前言

在项目开发过程中，经常会使用到checksum检验和，但是在不同的协议对接过程中发现checksum算法还是有差异的。本文的目的是对常用的checksum算法进行总结，并不断完善和补充，以备后期在项目中直接使用。

checksum算法

目前，使用到的checksum算法如下：

方式1：

/*方式-1
  注：测试网址：http://www.metools.info/code/c128.html
  */
void custom_checksum_1(void)
{
    int i = 0;
    int len = 0;
    unsigned short tempChecksum = 0;
    unsigned char checksum = 0;
    unsigned char data[8] = { 0 };

    /* 33 35 37 45 */
    data[0] = 0x33;
    data[1] = 0x35;
    data[2] = 0x37;
    data[3] = 0x45;

    len = strlen(data);
    for (i = 0; i < len; i++)
    {
        tempChecksum += data[i];
        printf("tempChecksum: %d(0x%X)\n", tempChecksum, tempChecksum);
    }
    printf("\n");

    if (tempChecksum > 0xFF)
    {
        //取补码
        tempChecksum = ~tempChecksum;
        tempChecksum += 1;
    }
    checksum = (unsigned char)tempChecksum;
    printf("checksum: %d(0x%X)\n", checksum, checksum);
}

方式2：

/* 方式-2 
  注：测试网址之前没记录，暂时找不到了，待补充
  */
void custom_checksum_2(void)
{
    int i = 0;
    int len = 0;
    unsigned short tempChecksum = 0;
    unsigned char checksum = 0;
    unsigned char data[8] = { 0 };

    /* 4A 55 93 E5 */
    data[0] = 0x4A;
    data[1] = 0x55;
    data[2] = 0x93;
    data[3] = 0xE5;
    
    len = strlen(data);
    for (i = 0; i < len; i++)
    {
        tempChecksum += data[i];
        printf("tempChecksum: %d(0x%X)\n", tempChecksum, tempChecksum);

        checksum = (unsigned char)tempChecksum; 
        if (tempChecksum > 0xFF)
        {
            checksum += 1;
        }
        printf("checksum: %d(0x%X)\n", checksum, checksum);

        tempChecksum = checksum;
    }
    printf("\n");

    printf("checksum: %d(0x%X)\n", checksum, checksum);
    checksum = ~checksum;
    printf("checksum: %d(0x%X)\n", checksum, checksum);
}