Hopefully fixed #8 by enforcing only one change per tile per tick

- TickerCoordinator.pendingChanges is now a HashSet
- TickerCoordinator now makes some best-effort checks to make sure the
world does not change during evaluation phase
- Extracted Cached{Block,Tile}Change
  - these classes implement hashCode() and equals() based on the
location of the block/tile they affect, thus ensuring that only one per
block/tile remain in the aforementioned set
- Similarly extracted PacketAffect{Block,Tile,Entity}
- Renamed a bunch of packets

src/main/java/ru/windcorp/progressia/client/comms/DefaultClientCommsListener.java

@@ -7,7 +7,7 @@ import ru.windcorp.progressia.common.comms.CommsListener;
 import ru.windcorp.progressia.common.comms.packets.Packet;
 import ru.windcorp.progressia.common.util.crash.CrashReports;
 import ru.windcorp.progressia.common.world.PacketSetLocalPlayer;
-import ru.windcorp.progressia.common.world.PacketWorldChange;
+import ru.windcorp.progressia.common.world.PacketAffectWorld;
 
 // TODO refactor with no mercy
 public class DefaultClientCommsListener implements CommsListener {
@@ -20,8 +20,8 @@ public class DefaultClientCommsListener implements CommsListener {
 
 	@Override
 	public void onPacketReceived(Packet packet) {
-		if (packet instanceof PacketWorldChange) {
-			((PacketWorldChange) packet).apply(
+		if (packet instanceof PacketAffectWorld) {
+			((PacketAffectWorld) packet).apply(
 					getClient().getWorld().getData()
 			);
 		} else if (packet instanceof PacketSetLocalPlayer) {

src/main/java/ru/windcorp/progressia/common/world/PacketAffectChunk.java

@@ -2,9 +2,9 @@ package ru.windcorp.progressia.common.world;
 
 import glm.vec._3.i.Vec3i;
 
-public abstract class PacketChunkChange extends PacketWorldChange {
+public abstract class PacketAffectChunk extends PacketAffectWorld {
 
-	public PacketChunkChange(String id) {
+	public PacketAffectChunk(String id) {
 		super(id);
 	}
 

src/main/java/ru/windcorp/progressia/common/world/PacketAffectWorld.java

@@ -2,9 +2,9 @@ package ru.windcorp.progressia.common.world;
 
 import ru.windcorp.progressia.common.comms.packets.Packet;
 
-public abstract class PacketWorldChange extends Packet {
+public abstract class PacketAffectWorld extends Packet {
 
-	public PacketWorldChange(String id) {
+	public PacketAffectWorld(String id) {
 		super(id);
 	}
 	

src/main/java/ru/windcorp/progressia/common/world/PacketRevokeChunk.java

@@ -6,7 +6,7 @@ import java.io.IOException;
 
 import glm.vec._3.i.Vec3i;
 
-public class PacketRevokeChunk extends PacketChunkChange {
+public class PacketRevokeChunk extends PacketAffectChunk {
 	
 	private final Vec3i position = new Vec3i();
 	

src/main/java/ru/windcorp/progressia/common/world/PacketSendChunk.java

@@ -10,7 +10,7 @@ import ru.windcorp.progressia.common.state.IOContext;
 import ru.windcorp.progressia.common.util.DataBuffer;
 import ru.windcorp.progressia.common.util.crash.CrashReports;
 
-public class PacketSendChunk extends PacketChunkChange {
+public class PacketSendChunk extends PacketAffectChunk {
 	
 	private final DataBuffer data = new DataBuffer();
 	private final Vec3i position = new Vec3i();

