征求对原始crc32校验和的意见

有人能给我解释一下为什么crc反转两次吗

/* ========================================================================
 * Table of CRC-32's of all single-byte values (made by make_crc_table)
 */// definicao de uint e uchar

 #include <stdio.h>

//#define TEST 1

#ifndef uchar
#define uchar unsigned char
#endif

#ifndef uint
#define uint unsigned int
#endif

#ifndef ushort
#define ushort unsigned short
#endif

static const uint crc_table[256] = {
  0x00000000L, 0x77073096L, 0xee0e612cL, 0x990951baL, 0x076dc419L,
  0x706af48fL, 0xe963a535L, 0x9e6495a3L, 0x0edb8832L, 0x79dcb8a4L,
  0xe0d5e91eL, 0x97d2d988L, 0x09b64c2bL, 0x7eb17cbdL, 0xe7b82d07L,
  0x90bf1d91L, 0x1db71064L, 0x6ab020f2L, 0xf3b97148L, 0x84be41deL,
  0x1adad47dL, 0x6ddde4ebL, 0xf4d4b551L, 0x83d385c7L, 0x136c9856L,
  0x646ba8c0L, 0xfd62f97aL, 0x8a65c9ecL, 0x14015c4fL, 0x63066cd9L,
  0xfa0f3d63L, 0x8d080df5L, 0x3b6e20c8L, 0x4c69105eL, 0xd56041e4L,
  0xa2677172L, 0x3c03e4d1L, 0x4b04d447L, 0xd20d85fdL, 0xa50ab56bL,
  0x35b5a8faL, 0x42b2986cL, 0xdbbbc9d6L, 0xacbcf940L, 0x32d86ce3L,
  0x45df5c75L, 0xdcd60dcfL, 0xabd13d59L, 0x26d930acL, 0x51de003aL,
  0xc8d75180L, 0xbfd06116L, 0x21b4f4b5L, 0x56b3c423L, 0xcfba9599L,
  0xb8bda50fL, 0x2802b89eL, 0x5f058808L, 0xc60cd9b2L, 0xb10be924L,
  0x2f6f7c87L, 0x58684c11L, 0xc1611dabL, 0xb6662d3dL, 0x76dc4190L,
  0x01db7106L, 0x98d220bcL, 0xefd5102aL, 0x71b18589L, 0x06b6b51fL,
  0x9fbfe4a5L, 0xe8b8d433L, 0x7807c9a2L, 0x0f00f934L, 0x9609a88eL,
  0xe10e9818L, 0x7f6a0dbbL, 0x086d3d2dL, 0x91646c97L, 0xe6635c01L,
  0x6b6b51f4L, 0x1c6c6162L, 0x856530d8L, 0xf262004eL, 0x6c0695edL,
  0x1b01a57bL, 0x8208f4c1L, 0xf50fc457L, 0x65b0d9c6L, 0x12b7e950L,
  0x8bbeb8eaL, 0xfcb9887cL, 0x62dd1ddfL, 0x15da2d49L, 0x8cd37cf3L,
  0xfbd44c65L, 0x4db26158L, 0x3ab551ceL, 0xa3bc0074L, 0xd4bb30e2L,
  0x4adfa541L, 0x3dd895d7L, 0xa4d1c46dL, 0xd3d6f4fbL, 0x4369e96aL,
  0x346ed9fcL, 0xad678846L, 0xda60b8d0L, 0x44042d73L, 0x33031de5L,
