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authordkf <donal.k.fellows@manchester.ac.uk>2010-02-17 21:58:08 (GMT)
committerdkf <donal.k.fellows@manchester.ac.uk>2010-02-17 21:58:08 (GMT)
commit75bf2508e3fdda7b0fb86b63d1cb8e14954cd880 (patch)
treeab11411b42bc7dae1ca8db2a091543842cce9d0a /generic/tclHash.c
parent0198d4b07964264dc869e7b3ec88e8b7fd25d18f (diff)
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Return to using the classic hash function. Now with *extensive* notes in the
comments about why this function is preferred.
Diffstat (limited to 'generic/tclHash.c')
-rw-r--r--generic/tclHash.c46
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;
}