diff options
Diffstat (limited to 'generic/tclObj.c')
| -rw-r--r-- | generic/tclObj.c | 48 |
1 files changed, 34 insertions, 14 deletions
diff --git a/generic/tclObj.c b/generic/tclObj.c index b45d1c8..721de46 100644 --- a/generic/tclObj.c +++ b/generic/tclObj.c @@ -13,7 +13,7 @@ * See the file "license.terms" for information on usage and redistribution of * this file, and for a DISCLAIMER OF ALL WARRANTIES. * - * RCS: @(#) $Id: tclObj.c,v 1.168 2010/02/10 16:29:49 dkf Exp $ + * RCS: @(#) $Id: tclObj.c,v 1.169 2010/02/17 21:58:11 dkf Exp $ */ #include "tclInt.h" @@ -4038,24 +4038,44 @@ TclHashObjKey( void *keyPtr) /* Key from which to compute hash value. */ { Tcl_Obj *objPtr = keyPtr; - register unsigned result = 0x811c9dc5; - const unsigned char *string = (unsigned char *) TclGetString(objPtr); - const unsigned char *last = string + objPtr->length; + const char *string = TclGetString(objPtr); + unsigned int result = 0; + const char *end = string + objPtr->length; /* - * This is the (32-bit) Fowler/Noll/Vo hash algorithm. This has the - * property of being a reasonably good non-cryptographic hash function for - * short string words, i.e., virtually all names used in practice. It is - * also faster than Tcl's original algorithm on Intel x86, where there is - * a fast built-in multiply assembly instruction. + * I tried a zillion different hash functions and asked many other people + * for advice. Many people had their own favorite functions, all + * different, but no-one had much idea why they were good ones. I chose + * the one below (multiply by 9 and add new character) because of the + * following reasons: * - * Derived from Public Domain implementation by Landon Curt Noll at: - * http://www.isthe.com/chongo/src/fnv/hash_32.c + * 1. Multiplying by 10 is perfect for keys that are decimal strings, and + * multiplying by 9 is just about as good. + * 2. Times-9 is (shift-left-3) plus (old). This means that each + * character's bits hang around in the low-order bits of the hash value + * for ever, plus they spread fairly rapidly up to the high-order bits + * to fill out the hash value. This seems works well both for decimal + * and non-decimal strings. + * + * Note that this function is very weak against malicious strings; it's + * very easy to generate multiple keys that have the same hashcode. On the + * other hand, that hardly ever actually occurs and this function *is* + * very cheap, even by comparison with industry-standard hashes like FNV. + * If real strength of hash is required though, use a custom hash based on + * Bob Jenkins's lookup3(), but be aware that it's significantly slower. + * Tcl does not use that level of strength because it typically does not + * need it (and some of the aspects of that strength are genuinely + * unnecessary given the rest of Tcl's hash machinery, and the fact that + * we do not either transfer hashes to another machine, use them as a true + * substitute for equality, or attempt to minimize work in rebuilding the + * hash table). + * + * See also HashStringKey in tclHash.c. + * See also HashString in tclLiteral.c. */ -#define FNV_32_PRIME ((unsigned) 0x01000193) - while (string < last) { - result = (result * FNV_32_PRIME) ^ (*string++); + while (string < end) { + result += (result << 3) + UCHAR(*string++); } return result; } |