit hash being
used for that too.
Given the much lower WSPR counts I would expect "valid" collisions to be pretty
rare.
Mike W9MDB
On Friday, August 13, 2021, 05:41:12 PM CDT, Phil Karn via wsjt-devel
<wsjt-devel@lists.sourceforge.net> wrote:
The hash function used in wspr is 15 bits wide, i.e., there can be
32,768 values. This may seem like a lot, but the "birthday paradox" says
that the probability of a collision grows faster than you might expect
as the set size grows. It comes from the fact that you only need ~23
people to have a 50% probability that two of them have the same birthday.
A very rough approximation is that the probability of a collision is 1/2
when the set size is equal to the square root of the hash size. For 15
bits, that's about 180. My hashtable.txt for 40m currently has 353 entries.
Has anyone seen a collision in practice? If one occurs, the most recent
duplicate entry is most likely the correct one. Requiring a match in the
first 4 characters of the grid square would also seem to greatly reduce
the problem.
Phil
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