I don't think I would be allowed to post it here. Copyright and all that...
Not that it is that complex. Fundamentally what happens is that it takes the digital switch control line, and then uses transition() to have some finite rise/fall time between 0 and 1. Then it uses pow() to logarithmically alter the resistance of the branch across the switch terminals.
i.e. along these lines:
Code:rout = roff*pow(ron/roff,transition(state, tconv, trise, tfall));
Hopefully that explains enough of the principles without posting the actual code here.
Andrew.