How would you solve this 'interview' question in D?

[Timon Gehr]

On 06/26/2013 10:51 PM, Gary Willoughby wrote:
> Just for a bit of fun, I saw this question posted on reddit the other
> day and wondered how *you* would solve this in D?
>
> http://stackoverflow.com/questions/731832/interview-question-ffn-n

int f(int x){ return x?(x&1?x:-x)+(x>0?1:-1):0; }

unittest{
     foreach(x;int.min..int.max)
         assert(f(f(x))==-x);
}