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

[John Colvin]

On Thursday, 27 June 2013 at 18:00:21 UTC, Steven Schveighoffer 
wrote:
> On Thu, 27 Jun 2013 13:43:24 -0400, Steven Schveighoffer 
> <schveiguy at yahoo.com> wrote:
>
>> On Wed, 26 Jun 2013 16:51:33 -0400, Gary Willoughby 
>> <dev at kalekold.net> 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 < 0 ? (x & 1 ? x+1 : -x+1) : (x & 1 ? x-1 : -x-1);
>> }
>>
>> works for all but int.min, but -int.min == int.min, so, I'm 
>> not sure what to do for that.  Either change argument type for 
>> long, in which case the result of f(f(int.min)) == int.max + 
>> 1LL, or special case int.min to return int.min.
>
> I was wrong, it doesn't work for -1 also.
>
> Hm...
>
> I could special case that too... but I like the simplicity :)
>
> -Steve

The trick is to move away from 0 in all cases (see my solution)