Computer Associates v. Altai
A 1992 US case that gave courts a three-step test for deciding when one program has copied another.
A programmer who moved from Computer Associates to Altai brought chunks of the source code for CA's scheduler component ADAPTER with him and folded them into Altai's OSCAR. Once that came out, Altai had the module rewritten from scratch by programmers who had never seen the original. CA sued over both versions.
The Second Circuit held Altai liable for the first version but cleared the rewrite, and in doing so laid out abstraction–filtration–comparison: break the program into levels of abstraction, filter out ideas, anything dictated by efficiency or external constraints, and public-domain material, then compare whatever is left. Usually not much. The test effectively replaced the far broader protection of Whelan and has been followed by most US courts since.