src/game/java/net/minecraft/util/datafix/fixes/BlockEntityBlockStateFix.java

package net.minecraft.util.datafix.fixes;

import com.mojang.datafixers.DSL;
import com.mojang.datafixers.OpticFinder;
import com.mojang.datafixers.Typed;
import com.mojang.datafixers.schemas.Schema;
import com.mojang.datafixers.types.Type;
import com.mojang.serialization.Dynamic;

public class BlockEntityBlockStateFix extends NamedEntityFix {
    public BlockEntityBlockStateFix(Schema p_14810_, boolean p_14811_) {
        super(p_14810_, p_14811_, "BlockEntityBlockStateFix", References.BLOCK_ENTITY, "minecraft:piston");
    }

    @Override
    protected Typed<?> fix(Typed<?> p_14814_) {
        Type<?> type = this.getOutputSchema().getChoiceType(References.BLOCK_ENTITY, "minecraft:piston");
        Type<?> type1 = type.findFieldType("blockState");
        OpticFinder<?> opticfinder = DSL.fieldFinder("blockState", type1);
        Dynamic<?> dynamic = p_14814_.get(DSL.remainderFinder());
        int i = dynamic.get("blockId").asInt(0);
        dynamic = dynamic.remove("blockId");
        int j = dynamic.get("blockData").asInt(0) & 15;
        dynamic = dynamic.remove("blockData");
        Dynamic<?> dynamic1 = BlockStateData.getTag(i << 4 | j);
        Typed<?> typed = type.pointTyped(p_14814_.getOps()).orElseThrow(() -> new IllegalStateException("Could not create new piston block entity."));
        return typed.set(DSL.remainderFinder(), dynamic)
            .set(
                opticfinder,
                type1.readTyped(dynamic1).result().orElseThrow(() -> new IllegalStateException("Could not parse newly created block state tag.")).getFirst()
            );
    }
}