Well, crystals form into regular geometrical patterns without any sort of over-arching goal. So that could be one way to do it, have some simple check each pie

Well, crystals form into regular geometrical patterns without any sort of over-arching goal. So that could be one way to do it, have some simple check each piece could perform (like observing neighbors), maybe some kind of random 'seed' selection to determine starting points and later some annealing so everything joins correctly. That is where I'd probably start.