[Nauty] canonical label for directed graphs

[Brendan McKay]

* Gordon Royle <gordon at csse.uwa.edu.au> [050803 22:24]:
> nauty already computes canonical labellings for directed graphs....   
> there is no need to modify the algorithm.
> 
> However the default settings for the "options" structure is to have  
> options.digraph set to FALSE - presumably because some speed can be  
> gained if the algorithm can rely on the input being undirected  
> without actually confirming that edge j-i is present when it notices  
> edge i-j.
> 
> So, just set options.digraph = TRUE before your call to nauty, and it  
> will treat it as a directed graph
 
If you are calling nauty from your own problem, I also recommend
   options.mininvarlevel = 1;
   options.maxinvarlevel = 99;      /* or something */
   options.invarproc = adjacencies;
For long jobs you might want to test whether this helps or hurts.
The values of these parameters can change the canonical labelling
so be consistent.

Brendan.