stereochemistry - How does Walter White make pure crystal meth using a non-stereospecific reaction?

In the fantastic TV series Breaking Bad, Walter White, the dying chemistry teacher, takes to making crystal meth (d-methamphetamine) by several routes.

Initially, he uses the common small-scale route starting with pseudo-ephedrine which uses a stereospecific starring material to make a stereospecific end product (d-methamphetamine is the active compound). But doing this on a large scale is limited because it is hard to get large quantities of the pseudo-ephedrine.

Walter uses his knowledge of chemistry to move to an alternative route starting with phenyl-acetone(or P2P as it often called):

There are several ways of doing the reduction to produce the final product (White first seems to use a Thorium dioxide dehydrocarboxylation in a tube furnace but later seems to favour the aluminium amalgam reduction).

But all the P2P methods produce racemic products. White clearly knows this as the issue of getting the right enantiomer is raised more than once in the show. His product is considered to be exceptionally pure.

So how does he get an enantiomerically pure product? Or perhaps, since the show might not want to tell us the answer, what possible methods could he have used?