New movement system for leashed entities (#4147)

* New movement system for leashed entities
Entities are accelerated towards the leashed to entity as if by a spring.
 * Mobs now pathfind close to but not directly to the leashing entity.
* Also minor comment changes

src/Mobs/Monster.cpp

@@ -22,10 +22,6 @@
 
 
 
-// Ticks to wait to do leash calculations
-#define LEASH_ACTIONS_TICK_STEP 10
-
-
 
 /** Map for eType <-> string
 Needs to be alpha-sorted by the strings, because binary search is used in StringToMobType()
@@ -239,28 +235,12 @@ void cMonster::MoveToWayPoint(cChunk & a_Chunk)
 		AddSpeedX(Distance.x);
 		AddSpeedZ(Distance.z);
 	}
-
-	// Speed up leashed mobs getting far from player
-	if (IsLeashed() && GetLeashedTo()->IsPlayer())
-	{
-		Distance = GetLeashedTo()->GetPosition() - GetPosition();
-		Distance.Normalize();
-		AddSpeedX(Distance.x);
-		AddSpeedZ(Distance.z);
-	}
-
 }
 
 
 
 
 
-
-
-
-
-
-
 void cMonster::MoveToPosition(const Vector3d & a_Position)
 {
 	m_FinalDestination = a_Position;
@@ -400,10 +380,7 @@ void cMonster::Tick(std::chrono::milliseconds a_Dt, cChunk & a_Chunk)
 	}  // switch (m_EMState)
 
 	// Leash calculations
-	if ((m_TicksAlive % LEASH_ACTIONS_TICK_STEP) == 0)
-	{
-		CalcLeashActions();
-	}
+	CalcLeashActions(a_Dt);
 
 	BroadcastMovementUpdate();
 
@@ -422,7 +399,7 @@ void cMonster::Tick(std::chrono::milliseconds a_Dt, cChunk & a_Chunk)
 
 
 
-void cMonster::CalcLeashActions()
+void cMonster::CalcLeashActions(std::chrono::milliseconds a_Dt)
 {
 	// This mob just spotted in the world and [m_LeashToPos not null] shows that should be leashed to a leash knot at m_LeashToPos.
 	// This keeps trying until knot is found. Leash knot may be in a different chunk that needn't or can't be loaded yet.
@@ -435,19 +412,54 @@ void cMonster::CalcLeashActions()
 			SetLeashToPos(nullptr);
 		}
 	}
-	else if (IsLeashed())  // Mob is already leashed to an entity: follow it.
-	{
-		// TODO: leashed mobs in vanilla can move around up to 5 blocks distance from leash origin
-		MoveToPosition(m_LeashedTo->GetPosition());
 
-		// If distance to target > 10 break leash
-		Vector3f a_Distance(m_LeashedTo->GetPosition() - GetPosition());
-		double Distance(a_Distance.Length());
-		if (Distance > 10.0)
+	if (!IsLeashed())
+	{
+		return;
+	}
+
+	static const double CloseFollowDistance = 1.8;   // The closest the mob will path towards the leashed to entity
+	static const double LeashNaturalLength  = 5.0;   // The closest the mob is actively pulled towards the entity
+	static const double LeashMaximumLength  = 10.0;  // Length where the leash breaks
+	static const double LeashSpringConstant = 20.0;  // How stiff the leash is
+
+	const auto LeashedToPos = m_LeashedTo->GetPosition();
+	const auto Displacement = LeashedToPos - GetPosition();
+	const auto Distance = Displacement.Length();
+	const auto Direction = Displacement.NormalizeCopy();
+
+	// If the leash is over-extended, break the leash:
+	if (Distance > LeashMaximumLength)
 	{
 		LOGD("Leash broken (distance)");
 		Unleash(false);
+		return;
 	}
+
+	// If the mob isn't following close enough, pull the mob towards the leashed to entity:
+	if (Distance > LeashNaturalLength)
+	{
+		// Accelerate monster towards the leashed to entity:
+		const auto Extension = Distance - LeashNaturalLength;
+		auto Acceleration = Direction * (Extension * LeashSpringConstant);
+
+		// Stop mobs from floating up when on the ground
+		if (IsOnGround() && (Acceleration.y < std::abs(GetGravity())))
+		{
+			Acceleration.y = 0.0;
+		}
+
+		// Apply the acceleration
+		using namespace std::chrono;
+		AddSpeed(Acceleration * duration_cast<duration<double>>(a_Dt).count());
+	}
+
+	// Passively follow the leashed to entity:
+	if (Distance > CloseFollowDistance)
+	{
+		const Vector3d TargetBlock((LeashedToPos - Direction * CloseFollowDistance).Floor());
+		// Move to centre of target block face
+		MoveToPosition(TargetBlock + Vector3d{ 0.5, 0.0, 0.5 });
 	}
 }
 

src/Mobs/Monster.h

@@ -326,6 +326,6 @@ private:
 	cPawn * m_Target;
 
 	/** Leash calculations inside Tick function */
-	void CalcLeashActions();
+	void CalcLeashActions(std::chrono::milliseconds a_Dt);
 
 } ;  // tolua_export