Starcraft AI blog

Today’s game is Steamhammer vs. WOPR by Soeren Klett on Andromeda. The game is not only epically long, it is epically clumsy on both sides.

WOPR plays a mech strategy that is slow with air defense. When Steamhammer opens with mutalisks, it flicks WOPR aside. This game, Steamhammer randomly chose to open with a low-economy lurker rush, and momentum went the opposite direction. WOPR had just enough scans to stop the initial lurker attack (a cleverer or more persistent attack might have succeeded). Then zerg strangely decided that its next tech choice should be hydralisks. Steamhammer had seen enough tanks that it should have known it was making a bad decision, so I think the error was not in analyzing the unit mix, but in choosing the tech target based on the analysis. Yesterday I rewrote StrategyBossZerg::chooseTechTarget() in a way that fixes 2 subtle bugs and improves the intended behavior, and I hope that will solve the problem. Anyway, WOPR eventually went on campaign and wiped out the zerg natural and main and the in-base mineral only, while cementing its huge lead by expanding across half the map. All that was left was to clean up a few underpopulated zerg expansions.

In the picture, the zerg economy remains weak and the army is hopelessly outmatched because it has suffered continual losses due to poor unit mix and poor tactics. In a human game, zerg could give up. 2 new terran expansions are visible on the minimap. Also notice that there are 3 evo chambers even though Steamhammer only knows how to upgrade with 2—it’s another bug that I fixed recently.

Like many old-school bots, WOPR believes it is finished when it has destroyed the enemy main. It does not seek out remaining bases, but sends its army toward (0,0) in the upper left. But WOPR does move its army to react to enemy units that it sees, and it continues to take bases and defend itself. Terran was maxed and zerg was extremely weak, with 8 surviving drones and the only tech a spire built at the last minute before the hive was destroyed. In the minimap, red is everywhere and blue survives in a corner. (Light blue on the minimap represents neutral buildings.)

Steamhammer knows how to recover. It slowly rebuilt its economy and tech and made mostly air units, especially scourge since it had seen battlecruisers. When the few zerg units showed themselves, an overwhelmingly strong column of terran units appeared and chased them away. Even constrained to ignore the zerg bases, terran had a big advantage. If zerg built up fully and also maxed its army, terran still had more bases and had been banking resources for a long time. WOPR only had to hold firm until the time limit was reached, and it would be awarded victory on points.

After the above picture, the battle started to get interesting. The scourge scored more victories than losses against the battlecruisers and cloaked wraiths. The first ultralisks were annihilated by tanks with +3 attack, but zerg was starting to do some damage. Of course WOPR had a huge reserve of minerals and gas, and it instantly replaced lost units. But then zerglings and one ultralisk found their way into the terran 9 o’clock base and destroyed it before it had quite mined out. Terran forces had been distracted by air activity at the center base, and most of the terran army remained guarding the unimportant upper left. It looked as though zerg had a chance after all; WOPR’s tactical reactions were not accurate enough. Notice the zerg supply continuing to increase. WOPR replaced many of its lost workers with combat units, which was correct play because terran had a big bank and didn’t need the income.

Steamhammer attacked the upper left natural and didn’t quite finish it off. Then the ray of hope started to dim. Steamhammer made too many guardians and was in danger of losing them all to the battlecruisers. Then hope brightened: WOPR played poorly and Steamhammer had the sense to follow up with hydralisks and scourge, and the battlecruisers exploded. WOPR targeted hydralisks though battlecruisers can one-shot the more dangerous scourge. Zerg destroyed the upper left natural. Then hope dimmed again: Steamhammer decided that the next target had to be, not an undefended base, but the upper left main where the giant terran army was hanging out. In the picture, the zerg army could not even get close before it was forced back by terran reinforcements appearing to its rear. Still, notice that zerg has retaken its main and mineral only and is now taking the formerly terran upper left natural. WOPR retook 9 o’clock and finished mining it out, but was otherwise not as economically active as it should have been. The zerg economy was in high gear and zerg was ahead in minerals but hurting for gas.

Both sides showed messy tactics and micro. Steamhammer made a few devourers for help against the battlecruisers and wraiths, but used them poorly. Zerg lost workers after taking the indefensible center base (which it had earlier kicked out of terran hands). Steamhammer also made many indecisive movements and often chose the wrong targets. WOPR played worse, though. It kept the goliaths in the corner surrounded by siege tanks, so the massive air defense was out of range of attacking guardians. Guardians slowly ate through the rows of tanks and finally killed the command center. Hydralisks, despite some confused behavior, kept terran reinforcements at bay though they couldn’t approach the mass of tanks.

Having killed the upper left command center, Steamhammer felt no need to engage the remaining terran army but set about the task of clearing the other terran bases. At least that part worked well. WOPR was less efficient and burned through its entire bank of minerals. Steamhammer won without ever maxing its army.

It was 46 minutes and full of events, many of which I didn’t mention. As one example, you can see on the minimap in the last couple pictures that Steamhammer built 2 macro hatcheries in a strange position outside its original base, where WOPR had taken the natural. I’m not sure what caused that.

By the way, this is the kind of game where either player would have benefited from being able to take island bases. If Steamhammer knew how to take islands, it could have made 2 more bases and would have recovered faster. If WOPR took islands and turreted them up, they could have been defended indefinitely by air units. To destroy a strongly defended island base you need a coordinated large-scale attack which no bot has the skills to pull off.