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zlib-src/adler32.c  view on Meta::CPAN

/* ========================================================================= */
local uLong adler32_combine_(uLong adler1, uLong adler2, z_off64_t len2) {
    unsigned long sum1;
    unsigned long sum2;
    unsigned rem;

    /* for negative len, return invalid adler32 as a clue for debugging */
    if (len2 < 0)
        return 0xffffffffUL;

    /* the derivation of this formula is left as an exercise for the reader */
    MOD63(len2);                /* assumes len2 >= 0 */
    rem = (unsigned)len2;
    sum1 = adler1 & 0xffff;
    sum2 = rem * sum1;
    MOD(sum2);
    sum1 += (adler2 & 0xffff) + BASE - 1;
    sum2 += ((adler1 >> 16) & 0xffff) + ((adler2 >> 16) & 0xffff) + BASE - rem;
    if (sum1 >= BASE) sum1 -= BASE;
    if (sum1 >= BASE) sum1 -= BASE;
    if (sum2 >= ((unsigned long)BASE << 1)) sum2 -= ((unsigned long)BASE << 1);