Starcraft AI blog

Humans build up a wealth of knowledge about each map they regularly play: The best proxy spots are here and there; this base is particularly vulnerable to muta harass; such-and-such expansion build is only safe in cross positions. Some of the knowledge is very specific: On Circuit Breaker, at a mineral only you can displace the resource depot toward the edge of the map and it’s a little safer, or protoss can place the nexus normally and provide some protection from vulture raids with a specific pylon wall. I eventually want to teach Steamhammer to collect strategy and tactics data per map and learn that kind of knowledge itself.

For now, though, bots have a simplified understanding of maps. The information that comes out of terrain analysis is a small fraction of what a human knows. So I asked myself: On a map with destructible neutral buildings, like say Hitchhiker, how should a bot decide which buildings to break down and when? Is there a rule of thumb that we can use as a first approximation of deep knowledge and tactical analysis?

One of the steps is to decide where you want to go. If you have a destination in mind, you can compare the different ways of reaching it and choose a set of paths to open (a set of neutral buildings to destroy or mineral blocks to mine out). Most bots today, I get the impression, don’t use this goal-oriented approach, but open a new path when they happen to find that it is nearby.

Two major reasons to open a closed path are to expand to a base through it, or to attack the enemy through it. Suppose you choose your next expansion by scoring the neutral bases and taking the one with the best score (or perhaps none if none passes a threshold). For a base that you have to open a path to reach, count the cost of opening the path against the score.

Opening a path for attack does not seem as simple. On Hitchhiker, you start with an open path down the center, but it is a narrow ravine that is easy for either player to defend. You can also destroy buildings to open side paths. A path you can attack through is also a path the enemy can attack you through, and if you open a side path and send your army there, what stops the enemy from countering down another path? How do you weigh risk and gain?

Maybe you can suggest a better idea, but here is my thought: If you are unable to attack down any existing path (the combat simulator keeps saying you’ll lose), and the reason is not that your army is puny because you are behind, then maybe you should open another path and attack there instead. That seems simple enough to implement and likely to be of some value.

Another reason to open a closed path is to create a shorter path to a place you can already reach. That’s the reason on SSCAIT maps; it applies on Heartbreak Ridge and Destination to the backdoor mineral blocks, and on Benzene to the backdoor neutral buildings. You want to open the path when it benefits you more than the enemy, which generally means that you can actually or potentially control the new path with your army. I think that strong current bots are not bad at doing this defensively for paths near their base, but I haven’t seen a bot open a path offensively.