Starcraft AI blog

There are two ways to make headway: You can execute your plan better, or you can execute a better plan. In bot development, I think it’s right to spend most effort on polishing what is already done, or adjusting your plan detail by detail. No matter what plan you follow, you’ll meet a huge number of bugs and weaknesses, and they have to be taken one at a time. All the roads are long.

But tscmoo has fun trying out different plans altogether, and tscmoo is a frequent tournament winner. Isn’t there a connection? When you try something new, your opponents will not be ready for it. And we know of strong plans which no bot tries.

Corsair-dark templar is an example. Sair-DT is for protoss versus zerg: Corsairs chase away or kill overlords in an area, and then the dark templar have free rein. I wouldn’t say that sair-DT is a strategy; it is a class of tactical plans which can be fit into a strategy.

I sometimes see bots with corsair skills clear an area of overlords, but the bots do not seem to realize the opportunity. There is a reason they don’t, of course; sair-DT play requires coordination. The corsairs and the dark templar must be ready at the same time to attack the same location, and they have to work together.

But defending against sair-DT also requires coordination, and bots are liable to be poor at it. Think of what Steamhammer does: It makes hydras and individually chases after each corsair that enters its base, scattering its forces. 4 dark templar, even if detected, could kill the scattered hydras one at a time and slip away safely because the pressure on the corsairs is released. Each unit supports the others; it’s not easy to implement, but by the same token it is not easy to defend against.

Here is a common pattern in human games. Zerg owns the natural and main of some start location. Corsairs are flying about, so zerg makes hydras for defense and holds them in front of the natural and collects overlords there. The hydras defend both bases by ground and defend the overlords, while the overlords detect any DTs that try to sneak past. There is still one weak point. Protoss sends corsairs through the main while at the same time dropping DTs from a shuttle in a far corner. When the sairs see that the defenses are weak enough, the dark templar move in and snipe any drones that don’t escape fast enough, then go to work on buildings. Zerg rushes to defend and will try to scourge the shuttle; if the shuttle lives, the DTs will load back up and get away free, counting their loot. I find it hard to imagine any current zerg bot defending well against such a sophisticated attack.

It was tempting, but I decided not to implement sair-DT myself, because it is too far off my path. Anybody could fork Steamhammer and get coding. Or if you have a protoss bot that already knows corsair skills....