The old approach was to loop over a small circular buffer of recently
seen addresses recomputing the CRC every time: this prevented the use of
a big cache, and 20 entries was too small in case of high traffic and
big range antennas.
@prog on ##rtlsdr suggested to use an alternative algorithm where
instead we compute the CRC of the message, and xor it to obtain the
address, that is later checked in our list of recently seen addresses.
This is a lot better and allows for bigger tables in O(1) lookup time.
I used this idea to implement a larger 1024 elements table. Instead of
writing a proper hash table I used the fact we are just dealing with a
cache, so I just hash the ICAO addess and overwrite the old entry at
that idex with the address-timestamp pair, not caring about collisions.
The bigger table makes a huge difference:
In a test vector of 113 seconds recording with 76 simultaneous aircrafts
the new algorithm detected around 10k more messages (!).
As @keenerd noted on ##rtlsdr, using an 8 bit magnitude vector is not
enough in order to distinguish every different I/Q pair.
With this commit a few more messages with good CRC are detected.
The demodulation algorithm now only skips the current message if the
CRC is valid, this improves the amount of messages detected with good
CRC by ~ 4% since sometimes we are just a few samples out of sync.
This also improves the amount of messages that can be fixed.
The --stats option was added in order to better evaluate how the
algorithm performs on the same input after some change. So if you run
the program with both --ifile and --stats no Mode S message is logged
at all, but at the end of the processing the program shows you the
amount of messages decoded with good, bad crc, bit corrections, valid
preambles and so forth.
