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  1. /* adler32.c -- compute the Adler-32 checksum of a data stream
  2.  * Copyright (C) 1995-2002 Mark Adler
  3.  * For conditions of distribution and use, see copyright notice in zlib.h
  4.  */
  5.  
  6. /* @(#) $Id$ */
  7.  
  8. #include "zlib.h"
  9.  
  10. #define BASE 65521L /* largest prime smaller than 65536 */
  11. #define NMAX 5552
  12. /* NMAX is the largest n such that 255n(n+1)/2 + (n+1)(BASE-1) <= 2^32-1 */
  13.  
  14. #define DO1(buf,i)  {s1 += buf[i]; s2 += s1;}
  15. #define DO2(buf,i)  DO1(buf,i); DO1(buf,i+1);
  16. #define DO4(buf,i)  DO2(buf,i); DO2(buf,i+2);
  17. #define DO8(buf,i)  DO4(buf,i); DO4(buf,i+4);
  18. #define DO16(buf)   DO8(buf,0); DO8(buf,8);
  19.  
  20. /* ========================================================================= */
  21. ZEXPORT(uLong) adler32( /* adler, buf, len) */
  22.     uLong adler,
  23.     const Bytef *buf,
  24.     uInt len )
  25. {
  26.     unsigned long s1 = adler & 0xffff;
  27.     unsigned long s2 = (adler >> 16) & 0xffff;
  28.     int k;
  29.  
  30.     if (buf == Z_NULL) return 1L;
  31.  
  32.     while (len > 0) {
  33.         k = len < NMAX ? len : NMAX;
  34.         len -= k;
  35.         while (k >= 16) {
  36.             DO16(buf);
  37.             buf += 16;
  38.             k -= 16;
  39.         }
  40.         if (k != 0) do {
  41.             s1 += *buf++;
  42.             s2 += s1;
  43.         } while (--k);
  44.         s1 %= BASE;
  45.         s2 %= BASE;
  46.     }
  47.     return (s2 << 16) | s1;
  48. }
  49.