MEDIUM: backend: Implement avalanche as a modifier of the hashing functions.
Summary:
Avalanche is supported not as a native hashing choice, but a modifier
on the hashing function. Note that this means that possible configs
written after 1.5-dev4 using "hash-type avalanche" will get an informative
error instead. But as discussed on the mailing list it seems nobody ever
used it anyway, so let's fix it before the final 1.5 release.
The default values were selected for backward compatibility with previous
releases, as discussed on the mailing list, which means that the consistent
hashing will still apply the avalanche hash by default when no explicit
algorithm is specified.
Examples
(default) hash-type map-based
Map based hashing using sdbm without avalanche
(default) hash-type consistent
Consistent hashing using sdbm with avalanche
Additional Examples:
(a) hash-type map-based sdbm
Same as default for map-based above
(b) hash-type map-based sdbm avalanche
Map based hashing using sdbm with avalanche
(c) hash-type map-based djb2
Map based hashing using djb2 without avalanche
(d) hash-type map-based djb2 avalanche
Map based hashing using djb2 with avalanche
(e) hash-type consistent sdbm avalanche
Same as default for consistent above
(f) hash-type consistent sdbm
Consistent hashing using sdbm without avalanche
(g) hash-type consistent djb2
Consistent hashing using djb2 without avalanche
(h) hash-type consistent djb2 avalanche
Consistent hashing using djb2 with avalanche
diff --git a/include/types/backend.h b/include/types/backend.h
index 1a433cf..57d17bd 100644
--- a/include/types/backend.h
+++ b/include/types/backend.h
@@ -105,8 +105,11 @@
/* hash types */
#define BE_LB_HASH_MAP 0x000000 /* map-based hash (default) */
#define BE_LB_HASH_CONS 0x100000 /* consistent hashbit to indicate a dynamic algorithm */
-#define BE_LB_HASH_AVAL 0x200000 /* run an avalanche hash before a map */
-#define BE_LB_HASH_TYPE 0x300000 /* get/clear hash types */
+#define BE_LB_HASH_TYPE 0x100000 /* get/clear hash types */
+
+/* additional modifier on top of the hash function (only avalanche right now) */
+#define BE_LB_HMOD_AVAL 0x200000 /* avalanche modifier */
+#define BE_LB_HASH_MOD 0x200000 /* get/clear hash modifier */
/* BE_LB_HFCN_* is the hash function, to be used with BE_LB_HASH_FUNC */
#define BE_LB_HFCN_SDBM 0x000000 /* sdbm hash */