RFC 1950       ZLIB Compressed Data Format Specification        May 1996


9. Appendix: Sample code

   The following C code computes the Adler-32 checksum of a data buffer.
   It is written for clarity, not for speed.  The sample code is in the
   ANSI C programming language. Non C users may find it easier to read
   with these hints:

      &      Bitwise AND operator.
      >>     Bitwise right shift operator. When applied to an
             unsigned quantity, as here, right shift inserts zero bit(s)
             at the left.
      <<     Bitwise left shift operator. Left shift inserts zero
             bit(s) at the right.
      ++     "n++" increments the variable n.
      %      modulo operator: a % b is the remainder of a divided by b.

      #define BASE 65521 /* largest prime smaller than 65536 */

      /*
         Update a running Adler-32 checksum with the bytes buf[0..len-1]
       and return the updated checksum. The Adler-32 checksum should be
       initialized to 1.

       Usage example:

         unsigned long adler = 1L;

         while (read_buffer(buffer, length) != EOF) {
           adler = update_adler32(adler, buffer, length);
         }
         if (adler != original_adler) error();
      */
      unsigned long update_adler32(unsigned long adler,
         unsigned char *buf, int len)
      {
        unsigned long s1 = adler & 0xffff;
        unsigned long s2 = (adler >> 16) & 0xffff;
        int n;

        for (n = 0; n < len; n++) {
          s1 = (s1 + buf[n]) % BASE;
          s2 = (s2 + s1)     % BASE;
        }
        return (s2 << 16) + s1;
      }

      /* Return the adler32 of the bytes buf[0..len-1] */





 
RFC 1950       ZLIB Compressed Data Format Specification        May 1996


      unsigned long adler32(unsigned char *buf, int len)
      {
        return update_adler32(1L, buf, len);
      }