cache transformer mixin addition

runelite-api/src/main/java/net/runelite/api/overlay/OverlayIndex.java

@@ -27,8 +27,10 @@ package net.runelite.api.overlay;
 import java.io.DataInputStream;
 import java.io.IOException;
 import java.io.InputStream;
+import java.util.HashMap;
 import java.util.HashSet;
 import java.util.Set;
+import java.util.function.Function;
 import lombok.Getter;
 import lombok.extern.slf4j.Slf4j;
 
@@ -38,6 +40,15 @@ public class OverlayIndex
 	@Getter
 	private static final Set<Integer> overlays = new HashSet<>();
 
+	/**
+	 * Stores transformer callbacks for given cache object.
+	 * The key is of format (indexId << 16 | archiveId).
+	 * The value is of format:
+	 * byte[] function(originalBytesFromCache){ if (doNothing) return originalBytesFromCache; else return customBytes; }
+	 */
+	@Getter
+	private static final HashMap<Integer, Function<byte[], byte[]>> cacheTransformers = new HashMap<>();
+
 	static
 	{
 		try (InputStream indexStream = OverlayIndex.class.getResourceAsStream("/runelite/index");
@@ -55,6 +66,16 @@ public class OverlayIndex
 		}
 	}
 
+	public static boolean hasCacheTransformer(int indexId, int archiveId)
+	{
+		return cacheTransformers.containsKey(indexId << 16 | archiveId);
+	}
+
+	public static Function<byte[], byte[]> getCacheTransformer(int indexId, int archiveId)
+	{
+		return cacheTransformers.get(indexId << 16 | archiveId);
+	}
+
 	public static boolean hasOverlay(int indexId, int archiveId)
 	{
 		return overlays.contains(indexId << 16 | archiveId);

runelite-mixins/src/main/java/net/runelite/mixins/RSAbstractArchiveMixin.java

@@ -45,6 +45,11 @@ public abstract class RSAbstractArchiveMixin implements RSAbstractArchive
 		final byte[] rsData = copy$getConfigData(groupId, fileId);
 		final int archiveId = ((RSArchive) this).getIndex();
 
+		if (OverlayIndex.hasCacheTransformer(archiveId, groupId))
+		{
+			return OverlayIndex.getCacheTransformer(archiveId, groupId).apply(rsData);
+		}
+
 		if (!OverlayIndex.hasOverlay(archiveId, groupId))
 		{
 			return rsData;