Starcraft AI blog

I have a feeling that some of the new behavior of Killerbot is designed specifically to counter Iron. Well, I could be wrong. It seemed interesting so I played some games between the two locally, with the 1 December version of Iron and the 27 November version of Killerbot. It was also a chance to see how the two cope with maps outside the SSCAIT pool.

This version of Iron scouts immediately, with one of its first SCVs. That’s a hit to its economy. I guess author Igor Dimitrijevic decided it was important to start adapting to the enemy’s build as soon as possible.

On many maps Iron tried its signature 3-vulture runby. When the runby succeeded, Killerbot could eventually clear the intruding vultures with mutalisks, but was far behind and lost every time. On the map Gladiator, which has an up-ramp out of the low-ground main, Iron felt the need to bunker its ramp and tried the runby later than usual. Killerbot had placed sunkens well and Iron aborted the runby in mid-flight (a skill in itself!). But then Killerbot was ahead and could even expand to the precariously-placed mineral only, with a winning advantage.

In some games, like on Match Point, Killerbot added a sunken in its main. I’m not sure what triggered the extra defense. That blunted the runby, even when the sunken was poorly placed, because Iron was not clever enough at working around it. On Match Point, Iron still gained some advantage and ended up winning a tense game.

I notice that Iron favors citadel defense: It builds heavy static defenses that cover a small area. With repair, the static defenses (bunker or turrets) are expected to hold without reinforcement. Pros usually favor the lightest possible static defense and reinforce with mobile units when needed—that’s more demanding, obviously.

Iron builds its first factory in a distant corner of its base. That way when it gets rushed early, the factory is likely to be overlooked and can get vultures out, which is usually enough to hold off the rush. It’s a clever trick that works well against bots. But later when Iron comes under mutalisk attack, the factory is outside the citadel of turrets and becomes hard to defend.

Killerbot’s mutalisks are attracted to turrets and do not fight them efficiently. In this picture most mutas are attacking the rightmost turret. They surrounded it and are taking fire from all sides, when they should have attacked from an angle where not all turrets were in range. Meanwhile, a couple mutas do not play well with others and attack different turrets on their own.

On Arcadia II, Killerbot surprised me with a strategy I hadn’t seen it play before. It opened with its usual 3 hatcheries and then, I can’t guess why, took drones off gas after 100 and went all-in speedlings. Iron saw it coming and bunkered its ramp, but it was not enough. The mass zerglings surged over everything. The red lines on the minimap are more zerglings on the way, far more than Iron can hope to save itself from after the bunker falls. I won’t be surprised if this strategy features more prominently in the next tournament version of Killerbot!

Killerbot depends on an easily-defensible natural expansion. On Outsider, a rather weird map, it didn’t even try to defend its wide natural and soon lost all drones. Fantasy and Jaedong played a famous game on this map in 2009. I think it is a difficult map for bots; you need to understand mineral jumping and drop play.

I even laid in a game on Jungle Story, an old-school map from before modern macro play was invented. The map was cool at the time but today is very non-standard. It has no close natural; the closest next base is an open mineral-only. The game was hilarious: Killerbot took the center gas expansion and defended it with 1 sunken. That base has 2 openings and Iron could have harassed through the opening not covered by the sunken. Or Iron could have bypassed the expansion and headed for the main. Instead Iron ran by the sunken, through the expansion, and toward the main, losing 2 vultures of the 3 for nothing! Apparently its decision-making is tuned for standard maps. Killerbot’s mutalisks erased the terran main.

Usually when Iron worked up to wraiths, Killerbot found itself unable to defend both its overlords and its mineral lines: Pick one, give up the other. Killerbot would benefit from scourge for air defense, or maybe a spore colony in the mineral line at some bases (it already makes an evo chamber).

Overall conclusion: It was fun because both bots are super-aggressive. Iron has better micro and a deadly runby tactic, and usually wins. But when Iron blunders, Killerbot takes over. Both bots could benefit from more geometry smarts in figuring paths and placements. I was surprised that Killerbot does some basic research very late. It fights more than I expected with slow zerglings and slow hydralisks.

Last year, Killerbot was clearly the best around. If it hasn’t kept up, that is a sign of how much progress we’ve seen this year. Here’s to another year of progress!