[atomicwrites]

That's called a clean room implementation and was the standard way to make x-compatible products (like for example, the bios on an IBM PC clone). Not sure what the current legal standing of that method is.

EDIT: Ninjad because I left the reply in a tab without posting.

[zymhan]

Reverse engineering is legal in the US, but you had better have detailed records proving no one who knew the insides of the original product ever influenced the clone. And be prepared to explain that in court.