Function crc32_combine_

extlibs/minizip/src/crc32.c:396–452

========================================================================= */

(crc1, crc2, len2)

/* ========================================================================= */
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;
}