Subtract option in 1.12.2

Hey,

this is my bounding box code for my block in 1.12.2:

@Override
        public AxisAlignedBB getBoundingBox(IBlockState state, IBlockAccess source, BlockPos pos) {
            switch ((EnumFacing) state.getValue(BlockHorizontal.FACING)) {
                case SOUTH :
                default :
                    return new AxisAlignedBB(0, 0, 0, 0.5, 1, 1).union(new AxisAlignedBB(0, 0, 0, 0.5, 1, 1));
                case NORTH :
                    return new AxisAlignedBB(0.5, 0, 0, 1, 1, 1).union(new AxisAlignedBB(0.5, 0, 0, 1, 1, 1));
                case EAST :
                    return new AxisAlignedBB(0, 0, 0.5, 1, 1, 1).union(new AxisAlignedBB(0, 0, 0.5, 1, 1, 1));
                case WEST :
                    return new AxisAlignedBB(0, 0, 0, 1, 1, 0.5).union(new AxisAlignedBB(0, 0, 0, 1, 1, 0.5));

Cause the subtract option is not working at all i need to edit it manually. Whats the line of code i need to add?