diff options
Diffstat (limited to 'generic/tclHash.c')
| -rw-r--r-- | generic/tclHash.c | 46 |
1 files changed, 32 insertions, 14 deletions
diff --git a/generic/tclHash.c b/generic/tclHash.c index 47d8fba..99c4b67 100644 --- a/generic/tclHash.c +++ b/generic/tclHash.c @@ -10,7 +10,7 @@ * See the file "license.terms" for information on usage and redistribution of * this file, and for a DISCLAIMER OF ALL WARRANTIES. * - * RCS: @(#) $Id: tclHash.c,v 1.42 2010/02/10 16:29:49 dkf Exp $ + * RCS: @(#) $Id: tclHash.c,v 1.43 2010/02/17 21:58:11 dkf Exp $ */ #include "tclInt.h" @@ -871,24 +871,42 @@ HashStringKey( Tcl_HashTable *tablePtr, /* Hash table. */ void *keyPtr) /* Key from which to compute hash value. */ { - const unsigned char *string = keyPtr; - unsigned result = 0x811c9dc5; - unsigned c; + register const char *string = (const char *) keyPtr; + register unsigned int result = 0; + register char c; /* - * 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 command and namespace names. 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, but isn't strong against maliciously-chosen + * keys. + * + * 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. + * Since Tcl command and namespace names are usually reasonably-named (the + * main use for string hashes in modern Tcl) speed is far more important + * than strength. + * + * See also HashString in tclLiteral.c. + * See also TclObjHashKey in tclObj.c. */ -#define FNV_32_PRIME ((unsigned) 0x01000193) - while ((c=*string++)) { - result = (result * FNV_32_PRIME) ^ c; + for (; (c=*string++) != 0 ;) { + result += (result<<3) + UCHAR(c); } return result; } |