text: /* adler32.c -- compute the Adler-32 checksum of a data stream
   * Copyright (C) 1995-2004 Mark Adler
   * For conditions of distribution and use, see copyright notice in zlib.h
   */
  
  /* @(#) $Id: adler32.c,v 1.2 2006-02-06 22:01:28 marcus Exp $ */
  
  #define ZLIB_INTERNAL
  #include "zlib.h"
  
  #define BASE 65521UL    /* largest prime smaller than 65536 */
  #define NMAX 5552
  /* NMAX is the largest n such that 255n(n+1)/2 + (n+1)(BASE-1) <= 2^32-1 */
  
  #define DO1(buf,i)  {adler += (buf)[i]; sum2 += adler;}
  #define DO2(buf,i)  DO1(buf,i); DO1(buf,i+1);
  #define DO4(buf,i)  DO2(buf,i); DO2(buf,i+2);
  #define DO8(buf,i)  DO4(buf,i); DO4(buf,i+4);
  #define DO16(buf)   DO8(buf,0); DO8(buf,8);
  
  /* use NO_DIVIDE if your processor does not do division in hardware */
  #ifdef NO_DIVIDE
  #  define MOD(a) \
      do { \
          if (a >= (BASE << 16)) a -= (BASE << 16); \
          if (a >= (BASE << 15)) a -= (BASE << 15); \
          if (a >= (BASE << 14)) a -= (BASE << 14); \
          if (a >= (BASE << 13)) a -= (BASE << 13); \
          if (a >= (BASE << 12)) a -= (BASE << 12); \
          if (a >= (BASE << 11)) a -= (BASE << 11); \
          if (a >= (BASE << 10)) a -= (BASE << 10); \
          if (a >= (BASE << 9)) a -= (BASE << 9); \
          if (a >= (BASE << 8)) a -= (BASE << 8); \
          if (a >= (BASE << 7)) a -= (BASE << 7); \
          if (a >= (BASE << 6)) a -= (BASE << 6); \
          if (a >= (BASE << 5)) a -= (BASE << 5); \
          if (a >= (BASE << 4)) a -= (BASE << 4); \
          if (a >= (BASE << 3)) a -= (BASE << 3); \
          if (a >= (BASE << 2)) a -= (BASE << 2); \
          if (a >= (BASE << 1)) a -= (BASE << 1); \
          if (a >= BASE) a -= BASE; \
      } while (0)
  #  define MOD4(a) \
      do { \
          if (a >= (BASE << 4)) a -= (BASE << 4); \
          if (a >= (BASE << 3)) a -= (BASE << 3); \
          if (a >= (BASE << 2)) a -= (BASE << 2); \
          if (a >= (BASE << 1)) a -= (BASE << 1); \
          if (a >= BASE) a -= BASE; \
      } while (0)
  #else
  #  define MOD(a) a %= BASE
  #  define MOD4(a) a %= BASE
  #endif
  
  /* ========================================================================= */
  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);
  }
  
  /* ========================================================================= */
  uLong ZEXPORT adler32_combine(adler1, adler2, len2)
      uLong adler1;
      uLong adler2;
      z_off_t len2;
  {
      unsigned long sum1;
      unsigned long sum2;
      unsigned rem;
  
      /* the derivation of this formula is left as an exercise for the reader */
      rem = (unsigned)(len2 % BASE);
      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 > (BASE << 1)) sum2 -= (BASE << 1);
      if (sum2 > BASE) sum2 -= BASE;
      return sum1 | (sum2 << 16);
  }
  