Starcraft AI blog

I decided to write up Steamhammer’s changes in parts. Scouting today, more stuff tomorrow. I wrote briefly about scouting improvements in September; here are the details. The scouting changes affect all races, though of course only zerg has an overlord to help. The overall effect of the changes is that Steamhammer collects more information sooner and makes better decisions, so it was worth the effort.

Steamhammer has suffered its usual ratings crash in the new version. I think the voters have the journalistic instinct to comfort the afflicted and afflict the comfortable, and top bots and new versions of well-known opponents count as comfortable. They say to themselves, “Let’s vote in a good challenge,” and in Steamhammer’s case succeed.

Even so, I’m satisfied with early results. I see plenty of familiar misbehaviors, and no surprise bugs. And play improvements are showing. For example, Microwave has opening learning, and learned that 5 pool usually beats Steamhammer’s default ZvZ opening mix. In Steamhammer-Microwave, Steamhammer played a gas-first opening that formerly would have lost instantly to the rush, but this time saw it coming and built a sunken in time to stay alive. It became a good game instead of a smash.

early game scouting

Classic Steamhammer scouting, from December 2016: Pick a base on the map, and send both the first overlord and the scouting drone to that base. It was a hack. The overlord waits there for the rest of its life whether the enemy is there or not, and the scouting drone keeps looking if necessary.

Good scouting: Send the overlord and drone to different bases. Redirect one or both depending on what is seen. Send later overlords to good locations to collect further information.

Steamhammer’s current scouting is in between. It uses only the first overlord and the scouting drone, and against terran doesn’t send the overlord because it still doesn’t understand how to keep it safe from marines. The overlord and drone go to different bases, so that the enemy is often found sooner. When the enemy base is located, the overlord changes its path if necessary to go there, so there is often a better view of the enemy’s play for a long time. If the overlord finds the base first, the drone either goes home or stops by to visit the base, depending on its orders.

I added only one clever feature, inference of the enemy zerg’s location from seeing its overlord. When Steamhammer sees an enemy overlord early in the game and the enemy base has not been located, it considers “what bases have I not yet scouted?” It calculates which of those bases the overlord might have come from, based on its position and top speed. If that narrows it down to one base, then “Bingo! Found you!” This doesn’t quite extract all the available information; it’s possible that seeing the overlord could eliminate one base and leave two possibilities, and Steamhammer doesn’t realize that. But that’s a rare case, and in practice the overlord sighting speeds up scouting fairly often. The feature was definitely worth adding, and it helps terran and protoss too.

I want a more general scout manager instead of the current one that is able to scout with one worker and one overlord. It should be: Dump whatever units you feel are free into the scout manager, and it will figure out how to employ them. I decided it was too much for now, but I’ll probably do it later.

middle game scouting

Most bots, I have noticed, scout only to see what’s up. Even Zia, which scouts in the middle game with a zergling, only sends the zergling to see stuff and doesn’t attack with it.

Purely out of orneriness, I decided to do middle game scouting differently. Steamhammer’s middle game scouting is reconnaissance in force.

When there are “enough” ground combat units for the main ground squad, Steamhammer redirects a small number of them into a squad named Recon. The choice of units depends on race, of course, but only certain ground unit types are considered suitable for Recon. The terran Recon squad can include medics, and there are actually remaining bugs in that. If the main squad loses too many units, Recon may be disbanded to join it.

Recon is another combat squad, and it gets orders to attack bases where the enemy is not known to be. Usually it discovers that the enemy is still not there, and it gets new attack orders. But if it sees enemy presence, then its orders remain the same; the combat simulator decides whether to attack or hold ground. Stray enemy units and naked expansions are at risk. If Recon holds ground too long, then its orders time out and it is given a new location to attack.

I added a couple parameters to squad behavior so that Recon can do its job better, while the main combat squad does its job as always. I’ll write that up tomorrow.

Recon usually moves around pretty fast and travels over most of the map, though it does get stuck sometimes holding ground. The effect of the Recon squad is that Steamhammer finds enemy expansions quickly and has much more vision of enemy army movement. It occasionally prevents expansions or catches enemy scouts or tears down misplaced buildings in the middle of the map, but my feeling is that that is less important. I’m curious to see how Recon copes with terrans that sow the map with spider mines. Recon does get dedicated detection when it is available, but the detector often lags behind. I may need to make changes if it struggles against spider mines.

other scouting-related stuff

• A new feature controls the damage of scouting misses, when the scout worker is lost before finding the enemy base. In past versions, that caused disastrously bad play when the early combat units came out: They scoured the map indiscriminately looking for enemy presence in unlikely corners, and were often out of position when the enemy came knocking. Now Steamhammer recognizes that is is likely to find the enemy in a starting base location, and looks there first. The enemy is found much sooner and the combat squad is out of position for a short time or not at all. It is more aggressive than Killerbot’s approach of leaving the combat units at home until the enemy is found.

• The gas steal works, though this version doesn’t use it. You can configure "go steal gas" and Steamhammer will find the enemy base and try to steal the geyser. Or "go scout" and "go steal gas", and the scout will steal gas and, if it still exists, continue to circle the enemy base. And some other combinations. Getting the gas steal to work for all races and cases was time consuming; the number of bugs was incredible.

• The previous Steamhammer version has a misbehavior when the scouting worker is on its way home: It stops to attack anything stationary that it runs across. It is the standard worker self-defense behavior popping up at an inappropriate time. Fixed.

• When doing worker harass on a map with other than 1 geyser in each main, there was a potential crash. It never happened, but I fixed it.