Programming

Don't copy the code directly from any company assets. There are plenty of ways to track code and data theft these days, so don't even attempt it. I am just saying that as a friendly reminder.

Honestly, there is not much that a company can do unless they specially own the business logic of what you are doing. Are there aspects to the code that apply to internal proprietary software? That probably isn't wise to share.

While I am not a lawyer, a general rule of thumb is that if you think you might be stealing something, you probably are. Anything you do on company time, is technically owned by that company.

If your previous work gets discarded by that company, never talk about it again. Never code it again for that company and just let the idea die, as far as that company is concerned. Independently resurrect the idea at a later date.

Nobody here can really tell you what to do, btw. Quite honestly, if you think that you can claim ownership of what you have, pay a few hundred bucks for a consultation with a lawyer.

If you rewrite it in a clean-room approach and another language it will most likely not be in breach of copyright.

But there are many other aspects where you may be at fault: breaking confidentiality, using trade secrets, non-disclosure, non-compete etc.

My advice would be to have an honest discussion with the company owner and ask for the permission to open the code under a permissive license. Be prepared to explain what the advantages would be for the company, beyond "the code is just sitting there". Be prepared to drop it if they say no.

If you go ahead it is quite possible you will be sued. Make sure you're willing to risk it and spend time and money defending your project.

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.