[chias]

It doesn't have to be. Onewayness says nothing about finding "the original x". If a function is one-way, it means that given y (a random n-bit string in the output space of h), it is hard to find any x such that h(x) = y. If your h() is just a modulo operation, this is trivial: just choose x = y.

[kbenson]

Ah, I see, way as in path, not way as in direction (which is I think the misinterpretation that leads people to the root of the misunderstanding). So, there's one path to get from x -> y, but it does not imply you can't get back to the original x. In that respect, "way" is an unfortunate word to use, at least as it's used in modern English.

[arjie]

Woah, I was completely wrong. You're right. A pseudo-inverse is sufficient. However, there are no functions known to be one-way. How interesting.