An algorithm I've heard of in multiplayer game programming, in which massive state must be transmitted to many clients ... xor the previous game state with the new game state on the server ... most of the result will be zeros because most of the new state is the same as the old state. Then compress it. All those zeros compress really really well which is great for network traffic. Then transmit the compressed xor'ed state, client decompresses and xors with current state to get new state. Don't remember where I first heard of this but it was probably 15 or 20 years ago.

Really clever. Never tried to implement such a thing though. Sounds fiddly and hard to debug.