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/* $OpenBSD: adler32.c,v 1.6 2005/07/20 15:56:40 millert Exp $ */ |
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/* adler32.c -- compute the Adler-32 checksum of a data stream |
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* Copyright (C) 1995-2004 Mark Adler |
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* For conditions of distribution and use, see copyright notice in zlib.h |
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*/ |
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#define ZLIB_INTERNAL |
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#include "zlib.h" |
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#define BASE 65521UL /* largest prime smaller than 65536 */ |
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#define NMAX 5552 |
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/* NMAX is the largest n such that 255n(n+1)/2 + (n+1)(BASE-1) <= 2^32-1 */ |
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#define DO1(buf,i) {adler += (buf)[i]; sum2 += adler;} |
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#define DO2(buf,i) DO1(buf,i); DO1(buf,i+1); |
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#define DO4(buf,i) DO2(buf,i); DO2(buf,i+2); |
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#define DO8(buf,i) DO4(buf,i); DO4(buf,i+4); |
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#define DO16(buf) DO8(buf,0); DO8(buf,8); |
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/* use NO_DIVIDE if your processor does not do division in hardware */ |
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#ifdef NO_DIVIDE |
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# define MOD(a) \ |
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do { \ |
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if (a >= (BASE << 16)) a -= (BASE << 16); \ |
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if (a >= (BASE << 15)) a -= (BASE << 15); \ |
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if (a >= (BASE << 14)) a -= (BASE << 14); \ |
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if (a >= (BASE << 13)) a -= (BASE << 13); \ |
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if (a >= (BASE << 12)) a -= (BASE << 12); \ |
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if (a >= (BASE << 11)) a -= (BASE << 11); \ |
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if (a >= (BASE << 10)) a -= (BASE << 10); \ |
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if (a >= (BASE << 9)) a -= (BASE << 9); \ |
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if (a >= (BASE << 8)) a -= (BASE << 8); \ |
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if (a >= (BASE << 7)) a -= (BASE << 7); \ |
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if (a >= (BASE << 6)) a -= (BASE << 6); \ |
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if (a >= (BASE << 5)) a -= (BASE << 5); \ |
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if (a >= (BASE << 4)) a -= (BASE << 4); \ |
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if (a >= (BASE << 3)) a -= (BASE << 3); \ |
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if (a >= (BASE << 2)) a -= (BASE << 2); \ |
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if (a >= (BASE << 1)) a -= (BASE << 1); \ |
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if (a >= BASE) a -= BASE; \ |
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} while (0) |
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# define MOD4(a) \ |
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do { \ |
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if (a >= (BASE << 4)) a -= (BASE << 4); \ |
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if (a >= (BASE << 3)) a -= (BASE << 3); \ |
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if (a >= (BASE << 2)) a -= (BASE << 2); \ |
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if (a >= (BASE << 1)) a -= (BASE << 1); \ |
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if (a >= BASE) a -= BASE; \ |
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} while (0) |
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#else |
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# define MOD(a) a %= BASE |
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# define MOD4(a) a %= BASE |
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#endif |
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/* ========================================================================= */ |
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uLong ZEXPORT adler32(adler, buf, len) |
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uLong adler; |
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const Bytef *buf; |
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uInt len; |
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{ |
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unsigned long sum2; |
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unsigned n; |
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/* split Adler-32 into component sums */ |
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sum2 = (adler >> 16) & 0xffff; |
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adler &= 0xffff; |
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/* in case user likes doing a byte at a time, keep it fast */ |
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✗✓ |
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if (len == 1) { |
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adler += buf[0]; |
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if (adler >= BASE) |
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adler -= BASE; |
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sum2 += adler; |
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if (sum2 >= BASE) |
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sum2 -= BASE; |
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return adler | (sum2 << 16); |
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} |
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/* initial Adler-32 value (deferred check for len == 1 speed) */ |
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✓✗ |
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if (buf == Z_NULL) |
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return 1L; |
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/* in case short lengths are provided, keep it somewhat fast */ |
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if (len < 16) { |
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while (len--) { |
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adler += *buf++; |
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sum2 += adler; |
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} |
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if (adler >= BASE) |
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adler -= BASE; |
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MOD4(sum2); /* only added so many BASE's */ |
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return adler | (sum2 << 16); |
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} |
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/* do length NMAX blocks -- requires just one modulo operation */ |
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while (len >= NMAX) { |
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len -= NMAX; |
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n = NMAX / 16; /* NMAX is divisible by 16 */ |
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do { |
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DO16(buf); /* 16 sums unrolled */ |
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buf += 16; |
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} while (--n); |
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MOD(adler); |
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MOD(sum2); |
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} |
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/* do remaining bytes (less than NMAX, still just one modulo) */ |
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if (len) { /* avoid modulos if none remaining */ |
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while (len >= 16) { |
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len -= 16; |
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DO16(buf); |
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buf += 16; |
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} |
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while (len--) { |
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adler += *buf++; |
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sum2 += adler; |
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} |
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MOD(adler); |
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MOD(sum2); |
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} |
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/* return recombined sums */ |
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return adler | (sum2 << 16); |
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} |
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/* ========================================================================= */ |
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uLong ZEXPORT adler32_combine(adler1, adler2, len2) |
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uLong adler1; |
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uLong adler2; |
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z_off_t len2; |
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{ |
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unsigned long sum1; |
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unsigned long sum2; |
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unsigned rem; |
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/* the derivation of this formula is left as an exercise for the reader */ |
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rem = (unsigned)(len2 % BASE); |
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sum1 = adler1 & 0xffff; |
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sum2 = rem * sum1; |
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MOD(sum2); |
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sum1 += (adler2 & 0xffff) + BASE - 1; |
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sum2 += ((adler1 >> 16) & 0xffff) + ((adler2 >> 16) & 0xffff) + BASE - rem; |
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if (sum1 > BASE) sum1 -= BASE; |
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if (sum1 > BASE) sum1 -= BASE; |
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if (sum2 > (BASE << 1)) sum2 -= (BASE << 1); |
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if (sum2 > BASE) sum2 -= BASE; |
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return sum1 | (sum2 << 16); |
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} |