[DRAFT] Random Permutations With Constant Space

You want to iterate over a random ordering of items without storing or needing to lookup already-used items. We can borrow an algorithm from cryptography called the Feistel Network to generate random permutations in constant space and time. Applications include generating unique gift card codes or credit card numbers for your customers without exposing already-generated codes/numbers, and efficiently implementing the fizzlefade effect from Wolfenstein 3D.