I have only superficially browsed vim sources some months ago. Vim source code is literally disgusting. The number of platforms vim runs on is close to Avogadro's number. It supports 5 or sth languages for customisation. I always had in my mind to fork the thing and remove all those languages, and strip off the compatibility stuff so it will be a pure Unix program, but I never got to do it.

Also, the source repository contains binary (.a, .dll) files, which I did not inspect. I always search the web for an alternative editor when I somehow browse the vim source repository.