src/main/java/ru/windcorp/progressia/common/world/block/PacketAffectBlock.java

@@ -0,0 +1,45 @@
+package ru.windcorp.progressia.common.world.block;
+
+import java.io.DataInput;
+import java.io.DataOutput;
+import java.io.IOException;
+
+import glm.vec._3.i.Vec3i;
+import ru.windcorp.progressia.common.world.Coordinates;
+import ru.windcorp.progressia.common.world.DecodingException;
+import ru.windcorp.progressia.common.world.PacketAffectChunk;
+
+public abstract class PacketAffectBlock extends PacketAffectChunk {
+	
+	private final Vec3i blockInWorld = new Vec3i();
+	
+	public PacketAffectBlock(String id) {
+		super(id);
+	}
+	
+	public Vec3i getBlockInWorld() {
+		return blockInWorld;
+	}
+	
+	public void set(Vec3i blockInWorld) {
+		this.blockInWorld.set(blockInWorld.x, blockInWorld.y, blockInWorld.z);
+	}
+
+	@Override
+	public void read(DataInput input) throws IOException, DecodingException {
+		this.blockInWorld.set(input.readInt(), input.readInt(), input.readInt());
+	}
+
+	@Override
+	public void write(DataOutput output) throws IOException {
+		output.writeInt(this.blockInWorld.x);
+		output.writeInt(this.blockInWorld.y);
+		output.writeInt(this.blockInWorld.z);
+	}
+
+	@Override
+	public void getAffectedChunk(Vec3i output) {
+		Coordinates.convertInWorldToChunk(this.blockInWorld, output);
+	}
+
+}

src/main/java/ru/windcorp/progressia/common/world/block/PacketSetBlock.java

@@ -5,15 +5,12 @@ import java.io.DataOutput;
 import java.io.IOException;
 
 import glm.vec._3.i.Vec3i;
-import ru.windcorp.progressia.common.world.Coordinates;
 import ru.windcorp.progressia.common.world.DecodingException;
-import ru.windcorp.progressia.common.world.PacketChunkChange;
 import ru.windcorp.progressia.common.world.WorldData;
 
-public class PacketSetBlock extends PacketChunkChange {
+public class PacketSetBlock extends PacketAffectBlock {
 	
-	private String id;
-	private final Vec3i blockInWorld = new Vec3i();
+	private String blockId;
 	
 	public PacketSetBlock() {
 		this("Core:SetBlock");
@@ -23,34 +20,31 @@ public class PacketSetBlock extends PacketChunkChange {
 		super(id);
 	}
 	
+	public String getBlockId() {
+		return blockId;
+	}
+	
 	public void set(BlockData block, Vec3i blockInWorld) {
-		this.id = block.getId();
-		this.blockInWorld.set(blockInWorld.x, blockInWorld.y, blockInWorld.z);
+		super.set(blockInWorld);
+		this.blockId = block.getId();
 	}
 
 	@Override
 	public void read(DataInput input) throws IOException, DecodingException {
-		this.id = input.readUTF();
-		this.blockInWorld.set(input.readInt(), input.readInt(), input.readInt());
+		super.read(input);
+		this.blockId = input.readUTF();
 	}
 
 	@Override
 	public void write(DataOutput output) throws IOException {
-		output.writeUTF(this.id);
-		output.writeInt(this.blockInWorld.x);
-		output.writeInt(this.blockInWorld.y);
-		output.writeInt(this.blockInWorld.z);
+		super.write(output);
+		output.writeUTF(this.blockId);
 	}
 
 	@Override
 	public void apply(WorldData world) {
-		BlockData block = BlockDataRegistry.getInstance().get(id);
-		world.setBlock(blockInWorld, block, true);
-	}
-
-	@Override
-	public void getAffectedChunk(Vec3i output) {
-		Coordinates.convertInWorldToChunk(this.blockInWorld, output);
+		BlockData block = BlockDataRegistry.getInstance().get(getBlockId());
+		world.setBlock(getBlockInWorld(), block, true);
 	}
 
 }

src/main/java/ru/windcorp/progressia/common/world/entity/PacketAffectEntity.java

@@ -0,0 +1,42 @@
+package ru.windcorp.progressia.common.world.entity;
+
+import java.io.DataInput;
+import java.io.DataOutput;
+import java.io.IOException;
+
+import ru.windcorp.progressia.common.world.DecodingException;
+import ru.windcorp.progressia.common.world.PacketAffectWorld;
+import ru.windcorp.progressia.common.world.WorldData;
+
+public class PacketAffectEntity extends PacketAffectWorld {
+	
+	private long entityId;
+
+	public PacketAffectEntity(String id) {
+		super(id);
+	}
+	
+	public long getEntityId() {
+		return entityId;
+	}
+	
+	public void set(long entityId) {
+		this.entityId = entityId;
+	}
+	
+	@Override
+	public void read(DataInput input) throws IOException, DecodingException {
+		this.entityId = input.readLong();
+	}
+	
+	@Override
+	public void write(DataOutput output) throws IOException {
+		output.writeLong(this.entityId);
+	}
+
+	@Override
+	public void apply(WorldData world) {
+		world.removeEntity(this.entityId);
+	}
+
+}

src/main/java/ru/windcorp/progressia/common/world/entity/PacketChangeEntity.java

@@ -8,44 +8,27 @@ import ru.windcorp.progressia.common.state.IOContext;
 import ru.windcorp.progressia.common.util.DataBuffer;
 import ru.windcorp.progressia.common.util.crash.CrashReports;
 import ru.windcorp.progressia.common.world.DecodingException;
-import ru.windcorp.progressia.common.world.PacketWorldChange;
 import ru.windcorp.progressia.common.world.WorldData;
 
