This is similar to Yarrow / Fortuna (internal state is a counter, output is the hash of the state) so I'm guessing it's not breakable, at least not trivially.
I guess it depends what you mean by “crack”. Given f(1), which I assume is public, you can predict all future outputs.
That is not what we mean by "crack". Read the article.
I said without knowing the seed, so f(1) is not public, only f(n) formula is.