Starcraft AI blog

TitanIron is, as all signs indicated, a fork of Iron. It forks from the latest Iron, the AIIDE 2017 Iron. The Iron that played in CIG 2018 was carried over from the previous CIG 2017 tournament, and is an earlier version.

#15 TitanIron crashed in 30% of its games. Its win rate was 51.46% overall, or 73.59% in non-crash games. #6 Iron itself (an earlier version) finished with 74.31% win rate, so TitanIron does not seem to be an improvement, even discounting poor code quality. Curious point: #9 LetaBot upset Iron, because LetaBot copes well with vulture and wraith harassment. But TitanIron upset LetaBot. Another curious point: TitanIron performed poorly on the map Andromeda and strongly on Destination, and about equally well on the other 3 maps. Andromeda seems a surprising map to have trouble with.

I watched some replays. In Iron-TitanIron games, the two played identical build orders until the first factory finished, when Iron made 1 vulture first while TitanIron immediately added a machine shop to get the vulture upgrades faster. The bigger difference came later, when Iron built a starport and made wraiths while TitanIron did not. I got the impression that TitanIron rarely or never goes air. The expense of going air puts TitanIron ahead in vultures for a while, so that it won some games, but it seemed that if the vulture pressure did not push Iron over the edge, then Iron would strike back and take the advantage. I watched only 1 game Locutus-TitanIron, because Locutus’s proxy pylon trick misled TitanIron just as it does Iron, and Locutus won easily. I watched a strange game against AIUR where TitanIron built a second command center far from its natural, slowly floated it over, left it in the air, and built a new command center underneath. Not all the bugs are crashing bugs. In the picture, TitanIron is losing to AIUR. Notice the nicely spaced tanks, the spider mines directly next to one tank, the barracks floating in an unhelpful position, and the spare command center in the air.

Overall, my impression is that TitanIron’s play is often similar to Iron’s. Unlike Iron, it does not make air units (it seems to have drop skills, but I didn’t run into any games with drop). Against protoss, TitanIron makes more tanks and uses them more cautiously and often clumsily. TitanIron also seems a bit fonder of expanding and growing its economy.

TitanIron adds over 4,000 lines of code to Iron. It was made by a team of 10, so that’s not an excessive amount of new code. The crash rate and the score suggest that the team was not disciplined enough in code quality and testing (of course Steamhammer crashed even more, so I don’t get to brag). Read on and you’ll see what most of the new lines of code do. I question the choices of where to spend effort. I’m not sure what the plan behind TitanIron was supposed to be.

openings

Iron does not play different openings as such. Conceptually, I see Iron as playing one opening which it varies reactively. TitanIron adds a directory opening with code which allows it to define specific build orders. The build order system is loosely modeled on Steamhammer’s, using similar names (which are not the same as UAlbertaBot’s names)—some members of the team have worked on Steamhammer forks.

TitanIron knows 3 specific build orders, named 8BB CC (1 barracks expand), SKT (tanks first), and 5BB (marines). Based on watching replays, TitanIron retains and uses Iron’s reactive opening, with modifications.

opponent-specific strategies

Iron does not recognize opponents by name. TitanIron recognizes 2 specific opponents: Locutus and PurpleSwarm. The zerg PurpleSwarm is a curious choice, since it did not play in CIG. Maybe they found it an interesting test opponent? In any case, Locutus is the main focus. It is recognized in 4 strategy classes, Locutus, SKT, TankAdvance, and Walling. In Iron’s codebase, any number of strategies can be active at the same time, and other parts of the code check by name which strategies are active to suit their actions to the situation.

	Locutus::Locutus()
	{
		std::string enemyName = him().Player()->getName();
		if (enemyName == “Locutus” || enemyName == “locutus”)
		{
			me().SetOpening(“SKT”);
			m_detected = true;
		}
	}

SKT (defined in opening/opening.cpp) builds a barracks and refinery on 11, then adds 2 factories and gets tanks before vultures. It sounds as though it should refer to the “SK terran” unit mix of marines and medics with science vessels and no tanks, but it doesn’t. The Locutus strategy turns itself off (if I understand the code’s intent correctly) after all 4 dark templar of Locutus’s DT drop are dead, or after frame 13,000. Various buildings (barracks, factory, e-bay, turret) recognize when the Locutus strategy is active and carry out scripted actions. The name “Locutus” also activates the TankAdvance strategy which seems to first guard the natural and then perform a tank push, and deactivates the Walling strategy after frame 11,000 or when above 12 marines, causing the barracks to lift and open the wall.

TitanIron scored a total of 1 win out of 125 games against Locutus, so the special attention does not seem to have paid off.

PurpleSwarm gets less attention. (The question is why it got any.)

	Purpleswarm::Purpleswarm()
	{
		std::string enemyName = him().Player()->getName();
		if (him().Race() == BWAPI::Races::Zerg &&
			(enemyName == “Purpleswarm” || enemyName == “purpleswarm” || enemyName == “PurpleSwarm”))
		{
			me().SetOpening(“5BB”);
			m_detected = true;
		}
	}

5BB (also defined in opening/opening.cpp) builds barracks on 10 and 12, later adding a third barracks and training marines up to 30. I don’t see any other cases where TitanIron uses this opening. The rest of the code has no special instructions for PurpleSwarm or 5BB.

other new files

Besides the opening directory, TitanIron adds 16 files in the strategy and behavior directories, defining 8 strategies and behaviors. The added strategies are:

• GuardNatural
• Locutus
• PurpleSwarm
• SKT
• TankAdvance

These are remarkable for being all and only the classes used when Locutus or PurpleSwarm is recognized. Do they have any other purpose? I didn’t dig into it, but I suspect that GuardNatural and TankAdvance may be used more widely against protoss.

The added unit behaviors are:

• GuardLoc - guard a location
• HangingBase - carry out drops
• SKTAttack - related to SKT

GuardLoc has some connection with GuardNatural, but seems to be a general-purpose behavior, as far as I can tell. I’m not sure how HangingBase got its name.

The new opening directory and the newly added strategy and behavior files account for about 2/3rds of the lines of code added to Iron. The rest is scattered through the code and not as easy to inventory, but surely much of it must be uses of the new openings, strategies, and behaviors. I do see a lot of changes related to expanding.