Starcraft AI blog

Steamhammer and Zia tied for 16-17th place in the round robin phase of SSCAIT 2016, and played a best-of-3 tiebreaker match to decide who would move on to the round of 16. The video is out with commentary, and here are the games. They’re all somewhat entertaining, if you can tolerate weak bot ZvZ. The caster seemed unfamiliar with bot play, so I’ll explain the reasons behind some of the events.

Game 1 was on Destination. Zia had learned from past games that its most successful opening was an aggressive 9 pool, and that’s how Zia opened. This version of Steamhammer follows a fixed 12 pool build, which is riskier on this 2 player map. When Zia’s initial 6 zerglings arrived at its base, Steamhammer was just about to produce its first 4 zerglings, so Steamhammer pulled drones to defend. With drones and 4 zerglings, Steamhammer should have defended safely. But Steamhammer tends to be overeager with its drone defense and pulls its drones too far out; it should defend in the mineral line, or at least on the creep, but it pulled drones most of the way to the ramp. (This is one of its most severe weaknesses, one step behind the absolutely-must-fix problems.) When Steamhammer’s zerglings followed the drones moments later, the drones got in their way, and too many drones were lost. Zia was far ahead, cautiously built up, and won easily.

Game 2 was on Circuit Breaker, a 4-player map where the aggressive 9 pool is less likely to work. Also, the players spawned in cross positions, so the rush distance was longer. Nevertheless, Zia had learned that it worked, so Zia played the aggressive opening. And sure enough, Zia arrived with more zerglings and Steamhammer again defended poorly, frittering away its lings against a larger force instead of massing for a short time before engaging. Zia was tearing down the natural hatchery while safely expanding itself, and the game seemed to be decided. Then Zia’s remaining zerglings settled on the wrong targets, one attacking an egg and another a larva. The hatchery was deep in the red and would have died if attacked by those 2 lings. Instead Steamhammer’s next wave cleared the invaders and the hatchery survived with 34 hit points, a sliver of red on its health bar. Then Steamhammer had both economic and tech advantages and went on to win. Steamhammer does not make the same targeting mistake, so you could say it was a deserved win, but it was no less a lucky win.

Game 3 was on Fighting Spirit. Zia’s learning algorithm said “uh oh, that didn’t work, better try another opening” and picked a defensive 9 pool instead. Zia massed its zerglings but held them on its ramp for safety. Steamhammer’s opening gave it more larvae, a stronger economy, and a faster tech path, all at the cost of fewer and later zerglings, so Zia’s choice to stay home was poor. Then Zia made another mistake when it wanted to expand to its natural. It should have swept its ling force out to secure the natural, but instead pulled back to the main hatchery to let the expansion drone out. Only after Steamhammer sniped a few expansion drones did Zia change its mind and attack across the map. Steamhammer defended with its characteristic incompetence, overeagerly pulling all drones and losing every one, but mutalisks were out and cleaned up.

Zia has a sneaky fallback when it repeatedly fails to take its natural: It takes a hidden base instead. This time it took the center base on Fighting Spirit. Zia is capable of rebuilding from a hidden base and the cash for a drone or two, but of course it takes a long time. Steamhammer’s mutalisks abolished Zia’s main and then set out across the map to find remaining buildings. Steamhammer’s search behavior (inherited from UAlbertaBot) is inefficient and looks strange, and the caster was understandably confused about it. The algorithm is simple. Steamhammer keeps track of when it explores each square of its map grid, and picks one of the squares that it has least recently explored as the next destination. It’s good enough.

Steamhammer will play LetaBot in the round of 16. The pairings are 1-16, 2-15 and so on, the arrangement with the least likelihood of upsets for the top finishers. Nevertheless, Steamhammer does defeat LetaBot occasionally and has a small chance to move on to the round of 8. If Steamhammer pulls off the upset, it will almost surely lose in the round of 8—it loses 100% against Wuli’s 9-9 gate zealot rush, and nearly as often against Overkill by Sijia Xu. I’ll borrow the bracket image.

The round robin phase of SSCAIT 2016 has just finished. Seconds ago, Matej Istenik beat OpprimoBot in the last remaining game. Did Steamhammer make it into the round of 16?

It was a nailbiter! Only fiction gives us closer decisions. As the end approached, Steamhammer and Zia were tied for 16-17th places, each with 1 game remaining to be played. One would be in and one out! As the last dozen or so games came up, Zia was scheduled to play MegaBot, and after 2 intervening games—just enough time to enjoy the tension—Steamhammer to play Tscmoo protoss.

