First (untested) look at hooking all the itemstack serializer/deserializer stuff together

this should address #5063 and related issues, if it works correctly.

src/data/bedrock/item/upgrade/R12ItemIdToBlockIdMap.php

@@ -0,0 +1,87 @@
+<?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\item\upgrade;
+
+use pocketmine\utils\AssumptionFailedError;
+use pocketmine\utils\SingletonTrait;
+use pocketmine\utils\Utils;
+use Webmozart\PathUtil\Path;
+use function array_search;
+use function file_get_contents;
+use function is_array;
+use function is_string;
+use function json_decode;
+use const pocketmine\BEDROCK_ITEM_UPGRADE_SCHEMA_PATH;
+
+/**
+ * Maps all known 1.12 and lower item IDs to their respective block IDs, where appropriate.
+ * If an item ID does not have a corresponding 1.12 block ID, assume the item is not a blockitem.
+ *
+ * This is only needed for deserializing blockitems from 1.8 and lower (or 1.12 and lower in the case of PM). In 1.9 and
+ * above, the blockstate NBT is stored in the itemstack NBT, and the item ID is not used.
+ */
+final class R12ItemIdToBlockIdMap{
+	use SingletonTrait;
+
+	private static function make() : self{
+		$map = json_decode(
+			Utils::assumeNotFalse(file_get_contents(Path::join(BEDROCK_ITEM_UPGRADE_SCHEMA_PATH, '1.12.0_item_id_to_block_id_map.json')), "Missing required resource file"),
+			associative: true,
+			flags: JSON_THROW_ON_ERROR
+		);
+		if(!is_array($map)){
+			throw new AssumptionFailedError("Invalid blockitem ID mapping table, expected array as root type");
+		}
+
+		$builtMap = [];
+		foreach($map as $itemId => $blockId){
+			if(!is_string($itemId)){
+				throw new AssumptionFailedError("Invalid blockitem ID mapping table, expected string as key");
+			}
+			if(!is_string($blockId)){
+				throw new AssumptionFailedError("Invalid blockitem ID mapping table, expected string as value");
+			}
+			$builtMap[$itemId] = $blockId;
+		}
+
+		return new self($builtMap);
+	}
+
+	/**
+	 * @param string[] $itemToBlock
+	 * @phpstan-param array<string, string> $itemToBlock
+	 */
+	public function __construct(private array $itemToBlock){}
+
+	public function itemIdToBlockId(string $itemId) : ?string{
+		return $this->itemToBlock[$itemId] ?? null;
+	}
+
+	public function blockIdToItemId(string $blockId) : ?string{
+		//we don't need this for any functionality, so we're not concerned about performance here
+		//however, it might be nice to have for debugging
+		$itemId = array_search($blockId, $this->itemToBlock, true);
+		return $itemId !== false ? $itemId : null;
+	}
+}