wrap up block_id_map in a class

src/data/bedrock/LegacyBlockIdToStringIdMap.php

@@ -0,0 +1,69 @@
+<?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\bedrock;
+
+use pocketmine\utils\AssumptionFailedError;
+use pocketmine\utils\SingletonTrait;
+use function file_get_contents;
+use function is_array;
+use function is_int;
+use function is_string;
+use function json_decode;
+
+final class LegacyBlockIdToStringIdMap{
+	use SingletonTrait;
+
+	/**
+	 * @var string[]
+	 * @phpstan-var array<int, string>
+	 */
+	private $legacyToString = [];
+	/**
+	 * @var int[]
+	 * @phpstan-var array<string, int>
+	 */
+	private $stringToLegacy = [];
+
+	public function __construct(){
+		$stringToLegacyId = json_decode(file_get_contents(\pocketmine\RESOURCE_PATH . 'vanilla/block_id_map.json'), true);
+		if(!is_array($stringToLegacyId)){
+			throw new AssumptionFailedError("Invalid format of block_id_map");
+		}
+		foreach($stringToLegacyId as $stringId => $legacyId){
+			if(!is_string($stringId) or !is_int($legacyId)){
+				throw new AssumptionFailedError("Block ID map should have string keys and int values");
+			}
+			$this->legacyToString[$legacyId] = $stringId;
+			$this->stringToLegacy[$stringId] = $legacyId;
+		}
+	}
+
+	public function legacyToString(int $legacy) : ?string{
+		return $this->legacyToString[$legacy] ?? null;
+	}
+
+	public function stringToLegacy(string $string) : ?int{
+		return $this->stringToLegacy[$string] ?? null;
+	}
+}