MEDIUM: backend: add support for the wt6 hash

This function was designed for haproxy while testing other functions
in the past. Initially it was not planned to be used given the not
very interesting numbers it showed on real URL data : it is not as
smooth as the other ones. But later tests showed that the other ones
are extremely sensible to the server count and the type of input data,
especially DJB2 which must not be used on numeric input. So in fact
this function is still a generally average performer and it can make
sense to merge it in the end, as it can provide an alternative to
sdbm+avalanche or djb2+avalanche for consistent hashing or when hashing
on numeric data such as a source IP address or a visitor identifier in
a URL parameter.

doc/configuration.txt

@@ -2548,6 +2548,14 @@ hash-type <method> <function> <modifier>
               poorly with numeric-only input or when the total server weight is
               a multiple of 33, unless the avalanche modifier is also used.
 
+       wt6    this function was designed for haproxy while testing other
+              functions in the past. It is not as smooth as the other ones, but
+              is much less sensible to the input data set or to the number of
+              servers. It can make sense as an alternative to sdbm+avalanche or
+              djb2+avalanche for consistent hashing or when hashing on numeric
+              data such as a source IP address or a visitor identifier in a URL
+              parameter.
+
     <modifier> indicates an optional method applied after hashing the key :
 
        avalanche   This directive indicates that the result from the hash

include/common/hash.h

@@ -23,6 +23,7 @@
 #define _COMMON_HASH_H_
 
 unsigned long hash_djb2(const char *key, int len);
+unsigned long hash_wt6(const char *key, int len);
 unsigned long hash_sdbm(const char *key, int len);
 
 #endif /* _COMMON_HASH_H_ */

include/types/backend.h

@@ -114,6 +114,7 @@
 /* BE_LB_HFCN_* is the hash function, to be used with BE_LB_HASH_FUNC */
 #define BE_LB_HFCN_SDBM   0x000000 /* sdbm hash */
 #define BE_LB_HFCN_DJB2   0x400000 /* djb2 hash */
+#define BE_LB_HFCN_WT6    0x800000 /* wt6 hash */
 #define BE_LB_HASH_FUNC   0xC00000 /* get/clear hash function */
 
 

src/backend.c

@@ -61,6 +61,9 @@ static unsigned long gen_hash(const struct proxy* px, const char* key, unsigned
 	case BE_LB_HFCN_DJB2:
 		hash = hash_djb2(key, len);
 		break;
+	case BE_LB_HFCN_WT6:
+		hash = hash_wt6(key, len);
+		break;
 	case BE_LB_HFCN_SDBM:
 		/* this is the default hash function */
 	default:

src/cfgparse.c

@@ -4128,9 +4128,11 @@ int cfg_parse_listen(const char *file, int linenum, char **args, int kwm)
 			}
 			else if (!strcmp(args[2], "djb2")) {
 				curproxy->lbprm.algo |= BE_LB_HFCN_DJB2;
+			} else if (!strcmp(args[2], "wt6")) {
+				curproxy->lbprm.algo |= BE_LB_HFCN_WT6;
 			}
 			else {
-				Alert("parsing [%s:%d] : '%s' only supports 'sdbm' and 'djb2' hash functions.\n", file, linenum, args[0]);
+				Alert("parsing [%s:%d] : '%s' only supports 'sdbm', 'djb2' or 'wt6' hash functions.\n", file, linenum, args[0]);
 				err_code |= ERR_ALERT | ERR_FATAL;
 				goto out;
 			}

src/hash.c

@@ -17,6 +17,33 @@
 #include <common/hash.h>
 
 
+unsigned long hash_wt6(const char *key, int len)
+{
+	unsigned h0 = 0xa53c965aUL;
+	unsigned h1 = 0x5ca6953aUL;
+	unsigned step0 = 6;
+	unsigned step1 = 18;
+
+	for (; len > 0; len--) {
+		unsigned int t;
+
+		t = ((unsigned int)*key);
+		key++;
+
+		h0 = ~(h0 ^ t);
+		h1 = ~(h1 + t);
+
+		t  = (h1 << step0) | (h1 >> (32-step0));
+		h1 = (h0 << step1) | (h0 >> (32-step1));
+		h0 = t;
+
+		t = ((h0 >> 16) ^ h1) & 0xffff;
+		step0 = t & 0x1F;
+		step1 = t >> 11;
+	}
+	return h0 ^ h1;
+}
+
 unsigned long hash_djb2(const char *key, int len)
 {
 	unsigned long hash = 5381;