1 /* adler32.c -- compute the Adler-32 checksum of a data stream 2 * Copyright (C) 1995-2004 Mark Adler 3 * For conditions of distribution and use, see copyright notice in zlib.h 4 */ 5 6 /* @(#) $Id$ */ 7 8 #define ZLIB_INTERNAL 9 #include "zlib.h" 10 11 #define BASE 65521UL /* largest prime smaller than 65536 */ 12 #define NMAX 5552 13 /* NMAX is the largest n such that 255n(n+1)/2 + (n+1)(BASE-1) <= 2^32-1 */ 14 15 #define DO1(buf,i) {adler += (buf)[i]; sum2 += adler;} 16 #define DO2(buf,i) DO1(buf,i); DO1(buf,i+1); 17 #define DO4(buf,i) DO2(buf,i); DO2(buf,i+2); 18 #define DO8(buf,i) DO4(buf,i); DO4(buf,i+4); 19 #define DO16(buf) DO8(buf,0); DO8(buf,8); 20 21 /* use NO_DIVIDE if your processor does not do division in hardware */ 22 #ifdef NO_DIVIDE 23 # define MOD(a) \ 24 do { \ 25 if (a >= (BASE << 16)) a -= (BASE << 16); \ 26 if (a >= (BASE << 15)) a -= (BASE << 15); \ 27 if (a >= (BASE << 14)) a -= (BASE << 14); \ 28 if (a >= (BASE << 13)) a -= (BASE << 13); \ 29 if (a >= (BASE << 12)) a -= (BASE << 12); \ 30 if (a >= (BASE << 11)) a -= (BASE << 11); \ 31 if (a >= (BASE << 10)) a -= (BASE << 10); \ 32 if (a >= (BASE << 9)) a -= (BASE << 9); \ 33 if (a >= (BASE << 8)) a -= (BASE << 8); \ 34 if (a >= (BASE << 7)) a -= (BASE << 7); \ 35 if (a >= (BASE << 6)) a -= (BASE << 6); \ 36 if (a >= (BASE << 5)) a -= (BASE << 5); \ 37 if (a >= (BASE << 4)) a -= (BASE << 4); \ 38 if (a >= (BASE << 3)) a -= (BASE << 3); \ 39 if (a >= (BASE << 2)) a -= (BASE << 2); \ 40 if (a >= (BASE << 1)) a -= (BASE << 1); \ 41 if (a >= BASE) a -= BASE; \ 42 } while (0) 43 # define MOD4(a) \ 44 do { \ 45 if (a >= (BASE << 4)) a -= (BASE << 4); \ 46 if (a >= (BASE << 3)) a -= (BASE << 3); \ 47 if (a >= (BASE << 2)) a -= (BASE << 2); \ 48 if (a >= (BASE << 1)) a -= (BASE << 1); \ 49 if (a >= BASE) a -= BASE; \ 50 } while (0) 51 #else 52 # define MOD(a) a %= BASE 53 # define MOD4(a) a %= BASE 54 #endif 55 56 /* ========================================================================= */ 57 uLong ZEXPORT adler32(adler, buf, len) 58 uLong adler; 59 const Bytef *buf; 60 uInt len; 61 { 62 unsigned long sum2; 63 unsigned n; 64 65 /* split Adler-32 into component sums */ 66 sum2 = (adler >> 16) & 0xffff; 67 adler &= 0xffff; 68 69 /* in case user likes doing a byte at a time, keep it fast */ 70 if (len == 1) { 71 adler += buf[0]; 72 if (adler >= BASE) 73 adler -= BASE; 74 sum2 += adler; 75 if (sum2 >= BASE) 76 sum2 -= BASE; 77 return adler | (sum2 << 16); 78 } 79 80 /* initial Adler-32 value (deferred check for len == 1 speed) */ 81 if (buf == Z_NULL) 82 return 1L; 83 84 /* in case short lengths are provided, keep it somewhat fast */ 85 if (len < 16) { 86 while (len--) { 87 adler += *buf++; 88 sum2 += adler; 89 } 90 if (adler >= BASE) 91 adler -= BASE; 92 MOD4(sum2); /* only added so many BASE's */ 93 return adler | (sum2 << 16); 94 } 95 96 /* do length NMAX blocks -- requires just one modulo operation */ 97 while (len >= NMAX) { 98 len -= NMAX; 99 n = NMAX / 16; /* NMAX is divisible by 16 */ 100 do { 101 DO16(buf); /* 16 sums unrolled */ 102 buf += 16; 103 } while (--n); 104 MOD(adler); 105 MOD(sum2); 106 } 107 108 /* do remaining bytes (less than NMAX, still just one modulo) */ 109 if (len) { /* avoid modulos if none remaining */ 110 while (len >= 16) { 111 len -= 16; 112 DO16(buf); 113 buf += 16; 114 } 115 while (len--) { 116 adler += *buf++; 117 sum2 += adler; 118 } 119 MOD(adler); 120 MOD(sum2); 121 } 122 123 /* return recombined sums */ 124 return adler | (sum2 << 16); 125 } 126