adler32.c

00001 /* adler32.c -- compute the Adler-32 checksum of a data stream
00002  * Copyright (C) 1995-2011 Mark Adler
00003  * For conditions of distribution and use, see copyright notice in zlib.h
00004  */
00005 
00006 /* @(#) $Id$ */
00007 
00008 #include "zutil.h"
00009 
00010 #define local static
00011 
00012 local uLong adler32_combine_ OF((uLong adler1, uLong adler2, z_off64_t len2));
00013 
00014 #define BASE 65521      /* largest prime smaller than 65536 */
00015 #define NMAX 5552
00016 /* NMAX is the largest n such that 255n(n+1)/2 + (n+1)(BASE-1) <= 2^32-1 */
00017 
00018 #define DO1(buf,i)  {adler += (buf)[i]; sum2 += adler;}
00019 #define DO2(buf,i)  DO1(buf,i); DO1(buf,i+1);
00020 #define DO4(buf,i)  DO2(buf,i); DO2(buf,i+2);
00021 #define DO8(buf,i)  DO4(buf,i); DO4(buf,i+4);
00022 #define DO16(buf)   DO8(buf,0); DO8(buf,8);
00023 
00024 /* use NO_DIVIDE if your processor does not do division in hardware --
00025    try it both ways to see which is faster */
00026 #ifdef NO_DIVIDE
00027 /* note that this assumes BASE is 65521, where 65536 % 65521 == 15
00028    (thank you to John Reiser for pointing this out) */
00029 #  define CHOP(a) \
00030     do { \
00031         unsigned long tmp = a >> 16; \
00032         a &= 0xffffUL; \
00033         a += (tmp << 4) - tmp; \
00034     } while (0)
00035 #  define MOD28(a) \
00036     do { \
00037         CHOP(a); \
00038         if (a >= BASE) a -= BASE; \
00039     } while (0)
00040 #  define MOD(a) \
00041     do { \
00042         CHOP(a); \
00043         MOD28(a); \
00044     } while (0)
00045 #  define MOD63(a) \
00046     do { /* this assumes a is not negative */ \
00047         z_off64_t tmp = a >> 32; \
00048         a &= 0xffffffffL; \
00049         a += (tmp << 8) - (tmp << 5) + tmp; \
00050         tmp = a >> 16; \
00051         a &= 0xffffL; \
00052         a += (tmp << 4) - tmp; \
00053         tmp = a >> 16; \
00054         a &= 0xffffL; \
00055         a += (tmp << 4) - tmp; \
00056         if (a >= BASE) a -= BASE; \
00057     } while (0)
00058 #else
00059 #  define MOD(a) a %= BASE
00060 #  define MOD28(a) a %= BASE
00061 #  define MOD63(a) a %= BASE
00062 #endif
00063 
00064 /* ========================================================================= */
00065 uLong ZEXPORT adler32(adler, buf, len)
00066     uLong adler;
00067     const Bytef *buf;
00068     uInt len;
00069 {
00070     unsigned long sum2;
00071     unsigned n;
00072 
00073     /* split Adler-32 into component sums */
00074     sum2 = (adler >> 16) & 0xffff;
00075     adler &= 0xffff;
00076 
00077     /* in case user likes doing a byte at a time, keep it fast */
00078     if (len == 1) {
00079         adler += buf[0];
00080         if (adler >= BASE)
00081             adler -= BASE;
00082         sum2 += adler;
00083         if (sum2 >= BASE)
00084             sum2 -= BASE;
00085         return adler | (sum2 << 16);
00086     }
00087 
00088     /* initial Adler-32 value (deferred check for len == 1 speed) */
00089     if (buf == Z_NULL)
00090         return 1L;
00091 
00092     /* in case short lengths are provided, keep it somewhat fast */
00093     if (len < 16) {
00094         while (len--) {
00095             adler += *buf++;
00096             sum2 += adler;
00097         }
00098         if (adler >= BASE)
00099             adler -= BASE;
00100         MOD28(sum2);            /* only added so many BASE's */
00101         return adler | (sum2 << 16);
00102     }
00103 
00104     /* do length NMAX blocks -- requires just one modulo operation */
00105     while (len >= NMAX) {
00106         len -= NMAX;
00107         n = NMAX / 16;          /* NMAX is divisible by 16 */
00108         do {
00109             DO16(buf);          /* 16 sums unrolled */
00110             buf += 16;
00111         } while (--n);
00112         MOD(adler);
00113         MOD(sum2);
00114     }
00115 
00116     /* do remaining bytes (less than NMAX, still just one modulo) */
00117     if (len) {                  /* avoid modulos if none remaining */
00118         while (len >= 16) {
00119             len -= 16;
00120             DO16(buf);
00121             buf += 16;
00122         }
00123         while (len--) {
00124             adler += *buf++;
00125             sum2 += adler;
00126         }
00127         MOD(adler);
00128         MOD(sum2);
00129     }
00130 
00131     /* return recombined sums */
00132     return adler | (sum2 << 16);
00133 }
00134 
00135 /* ========================================================================= */
00136 local uLong adler32_combine_(adler1, adler2, len2)
00137     uLong adler1;
00138     uLong adler2;
00139     z_off64_t len2;
00140 {
00141     unsigned long sum1;
00142     unsigned long sum2;
00143     unsigned rem;
00144 
00145     /* for negative len, return invalid adler32 as a clue for debugging */
00146     if (len2 < 0)
00147         return 0xffffffffUL;
00148 
00149     /* the derivation of this formula is left as an exercise for the reader */
00150     MOD63(len2);                /* assumes len2 >= 0 */
00151     rem = (unsigned)len2;
00152     sum1 = adler1 & 0xffff;
00153     sum2 = rem * sum1;
00154     MOD(sum2);
00155     sum1 += (adler2 & 0xffff) + BASE - 1;
00156     sum2 += ((adler1 >> 16) & 0xffff) + ((adler2 >> 16) & 0xffff) + BASE - rem;
00157     if (sum1 >= BASE) sum1 -= BASE;
00158     if (sum1 >= BASE) sum1 -= BASE;
00159     if (sum2 >= (BASE << 1)) sum2 -= (BASE << 1);
00160     if (sum2 >= BASE) sum2 -= BASE;
00161     return sum1 | (sum2 << 16);
00162 }
00163 
00164 /* ========================================================================= */
00165 uLong ZEXPORT adler32_combine(adler1, adler2, len2)
00166     uLong adler1;
00167     uLong adler2;
00168     z_off_t len2;
00169 {
00170     return adler32_combine_(adler1, adler2, len2);
00171 }
00172 
00173 uLong ZEXPORT adler32_combine64(adler1, adler2, len2)
00174     uLong adler1;
00175     uLong adler2;
00176     z_off64_t len2;
00177 {
00178     return adler32_combine_(adler1, adler2, len2);
00179 }