Zia had beaten MegaBot in their previous game, and easily won again. In the previous Steamhammer-Tscmoo game, Tscmoo had opened forge expand; Steamhammer did not know how to adapt its build and lost. (That is why Steamhammer opens three hatch before pool against XIMP and ZerGreenBot.) Tscmoo is unpredictable.... I thought the odds were poor, but I knew anything was possible. This game Tscmoo opened 14 nexus and then gas before gateway, a crazy greedy opening, and went down like a house of cards. What a way to deflate expectations of an exciting decision!

Steamhammer and Zia finished in a tie for 16-17th places. I don’t know how tiebreaking will work. Their result against each other was (I’m sure you already guessed this) 1-1, so that doesn’t help. Maybe they’ll have a playoff game. Either way, I’ll enjoy watching the finals and I’m happy that my bot performed so well after such a short development time. Watch out for the next version!

Next: Some of the bugs that came up in the round robin phase.

I had been hoping that Zia would start to choose between its openings, and now that it has I want to see how it’s doing. So I watched a bunch of replays. It’s using strategy learning, though I can’t say in what form. I predicted that choosing between its strategies would be advantageous, and it’s true to an extent.

With more variety, Zia has become more entertaining to watch. I like it. Zia plays these openings that I’ve seen:

• 5 pool
• 9 pool
• 9 pool speed
• 12 hatchery

I didn’t catch it playing overpool or 12 pool, which you might expect to be common.

Zia’s opening chat message gives a hint about its opponent model. It says “Nice to meet you!” for new opponents and “Hi again!” for opponents it has met before. And it either predicts a “harsh game” or claims “I may overwhelm you.” I think it picks the second message when it believes it has found a strong counter strategy.

Against opponents with a single strategy which is directly countered by one of these openings, like ZZZKbot’s 4 pool (hard countered by 9 pool plus a sunken so that the trickle of attacking lings has no chance whatever), Zia seems to learn the counter and should then win every game. Zia even managed to find a strategy that gives it a chance against IceBot—Zia won a game which brought out weaknesses in both bots, weaknesses I didn’t realize IceBot suffered from.

And I see signs that Zia adapts after the opening. For example, I saw it add a spire when it needed scourge for air defense. I get the impression that it decides flexibly between hydralisks, mutalisks, and lurkers for the middle game—at least it’s not hardwired, maybe it’s random, I hope it’s learned. I have seen it play 12 hatch, 11 pool, 10 gas and also 12 hatch, 11 gas, 10 pool; I hope it’s foreseeing how much gas it will want to boot up its future unit mix.

Playing many openings does have a disadvantage: It’s harder to play all of them well. It’s not enough to know the build, you have to know how to play it, and it adds up to a lot of knowledge. The worst is Zia’s 9 pool speed opening, which it plays in a strange way as a late zergling all-in: It makes 100% zerglings until it attacks around supply 20-24; if it fails, Zia doesn’t have enough economy for the middle game. (I expect a 9 hatch build would strike harder if you want to play that way.)

Zia still plays poorly overall, if you ask me. It needs to brush up on skills like not sending drones through the enemy army. It needs better scouting (it doesn’t send out its initial overlord), better tactics (no, don’t run up the ramp to fight the bunker! Hit the SCVs in the expansion first!), better engagement skills (big groups of zerglings should surround before attacking), and better micro (in lings versus zealots, retreat a ling that will die in 1 more hit). And stuff. It’s a hard game.

Zia’s current description is “Implementing more strats . . .” I guess the author has the most fun with that, which is all that really matters, but it’s not the way to a winning bot. Breadth of skills, not depth of skills: You gain more by reducing your weaknesses than by increasing your strengths. Zia already has relative strength in strategy, and will improve most with other skills.

Hmm, I should write a post about The Winning Attitude for authors of game programs. Only for those who seek the winning attitude, of course; it’s optional.

Tomorrow: Novelty maps.

Zia was reuploaded today. Its new description says “It has memory, and is trying to play like people do.” It sounds like it can now choose among its strategies as I hoped. It will take a lot of games to see how well it works....

Zia has followed a wandering course. The early versions opened 12-hatch and went mass mutalisks. Then it switched to a 9-pool opening. Then a fast rush opening. Now it is back to an economic opening, this time with mass lurkers and lings.