-public class PacketEntityChange extends PacketWorldChange {
+public class PacketChangeEntity extends PacketAffectEntity {
 
-	private long entityId;
 	private final DataBuffer buffer = new DataBuffer();
 
-	public PacketEntityChange() {
+	public PacketChangeEntity() {
 		super("Core:EntityChange");
 	}
 	
-	protected PacketEntityChange(String id) {
+	protected PacketChangeEntity(String id) {
 		super(id);
 	}
 	
-	public long getEntityId() {
-		return entityId;
-	}
-
-	public void setEntityId(long entityId) {
-		this.entityId = entityId;
-	}
-
 	public DataBuffer getBuffer() {
 		return buffer;
 	}
 	
-	public DataInput getReader() {
-		return buffer.getReader();
-	}
-
-	public DataOutput getWriter() {
-		return buffer.getWriter();
-	}
-	
 	public void set(EntityData entity) {
-		this.entityId = entity.getEntityId();
+		super.set(entity.getEntityId());
+		
 		try {
 			entity.write(this.buffer.getWriter(), IOContext.COMMS);
 		} catch (IOException e) {
@@ -55,13 +38,13 @@ public class PacketEntityChange extends PacketWorldChange {
 	
 	@Override
 	public void read(DataInput input) throws IOException, DecodingException {
-		this.entityId = input.readLong();
+		super.read(input);
 		this.buffer.fill(input, input.readInt());
 	}
 	
 	@Override
 	public void write(DataOutput output) throws IOException {
-		output.writeLong(this.entityId);
+		super.write(output);
 		output.writeInt(this.buffer.getSize());
 		this.buffer.flush(output);
 	}
@@ -75,7 +58,7 @@ public class PacketEntityChange extends PacketWorldChange {
 		}
 
 		try {
-			entity.read(getReader(), IOContext.COMMS);
+			entity.read(getBuffer().getReader(), IOContext.COMMS);
 		} catch (IOException e) {
 			throw CrashReports.report(e, "Entity could not be read");
 		}

src/main/java/ru/windcorp/progressia/common/world/entity/PacketRevokeEntity.java

@@ -4,12 +4,10 @@ import java.io.DataInput;
 import java.io.DataOutput;
 import java.io.IOException;
 
-import ru.windcorp.progressia.common.world.PacketWorldChange;
+import ru.windcorp.progressia.common.world.DecodingException;
 import ru.windcorp.progressia.common.world.WorldData;
 
-public class PacketRevokeEntity extends PacketWorldChange {
-	
-	private long entityId;
+public class PacketRevokeEntity extends PacketAffectEntity {
 	
 	public PacketRevokeEntity() {
 		this("Core:RevokeEntity");
@@ -19,23 +17,24 @@ public class PacketRevokeEntity extends PacketWorldChange {
 		super(id);
 	}
 	
+	@Override
 	public void set(long entityId) {
-		this.entityId = entityId;
+		super.set(entityId);
 	}
 	
 	@Override
-	public void read(DataInput input) throws IOException {
-		this.entityId = input.readLong();
+	public void read(DataInput input) throws IOException, DecodingException {
+		super.read(input);
 	}
 	
 	@Override
 	public void write(DataOutput output) throws IOException {
-		output.writeLong(this.entityId);
+		super.write(output);
 	}
 
 	@Override
 	public void apply(WorldData world) {
-		world.removeEntity(this.entityId);
+		world.removeEntity(getEntityId());
 	}
 
 }

src/main/java/ru/windcorp/progressia/common/world/entity/PacketSendEntity.java

@@ -8,13 +8,11 @@ import ru.windcorp.progressia.common.state.IOContext;
 import ru.windcorp.progressia.common.util.DataBuffer;
 import ru.windcorp.progressia.common.util.crash.CrashReports;
 import ru.windcorp.progressia.common.world.DecodingException;
-import ru.windcorp.progressia.common.world.PacketWorldChange;
 import ru.windcorp.progressia.common.world.WorldData;
 
