TL;DR - Only a lawyer can really answer that. That said, it really depends on what's in your employment contract, either explicitly or implied. Definitely sta

TL;DR - Only a lawyer can really answer that. That said, it really depends on what's in your employment contract, either explicitly or implied. Definitely start completely from scratch (use none of the code or even notes you wrote while an employee), but the other IP around this is where it gets tricky. Every jurisdiction has it's own subtleties. It's also important to note that in many places (you didn't say where you are) you could be sued even if they don't really have a case, forcing you to spend the time and money to defend yourself.