Upgrade Tcl's hash function to use the FNV-32 algorithm. This is marginally

faster and gives a bit better distribution of keys (especially in large hash
tables) but does change hash iteration order.

1 files changed, 13 insertions, 20 deletions

diff --git a/generic/tclHash.c b/generic/tclHash.c
index 9ed941e..f3c4b3a 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 1.39 2009/07/16 21:24:39 dgp Exp $
+ * RCS: @(#) $Id: tclHash.c,v 1.40 2010/02/07 09:10:33 dkf Exp $
  */
 
 #include "tclInt.h"
@@ -866,36 +866,29 @@ CompareStringKeys(
  *----------------------------------------------------------------------
  */
 
-static unsigned int
+static unsigned
 HashStringKey(
     Tcl_HashTable *tablePtr,	/* Hash table. */
     void *keyPtr)		/* Key from which to compute hash value. */
 {
     register const char *string = (const char *) keyPtr;
-    register unsigned int result;
+    register unsigned result = 0;
     register int c;
 
     /*
-     * 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:
+     * 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.
      *
-     * 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.
+     * Derived from Public Domain implementation by Landon Curt Noll at:
+     * http://www.isthe.com/chongo/src/fnv/hash_32.c
      */
 
-    result = 0;
-
-    for (c=*string++ ; c ; c=*string++) {
-	result += (result<<3) + c;
+#define FNV_32_PRIME	((unsigned) 0x01000193)
+    while ((c=*string++)) {
+	result = (result * FNV_32_PRIME) ^ c;
     }
     return result;
 }