Starcraft AI blog

The #2 most frustrating weakness in the play of Steamhammer 0.2 is its insane obsession with bunkers.

Here’s the kind of strongpoint Icebot builds when it sees Steamhammer’s mutalisks coming: A bunker, surrounded by turrets, with SCVs to repair, placed out in front where it can’t be overlooked. Objectively, it’s a stupid move, because the rest of the terran base is barely defended. Against an opponent that knows how to bypass strongpoints, like Overkill, Icebot collapses.

But against Steamhammer, Icebot holds and usually wins. Steamhammer sees that bunker and can’t take its eyes away. Steamhammer measures the air defense and realizes it can’t attack, so it pulls back a little, waits, collects some more mutalisks, pokes in to take another look, still doesn’t have enough, waits some more... soon Icebot has goliaths and valkyries and it is too late. Steamhammer’s opening is a low-econ rush, and the last thing it can afford is to wait.

It’s frustrating for two reasons. One, it’s intolerably foolish and must be fixed. And two, it’s not a simple bug that can be set right with an hour of coding. It’s an ingrained behavior with both tactical and micro aspects—it’s the result of a design attitude, and it has more than one cause.

I already fixed one cause in moving from Steamhammer 0.1 to the tournament 0.2 version. In 0.1, the mutalisks were grouped with zerglings in the same squad and followed the same path, so the mutas were constrained to ground paths and could not avoid front bunkers. In 0.2 the mutas are in their own squad and can fly as they will, so in some cases they fly a straight line path far from any bunker and ravage the terran. It helps noticeably, but not enough.

Another cause is targeting priorities. Steamhammer 0.2 believes that a bunker is more important to attack than a marine or a turret. When it collects enough mutalisks to fight, it should pick off turrets until the bunker is laid bare—the turrets are easier to destroy. Instead it ignores the turrets and goes straight for the bunker, which of course terran repairs. The development version of Steamhammer gets the priorities mostly right, after about 3 rounds of tinkering, so this cause is fixed too (well, fixed enough for now). New Steamhammer plays clearly better around bunkers.

But improved Steamhammer still shows obsessive tendencies. Given an unguarded command center and a bunker, instead of winning the game without risk it will too often divert to fight the bunker, depending on what happens to catch its attention.

The most fundamental cause is that the tactical system is not conceived with guerilla warfare in mind. I think many bots share this weakness, maybe most bots. Steamhammer, like its parent UAlbertaBot, draws a line from its base to the enemy’s and wants to progress down the line: Defeat the units in front of me, move forward, defeat the units in front of me, etc. It’s a super-simplified tactical framework. And for the guerilla warfare of air units, it is exactly backwards. The guerilla wants to bypass the units in front and hit wherever the defenders aren’t.

The simplification shows in how the combat simulator SparCraft works. SparCraft wargames out a battle under the assumption that both sides stand and fight, and predicts who will win: The side that is not annihilated. But in the fight of mutalisks versus terran base, only the terrans want to stand and fight. The mutalisks are happy to hit and run, as long as they don’t suffer too much. The mutalisks would prefer to ask SparCraft the question, “can I kill something—anything—and get away without any mutas dying?”

Anyway, I’m not going to fix this fundamental cause soon. Someday I hope to work on SparCraft and teach it to answer more questions. But that is at the tactics level, and I’m doing strategy first.

Tomorrow: The #1 most frustrating weakness: Retreating is hard to do.