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/* adler32.c -- compute the Adler-32 checksum of a data stream |
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* Copyright (C) 1995-2011 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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|
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/* @(#) $Id$ */ |
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|
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#include "zutil.h" |
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|
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#define local static |
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|
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local uLong adler32_combine_ OF((uLong adler1, uLong adler2, z_off64_t len2)); |
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|
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#define BASE 65521 /* 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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|
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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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|
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/* use NO_DIVIDE if your processor does not do division in hardware -- |
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try it both ways to see which is faster */ |
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#ifdef NO_DIVIDE |
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/* note that this assumes BASE is 65521, where 65536 % 65521 == 15 |
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(thank you to John Reiser for pointing this out) */ |
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# define CHOP(a) \ |
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do { \ |
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unsigned long tmp = a >> 16; \ |
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a &= 0xffffUL; \ |
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a += (tmp << 4) - tmp; \ |
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} while (0) |
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# define MOD28(a) \ |
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do { \ |
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CHOP(a); \ |
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if (a >= BASE) a -= BASE; \ |
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} while (0) |
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# define MOD(a) \ |
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do { \ |
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CHOP(a); \ |
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MOD28(a); \ |
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} while (0) |
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# define MOD63(a) \ |
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do { /* this assumes a is not negative */ \ |
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z_off64_t tmp = a >> 32; \ |
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a &= 0xffffffffL; \ |
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a += (tmp << 8) - (tmp << 5) + tmp; \ |
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tmp = a >> 16; \ |
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a &= 0xffffL; \ |
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a += (tmp << 4) - tmp; \ |
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tmp = a >> 16; \ |
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a &= 0xffffL; \ |
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a += (tmp << 4) - tmp; \ |
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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 MOD28(a) a %= BASE |
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# define MOD63(a) a %= BASE |
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#endif |
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|
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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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|
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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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|
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/* in case user likes doing a byte at a time, keep it fast */ |
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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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|
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/* initial Adler-32 value (deferred check for len == 1 speed) */ |
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if (buf == Z_NULL) |
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return 1L; |
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|
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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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MOD28(sum2); /* only added so many BASE's */ |
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return adler | (sum2 << 16); |
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} |
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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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|
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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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|
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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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/* ========================================================================= */ |
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local uLong adler32_combine_(adler1, adler2, len2) |
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uLong adler1; |
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uLong adler2; |
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z_off64_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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|
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/* for negative len, return invalid adler32 as a clue for debugging */ |
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if (len2 < 0) |
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return 0xffffffffUL; |
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|
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/* the derivation of this formula is left as an exercise for the reader */ |
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MOD63(len2); /* assumes len2 >= 0 */ |
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rem = (unsigned)len2; |
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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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} |
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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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return adler32_combine_(adler1, adler2, len2); |
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} |
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|
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uLong ZEXPORT adler32_combine64(adler1, adler2, len2) |
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uLong adler1; |
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uLong adler2; |
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z_off64_t len2; |
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{ |
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return adler32_combine_(adler1, adler2, len2); |
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} |
