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-rw-r--r--generic/tclObj.c48
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;
}