Starcraft AI blog

LetaBot’s wall code is hard to follow, so I’ll take it in bite-size pieces. Today, deciding where the wall should go. It’s likely to be a good location for other defenses, too.

LetaBot can calculate how to wall off its main base with a barracks and 2 depots. It’s the classic terran wall. Unfortunately it doesn’t seem to recognize cases where fewer buildings are enough. The call to calculate the building locations for the wall is BuildingManager::WallOff(). If successful, it stores the calculated locations in the object for later use. From the BuildingManager class declaration in BuildingManager.h:

	bool WallCalculated;//True when the map calculation is done
	bool WallSound;
	BWAPI::TilePosition BarracksWall;
	BWAPI::TilePosition SupplyWall1;
	BWAPI::TilePosition SupplyWall2;

Below, I left out some confusing redundant code plus code with unrelated purposes.

void BuildingManager::WallOff(){

	BWAPI::TilePosition TileChoke;
		
	TileChoke = BWAPI::TilePosition( InfoMan->PosMainChoke );

It’s looking up a location recorded in the InformationManager. Here’s how InformationManager calculates it, at first naming the target choke BunkerChoke.

	BWTA::Region* natRegion = BWTA::getRegion( PosOurNat );
	BWTA::Region* mainRegion = BWTA::getRegion( PosOurBase );

	BWTA::Chokepoint* BunkerChoke = NULL;
	BOOST_FOREACH( BWTA::Chokepoint*  choke, BWTA::getChokepoints() ){
	   if( (choke->getRegions().first == natRegion && choke->getRegions().second == mainRegion) ||
		   (choke->getRegions().second == natRegion && choke->getRegions().first == mainRegion) ){
			BunkerChoke = choke;
	   }
   }

It walks through the choke points as reported by the Brood War Terrain Analysis library BWTA until it finds one which is between the main and the natural. In theory there could be more than one, but it doesn’t worry about that. The natural (at PosOurNat) is calculated earlier as the gas base which is closest to the main by ground distance. That should be fine for standard competitive maps, including maps with an internal mineral-only like Andromeda and Electric Circuit.

	if( BunkerChoke == NULL && mainRegion != NULL && natRegion != NULL ){
		int closestNat = 99999;
		BOOST_FOREACH( BWTA::Chokepoint*  choke, natRegion->getChokepoints() ){
		   if( BWTA::getGroundDistance(  BWAPI::TilePosition(choke->getCenter() ), OurBase ) < closestNat ){
			   BunkerChoke = choke;
			   closestNat = BWTA::getGroundDistance(  BWAPI::TilePosition(choke->getCenter() ), OurBase );
		   }
	   }
   }

If that doesn’t succeed because the map has no choke between the main and natural, it tries again. It could happen if the natural is internal to the main’s region or if the closest gas natural is farther and there are intervening regions (which is likely if the closest next base is mineral-only, as on Nostalgia or Korhal of Ceres). (It also fails on an island or semi-island map, but presumably LetaBot is not intended to play on those.) The code considers the chokes to the natural. It walks through these chokes and picks the one that is closest to the start location (OurBase) by ground distance. I’m not sure how well it works, but that’s what it says. It may mess up if there is an intervening region between the main and natural and the intervening region has another entrance. My first thought is to consider the chokes out of the main and do a path analysis: Go through the choke to the next region; from there, compute shortest ground paths to the possible enemy base locations; if none of the paths involves returning back through the main, then that choke is toward the enemy and may be worth walling.

   if( BunkerChoke != NULL){
     PosMainChoke = BunkerChoke->getCenter();
   }

If one of the two methods worked, then PosMainChoke is the center of BunkerChoke, all done.

Discussion. It looks like some maps will confuse the wall locating code, especially non-standard and old-school maps. If there is an internal natural, or an entrance to the main other than through the natural, LetaBot may do something unhelpful. For example, Alchemist (as played in CIG 2016) has two entrances to each base. It’s a 3-player map arranged as a big circle, so that the enemy is closer to one of your base entrances than to the other. This code will choose the natural base with its wall location sometimes toward the enemy and sometimes away—it probably should put the wall toward and the natural away (there are bases outside both doors). In another example, the old tournament map Bifrost has a front entrance to the main plus an internal mineral-only with a back entrance. Hypothetically, if we gave the internal base a geyser, then this code would build an unhelpful wall between the main and the internal natural, allowing enemy units that came through the back entrance access to the ledge above your mineral line. The right plan is to block the front and back entrances separately.

Even choosing a wall location can be tricky! LetaBot is optimized for the usual case, and nothing is wrong with that. It would be extra work for little gain to handle all cases well.

The name BunkerChoke is a hint: A wall is only one form of defense. The same choke may be good for other defenses, whether static defense or units standing guard. Even smart bots sometimes misplace their early defense. For example, watch Zia on Andromeda place early zerglings on guard well back from the ramp in the “choke to the main,” because it treats the high-ground mineral-only as a separate region. Or watch Skynet on Benzene misplace its early zealots, because there are two entrances to the main and Skynet does not realize that one of the entrances is blocked by map buildings. It may seem like an easy decision, but it’s hard.