"I thought I'd demonstrate this functionality, with a bit of real world code. Postabon recommends deals to users based on a variety of factors such as the age of the deal, other people's votes, your preferences, and (relevant for this example) your distance from the deal. Most of the other factors can be computed asyncronously and just cached, but your distance from deals is computed a lot, and can't really be pre-computed since I have no way of knowing your location a priori (well, that's not strictly true, and we do do some memoization, but it has a relatively low hit rate)."
Yes. He has a bunch of existing points (the deals) and and a new point (user location). Deals are the known points, which can be represented any way you like. lat lon, vectors, cylindrical coords, you name it. The thing not known in advance is the user location. And don't umm... me. It's rude and stupid, especially when you don't seem to know what you're talking about.
"I thought I'd demonstrate this functionality, with a bit of real world code. Postabon recommends deals to users based on a variety of factors such as the age of the deal, other people's votes, your preferences, and (relevant for this example) your distance from the deal. Most of the other factors can be computed asyncronously and just cached, but your distance from deals is computed a lot, and can't really be pre-computed since I have no way of knowing your location a priori (well, that's not strictly true, and we do do some memoization, but it has a relatively low hit rate)."