Preserved merged tag order

- Merged datapack tags keep first-seen value order with duplicates removed instead of re-sorting, so world preset tag order is preserved
- NeoForge remove lists are kept as the deduplicated union of both tags instead of being dropped

adapters/modded-common/src/main/java/art/arcane/iris/modded/ModdedDatapackTagMerge.java

@@ -25,12 +25,13 @@ import java.io.IOException;
 import java.nio.charset.StandardCharsets;
 import java.nio.file.Files;
 import java.nio.file.Path;
+import java.util.LinkedHashMap;
 import java.util.Map;
-import java.util.TreeMap;
 
 final class ModdedDatapackTagMerge {
     private static final String REPLACE = "replace";
     private static final String VALUES = "values";
+    private static final String REMOVE = "remove";
 
     private ModdedDatapackTagMerge() {
     }
@@ -45,19 +46,27 @@ final class ModdedDatapackTagMerge {
     static void merge(Path source, Path destination) throws IOException {
         JSONObject incoming = read(source);
         JSONObject existing = read(destination);
-        Map<String, Object> values = new TreeMap<>();
-        collect(existing, values, destination);
-        collect(incoming, values, source);
-        JSONArray merged = new JSONArray();
-        for (Object value : values.values()) {
-            merged.put(value);
-        }
         JSONObject tag = new JSONObject();
         tag.put(REPLACE, existing.optBoolean(REPLACE, false) || incoming.optBoolean(REPLACE, false));
-        tag.put(VALUES, merged);
+        tag.put(VALUES, union(existing, incoming, VALUES, source, destination));
+        if (existing.has(REMOVE) || incoming.has(REMOVE)) {
+            tag.put(REMOVE, union(existing, incoming, REMOVE, source, destination));
+        }
         Files.writeString(destination, tag.toString(4), StandardCharsets.UTF_8);
     }
 
+    private static JSONArray union(JSONObject existing, JSONObject incoming, String field, Path source, Path destination)
+            throws IOException {
+        Map<String, Object> entries = new LinkedHashMap<>();
+        collect(existing, field, entries, destination);
+        collect(incoming, field, entries, source);
+        JSONArray merged = new JSONArray();
+        for (Object entry : entries.values()) {
+            merged.put(entry);
+        }
+        return merged;
+    }
+
     private static JSONObject read(Path tag) throws IOException {
         try {
             return new JSONObject(Files.readString(tag, StandardCharsets.UTF_8));
@@ -66,18 +75,18 @@ final class ModdedDatapackTagMerge {
         }
     }
 
-    private static void collect(JSONObject tag, Map<String, Object> values, Path origin) throws IOException {
-        JSONArray entries = tag.optJSONArray(VALUES);
-        if (entries == null) {
+    private static void collect(JSONObject tag, String field, Map<String, Object> entries, Path origin) throws IOException {
+        JSONArray values = tag.optJSONArray(field);
+        if (values == null) {
             return;
         }
-        for (int index = 0; index < entries.length(); index++) {
-            Object entry = entries.get(index);
+        for (int index = 0; index < values.length(); index++) {
+            Object entry = values.get(index);
             String key = entry instanceof JSONObject object ? object.optString("id", "") : entry.toString();
             if (key.isBlank()) {
-                throw new IOException("Iris cannot merge datapack tag " + origin + ": entry " + index + " has no id.");
+                throw new IOException("Iris cannot merge datapack tag " + origin + ": " + field + " entry " + index + " has no id.");
             }
-            values.putIfAbsent(key, entry);
+            entries.putIfAbsent(key, entry);
         }
     }
 }

adapters/modded-common/src/test/java/art/arcane/iris/modded/ModdedForcedDatapackTest.java

@@ -19,6 +19,7 @@
 package art.arcane.iris.modded;
 
 import art.arcane.volmlib.util.collection.KSet;
+import art.arcane.volmlib.util.json.JSONArray;
 import art.arcane.volmlib.util.json.JSONObject;
 import art.arcane.iris.pack.datapack.DataVersion;
 import art.arcane.iris.pack.datapack.v1217.DataFixerV1217;
@@ -291,6 +292,60 @@ public class ModdedForcedDatapackTest {
         }
     }
 
+    @Test
+    public void tagMergeKeepsFirstSeenOrderAndUnionsRemoveLists() throws IOException {
+        Path root = Files.createTempDirectory("iris-forced-pack-tag-order");
+        try {
+            Path tag = Path.of("data/minecraft/tags/worldgen/world_preset/normal.json");
+            Path source = root.resolve("source");
+            Path destination = root.resolve("destination");
+            Files.createDirectories(source.resolve(tag).getParent());
+            Files.createDirectories(destination.resolve(tag).getParent());
+            Files.writeString(destination.resolve(tag),
+                    "{\"replace\":false,\"values\":[\"irisworldgen:z\",\"irisworldgen:a\"],\"remove\":[\"minecraft:large_biomes\"]}",
+                    StandardCharsets.UTF_8);
+            Files.writeString(source.resolve(tag),
+                    "{\"replace\":false,\"values\":[\"irisworldgen:m\",\"irisworldgen:a\"],\"remove\":[\"minecraft:amplified\",\"minecraft:large_biomes\"]}",
+                    StandardCharsets.UTF_8);
+
+            ModdedForcedDatapack.mergeDirectory(source, destination);
+
+            JSONObject merged = new JSONObject(Files.readString(destination.resolve(tag), StandardCharsets.UTF_8));
+            assertEquals(List.of("irisworldgen:z", "irisworldgen:a", "irisworldgen:m"), strings(merged.getJSONArray("values")));
+            assertEquals(List.of("minecraft:large_biomes", "minecraft:amplified"), strings(merged.getJSONArray("remove")));
+        } finally {
+            deleteTree(root);
+        }
+    }
+
+    @Test
+    public void tagMergeWithoutRemoveListsWritesNone() throws IOException {
+        Path root = Files.createTempDirectory("iris-forced-pack-tag-no-remove");
+        try {
+            Path tag = Path.of("data/minecraft/tags/worldgen/biome/is_overworld.json");
+            for (String side : List.of("source", "destination")) {
+                Files.createDirectories(root.resolve(side).resolve(tag).getParent());
+                Files.writeString(root.resolve(side).resolve(tag),
+                        "{\"replace\":false,\"values\":[\"iris:biomes/" + side + "\"]}", StandardCharsets.UTF_8);
+            }
+
+            ModdedForcedDatapack.mergeDirectory(root.resolve("source"), root.resolve("destination"));
+
+            JSONObject merged = new JSONObject(Files.readString(root.resolve("destination").resolve(tag), StandardCharsets.UTF_8));
+            assertFalse(merged.has("remove"));
+        } finally {
+            deleteTree(root);
+        }
+    }
+
+    private static List<String> strings(JSONArray array) {
+        List<String> values = new ArrayList<>();
+        for (int index = 0; index < array.length(); index++) {
+            values.add(array.get(index).toString());
+        }
+        return values;
+    }
+
     @Test
     public void tagMergeKeepsStrictConflictsForNonTagFilesInTheSameMerge() throws IOException {
         Path root = Files.createTempDirectory("iris-forced-pack-tag-strict");