  0xaa0a4c5fL, 0xdd0d7cc9L, 0x5005713cL, 0x270241aaL, 0xbe0b1010L,
  0xc90c2086L, 0x5768b525L, 0x206f85b3L, 0xb966d409L, 0xce61e49fL,
  0x5edef90eL, 0x29d9c998L, 0xb0d09822L, 0xc7d7a8b4L, 0x59b33d17L,
  0x2eb40d81L, 0xb7bd5c3bL, 0xc0ba6cadL, 0xedb88320L, 0x9abfb3b6L,
  0x03b6e20cL, 0x74b1d29aL, 0xead54739L, 0x9dd277afL, 0x04db2615L,
  0x73dc1683L, 0xe3630b12L, 0x94643b84L, 0x0d6d6a3eL, 0x7a6a5aa8L,
  0xe40ecf0bL, 0x9309ff9dL, 0x0a00ae27L, 0x7d079eb1L, 0xf00f9344L,
  0x8708a3d2L, 0x1e01f268L, 0x6906c2feL, 0xf762575dL, 0x806567cbL,
  0x196c3671L, 0x6e6b06e7L, 0xfed41b76L, 0x89d32be0L, 0x10da7a5aL,
  0x67dd4accL, 0xf9b9df6fL, 0x8ebeeff9L, 0x17b7be43L, 0x60b08ed5L,
  0xd6d6a3e8L, 0xa1d1937eL, 0x38d8c2c4L, 0x4fdff252L, 0xd1bb67f1L,
  0xa6bc5767L, 0x3fb506ddL, 0x48b2364bL, 0xd80d2bdaL, 0xaf0a1b4cL,
  0x36034af6L, 0x41047a60L, 0xdf60efc3L, 0xa867df55L, 0x316e8eefL,
  0x4669be79L, 0xcb61b38cL, 0xbc66831aL, 0x256fd2a0L, 0x5268e236L,
  0xcc0c7795L, 0xbb0b4703L, 0x220216b9L, 0x5505262fL, 0xc5ba3bbeL,
  0xb2bd0b28L, 0x2bb45a92L, 0x5cb36a04L, 0xc2d7ffa7L, 0xb5d0cf31L,
  0x2cd99e8bL, 0x5bdeae1dL, 0x9b64c2b0L, 0xec63f226L, 0x756aa39cL,
  0x026d930aL, 0x9c0906a9L, 0xeb0e363fL, 0x72076785L, 0x05005713L,
  0x95bf4a82L, 0xe2b87a14L, 0x7bb12baeL, 0x0cb61b38L, 0x92d28e9bL,
  0xe5d5be0dL, 0x7cdcefb7L, 0x0bdbdf21L, 0x86d3d2d4L, 0xf1d4e242L,
  0x68ddb3f8L, 0x1fda836eL, 0x81be16cdL, 0xf6b9265bL, 0x6fb077e1L,
  0x18b74777L, 0x88085ae6L, 0xff0f6a70L, 0x66063bcaL, 0x11010b5cL,
  0x8f659effL, 0xf862ae69L, 0x616bffd3L, 0x166ccf45L, 0xa00ae278L,
  0xd70dd2eeL, 0x4e048354L, 0x3903b3c2L, 0xa7672661L, 0xd06016f7L,
  0x4969474dL, 0x3e6e77dbL, 0xaed16a4aL, 0xd9d65adcL, 0x40df0b66L,
  0x37d83bf0L, 0xa9bcae53L, 0xdebb9ec5L, 0x47b2cf7fL, 0x30b5ffe9L,
  0xbdbdf21cL, 0xcabac28aL, 0x53b39330L, 0x24b4a3a6L, 0xbad03605L,
  0xcdd70693L, 0x54de5729L, 0x23d967bfL, 0xb3667a2eL, 0xc4614ab8L,
  0x5d681b02L, 0x2a6f2b94L, 0xb40bbe37L, 0xc30c8ea1L, 0x5a05df1bL,
  0x2d02ef8dL
};


