有人可以向我解释一下Zlib library code中存在的CRC32联合收割机的工作情况。我阅读了一篇关于CRC计算和使用工作的优秀文章,该文章来自Zlib here,但它没有提到组合两个CRC。我想知道组合两个CRC的逻辑。
我在我的代码中尝试了它,因为它是通过复制粘贴,但我不确定是什么使它工作。
从here(截至今天)获取的代码如下:
#define GF2_DIM 32 /* dimension of GF(2) vectors (length of CRC) */
/* ========================================================================= */
local unsigned long gf2_matrix_times(mat, vec)
unsigned long *mat;
unsigned long vec;
{
unsigned long sum;
sum = 0;
while (vec) {
if (vec & 1)
sum ^= *mat;
vec >>= 1;
mat++;
}
return sum;
}
/* ========================================================================= */
local void gf2_matrix_square(square, mat)
unsigned long *square;
unsigned long *mat;
{
int n;
for (n = 0; n < GF2_DIM; n++)
square[n] = gf2_matrix_times(mat, mat[n]);
}
/* ========================================================================= */
local uLong crc32_combine_(crc1, crc2, len2)
uLong crc1;
uLong crc2;
z_off64_t len2;
{
int n;
unsigned long row;
unsigned long even[GF2_DIM]; /* even-power-of-two zeros operator */
unsigned long odd[GF2_DIM]; /* odd-power-of-two zeros operator */
/* degenerate case (also disallow negative lengths) */
if (len2 <= 0)
return crc1;
/* put operator for one zero bit in odd */
odd[0] = 0xedb88320UL; /* CRC-32 polynomial */
row = 1;
for (n = 1; n < GF2_DIM; n++) {
odd[n] = row;
row <<= 1;
}
/* put operator for two zero bits in even */
gf2_matrix_square(even, odd);
/* put operator for four zero bits in odd */
gf2_matrix_square(odd, even);
/* apply len2 zeros to crc1 (first square will put the operator for one
zero byte, eight zero bits, in even) */
do {
/* apply zeros operator for this bit of len2 */
gf2_matrix_square(even, odd);
if (len2 & 1)
crc1 = gf2_matrix_times(even, crc1);
len2 >>= 1;
/* if no more bits set, then done */
if (len2 == 0)
break;
/* another iteration of the loop with odd and even swapped */
gf2_matrix_square(odd, even);
if (len2 & 1)
crc1 = gf2_matrix_times(odd, crc1);
len2 >>= 1;
/* if no more bits set, then done */
} while (len2 != 0);
/* return combined crc */
crc1 ^= crc2;
return crc1;
}
谢谢!