Annotation of sys/lib/libz/adler32.c, Revision 1.1
1.1 ! nbrk 1: /* $OpenBSD: adler32.c,v 1.9 2006/07/20 11:14:10 mickey Exp $ */
! 2: /* adler32.c -- compute the Adler-32 checksum of a data stream
! 3: * Copyright (C) 1995-2004 Mark Adler
! 4: * For conditions of distribution and use, see copyright notice in zlib.h
! 5: */
! 6:
! 7: #define ZLIB_INTERNAL
! 8: #include "zlib.h"
! 9:
! 10: #define BASE 65521UL /* largest prime smaller than 65536 */
! 11: #define NMAX 5552
! 12: /* NMAX is the largest n such that 255n(n+1)/2 + (n+1)(BASE-1) <= 2^32-1 */
! 13:
! 14: #define DO1(buf,i) {adler += (buf)[i]; sum2 += adler;}
! 15: #define DO2(buf,i) DO1(buf,i); DO1(buf,i+1);
! 16: #define DO4(buf,i) DO2(buf,i); DO2(buf,i+2);
! 17: #define DO8(buf,i) DO4(buf,i); DO4(buf,i+4);
! 18: #define DO16(buf) DO8(buf,0); DO8(buf,8);
! 19:
! 20: /* use NO_DIVIDE if your processor does not do division in hardware */
! 21: #ifdef NO_DIVIDE
! 22: # define MOD(a) \
! 23: do { \
! 24: if (a >= (BASE << 16)) a -= (BASE << 16); \
! 25: if (a >= (BASE << 15)) a -= (BASE << 15); \
! 26: if (a >= (BASE << 14)) a -= (BASE << 14); \
! 27: if (a >= (BASE << 13)) a -= (BASE << 13); \
! 28: if (a >= (BASE << 12)) a -= (BASE << 12); \
! 29: if (a >= (BASE << 11)) a -= (BASE << 11); \
! 30: if (a >= (BASE << 10)) a -= (BASE << 10); \
! 31: if (a >= (BASE << 9)) a -= (BASE << 9); \
! 32: if (a >= (BASE << 8)) a -= (BASE << 8); \
! 33: if (a >= (BASE << 7)) a -= (BASE << 7); \
! 34: if (a >= (BASE << 6)) a -= (BASE << 6); \
! 35: if (a >= (BASE << 5)) a -= (BASE << 5); \
! 36: if (a >= (BASE << 4)) a -= (BASE << 4); \
! 37: if (a >= (BASE << 3)) a -= (BASE << 3); \
! 38: if (a >= (BASE << 2)) a -= (BASE << 2); \
! 39: if (a >= (BASE << 1)) a -= (BASE << 1); \
! 40: if (a >= BASE) a -= BASE; \
! 41: } while (0)
! 42: # define MOD4(a) \
! 43: do { \
! 44: if (a >= (BASE << 4)) a -= (BASE << 4); \
! 45: if (a >= (BASE << 3)) a -= (BASE << 3); \
! 46: if (a >= (BASE << 2)) a -= (BASE << 2); \
! 47: if (a >= (BASE << 1)) a -= (BASE << 1); \
! 48: if (a >= BASE) a -= BASE; \
! 49: } while (0)
! 50: #else
! 51: # define MOD(a) a %= BASE
! 52: # define MOD4(a) a %= BASE
! 53: #endif
! 54:
! 55: /* ========================================================================= */
! 56: uLong ZEXPORT adler32(adler, buf, len)
! 57: uLong adler;
! 58: const Bytef *buf;
! 59: uInt len;
! 60: {
! 61: unsigned long sum2;
! 62: unsigned n;
! 63:
! 64: /* split Adler-32 into component sums */
! 65: sum2 = (adler >> 16) & 0xffff;
! 66: adler &= 0xffff;
! 67:
! 68: /* in case user likes doing a byte at a time, keep it fast */
! 69: if (len == 1) {
! 70: adler += buf[0];
! 71: if (adler >= BASE)
! 72: adler -= BASE;
! 73: sum2 += adler;
! 74: if (sum2 >= BASE)
! 75: sum2 -= BASE;
! 76: return adler | (sum2 << 16);
! 77: }
! 78:
! 79: /* initial Adler-32 value (deferred check for len == 1 speed) */
! 80: if (buf == Z_NULL)
! 81: return 1L;
! 82:
! 83: /* in case short lengths are provided, keep it somewhat fast */
! 84: if (len < 16) {
! 85: while (len--) {
! 86: adler += *buf++;
! 87: sum2 += adler;
! 88: }
! 89: if (adler >= BASE)
! 90: adler -= BASE;
! 91: MOD4(sum2); /* only added so many BASE's */
! 92: return adler | (sum2 << 16);
! 93: }
! 94:
! 95: /* do length NMAX blocks -- requires just one modulo operation */
! 96: while (len >= NMAX) {
! 97: len -= NMAX;
! 98: n = NMAX / 16; /* NMAX is divisible by 16 */
! 99: do {
! 100: DO16(buf); /* 16 sums unrolled */
! 101: buf += 16;
! 102: } while (--n);
! 103: MOD(adler);
! 104: MOD(sum2);
! 105: }
! 106:
! 107: /* do remaining bytes (less than NMAX, still just one modulo) */
! 108: if (len) { /* avoid modulos if none remaining */
! 109: while (len >= 16) {
! 110: len -= 16;
! 111: DO16(buf);
! 112: buf += 16;
! 113: }
! 114: while (len--) {
! 115: adler += *buf++;
! 116: sum2 += adler;
! 117: }
! 118: MOD(adler);
! 119: MOD(sum2);
! 120: }
! 121:
! 122: /* return recombined sums */
! 123: return adler | (sum2 << 16);
! 124: }
! 125:
! 126: #ifdef ADLER32_COMBINE
! 127: /* ========================================================================= */
! 128: uLong ZEXPORT adler32_combine(adler1, adler2, len2)
! 129: uLong adler1;
! 130: uLong adler2;
! 131: z_off_t len2;
! 132: {
! 133: unsigned long sum1;
! 134: unsigned long sum2;
! 135: unsigned rem;
! 136:
! 137: /* the derivation of this formula is left as an exercise for the reader */
! 138: rem = (unsigned)(len2 % BASE);
! 139: sum1 = adler1 & 0xffff;
! 140: sum2 = rem * sum1;
! 141: MOD(sum2);
! 142: sum1 += (adler2 & 0xffff) + BASE - 1;
! 143: sum2 += ((adler1 >> 16) & 0xffff) + ((adler2 >> 16) & 0xffff) + BASE - rem;
! 144: if (sum1 > BASE) sum1 -= BASE;
! 145: if (sum1 > BASE) sum1 -= BASE;
! 146: if (sum2 > (BASE << 1)) sum2 -= (BASE << 1);
! 147: if (sum2 > BASE) sum2 -= BASE;
! 148: return sum1 | (sum2 << 16);
! 149: }
! 150: #endif /* ADLER32_COMBINE */
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