说明：  该源程序可以实现4位crc编码，用户输入需要发送的由0和1组成的字符串，程序运行结果可以显示出4位CRC编码，其生成函数为x4+x+1
(The source program can achieve four crc coding and user input needs to be sent by a string of 0 and 1. The result of the program is a crc code.The crc coding generation function is x4+x+1.)