Add in a remapping event for mods to consume.

src/main/java/cpw/mods/fml/common/Loader.java

@@ -47,6 +47,7 @@ import cpw.mods.fml.common.LoaderState.ModState;
 import cpw.mods.fml.common.discovery.ModDiscoverer;
 import cpw.mods.fml.common.event.FMLInterModComms;
 import cpw.mods.fml.common.event.FMLLoadEvent;
+import cpw.mods.fml.common.event.FMLModIdMappingEvent;
 import cpw.mods.fml.common.functions.ArtifactVersionNameFunction;
 import cpw.mods.fml.common.functions.ModIdFunction;
 import cpw.mods.fml.common.registry.GameData;
@@ -837,4 +838,16 @@ public class Loader
         FMLLog.info("Attempting connection with missing mods %s at %s", difference, side);
         return true;
     }
+
+    public void fireRemapEvent(Map<String, Integer[]> remaps)
+    {
+        if (remaps.isEmpty())
+        {
+            FMLLog.finest("Skipping remap event - no remaps occured");
+        }
+        else
+        {
+            modController.propogateStateMessage(new FMLModIdMappingEvent(remaps));
+        }
+    }
 }

src/main/java/cpw/mods/fml/common/event/FMLModIdMappingEvent.java

@@ -0,0 +1,48 @@
+package cpw.mods.fml.common.event;
+
+import java.util.Collections;
+import java.util.Comparator;
+import java.util.List;
+import java.util.Map;
+import java.util.Map.Entry;
+
+import com.google.common.collect.ImmutableList;
+import com.google.common.collect.Lists;
+
+public class FMLModIdMappingEvent extends FMLEvent {
+    public static enum RemapTarget { BLOCK, ITEM }
+    public class ModRemapping
+    {
+        public final int oldId;
+        public final int newId;
+        public final String tag;
+        public final RemapTarget remapTarget;
+        public ModRemapping(int oldId, int newId, String tag)
+        {
+            this.oldId = oldId;
+            this.newId = newId;
+            this.tag = tag.substring(1);
+            this.remapTarget = tag.charAt(0) == '\u0001' ? RemapTarget.BLOCK : RemapTarget.ITEM;
+        }
+
+    }
+    public final ImmutableList<ModRemapping> remappedIds;
+
+    public FMLModIdMappingEvent(Map<String, Integer[]> mappings)
+    {
+        List<ModRemapping> remappings = Lists.newArrayList();
+        for (Entry<String, Integer[]> mapping : mappings.entrySet())
+        {
+            remappings.add(new ModRemapping(mapping.getValue()[0], mapping.getValue()[1], mapping.getKey()));
+        }
+
+        Collections.sort(remappings, new Comparator<ModRemapping>() {
+            @Override
+            public int compare(ModRemapping o1, ModRemapping o2)
+            {
+                return Integer.compare(o1.newId,o2.newId);
+            }
+        });
+        remappedIds = ImmutableList.copyOf(remappings);
+    }
+}

src/main/java/cpw/mods/fml/common/registry/GameData.java

@@ -16,15 +16,12 @@ import java.io.File;
 import java.io.IOException;
 import java.util.Map;
 import java.util.Map.Entry;
-import java.util.Set;
 import java.util.logging.Level;
 
 import net.minecraft.block.Block;
 import net.minecraft.item.Item;
 import net.minecraft.item.ItemBlock;
 import net.minecraft.item.ItemStack;
-import net.minecraft.nbt.NBTTagCompound;
-import net.minecraft.nbt.NBTTagList;
 
 import com.google.common.base.Charsets;
 import com.google.common.base.Joiner;
@@ -32,7 +29,6 @@ import com.google.common.base.Joiner.MapJoiner;
 import com.google.common.collect.HashBasedTable;
 import com.google.common.collect.ImmutableListMultimap;
 import com.google.common.collect.Maps;
-import com.google.common.collect.Sets;
 import com.google.common.collect.Table;
 import com.google.common.io.Files;
 
@@ -215,6 +211,7 @@ public class GameData {
 
     public static void injectWorldIDMap(Map<String, Integer> dataList)
     {
+        Map<String, Integer[]> remaps = Maps.newHashMap();
         blockRegistry.beginIdSwap();
         itemRegistry.beginIdSwap();
         for (Entry<String, Integer> entry : dataList.entrySet())
@@ -237,6 +234,7 @@ public class GameData {
             if (currId != newId)
             {
                 FMLLog.info("Found %s id mismatch %s : %d %d", isBlock ? "block" : "item", itemName, currId, newId);
+                remaps.put(itemName, new Integer[] { currId, newId });
             }
 
             if (isBlock)
@@ -251,5 +249,6 @@ public class GameData {
 
         blockRegistry.completeIdSwap();
         itemRegistry.completeIdSwap();
+        Loader.instance().fireRemapEvent(remaps);
     }
 }