Starcraft AI blog

Some bots, like Willbot, take a passive line and build up a large force before taking any offensive action. Others, like Steamhammer, prefer to keep the pressure on and attack whenever possible, even at high risk.

Neither style of play shows real understanding of strategy. Here are 2 simple principles that should be uncontroversial:

1. The side with more stuff has an advantage. If you have more units, or better upgrades, or higher tech, that is an advantage. You get those things by spending minerals and gas, so ultimately the side with more resources has an advantage.

2. The side on the defensive has an advantage. If you have static defense built, tanks sieged, lurkers burrowed, or simply units deployed in good position to engage, that is an advantage. If you have a shorter route to your production buildings, then reinforcements arrive sooner and that is an advantage. If you can stop or delay or channel attacking forces with blocking buildings or chosen terrain, that is an advantage.

From these 2 axioms, we can act like Euclid and derive a theorem: If you control more resources, you don’t have to attack. You can attack if you spot a weakness, but you don’t need to. You can hang back, in a safe defensive position, until your theoretical resource advantage manifests as a practical battlefield advantage, and then attack. At least you can wait until you are maxed at 200 supply.

Containment is the common case we see in bot play. When Steamhammer has the opponent contained, it constantly tries to press forward to notice and immediately exploit any weakness. It’s usually a mistake. Typically you should contain at the best defensive location you can, take care with the forward units you risk for vision, and scout to make sure the opponent can’t sneak an expansion or bypass the containment with drops or air play. You control the resources on the rest of the map, and that is an advantage.

The general rule is: If you can take expansions and deny expansions to the opponent, you will win. You can win without ever entering a fair fight. If the opponent takes an undefended expansion, smash it. If the opponent moves their army to defend a new expansion, smash the enemy natural instead. At worst, you force the opponent to allocate forces accurately to defend all threats. Tscmoo is the bot which implements this rule the best, though it still seems a bit crude to me.

The extreme case is a map split, where each side ends up controlling about half the map—except one side controls an extra base or two. Humans sometimes play from the beginning of the game for a favorable map split. I don’t think any bot understands the idea.

How do we get from here, strategic ignorance, to there, understanding tactical force allocation risks and tradeoffs to meet the strategic goal? Well, I mis-stated it; bots don’t have to understand, they only have to take the right actions. Bots today that contain the enemy don’t (it seems to me) understand what they are doing. They are following rules that produce containments as an emergent behavior. It’s a valid approach. But I recommend more explicit knowledge representations, because I think it will lead to faster progress.

Sometime this year Steamhammer will get an evaluation function that tells it how good or bad a situation is. The first version may be a simple hand-written evaluator that is used for a few decisions. In time, I hope to create an accurate evaluator by machine learning, good for decisions throughout the game. Then the same underlying knowledge, encoded the evaluator, will let Steamhammer adapt its openings moment by moment, choose its unit mix, and maneuver its forces.