Starcraft AI blog

LetaBot suggested I should write about the decline of the rushbots. He pointed out that ZZZKBot by Chris Coxe (the best of them) is no longer a top bot on SSCAIT, but has fallen to the middle of the pack, and that UAlbertaBot has lost ground too. I’ll add that cannon rushes were never reliable, but remain moderately successful.

It would be interesting to examine the decline in historical data, but I don’t have the historical data. I once thought of setting up a job to scrape the SSCAIT page once a day, to save the game results for analysis, but I decided there were better uses for my time.

The rushbots are failing because rushbots have not changed and their opposition has improved in defense. The defense improvements include safer openings, better worker micro, and more cautious play with early military units (such as Tscmoo stationing early units defensively in its mineral line). Also the SCV rushbot Stone has been replaced with the non-rushing Iron—if Stone were still playing on SSCAIT, maybe its results would still be good. Or maybe not, we don’t know; its disappearance reduced pressure to improve rush defense. The fact that SSCAIT admins are tired of zerg rushbots (as we all are) and have disabled many of them also reduces pressure to improve rush defense.

This is going on a bit of a tangent, but I’ll tell a story. Once upon a time I was playing (on Lost Temple, it was that long ago) as zerg versus a protoss who was clearly better than me (which wasn’t saying much). I saw the protoss army move out toward my mineral-only. I had prepared a backstab: I ran a dozen speedlings past the protoss natural into the undefended main mineral line. Then I had to turn my full attention to defense, narrowly holding the attack, and I didn’t see what happened with the backstab. After I lost the game I watched the replay. I was astonished to find that the protoss had not lost a single probe to my backstab, but had microed probes to glitch out my lings and kill them safely. I hadn’t known it was possible. It was a memorable lesson: Micro can make the difference between losing everything and losing nothing.

Worker micro is a key skill, and it’s basic to good play. Even today’s best bots have visible flaws in their worker micro. As long as that remains true, the zerg rushbots have a chance to win even against some safe strategies, and they have a role to play as teachers. But if they want to keep that role they’ll have to improve their zergling micro. They need to know how to avoid defenders while attacking vulnerable workers, when to step back and change targets, and when to gather up a force before striking. At least they need to go around the bunker instead of running straight into it and dying.

For the rest of the post, I’ll concentrate on zergling rushes.

4 pool and 5 pool are objectively sound, and can be played at pro level. Like any playable opening, they counter some enemy strategies and are countered by others. If a pro plays a safe opening, then a 4 pool or 5 pool will lose. But pros are always looking for an edge, and they will correctly play unsafe greedy openings to get one—if you never play an unsafe opening, you are leaving money on the table. And if your opponent plays unsafe openings often enough, then it is correct to make them pay the cost for the risk they are taking. Also some openings are in-between, safe-ish but vulnerable to better micro or better luck.

4 pool comes in two variations. The build order for the more aggressive variant is: 4 pool, 6 lings. The other variant is: 4 pool, 1 drone, 6 lings; with the extra drone you have more economy but you can get 2 fewer lings before the first overlord. There is not much difference between the variants; they are both do or die. 5 pool is a little slower, but your stronger economy means that you need to do less damage to the enemy to break even. You can accumulate minerals while attacking, which gives options to transition into another strategy. 5 pool can be played against any race, but experts don’t 4 pool against protoss because protoss will normally survive and you need that touch of extra economy to make it to the middle game.

This old Team Liquid post lists pro games and statistics for 4 and 5 pool games between 2004 and 2009. There are not many; play them often and you’ll lose often; play them occasionally to punish greed and you’ll do well. 5 pool was played as recently as 17 July in Effort vs. Shuttle on the first day of the ASL (Afreeca Starcraft League). (It’s the first game of the cast, starting at 9:37 into the video. The English commentators didn’t count the drones and never realized it was a 5 pool.) Watch the game if you want to see the economic risk of a zergling rush. Effort got lucky and scouted the right way with his overlord, but Shuttle also got lucky and saw the lings in time to make a shield battery and pull the minimum number of probes to hold. Effort struggled for the rest of the game to catch up, but despite strong aggressive play in the middle game never drew even.

6 pool, 7 pool and 8 pool are considered inferior. They are seen as giving up economy without enough payback: The zergling attack at the later timing is too weak to be threatening.

9 pool is no longer a rush but a mainstream build. With 9 pool and zergling speed, the lings have greater potential to run by defenses, they can catch and kill fleeing workers, they can switch between targets with agility, and they’re generally more dangerous. And even this non-rush build is economically risky except in ZvZ. Overpool (overlord at 9 drones, then spawning pool at 9 while the overlord is in the egg) is the fastest build that is common outside ZvZ.

If you always play the same strategy, then you set yourself up for a counter. Bots with strategy learning will learn to counter you. If you always play the same cheese strategy like 4 pool or 5 pool, with standard hard counters, then you set yourself up to lose every game. Zergling rushes are sound, but not as your only opening.

If your bot has strategy learning, then I think it deserves at least one cheese opening to punish weak or greedy opponents. Zergling rushes are only one possibility—they are vanilla cheese, if the metaphor doesn’t make you gag. Tomorrow: More cheesy goodness. Cheese is great, cheese is good, and we thank it as our food.