/* =========================================================================
 * This function can be used by asm versions of crc32()
 */
const uint *   get_crc_table()
{
#ifdef DYNAMIC_CRC_TABLE
  if (crc_table_empty) make_crc_table();
#endif
  return (const uint *)crc_table;
}

/* ========================================================================= */
#define DO1(buf) crc = crc_table[((int)crc ^ (*buf++)) & 0xff] ^ (crc >> 8);
#define DO2(buf)  DO1(buf); DO1(buf);
#define DO4(buf)  DO2(buf); DO2(buf);
#define DO8(buf)  DO4(buf); DO4(buf);

/* ========================================================================= */
uint   crc32(
    uint crc,
    const uchar *buf,
    uint len)
{
    if (buf == NULL) return 0L;
#ifdef DYNAMIC_CRC_TABLE
    if (crc_table_empty)
      make_crc_table();
#endif
    crc = crc ^ 0xffffffffL;
    while (len >= 8)
    {
      DO8(buf);
      len -= 8;
    }
    if (len) do {
      DO1(buf);
    } while (--len);
    return crc ^ 0xffffffffL;
}


#ifdef TEST

int main()
{

    printf("Testando crc \n");
        printf("%0.8x \n ", crc32(0,"ala", 1));



}


#endif

/*========================================================================
*所有单字节值的CRC-32表（由make_CRC_表制作）
*///明确规定
#包括
//#定义测试1
#伊芬德夫·乌查尔
#定义uchar无符号字符
#恩迪夫
#ifndef单元
#定义uint unsigned int
#恩迪夫
#伊芬德夫乌斯波特酒店
#定义ushort无符号短
#恩迪夫
静态常数crc_表[256]={
0x00000000L、0x7707096L、0xee0e612cL、0x990951baL、0x076dc419L、，
0x706af48fL、0xe963a535L、0x9e6495a3L、0x0edb8832L、0x79dcb8a4L、，
0xe0d5e91eL、0x97d2d988L、0x09b64c2bL、0x7eb17cbdL、0xe7b82d07L、，
0x90bf1d91L、0x1db71064L、0x6ab020f2L、0xf3b97148L、0x84be41deL、，
0x1adad47dL、0x6ddde4ebL、0xf4d4b551L、0x83d385c7L、0x136c9856L、，
0x646ba8c0L、0xfd62f97aL、0x8a65c9ecL、0x14015c4fL、0x63066cd9L、，
0xfa0f3d63L、0x8d080df5L、0x3b6e20c8L、0x4c69105eL、0xd56041e4L、，
0xa2677172L、0x3c03e4d1L、0x4B04447L、0xd20d85fdL、0xa50ab56bL、，
0x35b5a8faL、0x42b2986cL、0xdbbbc9d6L、0xacbcf940L、0x32d86ce3L、，
0x45df5c75L、0xdcd60dcfL、0xabd13d59L、0x26d930acL、0x51de003aL、，
0xc8d75180L、0xbfd06116L、0x21b4f4b5L、0x56b3c423L、0xcfba9599L、，
0xb8bda50fL、0x2802b89eL、0x5f058808L、0xc60cd9b2L、0xb10be924L、，
0x2f6f7c87L、0x58684c11L、0xc1611dabL、0xb6662d3dL、0x76dc4190L、，
0x01db7106L、0x98d220bcL、0xefd5102aL、0x71b18589L、0x06b6b51fL、，
0x9fbfe4a5L、0xe8b8d433L、0x7807c9a2L、0x0f00f934L、0x9609a88eL、，
0xe10e9818L、0x7f6a0dbbL、0x086d3d2dL、0x91646c97L、0xe6635c01L、，
0x6b6b51f4L、0x1c6c6162L、0x856530d8L、0xf262004eL、0x6c0695edL、，
0x1b01a57bL、0x8208f4c1L、0xf50fc457L、0x65b0d9c6L、0x12b7e950L、，
0x8bbeb8eaL、0xfcb9887cL、0x62dd1ddfL、0x15da2d49L、0x8cd37cf3L、，
0xfbd44c65L、0x4db26158L、0x3ab551ceL、0xa3bc0074L、0xD4B30E2L、，
0x4adfa541L、0x3dd895d7L、0xa4d1c46dL、0xd3d6f4fbL、0x4399E96AL、，
0x346ed9fcL、0xad678846L、0xda60b8d0L、0x44042d73L、0x33031de5L、，
0xaa0a4c5fL、0xdd0d7cc9L、0x5005713cL、0x270241aaL、0xbe0b1010L、，
0xc90c2086L、0x5768b525L、0x206f85b3L、0xb966d409L、0xce61e49fL、，
0x5edef90eL、0x29d9c998L、0xb0d09822L、0xc7d7a8b4L、0x59b33d17L、，
0x2eb40d81L、0xb7bd5c3bL、0xc0ba6cadL、0xedb88320L、0x9abfb3b6L、，
