This simulator doesn’t work quite as well as you’d hope but it’s a library check-in/check-out simulator that’s intended to help you figure out how long you have to wait until a book you have on hold comes in.
I found this simulator on a MetaFilter thread where a patron was asking exactly that question – if there was an easy formula to predict how long until a book you have on hold comes in.
Turns out there’s also a known mathematical formula known as Kleinrock’s Queuing Theory that helps you predict how long until your book comes in. But that formula was originally created as part of one computer scientist’s work around the queuing of network packets on the Internet – who knew?