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 }