4 files changed, 63 insertions, 15 deletions

diff --git a/src/Blocks/BlockComparator.h b/src/Blocks/BlockComparator.h
index 394b53a15..ffd1bde61 100644
--- a/src/Blocks/BlockComparator.h
+++ b/src/Blocks/BlockComparator.h
@@ -60,6 +60,11 @@ public:
 		return true;
 	}
 
+	inline static bool IsOn(NIBBLETYPE a_Meta)
+	{
+		return ((a_Meta & 0x8) == 0x8);
+	}
+
 	inline static Vector3i GetSideCoordinate(const Vector3i & a_Position, NIBBLETYPE a_Meta, bool a_bInverse)
 	{
 		auto Position = a_Position;
diff --git a/src/Blocks/BlockPiston.h b/src/Blocks/BlockPiston.h
index 30ccc168d..18b688256 100644
--- a/src/Blocks/BlockPiston.h
+++ b/src/Blocks/BlockPiston.h
@@ -93,6 +93,9 @@ public:
 		return 11;
 	}
 
+	/** Returns true if the piston (with the specified meta) is extended */
+	static inline bool IsExtended(NIBBLETYPE a_PistonMeta) { return ((a_PistonMeta & 0x8) != 0x0); }
+
 private:
 
 	typedef std::unordered_set<Vector3i, VectorHasher<int>> Vector3iSet;
@@ -100,9 +103,6 @@ private:
 	/** Returns true if the piston (specified by blocktype) is a sticky piston */
 	static inline bool IsSticky(BLOCKTYPE a_BlockType) { return (a_BlockType == E_BLOCK_STICKY_PISTON); }
 
-	/** Returns true if the piston (with the specified meta) is extended */
-	static inline bool IsExtended(NIBBLETYPE a_PistonMeta) { return ((a_PistonMeta & 0x8) != 0x0); }
-
 	/** Returns true if the specified block can be pushed by a piston (and left intact) */
 	static inline bool CanPush(BLOCKTYPE a_BlockType, NIBBLETYPE a_BlockMeta)
 	{
diff --git a/src/Simulator/IncrementalRedstoneSimulator/PistonHandler.h b/src/Simulator/IncrementalRedstoneSimulator/PistonHandler.h
index 1ee68e521..f82ebdf94 100644
--- a/src/Simulator/IncrementalRedstoneSimulator/PistonHandler.h
+++ b/src/Simulator/IncrementalRedstoneSimulator/PistonHandler.h
@@ -39,14 +39,37 @@ public:
 	virtual cVector3iArray Update(const Vector3i & a_Position, BLOCKTYPE a_BlockType, NIBBLETYPE a_Meta, PoweringData a_PoweringData) override
 	{
 		// LOGD("Evaluating pisty the piston (%d %d %d)", a_Position.x, a_Position.y, a_Position.z);
+		auto Data = static_cast<cIncrementalRedstoneSimulator *>(m_World.GetRedstoneSimulator())->GetChunkData();
+		auto DelayInfo = Data->GetMechanismDelayInfo(a_Position);
 
-		if (a_PoweringData.PowerLevel > 0)
+		// Delay is used here to prevent an infinite loop (#3168)
+		if (DelayInfo == nullptr)
 		{
-			cBlockPistonHandler::ExtendPiston(a_Position, &m_World);
+			bool ShouldBeExtended = (a_PoweringData.PowerLevel != 0);
+			if (ShouldBeExtended != cBlockPistonHandler::IsExtended(a_Meta))
+			{
+				Data->m_MechanismDelays[a_Position] = std::make_pair(1, ShouldBeExtended);
+			}
 		}
 		else
 		{
-			cBlockPistonHandler::RetractPiston(a_Position, &m_World);
+			int DelayTicks;
+			bool ShouldBeExtended;
+			std::tie(DelayTicks, ShouldBeExtended) = *DelayInfo;
+
+			if (DelayTicks == 0)
+			{
+				if (ShouldBeExtended)
+				{
+					cBlockPistonHandler::ExtendPiston(a_Position, &m_World);
+				}
+				else
+				{
+					cBlockPistonHandler::RetractPiston(a_Position, &m_World);
+				}
+
+				Data->m_MechanismDelays.erase(a_Position);
+			}
 		}
 
