Fixed #1437.

AI/VCAI/VCAI.cpp

@@ -3233,10 +3233,15 @@ TSubgoal CGoal::whatToDoToAchieve()
 				auto creature = VLC->creh->creatures[objid];
 				if (t->subID == creature->faction) //TODO: how to force AI to build unupgraded creatures? :O
 				{
-					auto creatures = t->town->creatures[creature->level];
-					int upgradeNumber = std::find(creatures.begin(), creatures.end(), creature->idNumber) - creatures.begin();
+					auto creatures = vstd::tryAt(t->town->creatures, creature->level - 1);
+					if(!creatures)
+						continue; 
 
-					BuildingID bid(BuildingID::DWELL_FIRST + creature->level + upgradeNumber * GameConstants::CREATURES_PER_TOWN);
+					int upgradeNumber = vstd::find_pos(*creatures, creature->idNumber);
+					if(upgradeNumber < 0)
+						continue;
+
+					BuildingID bid(BuildingID::DWELL_FIRST + creature->level - 1 + upgradeNumber * GameConstants::CREATURES_PER_TOWN);
 					if (t->hasBuilt(bid)) //this assumes only creatures with dwellings are assigned to faction
 					{
 						dwellings.push_back(t);

Global.h

@@ -601,6 +601,34 @@ namespace vstd
 			return nullptr;
 	}
 
+	template <typename Container, typename Index>
+	bool isValidIndex(const Container &c, Index i)
+	{
+		return i >= 0  &&  i < c.size();
+	}
+
+	template <typename Container, typename Index>
+	boost::optional<typename Container::const_reference> tryAt(const Container &c, Index i)
+	{
+		if(isValidIndex(c, i))
+		{
+			auto itr = c.begin();
+			std::advance(itr, i);
+			return *itr;
+		}
+		return boost::none;
+	}
+
+	template <typename Container, typename Pred>
+	static boost::optional<typename Container::const_reference> tryFindIf(const Container &r, const Pred &t)
+	{
+		auto pos = range::find_if(r, t);
+		if(pos == boost::end(r))
+			return boost::none;
+		else
+			return *pos;
+	}
+
 	using boost::math::round;
 }
 using vstd::operator-=;

lib/CCreatureHandler.h

@@ -33,7 +33,7 @@ public:
 
 	CreatureID idNumber;
 	TFaction faction;
-	ui8 level; // 0 - unknown
+	ui8 level; // 0 - unknown; 1-7 for "usual" creatures
 
 	//stats that are not handled by bonus system
 	ui32 fightValue, AIValue, growth, hordeGrowth;