Is this legal?

[LiquidSharp]

Is it legal to base one's software off of another's API documentation?

I just started designing a small game engine and while I was laying out the overall architecture of it, I came across the C4 engine API docs. I really like how it is organized, and I have been thinking about organizing mine in the same way. Thus, I would probably have many of the same "packages," and they would probably contain similar classes. Is that legal? (I have not purchased a liscense)

Moreover, would it be legal for me to add the same functions to one of my classes, that are in one of the classes in this engine? For instance, I too have a Vector4D class in my engine, but I noticed how in the C4 engine there is a lot more functionality. Is it legal for me to implement my class in an almost identical way that the C4 developers have implemented theirs?

(And I realize that in any engine, vectors will have to have similar functionality, but is there a problem if one uses one's implementation to implement her/his class?)

I guess this might seem like a pretty stupid question, but I just want to make sure I am not crossing any boundaries.

Thanks for the help.

[.oisyn]

I think it's legal as long as you stick to the global interface and don't copy/paste any code or algorithms. But I'm not a jurist so my opinion isn't that valuable ;)

[GroundKeeper]

I don't think they have gotten around to patent general structures like the ones you describe. But the name of some modules/interfaces can be somewhat problematic as it might refer to a patent or copyrighted content. But if you want to go commersial with your software it is always good to let a lawyer have a look at it.