 		return {};
diff --git a/src/Simulator/IncrementalRedstoneSimulator/RedstoneComparatorHandler.h b/src/Simulator/IncrementalRedstoneSimulator/RedstoneComparatorHandler.h
index 95881990d..9a250a1fe 100644
--- a/src/Simulator/IncrementalRedstoneSimulator/RedstoneComparatorHandler.h
+++ b/src/Simulator/IncrementalRedstoneSimulator/RedstoneComparatorHandler.h
@@ -99,22 +99,42 @@ public:
 
 	virtual cVector3iArray Update(const Vector3i & a_Position, BLOCKTYPE a_BlockType, NIBBLETYPE a_Meta, PoweringData a_PoweringData) override
 	{
+		// Note that a_PoweringData here contains the maximum *side* power level, as specified by GetValidSourcePositions
 		// LOGD("Evaluating ALU the comparator (%d %d %d)", a_Position.x, a_Position.y, a_Position.z);
+		auto Data = static_cast<cIncrementalRedstoneSimulator *>(m_World.GetRedstoneSimulator())->GetChunkData();
+		auto DelayInfo = Data->GetMechanismDelayInfo(a_Position);
 
-		if (GetPowerLevel(a_Position, a_BlockType, a_Meta) > 0)
+		// Delay is used here to prevent an infinite loop (#3168)
+		if (DelayInfo == nullptr)
 		{
-			m_World.SetBlockMeta(a_Position, a_Meta | 0x8);
+			auto Power = GetFrontPowerLevel(a_Position, a_BlockType, a_Meta, a_PoweringData.PowerLevel);
+			auto PreviousFrontPower = static_cast<cIncrementalRedstoneSimulator *>(m_World.GetRedstoneSimulator())->GetChunkData()->ExchangeUpdateOncePowerData(a_Position, PoweringData(a_PoweringData.PoweringBlock, Power));
+
+			bool ShouldBeOn = (GetPowerLevel(a_Position, a_BlockType, a_Meta) > 0);  // Provide visual indication by examining *rear* power level
+			bool ShouldUpdate = (Power != PreviousFrontPower.PowerLevel);  // "Business logic" (:P) - determine by examining *side* power levels
+
+			if (ShouldUpdate || (ShouldBeOn != cBlockComparatorHandler::IsOn(a_Meta)))
+			{
+				Data->m_MechanismDelays[a_Position] = std::make_pair(1, ShouldBeOn);
+			}
 		}
 		else
 		{
-			m_World.SetBlockMeta(a_Position, a_Meta & 0x7);
-		}
+			int DelayTicks;
+			bool ShouldPowerOn;
+			std::tie(DelayTicks, ShouldPowerOn) = *DelayInfo;
 
-		auto Power = GetFrontPowerLevel(a_Position, a_BlockType, a_Meta, a_PoweringData.PowerLevel);
-		auto PreviousFrontPower = static_cast<cIncrementalRedstoneSimulator *>(m_World.GetRedstoneSimulator())->GetChunkData()->ExchangeUpdateOncePowerData(a_Position, PoweringData(a_PoweringData.PoweringBlock, Power));
-		if (Power != PreviousFrontPower.PowerLevel)
-		{
-			return GetAdjustedRelatives(a_Position, GetRelativeLaterals());
+			if (DelayTicks == 0)
+			{
+				m_World.SetBlockMeta(a_Position, ShouldPowerOn ? (a_Meta | 0x8) : (a_Meta & 0x7));
+				Data->m_MechanismDelays.erase(a_Position);
+
+				// Assume that an update (to front power) is needed.
+				// Note: potential inconsistencies will arise as power data is updated before-delay due to limitations of the power data caching functionality (only stores one bool)
+				// This means that other mechanisms like wires may get our new power data before our delay has finished
+				// This also means that we have to manually update ourselves to be aware of any changes that happened in the previous redstone tick
+				return StaticAppend(GetAdjustedRelatives(a_Position, GetRelativeLaterals()), { a_Position });
+			}
 		}
 
 		return {};