-public class PacketSendEntity extends PacketWorldChange {
+public class PacketSendEntity extends PacketAffectEntity {
 	
-	private String id;
-	private long entityId;
+	private String entityTypeId;
 	private final DataBuffer buffer = new DataBuffer();
 	
 	public PacketSendEntity() {
@@ -25,9 +23,23 @@ public class PacketSendEntity extends PacketWorldChange {
 		super(id);
 	}
 	
+	/**
+	 * Returns the text ID of the entity added by this packet.
+	 * @return text ID
+	 * @see #getEntityId()
+	 */
+	public String getEntityTypeId() {
+		return entityTypeId;
+	}
+	
+	public DataBuffer getBuffer() {
+		return buffer;
+	}
+	
 	public void set(EntityData entity) {
-		this.id = entity.getId();
-		this.entityId = entity.getEntityId();
+		super.set(entity.getEntityId());
+		
+		this.entityTypeId = entity.getId();
 		
 		try {
 			entity.write(this.buffer.getWriter(), IOContext.COMMS);
@@ -38,26 +50,28 @@ public class PacketSendEntity extends PacketWorldChange {
 
 	@Override
 	public void read(DataInput input) throws IOException, DecodingException {
-		this.id = input.readUTF();
-		this.entityId = input.readLong();
+		super.read(input);
+		
+		this.entityTypeId = input.readUTF();
 		this.buffer.fill(input, input.readInt());
 	}
 
 	@Override
 	public void write(DataOutput output) throws IOException {
-		output.writeUTF(this.id);
-		output.writeLong(this.entityId);
+		super.write(output);
+		
+		output.writeUTF(this.entityTypeId);
 		output.writeInt(this.buffer.getSize());
 		this.buffer.flush(output);
 	}
 	
 	@Override
 	public void apply(WorldData world) {
-		EntityData entity = EntityDataRegistry.getInstance().create(this.id);
+		EntityData entity = EntityDataRegistry.getInstance().create(getEntityTypeId());
 		
-		entity.setEntityId(this.entityId);
+		entity.setEntityId(getEntityId());
 		try {
-			entity.read(this.buffer.getReader(), IOContext.COMMS);
+			entity.read(getBuffer().getReader(), IOContext.COMMS);
 		} catch (IOException e) {
 			throw CrashReports.report(e, "Could not read an entity from an internal buffer");
 		}

src/main/java/ru/windcorp/progressia/common/world/tile/PacketAddTile.java

@@ -5,17 +5,13 @@ import java.io.DataOutput;
 import java.io.IOException;
 
 import glm.vec._3.i.Vec3i;
-import ru.windcorp.progressia.common.world.Coordinates;
 import ru.windcorp.progressia.common.world.DecodingException;
-import ru.windcorp.progressia.common.world.PacketChunkChange;
 import ru.windcorp.progressia.common.world.WorldData;
 import ru.windcorp.progressia.common.world.block.BlockFace;
 
-public class PacketAddTile extends PacketChunkChange {
+public class PacketAddTile extends PacketAffectTile {
 	
-	private String id;
-	private final Vec3i blockInWorld = new Vec3i();
-	private BlockFace face;
+	private String tileId;
 	
 	public PacketAddTile() {
 		this("Core:AddTile");
@@ -25,37 +21,31 @@ public class PacketAddTile extends PacketChunkChange {
 		super(id);
 	}
 	
+	public String getTileId() {
+		return tileId;
+	}
+	
 	public void set(TileData tile, Vec3i blockInWorld, BlockFace face) {
-		this.id = tile.getId();
-		this.blockInWorld.set(blockInWorld.x, blockInWorld.y, blockInWorld.z);
-		this.face = face;
+		super.set(blockInWorld, face, -1);
+		this.tileId = tile.getId();
 	}
 	
 	@Override
 	public void read(DataInput input) throws IOException, DecodingException {
-		this.id = input.readUTF();
-		this.blockInWorld.set(input.readInt(), input.readInt(), input.readInt());
-		this.face = BlockFace.getFaces().get(input.readByte());
+		super.read(input);
+		this.tileId = input.readUTF();
 	}
 	
 	@Override
 	public void write(DataOutput output) throws IOException {
-		output.writeUTF(this.id);
-		output.writeInt(this.blockInWorld.x);
-		output.writeInt(this.blockInWorld.y);
-		output.writeInt(this.blockInWorld.z);
-		output.writeByte(this.face.getId());
+		super.write(output);
+		output.writeUTF(this.tileId);
 	}
 