I suspect that Zia is collecting strategies. Some future version may have all the strategies available as choices. If it selects well, it will become a force to reckon with.

Zia’s mutalisk cloud is scary when it gets big. Eventually the mutas not only one-shot the units that they target, but their bounces instantly kill nearby units. The mutalisks sweep a path of destruction. But think about it—is that efficient? If mutalisk bounces at 1/3 power kill instantly, then the main attack must usually be gross overkill. Most of the firepower is wasted.

The idea of individual mutalisk control, as introduced by the Berkeley Overmind and copied by other zergs since, is to waste no firepower. Each flier independently dances in and out for safety and ideally attacks at near its maximum rate. But watch how Tscmoo zerg implements this: Its mutalisk cloud is also scary when it gets large, but usually not as scary as it could be, because it spreads out too much. Sometimes half the mutas are posing for pictures with the ground army while half are on the job. And the attackers often pick some targets over here, some over there, and don’t kill either as fast as they should. Tscmoo doesn’t focus its fire enough; it’s the opposite mistake from Zia.

Causing damage does not win games. Maximizing your damage output is not the winning move. You want to balance between killing the most important enemies and staying alive.

Try to imagine PerfectBot’s muta micro. Even PerfectBot can’t truly play perfectly, because calculating optimal micro is infeasible. But surely PerfectBot focuses fire efficiently, switching mutas fluidly between targets, taking into account importance and time to kill based on distance and damage rate and expected losses, to reduce overkill to near zero and spend less time flying between targets and strike a good balance between killing the most important stuff fast and staying alive. “This takes 5 more shots to kill, 12 are shooting, might lose 1, so switch 6 to new targets.” Zia and Tscmoo zerg are no competition for Jaedong, but I think Jaedong would boggle at PerfectBot’s mutalisks.

How close can we get to PerfectBot micro today? 1. Given a set of targets in priority order, calculating how to focus them down efficiently with minimal waste seems intricate but ultimately not that hard. 2. Folding in a desire to also minimize losses makes optimal decisions computationally infeasible. Even approximations seem tough. 3. Prioritizing the targets depends on the total game situation and will have to be done heuristically. For now I guess we’ll have to settle for a simplified algorithm.

Watching Zia last week, I thought it picked targets usually one at a time (simple 3) and once the target was chosen ignored damage taken while chasing it down (very simple 2), so the intricate-but-not-hard efficient killing calculation by itself should be a big improvement. Zia-this-week has been updated and has fancier micro than Zia-last-week, so I’m already behind the times! I got the impression that Zia-this-week is better about picking targets and switching targets and avoiding damage, but that it still wastes shots with too much overkill.

Speaking of zerg being easiest, I’m pleased with the aggressive new zerg bot Zia (games cast by Nepeta). Zia’s description says it’s “just for fun.”

Zia goes for middle game play with mutalisks and zerglings, complete with upgrades that eventually reach 2-2. (One game in the video also has a hydralisk switch which did not seem advisable.) Other mutalisk bots follow the lead of the Berkeley Overmind and spread out their mutas for (what is advertised as) maximum overall damage rate. Zia doesn’t seem to care about bot fashion. Zia concentrates its mutas into a bunch more like a human player, which may be less efficient in creating damage from an operations research point of view but is aggressive and can be more efficient at actually killing targets—and that’s how you win games.

Attack-oriented opponents, even strong ones like Iron and tscmoo terran, seemed unable to defend against so many mutas flying so aggressively and backed by lings. Defense-oriented bots like LetaBot (which holds its base with active defense and saves up its strength for mighty strikes) didn’t have much trouble, though. I also saw Krasi0, a hardened veteran that learned mutalisk defense facing the Berkeley Overmind, adapt effortlessly and win.

I like Zia, but by now you must realize that I’m not psychologically capable of finishing a post without asking for more. Mutalisk targeting looks a little haphazard—too much zipping about, hitting the wrong enemies, and taking incidental damage. When you concentrate your force, it becomes much more important to concentrate it in the right place! One thing the Berkeley Overmind did right was to fly around the perimeter of the defended area looking for weak points. Also the lings are not attacking as much as they should, as if they were sometimes ordered to move while next to a target. Would that be fun stuff to improve?

Update: Zia starts +1 attack for its mutalisks before spending on the first mutalisk. That’s dedication!