code: function(a, b, c, d) {
        d = Buffer.from(d);
        var $buf = Buffer.allocUnsafe(20 + d.length);
        a.copy($buf, 0, 0, 5);
        b.copy($buf, 5, 0, 10);
        $buf.write(c, 15, 5);
        Buffer.from(d).copy($buf, 20, 0);
        return $buf;
      }
docs: 编码器