add comment to avoid future confusion.

rts/Sim/Path/HAPFS/PathDataTypes.h

@@ -39,8 +39,11 @@ struct PathNode {
 /// functor to define node priority
 /// This needs to guarantee that the sorting is stable.
 struct lessCost {
-	inline bool operator() (const PathNode* x, const PathNode* y) const {
-		return std::tie(x->fCost, y->gCost, x->nodeNum) > std::tie(y->fCost, x->gCost, y->nodeNum);
+	inline bool operator() (const PathNode* lhs, const PathNode* rhs) const {
+		// fCost == gCost + hCost.
+		// When fCosts are the same, prioritize the node closest the goal. Since we don't have hCost to hand, we know
+		// that hCost == fCost - gCost. This is why we invert the left/right side for the gCost comparison.
+		return std::tie(lhs->fCost, rhs->gCost, lhs->nodeNum) > std::tie(rhs->fCost, lhs->gCost, rhs->nodeNum);
 	}
 };