I found an algorithm described in my old math notes that I wanted to dive into further, but the name I noted is apparently completely wrong ("Bolson Adding"), so now I am trying to find the actual name of it. You have an array of digits 0-9 and add them up in pairs, starting with first+second, then third+fourth, etc., and create a new array from that. Results above 9 are subtracted 10, to keep it single digits. Next, you do the same, but with second+third, fourth+fifth, etc., finishing with last+first. You attach that array at the end of the first and get a new array the same size as the original (it must be an even amount of digits). You can then repeat the process on this new array, and so on and so on. You can always get from the original to the result, but you (supposedly!) cannot get from the result back to the original. Does anyone know the name of this?
Without a key or the ability to decrypt this isn't a cipher.
Possibly the second part of the string of digits could be a key, but in that case just performing modulo addition would result in the Vernam cipher and you would not need the whole construction.
If it were a hash then it is a terrible one, just start with a string of all zero digits to see why. The output would also not be compressed.