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.
Doesn’t matter if you write it in code or chisel it on a stone tablet. It is still the companies intellectual property.
Think of it this way: You film a movie which for whatever reason doesn’t get published. This doesn’t give you the permission to write a book containing the same story, just in writing. The story is still owned by the film studio. The same reason applies to published material: You are not allowed to write a Star Wars story without approval from Disney, the copyright holder. Fan fiction exists in a gray zone for exact this reason.