fix AI jamming up its base by enforcing 1-cell gaps

OpenRA.Mods.RA/HackyAI.cs

@@ -169,6 +169,30 @@ ActorInfo ChooseDefenseToBuild(ProductionQueue queue)
 			return null;
 		}
 
+		IEnumerable<int2> Neighbours(int2 c)
+		{
+			/* only return 4-neighbors for now, maybe add 8s later. */
+			yield return c;
+			yield return new int2(c.X - 1, c.Y);
+			yield return new int2(c.X + 1, c.Y);
+			yield return new int2(c.X, c.Y - 1);
+			yield return new int2(c.X, c.Y + 1);
+		}
+
+		IEnumerable<int2> ExpandFootprint(IEnumerable<int2> cells)
+		{
+			var result = new Dictionary<int2, bool>();
+			foreach (var c in cells.SelectMany(c => Neighbours(c)))
+				result[c] = true;
+			return result.Keys;
+		}
+
+		bool NoBuildingsUnder(IEnumerable<int2> cells)
+		{
+			var bi = world.WorldActor.Trait<BuildingInfluence>();
+			return cells.All(c => bi.GetBuildingAt(c) == null);
+		}
+
 		int2? ChooseBuildLocation(ProductionItem item)
 		{
 			var bi = Rules.Info[item.Item].Traits.Get<BuildingInfo>();
@@ -177,7 +201,9 @@ ActorInfo ChooseDefenseToBuild(ProductionQueue queue)
 				foreach (var t in world.FindTilesInCircle(baseCenter, k))
 					if (world.CanPlaceBuilding(item.Item, bi, t, null))
 						if (bi.IsCloseEnoughToBase(world, p, item.Item, t))
-							return t;
+							if (NoBuildingsUnder(ExpandFootprint( 
+								FootprintUtils.Tiles( item.Item, bi, t ))))
+								return t;
 
 			return null;		// i don't know where to put it.
 		}