[KaVir]

Here's an article you may find of interest:

In particular, the first and last paragraphs of the Piecewise Reimplementation section:

"Many people have reimplemented computer programs by rewriting them to replace the source code with code of their own writing. There is no reason to believe that this would not be a copyright infringement, particularly if the reimplementer had access to the source code of the original program, even if none of the original source code remains."

"Even if you completely replace the program with new code, nonliteral elements also protected by the original program’s copyright are likely to remain and infringe – elements like the overall program structure or architecture and data structures that are not dictated by external or efficiency considerations. Although there is no case law on this point, it would seem that the only way to break the chain of infringing works is by some extraordinary act, such as a clean room implementation."