Optimize tag class and supplier lookup

This breaks registering of custom types, but /shrug

src/main/java/com/github/steveice10/opennbt/NBTIO.java

@@ -1,8 +1,6 @@
 package com.github.steveice10.opennbt;
 
-import com.github.steveice10.opennbt.tag.TagRegistry;
 import com.github.steveice10.opennbt.tag.builtin.CompoundTag;
-import com.github.steveice10.opennbt.tag.builtin.Tag;
 
 import java.io.DataInput;
 import java.io.DataInputStream;
@@ -75,12 +73,12 @@ public class NBTIO {
             in = new GZIPInputStream(in);
         }
 
-        Tag tag = readTag(in, littleEndian);
+        CompoundTag tag = readTag(in, littleEndian);
         if(!(tag instanceof CompoundTag)) {
             throw new IOException("Root tag is not a CompoundTag!");
         }
 
-        return (CompoundTag) tag;
+        return tag;
     }
 
     /**

src/main/java/com/github/steveice10/opennbt/tag/TagRegistry.java

@@ -13,22 +13,20 @@ import com.github.steveice10.opennbt.tag.builtin.LongTag;
 import com.github.steveice10.opennbt.tag.builtin.ShortTag;
 import com.github.steveice10.opennbt.tag.builtin.StringTag;
 import com.github.steveice10.opennbt.tag.builtin.Tag;
-import it.unimi.dsi.fastutil.ints.Int2ObjectMap;
-import it.unimi.dsi.fastutil.ints.Int2ObjectOpenHashMap;
 import it.unimi.dsi.fastutil.objects.Object2IntMap;
 import it.unimi.dsi.fastutil.objects.Object2IntOpenHashMap;
 import org.jetbrains.annotations.Nullable;
 
 import java.util.function.Supplier;
 
-
 /**
  * A registry containing different tag classes.
  */
-public class TagRegistry {
-    private static final Int2ObjectMap<Class<? extends Tag>> idToTag = new Int2ObjectOpenHashMap<>();
+public final class TagRegistry {
+    private static final int HIGHEST_ID = LongArrayTag.ID;
+    private static final Class<? extends Tag>[] idToTag = new Class[HIGHEST_ID + 1];
+    private static final Supplier<? extends Tag>[] instanceSuppliers = new Supplier[HIGHEST_ID + 1];
     private static final Object2IntMap<Class<? extends Tag>> tagToId = new Object2IntOpenHashMap<>();
-    private static final Int2ObjectMap<Supplier<? extends Tag>> instanceSuppliers = new Int2ObjectOpenHashMap<>();
 
     static {
         tagToId.defaultReturnValue(-1);
@@ -55,16 +53,18 @@ public class TagRegistry {
      * @throws TagRegisterException If an error occurs while registering the tag.
      */
     public static void register(int id, Class<? extends Tag> tag, Supplier<? extends Tag> supplier) throws TagRegisterException {
-        if(idToTag.containsKey(id)) {
+        if(id < 0 || id > HIGHEST_ID) {
+            throw new TagRegisterException("Tag ID must be between 0 and " + HIGHEST_ID);
+        }
+        if(idToTag[id] != null) {
             throw new TagRegisterException("Tag ID \"" + id + "\" is already in use.");
         }
-
         if(tagToId.containsKey(tag)) {
             throw new TagRegisterException("Tag \"" + tag.getSimpleName() + "\" is already registered.");
         }
 
-        instanceSuppliers.put(id, supplier);
-        idToTag.put(id, tag);
+        instanceSuppliers[id] = supplier;
+        idToTag[id] = tag;
         tagToId.put(tag, id);
     }
 
@@ -75,7 +75,8 @@ public class TagRegistry {
      */
     public static void unregister(int id) {
         tagToId.removeInt(getClassFor(id));
-        idToTag.remove(id);
+        idToTag[id] = null;
+        instanceSuppliers[id] = null;
     }
 
     /**
@@ -86,7 +87,7 @@ public class TagRegistry {
      */
     @Nullable
     public static Class<? extends Tag> getClassFor(int id) {
-        return idToTag.get(id);
+        return id >= 0 && id < idToTag.length ? idToTag[id] : null;
     }
 
     /**
@@ -107,7 +108,7 @@ public class TagRegistry {
      * @throws TagCreateException If an error occurs while creating the tag.
      */
     public static Tag createInstance(int id) throws TagCreateException {
-        Supplier<? extends Tag> supplier = instanceSuppliers.get(id);
+        Supplier<? extends Tag> supplier = id > 0 && id < instanceSuppliers.length ? instanceSuppliers[id] : null;
         if(supplier == null) {
             throw new TagCreateException("Could not find tag with ID \"" + id + "\".");
         }

src/main/java/com/github/steveice10/opennbt/tag/builtin/ByteArrayTag.java

@@ -10,13 +10,14 @@ import java.util.Arrays;
  */
 public class ByteArrayTag extends Tag {
     public static final int ID = 7;
+    private static final byte[] EMPTY_ARRAY = new byte[0];
     private byte[] value;
 
     /**
      * Creates a tag.
      */
     public ByteArrayTag() {
-        this(new byte[0]);
+        this(EMPTY_ARRAY);
     }
 
     /**

src/main/java/com/github/steveice10/opennbt/tag/builtin/CompoundTag.java

@@ -196,7 +196,7 @@ public class CompoundTag extends Tag implements Iterable<Entry<String, Tag>> {
             }
         } catch(TagCreateException e) {
             throw new IOException("Failed to create tag.", e);
-        } catch(EOFException e) {
+        } catch(EOFException ignored) {
             throw new IOException("Closing tag was not found!");
         }
     }

src/main/java/com/github/steveice10/opennbt/tag/builtin/IntArrayTag.java

@@ -12,13 +12,14 @@ import java.util.Arrays;
  */
 public class IntArrayTag extends Tag {
     public static final int ID = 11;
+    private static final int[] EMPTY_ARRAY = new int[0];
     private int[] value;
 
     /**
      * Creates a tag.
      */
     public IntArrayTag() {
-        this(new int[0]);
+        this(EMPTY_ARRAY);
     }
 
     /**

src/main/java/com/github/steveice10/opennbt/tag/builtin/LongArrayTag.java

@@ -12,13 +12,14 @@ import java.util.Arrays;
  */
 public class LongArrayTag extends Tag {
     public static final int ID = 12;
+    private static final long[] EMPTY_ARRAY = new long[0];
     private long[] value;
 
     /**
      * Creates a tag.
      */
     public LongArrayTag() {
-        this(new long[0]);
+        this(EMPTY_ARRAY);
     }
 
     /**