 	@Override
 	public void apply(WorldData world) {
-		TileData tile = TileDataRegistry.getInstance().get(id);
-		world.getTiles(blockInWorld, face).add(tile);
-	}
-	
-	@Override
-	public void getAffectedChunk(Vec3i output) {
-		Coordinates.convertInWorldToChunk(this.blockInWorld, output);
+		TileData tile = TileDataRegistry.getInstance().get(getTileId());
+		world.getTiles(getBlockInWorld(), getFace()).add(tile);
 	}
 
 }

src/main/java/ru/windcorp/progressia/common/world/tile/PacketAffectTile.java

@@ -0,0 +1,62 @@
+package ru.windcorp.progressia.common.world.tile;
+
+import java.io.DataInput;
+import java.io.DataOutput;
+import java.io.IOException;
+
+import glm.vec._3.i.Vec3i;
+import ru.windcorp.progressia.common.world.Coordinates;
+import ru.windcorp.progressia.common.world.DecodingException;
+import ru.windcorp.progressia.common.world.PacketAffectChunk;
+import ru.windcorp.progressia.common.world.block.BlockFace;
+
+public abstract class PacketAffectTile extends PacketAffectChunk {
+	
+	private final Vec3i blockInWorld = new Vec3i();
+	private BlockFace face;
+	private int tag;
+	
+	public PacketAffectTile(String id) {
+		super(id);
+	}
+	
+	public Vec3i getBlockInWorld() {
+		return blockInWorld;
+	}
+	
+	public BlockFace getFace() {
+		return face;
+	}
+	
+	public int getTag() {
+		return tag;
+	}
+	
+	public void set(Vec3i blockInWorld, BlockFace face, int tag) {
+		this.blockInWorld.set(blockInWorld.x, blockInWorld.y, blockInWorld.z);
+		this.face = face;
+		this.tag = tag;
+	}
+	
+	@Override
+	public void read(DataInput input) throws IOException, DecodingException {
+		this.blockInWorld.set(input.readInt(), input.readInt(), input.readInt());
+		this.face = BlockFace.getFaces().get(input.readByte());
+		this.tag = input.readInt();
+	}
+	
+	@Override
+	public void write(DataOutput output) throws IOException {
+		output.writeInt(this.blockInWorld.x);
+		output.writeInt(this.blockInWorld.y);
+		output.writeInt(this.blockInWorld.z);
+		output.writeByte(this.face.getId());
+		output.writeInt(this.tag);
+	}
+	
+	@Override
+	public void getAffectedChunk(Vec3i output) {
+		Coordinates.convertInWorldToChunk(this.blockInWorld, output);
+	}
+
+}

src/main/java/ru/windcorp/progressia/common/world/tile/PacketRemoveTile.java

@@ -5,17 +5,12 @@ import java.io.DataOutput;
 import java.io.IOException;
 
 import glm.vec._3.i.Vec3i;
-import ru.windcorp.progressia.common.world.Coordinates;
+import ru.windcorp.progressia.common.util.crash.CrashReports;
 import ru.windcorp.progressia.common.world.DecodingException;
-import ru.windcorp.progressia.common.world.PacketChunkChange;
 import ru.windcorp.progressia.common.world.WorldData;
 import ru.windcorp.progressia.common.world.block.BlockFace;
 
-public class PacketRemoveTile extends PacketChunkChange {
-	
-	private final Vec3i blockInWorld = new Vec3i();
-	private BlockFace face;
-	private int tag;
+public class PacketRemoveTile extends PacketAffectTile {
 	
 	public PacketRemoveTile() {
 		this("Core:RemoveTile");
@@ -25,37 +20,37 @@ public class PacketRemoveTile extends PacketChunkChange {
 		super(id);
 	}
 	
+	@Override
 	public void set(Vec3i blockInWorld, BlockFace face, int tag) {
-		this.blockInWorld.set(blockInWorld.x, blockInWorld.y, blockInWorld.z);
-		this.face = face;
-		this.tag = tag;
+		super.set(blockInWorld, face, tag);
 	}
 	
 	@Override
 	public void read(DataInput input) throws IOException, DecodingException {
-		this.blockInWorld.set(input.readInt(), input.readInt(), input.readInt());
-		this.face = BlockFace.getFaces().get(input.readByte());
-		this.tag = input.readInt();
+		super.read(input);
 	}
 	
