Just In Time Stock Control
Yeah, I wrote a Just In Time Stock Control and ordering system for the ZX81. It needed a minimum of 16KB, 32KB was better. Lots of hacks to save memory: PI/PI for zero, Peeks and Pokes everywhere.
The data was held on tape and needed loading into the arrays which could take 5 minutes to 10 minutes or longer.
Amazingly, it worked and reduced our stock holding nearly 70%! I seem to remember even selling it to a number of companies via ads in the computer rags.