Starcraft AI blog

As some people have noticed, Steamhammer’s “secret weapon” ZvZ opening, which when I first tested it scored overwhelmingly against all opponents including Steamhammer itself, is simply overhatch.

Overhatch means making up to 9 drones, spawning an overlord, and then starting a hatchery while still on 9 supply. Steamhammer’s version is overhatch 9 pool 9 gas (with drones in between buildings), parallel to its faster 9 hatch 9 pool 9 gas build. The overlord lets zerg get in a couple extra drones, so that when the spawning pool finishes the bot has 10 drones and can immediately spawn 8 zerglings. The timing is fast enough to hold off 5 pool except on 2-player maps. It puts the player in a reasonable economic position (by ZvZ standards), but also commits to zergling-heavy play because the lair will be later.

Overhatch is a rare opening and it is not written up on Liquipedia, and that is for a good reason: It is dominated by 10 hatch. In a ZvZ 10 hatch opening, you go up to 9 drones, spawn an overlord, and do the extractor trick to get a 10th drone. The 10th drone can mine for a while before minerals are available to morph the hatchery, so 10 hatch is always ahead of overhatch in minerals. There is no disadvantage to 10 hatch; it can only be better than overhatch (though only by a little). Steamhammer plays overhatch because it still does not work around the extractor bug (I have more important bugs to fix first).

results

The opening is strong against bots. Results on the SSCAIT server have been good, though less convincing than my original tests.

Steamhammer has lost several games by failing to build an extractor when it came up in the build order. It is a bug that I didn’t see in testing and haven’t solved. For some reason it hits overhatch much more often than other builds.

Steamhammer has also lost a game to Microwave’s 9 pool and another to Dawid Loranc’s 5 pool. Steamhammer’s zerglings were in time to defend with correct play, but Steamhammer pulled its drones too soon and lost some. It’s a severe defensive error. At some point I will teach Steamhammer how to defend with a drill, and then these losses won’t happen.

Steamhammer has also lost occasional games to strong opponents like Killerbot by Marian Devecka due to tactical errors. It does win most, though, even against opponents like Arrakhammer that know about the overhatch opening and are prepared to meet it.

counters

Like every opening, overhatch can be countered provided you know it is coming: You can safely open 12 hatch to end up equal in hatcheries, ahead in economy, and behind in nothing important. So Steamhammer doesn’t play overhatch every game. Steamhammer is set to play overhatch in half of games. Its other openings counter 12 hatch, which is a slow and risky opening in ZvZ. The random openings ensure that no opponent build can give a sure win; opponents have to either play better or get lucky.

If you want to counter overhatch safely, Liquipedia suggests that 12 pool gives a slight advantage. 12 pool is a safe ZvZ opening if played well. But I learned by experience that it is tough to teach a bot to play it well.

other matchups

The classic use of 10 hatch is in ZvP, where in old days it was considered a strong counter to 2 gate zealot play. 2 gate play is popular with protoss bots, so I tried overhatch against protoss too. It seems effective. I think it is doing better than overpool, which was already successful.

In ZvT, Steamhammer up through the current version has always played either zergling openings with heavy early pressure, or mutalisk openings with minimal zerglings and a heavy air attack. Thinking it through again after working on surviving early terran attacks, I decided to add a compromise opening for the next version. I picked an overhatch opening with early zergling pressure followed by a later, lighter air attack, at the cost of a weaker economy in the long run. I think it is objectively worse, but other zerg bots have been successful with similar approaches and it may work well against bots. It’s an idea that should be tried.

Tomorrow: Arrakhammer’s attempt to counter overhatch