 	@Override
 	public void write(DataOutput output) throws IOException {
-		output.writeInt(this.blockInWorld.x);
-		output.writeInt(this.blockInWorld.y);
-		output.writeInt(this.blockInWorld.z);
-		output.writeByte(this.face.getId());
-		output.writeInt(this.tag);
+		super.write(output);
 	}
 
 	@Override
 	public void apply(WorldData world) {
-		TileDataStack stack = world.getTiles(blockInWorld, face);
-		stack.remove(stack.getIndexByTag(tag));
+		TileDataStack stack = world.getTiles(getBlockInWorld(), getFace());
+		
+		int index = stack.getIndexByTag(getTag());
+		
+		if (index < 0) {
+			throw CrashReports.report(null,
+					"Could not find tile with tag %d at (%d; %d; %d; %s)",
+					getTag(),
+					getBlockInWorld().x, getBlockInWorld().y, getBlockInWorld().z,
+					getFace()
+			);
 		}
 		
-	@Override
-	public void getAffectedChunk(Vec3i output) {
-		Coordinates.convertInWorldToChunk(this.blockInWorld, output);
+		stack.remove(index);
 	}
 
 }

src/main/java/ru/windcorp/progressia/server/world/TickAndUpdateUtil.java

@@ -22,7 +22,7 @@ public class TickAndUpdateUtil {
 		try {
 			block.tick(context);
 		} catch (Exception e) {
-			throw CrashReports.report(e, "Could not tick block {}", block);
+			throw CrashReports.report(e, "Could not tick block %s", block);
 		}
 	}
 	
@@ -38,7 +38,7 @@ public class TickAndUpdateUtil {
 		try {
 			tile.tick(context);
 		} catch (Exception e) {
-			throw CrashReports.report(e, "Could not tick tile {}", tile);
+			throw CrashReports.report(e, "Could not tick tile %s", tile);
 		}
 	}
 	

src/main/java/ru/windcorp/progressia/server/world/tasks/AddTile.java

@@ -4,7 +4,7 @@ import java.util.function.Consumer;
 
 import ru.windcorp.progressia.common.world.tile.PacketAddTile;
 
-class AddTile extends CachedChunkChange<PacketAddTile> {
+class AddTile extends CachedTileChange<PacketAddTile> {
 
 	public AddTile(Consumer<? super CachedChange> disposer) {
 		super(disposer, new PacketAddTile());

src/main/java/ru/windcorp/progressia/server/world/tasks/CachedBlockChange.java

@@ -0,0 +1,45 @@
+package ru.windcorp.progressia.server.world.tasks;
+
+import java.util.function.Consumer;
+
+import glm.Glm;
+import glm.vec._3.i.Vec3i;
+import ru.windcorp.progressia.common.world.block.PacketAffectBlock;
+
+public class CachedBlockChange<P extends PacketAffectBlock> extends CachedChunkChange<P> {
+
+	public CachedBlockChange(Consumer<? super CachedChange> disposer, P packet) {
+		super(disposer, packet);
+	}
+	
+	@Override
+	public int hashCode() {
+		PacketAffectBlock packet = getPacket();
+		Vec3i biw = packet.getBlockInWorld();
+		
+		final int prime = 31;
+		int result = 1;
+		result = prime * result + biw.x;
+		result = prime * result + biw.y;
+		result = prime * result + biw.z;
+		return result;
+	}
+	
+	@Override
+	public boolean equals(Object obj) {
+		if (obj == null || getClass() != obj.getClass()) return false;
+		
+		PacketAffectBlock my = getPacket();
+		PacketAffectBlock other = ((CachedBlockChange<?>) obj).getPacket();
+		
+		return Glm.equals(my.getBlockInWorld(), other.getBlockInWorld());
+	}
+	
+	@Override
+	public String toString() {
+		Vec3i biw = getPacket().getBlockInWorld();
+		
+		return getClass().getSimpleName() + " (" + biw.x + "; " + biw.y + "; " + biw.z + ")";
+	}
+
+}

src/main/java/ru/windcorp/progressia/server/world/tasks/CachedChunkChange.java

@@ -3,9 +3,9 @@ package ru.windcorp.progressia.server.world.tasks;
 import java.util.function.Consumer;
 
 import glm.vec._3.i.Vec3i;
-import ru.windcorp.progressia.common.world.PacketChunkChange;
+import ru.windcorp.progressia.common.world.PacketAffectChunk;
 
