Separate ID handling from GameMode

the aliases of 0,1,2,3 remain for user-interface level compatibility.

src/data/java/GameModeIdMap.php

@@ -0,0 +1,67 @@
+<?php
+
+/*
+ *
+ *  ____            _        _   __  __ _                  __  __ ____
+ * |  _ \ ___   ___| | _____| |_|  \/  (_)_ __   ___      |  \/  |  _ \
+ * | |_) / _ \ / __| |/ / _ \ __| |\/| | | '_ \ / _ \_____| |\/| | |_) |
+ * |  __/ (_) | (__|   <  __/ |_| |  | | | | | |  __/_____| |  | |  __/
+ * |_|   \___/ \___|_|\_\___|\__|_|  |_|_|_| |_|\___|     |_|  |_|_|
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU Lesser General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * @author PocketMine Team
+ * @link http://www.pocketmine.net/
+ *
+ *
+*/
+
+declare(strict_types=1);
+
+namespace pocketmine\data\java;
+
+use pocketmine\player\GameMode;
+use pocketmine\utils\SingletonTrait;
+use function array_key_exists;
+
+final class GameModeIdMap{
+	use SingletonTrait;
+
+	/**
+	 * @var GameMode[]
+	 * @phpstan-var array<int, GameMode>
+	 */
+	private array $idToEnum = [];
+
+	/**
+	 * @var int[]
+	 * @phpstan-var array<int, int>
+	 */
+	private array $enumToId = [];
+
+	public function __construct(){
+		$this->register(0, GameMode::SURVIVAL());
+		$this->register(1, GameMode::CREATIVE());
+		$this->register(2, GameMode::ADVENTURE());
+		$this->register(3, GameMode::SPECTATOR());
+	}
+
+	private function register(int $id, GameMode $type) : void{
+		$this->idToEnum[$id] = $type;
+		$this->enumToId[$type->id()] = $id;
+	}
+
+	public function fromId(int $id) : ?GameMode{
+		return $this->idToEnum[$id] ?? null;
+	}
+
+	public function toId(GameMode $type) : int{
+		if(!array_key_exists($type->id(), $this->enumToId)){
+			throw new \InvalidArgumentException("Game mode does not have a mapped ID"); //this should never happen
+		}
+		return $this->enumToId[$type->id()];
+	}
+}