The idiomatic calculator interface is actually very hard to implement without bugs https://mitadmissions.org/blogs/entry/designing-a-calculator-with-fsm-logic1

The idiomatic calculator interface is actually very hard to implement without bugs https://mitadmissions.org/blogs/entry/designing-a-calculator-with-fsm-logic1/ https://mitadmissions.org/blogs/entry/designing-a-calculator... FSM don't help with the bugs. You need analysis on top to actually make a dent in the bug count