-public abstract class CachedChunkChange<P extends PacketChunkChange> extends CachedWorldChange<P> {
+public abstract class CachedChunkChange<P extends PacketAffectChunk> extends CachedWorldChange<P> {
 
 	public CachedChunkChange(Consumer<? super CachedChange> disposer, P packet) {
 		super(disposer, packet);

src/main/java/ru/windcorp/progressia/server/world/tasks/CachedTileChange.java

@@ -0,0 +1,54 @@
+package ru.windcorp.progressia.server.world.tasks;
+
+import java.util.Objects;
+import java.util.function.Consumer;
+
+import glm.Glm;
+import glm.vec._3.i.Vec3i;
+import ru.windcorp.progressia.common.world.tile.PacketAffectTile;
+
+public class CachedTileChange<P extends PacketAffectTile> extends CachedChunkChange<P> {
+
+	public CachedTileChange(Consumer<? super CachedChange> disposer, P packet) {
+		super(disposer, packet);
+	}
+	
+	@Override
+	public int hashCode() {
+		PacketAffectTile packet = getPacket();
+		Vec3i biw = packet.getBlockInWorld();
+		
+		final int prime = 31;
+		int result = 1;
+		result = prime * result + biw.x;
+		result = prime * result + biw.y;
+		result = prime * result + biw.z;
+		result = prime * result + Objects.hashCode(packet.getFace());
+		result = prime * result + packet.getTag();
+		return result;
+	}
+	
+	@Override
+	public boolean equals(Object obj) {
+		if (!(obj instanceof CachedTileChange<?>)) return false;
+		
+		PacketAffectTile my = getPacket();
+		PacketAffectTile other = ((CachedTileChange<?>) obj).getPacket();
+		
+		// Tag of -1 signals that we should ignore it
+		if (my.getTag() == -1 || other.getTag() == -1) return false;
+		
+		return Glm.equals(my.getBlockInWorld(), other.getBlockInWorld())
+				&& (my.getFace() == other.getFace())
+				&& (my.getTag() == other.getTag());
+	}
+	
+	@Override
+	public String toString() {
+		PacketAffectTile packet = getPacket();
+		Vec3i biw = packet.getBlockInWorld();
+		
+		return getClass().getSimpleName() + " (" + biw.x + "; " + biw.y + "; " + biw.z + "; " + packet.getFace() + "; tag: " + packet.getTag() + ")";
+	}
+
+}

src/main/java/ru/windcorp/progressia/server/world/tasks/CachedWorldChange.java

@@ -4,10 +4,10 @@ import java.util.function.Consumer;
 
 import glm.vec._3.i.Vec3i;
 import ru.windcorp.progressia.common.util.Vectors;
-import ru.windcorp.progressia.common.world.PacketWorldChange;
+import ru.windcorp.progressia.common.world.PacketAffectWorld;
 import ru.windcorp.progressia.server.Server;
 
-public abstract class CachedWorldChange<P extends PacketWorldChange> extends CachedChange {
+public abstract class CachedWorldChange<P extends PacketAffectWorld> extends CachedChange {
 	
 	private final P packet;
 

src/main/java/ru/windcorp/progressia/server/world/tasks/ChangeEntity.java

@@ -4,7 +4,7 @@ import java.util.function.Consumer;
 
 import glm.vec._3.i.Vec3i;
 import ru.windcorp.progressia.common.world.entity.EntityData;
-import ru.windcorp.progressia.common.world.entity.PacketEntityChange;
+import ru.windcorp.progressia.common.world.entity.PacketChangeEntity;
 import ru.windcorp.progressia.server.Server;
 
 class ChangeEntity extends CachedChange {
@@ -12,7 +12,7 @@ class ChangeEntity extends CachedChange {
 	private EntityData entity;
 	private StateChange<?> change;
 
-	private final PacketEntityChange packet = new PacketEntityChange();
+	private final PacketChangeEntity packet = new PacketChangeEntity();
 
 	public ChangeEntity(Consumer<? super CachedChange> disposer) {
 		super(disposer);
@@ -56,4 +56,15 @@ class ChangeEntity extends CachedChange {
 		this.change = null;
 	}
 	
