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.