SA-MP's GPCI function implemented in C#, based on Renegade334's implementation in Javascript
The full name for this function is apparently GetPlayerComputerId. But this was never documented, nor was the function itself.
For some reason, SA-MP's creator at the time implemented this function and kept it as a secret.
The idea behind this was to be a sort of unique ID to identify a player. Although that wasn't really the case as time went by, because people started figuring out how it worked. So multiple players ended up having the same hash.
Anyhow, this, and an IP were the only things used in SA-MP at the time to actually keep bad actors from rejoining servers.
Well, as in the game, this was also useful to identify players from outside of the gameserver. When for example, using a launcher or an external anti-cheat.
I also have this implementation ported for GO and PHP as a Gist: https://gist.github.com/VIRUXE/80a27dca1604ddc8d205416ace911914
There's also the original implementation in Javascript, from Renegade334: https://github.com/Renegade334/gpci-js