+	@Override
+	public int hashCode() {
+		return System.identityHashCode(entity);
+	}
+	
+	@Override
+	public boolean equals(Object obj) {
+		if (!(obj instanceof ChangeEntity)) return false;
+		return ((ChangeEntity) obj).entity == entity;
+	}
+
 }

src/main/java/ru/windcorp/progressia/server/world/tasks/RemoveTile.java

@@ -4,7 +4,7 @@ import java.util.function.Consumer;
 
 import ru.windcorp.progressia.common.world.tile.PacketRemoveTile;
 
-class RemoveTile extends CachedChunkChange<PacketRemoveTile> {
+class RemoveTile extends CachedTileChange<PacketRemoveTile> {
 
 	public RemoveTile(Consumer<? super CachedChange> disposer) {
 		super(disposer, new PacketRemoveTile());

src/main/java/ru/windcorp/progressia/server/world/tasks/SetBlock.java

@@ -4,7 +4,7 @@ import java.util.function.Consumer;
 
 import ru.windcorp.progressia.common.world.block.PacketSetBlock;
 
-class SetBlock extends CachedChunkChange<PacketSetBlock> {
+class SetBlock extends CachedBlockChange<PacketSetBlock> {
 
 	public SetBlock(Consumer<? super CachedChange> disposer) {
 		super(disposer, new PacketSetBlock());

src/main/java/ru/windcorp/progressia/server/world/ticking/Change.java

@@ -24,4 +24,16 @@ public abstract class Change extends TickerTask {
 		affect(server);
 	}
 	
+	@Override
+	public int hashCode() {
+		// Use instance hash code by default
+		return super.hashCode();
+	}
+	
+	@Override
+	public boolean equals(Object obj) {
+		// Use instance-based equals() by default
+		return super.equals(obj);
+	}
+
 }

src/main/java/ru/windcorp/progressia/server/world/ticking/TickerCoordinator.java

@@ -3,7 +3,9 @@ package ru.windcorp.progressia.server.world.ticking;
 import java.util.ArrayList;
 import java.util.Collection;
 import java.util.ConcurrentModificationException;
+import java.util.HashSet;
 import java.util.Objects;
+import java.util.concurrent.atomic.AtomicBoolean;
 import java.util.concurrent.atomic.AtomicInteger;
 
 import org.apache.logging.log4j.LogManager;
@@ -14,6 +16,9 @@ import com.google.common.collect.ImmutableList;
 
 import ru.windcorp.progressia.common.Units;
 import ru.windcorp.progressia.common.util.crash.CrashReports;
+import ru.windcorp.progressia.common.world.ChunkData;
+import ru.windcorp.progressia.common.world.ChunkDataListener;
+import ru.windcorp.progressia.common.world.ChunkDataListeners;
 import ru.windcorp.progressia.server.Server;
 
 /**
@@ -28,7 +33,7 @@ public class TickerCoordinator {
 	private final Server server;
 	
 	// Synchronized manually
-	private final Collection<Change> pendingChanges = new ArrayList<>(INITIAL_QUEUE_SIZE);
+	private final Collection<Change> pendingChanges = new HashSet<>(INITIAL_QUEUE_SIZE);
 	// Synchronized manually
 	private final Collection<Evaluation> pendingEvaluations = new ArrayList<>(INITIAL_QUEUE_SIZE);
 	
@@ -49,6 +54,8 @@ public class TickerCoordinator {
 	
 	private final AtomicInteger workingTickers = new AtomicInteger();
 	
+	private final AtomicBoolean canChange = new AtomicBoolean(true);
+	
 	private boolean isTickStartSet = false;
 	private long tickStart = -1;
 	private double tickLength = 1.0 / 20; // Do something about it
@@ -66,6 +73,15 @@ public class TickerCoordinator {
 		
 		this.tickers = ImmutableList.copyOf(tickerCollection);
 		this.threads = Collections2.transform(this.tickers, Ticker::getThread); // Immutable because it is a view
+		
+		server.getWorld().getData().addListener(ChunkDataListeners.createAdder(new ChunkDataListener() {
+			@Override
+			public void onChunkChanged(ChunkData chunk) {
+				if (!canChange.get()) {
+					throw CrashReports.report(null, "A change has been detected during evaluation phase");
+				}
+			}
+		}));
 	}
 	
 	/*
@@ -157,7 +173,9 @@ public class TickerCoordinator {
 	}
 	
 	private synchronized void runOnePass() throws InterruptedException {
+		canChange.set(false);
 		runPassStage(pendingEvaluations, "EVALUATION");
+		canChange.set(true);
 		runPassStage(pendingChanges, "CHANGE");
 	}