I told my boss I had an idea for a program that could improve efficiency across much of the business, and he let me build it on company time. In the long term, he wanted to be able to sell it to other companies. However, the program never got implemented due to personnel mismanagement, and I’d rather be able to post it on my github under a free licence so I can use it as a resume item, and at least someone would have the chance to actually use it. It’s all still in my head, and I could write it again if I wanted. If I do, is it illegal to publish it? What if I write it in a different language? Do I need to change the variable names? I did plenty of research and planning on company time to build it, and it’s not like I can research it again, it’s all still in my head.
IANAL and you really should ask a lawyer about this. The answer very much depends on your work contract and country of residence (the latter due to the fact that some generic contracts’ statements might be legally unenforceable in specific jurisdictions).
I’ll throw in a random fact: the contract might say that whatever you write as a programmer is still company’s property even off the clock and it will be legal in some US states.