I am currently creating something that has been inspired by another library. It is a complete rewrite on another platform – yet, has the same purpose and functionality and is written in the same language. As far as I understand it, this is still considered a derivative work.
The original piece is licensed under the Apache 2.0 license.
Which steps do I need to take now when publishing my source under the Apache 2.0 license?
Do I have to maintain copyright notices?
If so, it can’t obviously be in the source files (which are all new and cannot be mapped to the original piece).
What do I have to do to be on the safe side? What do I have to include when I distribute the sources?
1
If you only copied ideas from the original library and no actual source code, then your library is not a derived work as far as copyrights are concerned.
Copyrights are about the actual source code, not about the ideas/concepts that are represented in that source code. To protect those ideas/concepts, you need to turn to other intellectual property laws, such as trademarks and patents.
So, if your library is really a full rewrite (no traces of the original can be found), then you can release your library under any license you want, because you own the copyrights to it.
To be nice, you could mention the original library (possibly with a link) in your documentation as being your inspiration.
2
One of the key differences between patents and copyright is originality.
In copyright, if you can prove originality, then you can not infringe on someone else’s work no matter how similar (or even identical).
Patents are just the opposite, once a patent has been granted, originality means nothing. If today you register a patent, and tomorrow someone wakes up and totally independently comes up with the exact same design in every particular, any usage of that design infringes upon your patent.
Patents are intended to reward sharing inventions, and is intended to prevent people from claiming afterwards that they invented it and didn’t share, but should still benefit because they are willing to share now. No patent, no protection, no patent if someone else has already patented.
Copyright is intended to reward sharing creative work, by allowing the creator to benefit from the distribution of their work. As such, if you can prove the work was created by you, you get to benefit from the distribution. Identical valid claims would result in independent rights to authorize distribution.
That said, proving originality can sometimes be hard and expensive, and the presumption is going to be that the greater the similarity, the less likely it is original.