Idempotent partition around median of 5?

[Era Scarecrow via Digitalmars-d]

On Thursday, 4 February 2016 at 01:24:15 UTC, Andrei Alexandrescu 
wrote:
> This appears a simple problem: given numbers a, b, c, d, e, 
> swap them around so as to place the median in c and partition 
> the others around it. I.e. the postcondition is: a <= c && b <= 
> c && c <= d && c <= e.
>
> <snip>
>
> So there's got to be a better solution. Your challenge - should 
> you choose to accept it :o) - is an algorithm that does the 
> partitioning in 6 comparisons and <= 9 swaps, which is 
> idempotent: when applied twice, it always does 0 swaps.

  Well if we look at it, the max compares for optimal sorting 
algorithm is the log2 of the max combinations in how the elements 
could be arranged; !5 = 120 or 7 compares.

  I'm not sure of the max swaps, seems like it could be 4 if done 
right, but that might be impractical.