ABMs: Make catch-up behaviour optional

Default is true for backwards compatibility
Update lua_api.txt

doc/lua_api.txt

@@ -3063,10 +3063,14 @@ Definition tables
     {
     --  In the following two fields, also group:groupname will work.
         nodenames = {"default:lava_source"},
-        neighbors = {"default:water_source", "default:water_flowing"}, -- (any of these)
-    --  ^ If left out or empty, any neighbor will do
-        interval = 1.0, -- (operation interval)
-        chance = 1, -- (chance of trigger is 1.0/this)
+        neighbors = {"default:water_source", "default:water_flowing"}, -- Any of these --[[
+        ^ If left out or empty, any neighbor will do ]]
+        interval = 1.0, -- Operation interval in seconds
+        chance = 1, -- Chance of trigger per-node per-interval is 1.0 / this
+        catch_up = true, -- If true, catch-up behaviour is enabled --[[
+        ^ The chance value is temporarily reduced when returning to
+          an area to simulate time lost by the area being unattended.
+        ^ Note chance value can often be reduced to 1 ]]
         action = func(pos, node, active_object_count, active_object_count_wider),
     }
 

src/environment.cpp

@@ -577,17 +577,21 @@ class ABMHandler
 				i->timer -= trigger_interval;
 				actual_interval = trigger_interval;
 			}
-			float intervals = actual_interval / trigger_interval;
-			if(intervals == 0)
-				continue;
 			float chance = abm->getTriggerChance();
 			if(chance == 0)
 				chance = 1;
 			ActiveABM aabm;
 			aabm.abm = abm;
-			aabm.chance = chance / intervals;
-			if(aabm.chance == 0)
-				aabm.chance = 1;
+			if(abm->getSimpleCatchUp()) {
+				float intervals = actual_interval / trigger_interval;
+				if(intervals == 0)
+					continue;
+				aabm.chance = chance / intervals;
+				if(aabm.chance == 0)
+					aabm.chance = 1;
+			} else {
+				aabm.chance = chance;
+			}
 			// Trigger neighbors
 			std::set<std::string> required_neighbors_s
 					= abm->getRequiredNeighbors();

src/environment.h

@@ -155,6 +155,8 @@ class ActiveBlockModifier
 	virtual float getTriggerInterval() = 0;
 	// Random chance of (1 / return value), 0 is disallowed
 	virtual u32 getTriggerChance() = 0;
+	// Whether to modify chance to simulate time lost by an unnattended block
+	virtual bool getSimpleCatchUp() = 0;
 	// This is called usually at interval for 1/chance of the nodes
 	virtual void trigger(ServerEnvironment *env, v3s16 p, MapNode n){};
 	virtual void trigger(ServerEnvironment *env, v3s16 p, MapNode n,

src/script/cpp_api/s_env.cpp

@@ -143,8 +143,11 @@ void ScriptApiEnv::initializeEnvironment(ServerEnvironment *env)
 			int trigger_chance = 50;
 			getintfield(L, current_abm, "chance", trigger_chance);
 
-			LuaABM *abm = new LuaABM(L, id, trigger_contents,
-					required_neighbors, trigger_interval, trigger_chance);
+			bool simple_catch_up = true;
+			getboolfield(L, current_abm, "catch_up", simple_catch_up);
+
+			LuaABM *abm = new LuaABM(L, id, trigger_contents, required_neighbors,
+				trigger_interval, trigger_chance, simple_catch_up);
 
 			env->addActiveBlockModifier(abm);
 

src/script/lua_api/l_env.h

@@ -184,16 +184,18 @@ class LuaABM : public ActiveBlockModifier
 	std::set<std::string> m_required_neighbors;
 	float m_trigger_interval;
 	u32 m_trigger_chance;
+	bool m_simple_catch_up;
 public:
 	LuaABM(lua_State *L, int id,
 			const std::set<std::string> &trigger_contents,
 			const std::set<std::string> &required_neighbors,
-			float trigger_interval, u32 trigger_chance):
+			float trigger_interval, u32 trigger_chance, bool simple_catch_up):
 		m_id(id),
 		m_trigger_contents(trigger_contents),
 		m_required_neighbors(required_neighbors),
 		m_trigger_interval(trigger_interval),
-		m_trigger_chance(trigger_chance)
+		m_trigger_chance(trigger_chance),
+		m_simple_catch_up(simple_catch_up)
 	{
 	}
 	virtual std::set<std::string> getTriggerContents()
@@ -212,6 +214,10 @@ class LuaABM : public ActiveBlockModifier
 	{
 		return m_trigger_chance;
 	}
+	virtual bool getSimpleCatchUp()
+	{
+		return m_simple_catch_up;
+	}
 	virtual void trigger(ServerEnvironment *env, v3s16 p, MapNode n,
 			u32 active_object_count, u32 active_object_count_wider);
 };