Starcraft AI blog

Finishing off the enemy just means destroying all their buildings. It sounds simple, but it is a sophisticated skill, and there are a lot of ways to go wrong. Steamhammer has a number of special provisions for quickly finding the last enemy remnants, but small loopholes persist and occasionally a game slips through one.

PurpleSpirit-Steamhammer on La Mancha is an example. It’s an entertaining game, thanks to the purple habit of playing all over the map, but I want to focus on the end, after PurpleSpirit has lost, when Steamhammer fails to destroy the floating terran buildings that are right over its head. That part is entertaining too, but not for the same reason.

Everything terran on the ground is destroyed, except one command center which was infested instead. The remaining terran force is 2 full-strength battlecruisers, and the remaining terran infrastructure is 2 floating ebays. Steamhammer is maxed and banking resources, but its only anti-air units are 8 scourge, plus a defiler with plague. Notice how much game time is left?

One of Steamhammer’s special game-finishing skills is that it makes mutalisks to chase down the residue of the enemy. The condition is, if the enemy has no known bases and no known anti-air units, then Steamhammer will tech to mutalisks and make mutas its primary unit. The mutas scout faster than ground units, and can find floating buildings and island bases that ground units can’t reach. But here terran still has battlecruisers, so the mutalisk rule does not kick in. First the terran army, such as it is, must be defeated.

Some scourge have hit, and the battlecruisers are no longer at full HP. But Steamhammer has been replacing losses primarily with more zerglings and ultralisks, which are of no use. Oops, the unit mix is wrong. Now there are only 2 scourge, and a battlecruiser can kill a scourge in one shot—2 battlecruisers, if not distracted by other targets, are safe from 2 scourge. The ebays choose to park over the terran natural, and zerg units have congregated there. The battlecruisers seek zerg stuff to shoot, and the defiler responds by blanketing the area in defensive swarm, consuming zerglings like crazy.

Well, the 2 scourge hit one of the battlecruisers, which was distracted seeking zerg units that strayed from under dark swarm. And Steamhammer is now making 3 new pairs of scourge to replace various losses; if it can keep this up, the battlecruisers will eventually fall. Best of all, the defiler has plagued the terran buildings, despite the zerg units underneath. That will put the ebays into the yellow. One more plague should put them into the red, after which they will burn down.

Whew, finally the battlecruisers are shot down by scourge.

But that’s all she wrote. The swarms wore off and there was no need to renew them. The defiler did not plague again because it thought the zerg units underneath were more valuable. Scourge are coded to avoid floating buildings, because it is usually wasteful to spend gas destroying them. The mutalisk rule is engaged, and there is supply to build 1 mutalisk, but Steamhammer happened to choose to spawn zerglings first, and after that there was no supply to make a mutalisk. The game timed out with no more progress.

Finishing off the enemy can be hard. In this case, Steamhammer had the wrong unit mix; to make zerglings and ultralisks when all enemies were in the air was no good. The mutalisk rule should make mutalisks only, not mix them with other units. The scourge might have understood that when only floating buildings are left, they are good targets. Also the zerg ground units that can’t shoot up might have known better than to chase floating buildings (though it can be useful when they track a building trying to escape), and the defiler might have realized that damage to its own units was irrelevant when it could eliminate the enemy. That is a lot of flaws, and yet Steamhammer rarely fails to finish a game!

And fixing all the problems would only narrow the loophole, not eliminate it. In the worst case, Steamhammer would need to be able to destroy some of its own units to clear supply to make mutalisks to finish off the enemy. And that’s a high-end skill that I am in no hurry to add.

Next: The start of CoG 2019 analysis.