0x03b6e20cL、0x74b1d29aL、0xead54739L、0x9dd277afL、0x04db2615L、，
0x73dc1683L、0xe3630b12L、0x94643b84L、0x0d6d6a3eL、0x7a6a5aa8L、，
0xe40ecf0bL、0x9309ff9dL、0x0a00ae27L、0x7d079eb1L、0xf00f9344L、，
0x8708a3d2L、0x1e01f268L、0x6906c2feL、0xF76275DL、0x806567cbL、，
0x196c3671L、0x6e6b06e7L、0xfed41b76L、0x89d32be0L、0x10da7a5aL、，
0x67dd4accL、0xf9b9df6fL、0x8ebeeff9L、0x17b7be43L、0x60b08ed5L、，
0xd6d6a3e8L、0xa1d1937eL、0x38d8c2c4L、0x4fdff252L、0xd1bb67f1L、，
0xa6bc5767L、0x3fb506ddL、0x48b2364bL、0xd80d2bdaL、0xaf0a1b4cL、，
0x36034af6L、0x41047a60L、0xdf60efc3L、0xa867df55L、0x316e8eefL、，
0x4669be79L、0xcb61b38cL、0xbc66831aL、0x256fd2a0L、0x5268e236L、，
0xcc0c7795L、0xbb0b4703L、0x220216b9L、0x5505262fL、0xc5ba3bbeL、，
0xb2bd0b28L、0x2bb45a92L、0x5cb36a04L、0xc2d7ffa7L、0xb5d0cf31L、，
0x2cd99e8bL、0x5BDEA1DL、0x9b64c2b0L、0xec63f226L、0x756aa39cL、，
0x026d930aL、0x9c0906a9L、0xeb0e363fL、0x72076785L、0x05005713L、，
0x95bf4a82L、0xe2b87a14L、0x7bb12baeL、0x0cb61b38L、0x92d28e9bL、，
0xe5d5be0dL、0x7cdcefb7L、0x0bdbdf21L、0x86d3d2d4L、0xf1d4e242L、，
0x68ddb3f8L、0x1fda836eL、0x81be16cdL、0xf6b9265bL、0x6fb077e1L、，
0x18b74777L、0x88085ae6L、0xff0f6a70L、0x66063bcaL、0x11010b5cL、，
0x8f659effL、0xf862ae69L、0x616BFD3L、0x166ccf45L、0xa00ae278L、，
0xd70dd2eeL、0x4e048354L、0x3903b3c2L、0xa7672661L、0xd06016f7L、，
0x4969474dL、0x3e6e77dbL、0xaed16a4aL、0xd9d65adcL、0x40df0b66L、，
0x37d83bf0L、0xa9bcae53L、0xdebb9ec5L、0x47b2cf7fL、0x30b5ffe9L、，
0xbdbdf21cL、0xcabac28aL、0x53b39330L、0x24b4a3a6L、0xBAD0365L、，
0xcdd70693L、0x54de5729L、0x23d967bfL、0xb3667a2eL、0xc4614ab8L、，
0x5d681b02L、0x2a6f2b94L、0xb40bbe37L、0xc30c8ea1L、0x5a05df1bL、，
0x2d02ef8dL
};
/* =========================================================================
*此功能可由asm版本的crc32（）使用
*/
consuint*get_crc_table（）
{
#ifdef动态CRC表
如果（crc_table_empty）使_crc_table（）；
#恩迪夫
返回（const uint*）crc_表；
}
/* ========================================================================= */
#定义DO1（buf）crc=crc_表[（int）crc^（*buf++）和0xff]^（crc>>8）；
#定义DO2（buf）DO1（buf）；DO1（buf）；
#定义DO4（buf）DO2（buf）；DO2（buf）；
#定义DO8（buf）DO4（buf）；DO4（buf）；
/* ========================================================================= */
uint crc32(
uint crc，
const uchar*buf，
乌因特透镜）
{
if（buf==NULL）返回0L；
#ifdef动态CRC表
如果（crc\u表\u为空）
制作crc表格（）；
#恩迪夫
crc=crc^0xffffffffL；
而（len>=8）
{
DO8（buf）；
len-=8；
}
如果（len）做{
DO1（buf）；
}而（--len）；
返回crc^0xffffffffL；
}
#ifdef试验
int main（）
{
printf（“Testando crc\n”）；
printf（“%0.8x\n”，crc32（0，“ala”，1））；
}
#恩迪夫

第一次反转完成，因为否则，将前导零位添加到缓冲区不会改变产生的CRC值（假设初始CRC值为零）

我相信最后的反演是这样做的，这样这个函数就可以被增量调用：不必一次调用整个数据，你可以分块调用它，两个反演将在相邻的块上相互抵消

---

请注意，并非在所有CRC32实现上都进行这些反转。

如果您对CRC校验和感兴趣，则必须阅读以下内容：

虽然我已经很久没有读过它了，所以我不确定它是否能回答你的具体问题

编辑：至少它包含参数化

Here is the specification for the CRC-32 algorithm which is reportedly used in PKZip, AUTODIN II, Ethernet, and FDDI. Name : "CRC-32" Width : 32 Poly : 04C11DB7 Init : FFFFFFFF RefIn : True RefOut : True XorOut : FFFFFFFF Check : CBF43926 下面是CRC-32算法的规范，据报道，该算法是 用于PKZip、AUTODIN II、以太网和FDDI。 名称：“CRC-32” 宽度：32 Poly:04C11DB7 初始化：FFF