code of adler 32

This is a discussion on code of adler 32 within the C Programming forums, part of the General Programming Boards category; Code: uLong ZEXPORT adler32(adler, buf, len) uLong adler; const Bytef *buf; uInt len; { unsigned long sum2; unsigned n; /* ...

  1. #1
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    code of adler 32

    Code:
    uLong ZEXPORT adler32(adler, buf, len)
        uLong adler;
        const Bytef *buf;
        uInt len;
    {
        unsigned long sum2;
        unsigned n;
    
        /* split Adler-32 into component sums */
        sum2 = (adler >> 16) & 0xffff;
        adler &= 0xffff;
    
        /* in case user likes doing a byte at a time, keep it fast */
        if (len == 1) {
            adler += buf[0];
            if (adler >= BASE)
                adler -= BASE;
            sum2 += adler;
            if (sum2 >= BASE)
                sum2 -= BASE;
            return adler | (sum2 << 16);
        }
    
        /* initial Adler-32 value (deferred check for len == 1 speed) */
        if (buf == Z_NULL)
            return 1L;
    
        /* in case short lengths are provided, keep it somewhat fast */
        if (len < 16) {
            while (len--) {
                adler += *buf++;
                sum2 += adler;
            }
            if (adler >= BASE)
                adler -= BASE;
            MOD4(sum2);             /* only added so many BASE's */
            return adler | (sum2 << 16);
        }
    
        /* do length NMAX blocks -- requires just one modulo operation */
        while (len >= NMAX) {
            len -= NMAX;
            n = NMAX / 16;          /* NMAX is divisible by 16 */
            do {
                DO16(buf);          /* 16 sums unrolled */
                buf += 16;
            } while (--n);
            MOD(adler);
            MOD(sum2);
        }
    
        /* do remaining bytes (less than NMAX, still just one modulo) */
        if (len) {                  /* avoid modulos if none remaining */
            while (len >= 16) {
                len -= 16;
                DO16(buf);
                buf += 16;
            }
            while (len--) {
                adler += *buf++;
                sum2 += adler;
            }
            MOD(adler);
            MOD(sum2);
        }
    
        /* return recombined sums */
        return adler | (sum2 << 16);
    }
    Write the meaning of each line of the code

  2. #2
    C++ Witch laserlight's Avatar
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    Please refer to the homework policy.
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  3. #3
    and the hat of wrongness Salem's Avatar
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    OK, now you're just getting annoying.

    C code
    zlib source code

    You're just not going to get someone here to give you a long wax crayon description of what every line of code in zlib does when you basically don't have a clue about either compression or C programming.

    And since it's only been a couple of days since you last tried this trick, it means that you've not been paying any attention, nor had chance to get down to some solid research of your own.
    If you dance barefoot on the broken glass of undefined behaviour, you've got to expect the occasional cut.
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    prefer kanthane book

  5. #5
    Kernel hacker
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    In thread 109441 I posted a link to the Wiki page of Adler32. This code is not a sample implementation of Adler32, but rather an optimized version that contains different fast variants for different use-cases of the code.

    If there is a particular section of the code you want an explanation of, please feel free to ask SPECIFIC questions, but do not expect someone to explain 50 lines of rather complicated and not exactly written for readability.

    As Salem explained, if you are trying to learn about compression and have little understanding of C as a language, you need to start by learning the language, and then study the art of compression - neither is something you'd expect to learn in a few days.

    --
    Mats
    Compilers can produce warnings - make the compiler programmers happy: Use them!
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