runelite-api: add ability to get npc index

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

@@ -65,6 +65,7 @@ import net.runelite.rs.api.RSDeque;
 import net.runelite.rs.api.RSHashTable;
 import net.runelite.rs.api.RSIndexedSprite;
 import net.runelite.rs.api.RSItemContainer;
+import net.runelite.rs.api.RSNPC;
 import net.runelite.rs.api.RSName;
 import net.runelite.rs.api.RSWidget;
 
@@ -494,6 +495,24 @@ public abstract class RSClientMixin implements RSClient
 		eventBus.post(gameStateChange);
 	}
 
+
+	@FieldHook("cachedNPCs")
+	@Inject
+	public static void cachedNPCsChanged(int idx)
+	{
+		RSNPC[] cachedNPCs = client.getCachedNPCs();
+		if (idx < 0 || idx >= cachedNPCs.length)
+		{
+			return;
+		}
+
+		RSNPC npc = cachedNPCs[idx];
+		if (npc != null)
+		{
+			npc.setIndex(idx);
+		}
+	}
+
 	@Inject
 	@FieldHook("grandExchangeOffers")
 	public static void onGrandExchangeOffersChanged(int idx)

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

@@ -32,6 +32,9 @@ import net.runelite.rs.api.RSNPCComposition;
 @Mixin(RSNPC.class)
 public abstract class RSNPCMixin implements RSNPC
 {
+	@Inject
+	private int npcIndex;
+
 	@Inject
 	@Override
 	public int getId()
@@ -55,4 +58,18 @@ public abstract class RSNPCMixin implements RSNPC
 		RSNPCComposition composition = getComposition();
 		return composition == null ? -1 : composition.getCombatLevel();
 	}
+
+	@Inject
+	@Override
+	public int getIndex()
+	{
+		return npcIndex;
+	}
+
+	@Inject
+	@Override
+	public void setIndex(int id)
+	{
+		npcIndex = id;
+	}
 }