Starcraft AI blog

Steamhammer divides its units into squads (like many bots), and gives each combat squad orders to try to reach a target position, such as an enemy base. The system is inherited from UAlbertaBot. A squad may have units all over the map, such as some at the front line and some just spawned, and units in different places face different situations. So Steamhammer divides a squad into clusters of units and tries to make appropriate decisions for each cluster, depending on whether enemy units are near and so on. The clustering has been in place since fall 2018; see Steamhammer 2.0 squad unit clustering for the description, which is still up to date in everything it says, though some details have changed behind the scenes.

I want Steamhammer’s clusters to join up when they can, so that the army fights together. So it uses a follow the leader rule: The frontmost cluster, the one closest to the target position, is the leader. Other clusters, if they are not distracted by enemies nearby, try to join up with nearby clusters that are more forward, and ultimately with the leader. In other words, clusters don’t independently seek out the target position; their first priority is to merge into a big scary army that will win. Compare Steamhammer’s tactical habits with, say, Dragon, which feels freer to scatter its units across the map.

Thinking about nydus canals, I noticed that following the leader doesn’t work in all cases.

The enemy is attacking a base with a nydus. A squad is ordered to defend. Cluster 1 is closest, because it can jump through the nearby nydus to reach the base. Cluster 2 does not want to follow the leader; it wants to travel by ground, not by nydus, because that way is closer for it. Cluster 2, to follow its defense order, should attack (or at least threaten) the enemy from the rear.

Nydus canal is not the only way to break follow-the-leader. Imagine a target position with two separated entrances, like an edge base on Fighting Spirit. If two clusters find themselves at different entrances and want to attack, each should probably attack its own entrance.

I don’t know how I’ll end up fixing this. There is time to think about it. A related and more immediate issue is how to attack the same enemy position from two directions at once, which Steamhammer currently can’t do except by accident. I’ll solve that by analyzing which clusters are attacking the same enemy units and doing a joint combat sim, so that all attack together or all retreat.