Expand the remapping stuff.

src/main/java/com/velocitypowered/proxy/protocol/remap/EntityIdRemapper.java

@@ -24,6 +24,6 @@ public interface EntityIdRemapper {
     void setServerEntityId(int id);
 
     static EntityIdRemapper getMapper(int eid, int protocolVersion) {
-        return DummyEntityIdRemapper.INSTANCE;
+        return NoopEntityIdRemapper.INSTANCE;
     }
 }

src/main/java/com/velocitypowered/proxy/protocol/remap/Minecraft18EntityIdRemapper.java

@@ -0,0 +1,40 @@
+package com.velocitypowered.proxy.protocol.remap;
+
+import com.velocitypowered.proxy.protocol.ProtocolConstants;
+import io.netty.buffer.ByteBuf;
+
+public class Minecraft18EntityIdRemapper implements EntityIdRemapper {
+    private final int clientId;
+    private int serverId;
+
+    public Minecraft18EntityIdRemapper(int clientId, int serverId) {
+        this.clientId = clientId;
+        this.serverId = serverId;
+    }
+
+    @Override
+    public ByteBuf remap(ByteBuf original, ProtocolConstants.Direction direction) {
+        if (clientId == serverId) {
+            // If these are equal (i.e. first connection), no remapping is required.
+            return original.retain();
+        }
+
+        // TODO: Implement.
+        throw new UnsupportedOperationException("1.8 doesn't allow switching servers.");
+    }
+
+    @Override
+    public int getClientEntityId() {
+        return clientId;
+    }
+
+    @Override
+    public int getServerEntityId() {
+        return serverId;
+    }
+
+    @Override
+    public void setServerEntityId(int id) {
+        this.serverId = id;
+    }
+}

src/main/java/com/velocitypowered/proxy/protocol/remap/NoopEntityIdRemapper.java

@@ -3,10 +3,10 @@ package com.velocitypowered.proxy.protocol.remap;
 import com.velocitypowered.proxy.protocol.ProtocolConstants;
 import io.netty.buffer.ByteBuf;
 
-public class DummyEntityIdRemapper implements EntityIdRemapper {
-    public static final DummyEntityIdRemapper INSTANCE = new DummyEntityIdRemapper();
+public class NoopEntityIdRemapper implements EntityIdRemapper {
+    public static final NoopEntityIdRemapper INSTANCE = new NoopEntityIdRemapper();
 
-    private DummyEntityIdRemapper() {
+    private NoopEntityIdRemapper() {
 
     }