Starcraft AI blog

Here is Steamhammer’s most hilarious bug. When facing Jakub Trancik’s cannon bot, Steamhammer opens with 9 pool, a sunken, and then an expansion. The idea is that 9 pool counters the cannon rush; zerglings should be out before any cannons finish. If that somehow fails, the sunken prevents cannons from completely destroying the base, and the expansion gives zerg a foothold on the other side of the cannons; Steamhammer should be able to go destroy the protoss main. I hadn’t tested it, but with 3 separate counters to proxy cannons I felt confident. I was wrong, though. As it turned out, Steamhammer went 1-1 versus Jakub Trancik in the round robin phase of SSCAIT 2016.

In the losing game, here’s the probe trying to cannon up the zerg base. The probe took 1 sunken hit, escaped and started a pylon, then the probe and pylon immediately died. The defensive plan worked exactly as intended.

So how did Steamhammer lose? In the minimap, you can see Steamhammer’s overlord headed toward the northwest and drone scouting the southwest. The protoss main hadn’t been seen yet. When it saw a pylon warping in its base, Steamhammer applied a heuristic: I see an enemy building in this base location, so—this must be the enemy base! The scouting probe immediately returned home and starting running loops around the home base, because it was the enemy base too, and the scout never saw the enemy’s real base. It’s not entirely foolish behavior if your enemy likes to proxy! Zerglings explored around and found “another” enemy base filling up with cannons, and started to build up numbers in preparation for an eventual attack. The mistake didn’t look too bad so far.

Then Steamhammer built an extractor on its gas. Since the extractor was in enemy territory, it was obviously a gas steal, so the bot did not collect any gas. The drones will die in the enemy base, right? The zerg build was not able to progress, Steamhammer stopped building anything, and Jakub Trancik eventually did its zealot switch and won, gg.

Here is the unused “gas steal” extractor. That drone moving up the right is the scouting drone, still circling the “enemy” base. The minimap shows zerglings out exploring, looking for the “last remnants” of the protoss. You can see 2 zerg expansions; Steamhammer 0.2 is slow at it, but it set about methodically building up.

All those consequences from 1 flaky heuristic! I’m still laughing over it—I get to laugh because the fix is a 1-liner and I already made it in December. The heuristic and the bug are inherited from UAlbertaBot, but I have never seen the bug in UAlbertaBot’s play. I’ve also never seen the heuristic fail any other way, although it can. On the one hand, it’s a critical bug that can cause losses; on the other hand, it’s not that important to fix because it’s rare. Bugs are weird that way.

Next: More bugs and weaknesses (they won’t all get such a thorough writeup, though).