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.
The only real answer here is talk to an actual lawyer rather than a bunch of Joe Blows on the internet. Case in point: Laws will vary depending on country and you haven’t specified what country you’re in. The set of laws you’re subject to are possibly entirely different than the set of laws each commenter here is familiar with. Never take legal advice from the internet.
But if you’re only looking to publish the source code as a resume item, it’s not worth the legal exposure or time/money to talk to a lawyer. Find something else to write and put on your resume.