If you have plenty of code space, you could just use a lookup table.
Crude but effective, it'll be quick, and has the advantage that all lookups will take the same time.
You could probably reduce the table size by discarding some of the least significant bits from the input.

But I think Moses' suggestion with the 18-record table is really neat!
http://www.8052.com/forum/read.phtml?id=17711&top=