[vladimirralev]

The search space is not that big. It depends on the granularity of the decisions, but in SC2 most algorithms will work on the supply scale (with resources as constraint). Meaning you only have 3-5 decisions to make at any supply count. In this case considering the extractor cancel is a nice touch and I assume it was a special case because it's a known trick which makes no sense to consider for any other buildings.

[veemjeem]

Maybe the search space is limited in the first few minutes of the game when your opponent has also has a limited set of moves, but as the game goes on, the complexity increases exponentially. The 7 roach rush could be calculated here because the algorithm essentially completes when it reaches 7 roaches, but longer more complicated builds would be significantly harder -- like a 2-base blink all-in.