Starcraft AI blog

You’ve located an enemy refinery building and you want to know whether your ground units can walk there to attack it. Is it on ground that they can reach, or on an island? For other buildings, Steamhammer solves the reachability problem with a partition map: Areas reachable by ground from each other are in the same partition, and 2 simple lookups tell it whether one point is reachable from another.

Refineries are trickier. First of all, a geyser is not walkable ground, so properly speaking it doesn’t belong to any partition. The same for a refinery built on it; every other building is built on ground that can be walked over after the building is destroyed. This actually causes a rare bug in Steamhammer, where the ground squad understands that it cannot attack an island—except for any refinery that may be on the island, it may attempt to attack that! It knows that a refinery building is a special case, but it doesn’t understand the special case correctly.

Of course, the problem can be solved by filling in the partition map for the refinery with the partition that the geyser is in. “Of course”? No, actually it can’t: The geyser itself might be part of the unwalkable terrain between partitions. Imagine a wall on the left and a wall on the right, and a geyser plugging the gap between them. North of the geyser is one partition, south might be a different partition, and there is no way to walk from one to the other. The geyser, and any refinery you build on it, is adjacent to 2 partitions but doesn’t belong to either.

This actually happens in an even more complicated way on maps like Gold Rush and a number of others, where two assimilators form a gate. An assimilator is smaller than a geyser (protoss space warp tech, no doubt), so if you destroy the eggs between the assimilators, units can pass through. But if you destroy the assimilators the gap disappears and the bare geysers block passage. A bot needs sophisticated knowledge of the game to understand the effects.

I haven’t chosen a way to fix it yet. But it’s clear that geysers and refineries have to be treated with special care.