Did he even ask you to describe how you would likely tackle the problem while you were in their office? Things like design ideas, architectures, technologies, p

Did he even ask you to describe how you would likely tackle the problem while you were in their office? Things like design ideas, architectures, technologies, pros and cons, etc? If all he did was ask you to "do this" at their office without picking your brains as to your thought process, then regardless of the legitimacy of the offer, I would think twice about working there. The fact that he's asking for a solution that works on all of the data rather than a sample of it is also highly suspect. Either he is not being frank with you, or he doesn't have a firm grasp of scope (most likely the latter). Either way, he's probably not the sort of person you'd want to work for.