How do you approach generating "random" unique numbers/strings ? Exactly, when you have to be sure the generated stuff is unique overtime? Eg. as few collisions in future as possible.

Now I don't mean UUIDs but when there is a functionality that needs some length defined, symbol specific and definitely unique data, every time it does it's stuff.

TLDR STORY: Generating 8 digits long numbers so they are (deterministically - wink wink) unique is hard but Format Preserving Encryption saves the day. (for me)

FULL STORY:
I had to deal with both strings and codes today.
One was to generate shortlink word for url, luckily found a library that does exactly this. (Hashids)
BUT generating 8 digits long, somewhat random number was harder then I thought, found out on SO something like "sha256(seed) => bytes => ascii/numbers mangling" but that had a lot of collisions because of how the hash got mangled to actually output numbers and also to fit the length.
After some hours I stumbled upon Format Preserving encryption (pyffx) and man it did what I wanted and it had max 2 collisions in 100k values. Still the solution with this feels hacky af. (encrypting straddled unix timestamp with lots of decimals)