text: /* adler32.c -- compute the Adler-32 checksum of a data stream
   * Copyright (C) 1995-2011, 2016 Mark Adler
   * For conditions of distribution and use, see copyright notice in zlib.h
   */
  
  /* @(#) $Id$ */
  
  #include "zutil.h"
  
  local uLong adler32_combine_ OF((uLong adler1, uLong adler2, z_off64_t len2));
  
  #define BASE 65521U /* 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 --
     try it both ways to see which is faster */
  #ifdef NO_DIVIDE
  /* note that this assumes BASE is 65521, where 65536 % 65521 == 15
     (thank you to John Reiser for pointing this out) */
  #define CHOP(a)                      \
      do {                             \
          unsigned long tmp = a >> 16; \
          a &= 0xffffUL;               \
          a += (tmp << 4) - tmp;       \
      } while (0)
  #define MOD28(a)                  \
      do {                          \
          CHOP(a);                  \
          if (a >= BASE) a -= BASE; \
      } while (0)
  #define MOD(a)    \
      do {          \
          CHOP(a);  \
          MOD28(a); \
      } while (0)
  #define MOD63(a)                              \
      do { /* this assumes a is not negative */ \
          z_off64_t tmp = a >> 32;              \
          a &= 0xffffffffL;                     \
          a += (tmp << 8) - (tmp << 5) + tmp;   \
          tmp = a >> 16;                        \
          a &= 0xffffL;                         \
          a += (tmp << 4) - tmp;                \
          tmp = a >> 16;                        \
          a &= 0xffffL;                         \
          a += (tmp << 4) - tmp;                \
          if (a >= BASE) a -= BASE;             \
      } while (0)
  #else
  #define MOD(a) a %= BASE
  #define MOD28(a) a %= BASE
  #define MOD63(a) a %= BASE
  #endif
  
  /* ========================================================================= */
  uLong ZEXPORT adler32_z(adler, buf, len) uLong adler;
  const Bytef* buf;
  z_size_t 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;
          MOD28(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(adler, buf, len) uLong adler;
  const Bytef* buf;
  uInt len;
  { return adler32_z(adler, buf, len); }
  
  /* ========================================================================= */
  local uLong adler32_combine_(adler1, adler2, len2) uLong adler1;
  uLong adler2;
  z_off64_t len2;
  {
      unsigned long sum1;
      unsigned long sum2;
      unsigned rem;
  
      /* for negative len, return invalid adler32 as a clue for debugging */
      if (len2 < 0) return 0xffffffffUL;
  
      /* the derivation of this formula is left as an exercise for the reader */
      MOD63(len2); /* assumes len2 >= 0 */
      rem = (unsigned)len2;
      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 >= ((unsigned long)BASE << 1)) sum2 -= ((unsigned long)BASE << 1);
      if (sum2 >= BASE) sum2 -= BASE;
      return sum1 | (sum2 << 16);
  }
  
  /* ========================================================================= */
  uLong ZEXPORT adler32_combine(adler1, adler2, len2) uLong adler1;
  uLong adler2;
  z_off_t len2;
  { return adler32_combine_(adler1, adler2, len2); }
  
  uLong ZEXPORT adler32_combine64(adler1, adler2, len2) uLong adler1;
  uLong adler2;
  z_off64_t len2;
  { return adler32_combine_(adler1, adler2, len2); }
  