using 'lastfree == NULL' to signal that table is using the dummy

node for its hash part + new macro 'allocsizenode'

ltable.h

@@ -1,5 +1,5 @@
 /*
-** $Id: ltable.h,v 2.20 2014/09/04 18:15:29 roberto Exp roberto $
+** $Id: ltable.h,v 2.21 2015/11/03 15:47:30 roberto Exp roberto $
 ** Lua tables (hash)
 ** See Copyright Notice in lua.h
 */
@@ -27,6 +27,14 @@
 #define invalidateTMcache(t)	((t)->flags = 0)
 
 
+/* true when 't' is using 'dummynode' as its hash part */
+#define isdummy(t)		((t)->lastfree == NULL)
+
+
+/* allocated size for hash nodes */
+#define allocsizenode(t)	(isdummy(t) ? 0 : sizenode(t))
+
+
 /* returns the key, given the value of a table entry */
 #define keyfromval(v) \
   (gkey(cast(Node *, cast(char *, (v)) - offsetof(Node, i_val))))
@@ -51,7 +59,7 @@ LUAI_FUNC int luaH_getn (Table *t);
 
 #if defined(LUA_DEBUG)
 LUAI_FUNC Node *luaH_mainposition (const Table *t, const TValue *key);
-LUAI_FUNC int luaH_isdummy (Node *n);
+LUAI_FUNC int luaH_isdummy (const Table *t);
 #endif