diff options
Diffstat (limited to 'adler32.c')
| -rw-r--r-- | adler32.c | 68 |
1 files changed, 39 insertions, 29 deletions
@@ -1,5 +1,5 @@ /* adler32.c -- compute the Adler-32 checksum of a data stream - * Copyright (C) 1995-2007 Mark Adler + * Copyright (C) 1995-2011 Mark Adler * For conditions of distribution and use, see copyright notice in zlib.h */ @@ -9,9 +9,9 @@ #define local static -local uLong adler32_combine_(uLong adler1, uLong adler2, z_off64_t len2); +local uLong adler32_combine_ OF((uLong adler1, uLong adler2, z_off64_t len2)); -#define BASE 65521UL /* largest prime smaller than 65536 */ +#define BASE 65521 /* largest prime smaller than 65536 */ #define NMAX 5552 /* NMAX is the largest n such that 255n(n+1)/2 + (n+1)(BASE-1) <= 2^32-1 */ @@ -21,39 +21,44 @@ local uLong adler32_combine_(uLong adler1, uLong adler2, z_off64_t len2); #define DO8(buf,i) DO4(buf,i); DO4(buf,i+4); #define DO16(buf) DO8(buf,0); DO8(buf,8); -/* use NO_DIVIDE if your processor does not do division in hardware */ +/* use NO_DIVIDE if your processor does not do division in hardware -- + try it both ways to see which is faster */ #ifdef NO_DIVIDE -# define MOD(a) \ +/* note that this assumes BASE is 65521, where 65536 % 65521 == 15 + (thank you to John Reiser for pointing this out) */ +# define CHOP(a) \ + do { \ + unsigned long tmp = a >> 16; \ + a &= 0xffffUL; \ + a += (tmp << 4) - tmp; \ + } while (0) +# define MOD28(a) \ do { \ - if (a >= (BASE << 16)) a -= (BASE << 16); \ - if (a >= (BASE << 15)) a -= (BASE << 15); \ - if (a >= (BASE << 14)) a -= (BASE << 14); \ - if (a >= (BASE << 13)) a -= (BASE << 13); \ - if (a >= (BASE << 12)) a -= (BASE << 12); \ - if (a >= (BASE << 11)) a -= (BASE << 11); \ - if (a >= (BASE << 10)) a -= (BASE << 10); \ - if (a >= (BASE << 9)) a -= (BASE << 9); \ - if (a >= (BASE << 8)) a -= (BASE << 8); \ - if (a >= (BASE << 7)) a -= (BASE << 7); \ - if (a >= (BASE << 6)) a -= (BASE << 6); \ - if (a >= (BASE << 5)) a -= (BASE << 5); \ - if (a >= (BASE << 4)) a -= (BASE << 4); \ - if (a >= (BASE << 3)) a -= (BASE << 3); \ - if (a >= (BASE << 2)) a -= (BASE << 2); \ - if (a >= (BASE << 1)) a -= (BASE << 1); \ + CHOP(a); \ if (a >= BASE) a -= BASE; \ } while (0) -# define MOD4(a) \ +# define MOD(a) \ do { \ - if (a >= (BASE << 4)) a -= (BASE << 4); \ - if (a >= (BASE << 3)) a -= (BASE << 3); \ - if (a >= (BASE << 2)) a -= (BASE << 2); \ - if (a >= (BASE << 1)) a -= (BASE << 1); \ + CHOP(a); \ + MOD28(a); \ + } while (0) +# define MOD63(a) \ + do { /* this assumes a is not negative */ \ + z_off64_t tmp = a >> 32; \ + a &= 0xffffffffL; \ + a += (tmp << 8) - (tmp << 5) + tmp; \ + tmp = a >> 16; \ + a &= 0xffffL; \ + a += (tmp << 4) - tmp; \ + tmp = a >> 16; \ + a &= 0xffffL; \ + a += (tmp << 4) - tmp; \ if (a >= BASE) a -= BASE; \ } while (0) #else # define MOD(a) a %= BASE -# define MOD4(a) a %= BASE +# define MOD28(a) a %= BASE +# define MOD63(a) a %= BASE #endif /* ========================================================================= */ @@ -92,7 +97,7 @@ uLong ZEXPORT adler32(adler, buf, len) } if (adler >= BASE) adler -= BASE; - MOD4(sum2); /* only added so many BASE's */ + MOD28(sum2); /* only added so many BASE's */ return adler | (sum2 << 16); } @@ -137,8 +142,13 @@ local uLong adler32_combine_(adler1, adler2, len2) unsigned long sum2; unsigned rem; + /* for negative len, return invalid adler32 as a clue for debugging */ + if (len2 < 0) + return 0xffffffffUL; + /* the derivation of this formula is left as an exercise for the reader */ - rem = (unsigned)(len2 % BASE); + MOD63(len2); /* assumes len2 >= 0 */ + rem = (unsigned)len2; sum1 = adler1 & 0xffff; sum2 = rem * sum1; MOD(sum2); |