update doc

src/dict.c

 /*  dict.c is Copyright (c) 2015 Chen Wei <weichen302@gmail.com>
 
-   Use a dictionary like structure to store config options for fast lookup
+    Use cascade of open addressing hash tables to store config options that
+    involve domain names.
+
+                       root
+                        |
+             +---------------------+
+            com                   org
+             |                     |
+    +------------------+     +-------------+
+    yahoo google twitter   debian       freebsd
+      |      |               |             |
+     www    mail          +---------+     www
+                          cn jp uk us
+                          |
+                         ftp
+
+    The lookup steps over domain name hierarchy top-down. All labels are stored
+    in open addressing hash tables. Sub-level labels that belong to different
+    parent nodes are stored separately. e.g. yahoo, google, and twitter are in
+    one hash table, while debian and freebsd are in another. 
+
+    The hash table size is power of 2, two hash functions are used to compute
+    hash bucket. For locating a particular label from hash table, two hash
+    values are compared first, only if they are match, should the more
+    expensive string comparison be used to confirm the search. 
+
+    The search should take constant time regardless the size of --ipset and
+    --server rules.
+
 
     This program is free software; you can redistribute it and/or modify
     it under the terms of the GNU General Public License as published by
@@ -21,7 +49,6 @@
 #define OPEN_ADDRESSING_MAXJUMP 7                /* no reason, just like 7 */
 #define OPEN_ADDRESSING_DEFAULT_SLOT 4
 #define FNV1_32A_INIT ((uint32_t)0x811c9dc5)
-#define FNV_32_PRIME  ((uint32_t)0x01000193)
 #define max(A, B) ((A) > (B) ? (A) : (B))
 
 static char buf[MAXDNAME];