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(uLong adler, const Bytef *buf, uInt len) 58 { 59 unsigned long sum2; 60 unsigned n; 61 62 /* split Adler-32 into component sums */ 63 sum2 = (adler >> 16) & 0xffff; 64 adler &= 0xffff; 65 66 /* in case user likes doing a byte at a time, keep it fast */ 67 if (len == 1) { 68 adler += buf[0]; 69 if (adler >= BASE) 70 adler -= BASE; 71 sum2 += adler; 72 if (sum2 >= BASE) 73 sum2 -= BASE; 74 return adler | (sum2 << 16); 75 } 76 77 /* initial Adler-32 value (deferred check for len == 1 speed) */ 78 if (buf == Z_NULL) 79 return 1L; 80 81 /* in case short lengths are provided, keep it somewhat fast */ 82 if (len < 16) { 83 while (len--) { 84 adler += *buf++; 85 sum2 += adler; 86 } 87 if (adler >= BASE) 88 adler -= BASE; 89 MOD4(sum2); /* only added so many BASE's */ 90 return adler | (sum2 << 16); 91 } 92 93 /* do length NMAX blocks -- requires just one modulo operation */ 94 while (len >= NMAX) { 95 len -= NMAX; 96 n = NMAX / 16; /* NMAX is divisible by 16 */ 97 do { 98 DO16(buf); /* 16 sums unrolled */ 99 buf += 16; 100 } while (--n); 101 MOD(adler); 102 MOD(sum2); 103 } 104 105 /* do remaining bytes (less than NMAX, still just one modulo) */ 106 if (len) { /* avoid modulos if none remaining */ 107 while (len >= 16) { 108 len -= 16; 109 DO16(buf); 110 buf += 16; 111 } 112 while (len--) { 113 adler += *buf++; 114 sum2 += adler; 115 } 116 MOD(adler); 117 MOD(sum2); 118 } 119 120 /* return recombined sums */ 121 return adler | (sum2 << 16); 122 } 123