BlockMixin.java

import it.unimi.dsi.fastutil.longs.Long2ByteLinkedOpenHashMap;
import net.fabricmc.api.EnvType;
import net.fabricmc.api.Environment;
import net.minecraft.block.Block;
import net.minecraft.block.BlockState;
import net.minecraft.util.BooleanBiFunction;
import net.minecraft.util.math.BlockPos;
import net.minecraft.util.math.Direction;
import net.minecraft.util.shape.VoxelShape;
import net.minecraft.util.shape.VoxelShapes;
import net.minecraft.world.BlockView;
import org.spongepowered.asm.mixin.Mixin;
import org.spongepowered.asm.mixin.Overwrite;

@Mixin(value = Block.class, priority = 500)
public class BlockMixin {
    private static final ThreadLocal<Long2ByteLinkedOpenHashMap> FACE_CULL_MAP_ = ThreadLocal.withInitial(() -> {
        Long2ByteLinkedOpenHashMap map = new Long2ByteLinkedOpenHashMap(1000) {
            @Override
            protected void rehash(int newN) {}
        }
        map.defaultReturnValue((byte) 127);
        return map;
    });

    /**
     * Don't use NeighborGroups, calculating its hashCode is slow. Instead, calculate a long cache key based
     * on the state, neighbor state and side.
     */
    @Overwrite
    @Environment(EnvType.CLIENT)
    public static boolean shouldDrawSide(BlockState state, BlockView world, BlockPos pos, Direction side) {
        BlockPos neighbor = pos.offset(side);
        BlockState neighborState = world.getBlockState(neighbor);
        if (state.skipRenderingSide(neighborState, side)) {
            return false;
        }

        if (!neighborState.isFullBoundsCubeForCulling()) {
            return true;
        }

        Long2ByteLinkedOpenHashMap cullMap = FACE_CULL_MAP_.get();
        // hashCode is unique, see AbstractPropertyContainerMixin
        long cacheKey = ((long) state.hashCode() << 32 + 3) + ((long) neighborState.hashCode() << 3) + side.ordinal();
        byte cached = cullMap.getAndMoveToFirst(cacheKey);
        if (cached != 127) {
            return cached != 0;
        }

        VoxelShape shape = state.getCullShape(world, pos, side);
        VoxelShape neighborShape = neighborState.getCullShape(world, neighbor, side.getOpposite());
        boolean shouldDraw = VoxelShapes.matchesAnywhere(shape, neighborShape, BooleanBiFunction.ONLY_FIRST);

        if (cullMap.size() == 1000) {
            cullMap.removeLastByte();
        }
        cullMap.putAndMoveToFirst(cacheKey, (byte) (shouldDraw ? 1 : 0));

        return shouldDraw;
    }
}