Original author here. Thanks for the spacial data structures link, I'll look into it. The correlations are done in the frequency domain. And, although I didn't

Original author here. Thanks for the spacial data structures link, I'll look into it. The correlations are done in the frequency domain. And, although I didn't look into it, I suspect they have pretty good algorithms to compute the correlations themselves. At least they have very smart people working on that.