New function 'luaL_addgsub'

Added a new function 'luaL_addgsub', similar to 'luaL_gsub' but that
adds its result directly to a preexisting buffer, avoiding the creation
of one extra intermediate string. Also added two simple macros,
'luaL_bufflen' and 'luaL_buffaddr', to query the current length
and the contents address of a buffer.

lauxlib.c

@@ -951,18 +951,24 @@ LUALIB_API void luaL_requiref (lua_State *L, const char *modname,
 }
 
 
-LUALIB_API const char *luaL_gsub (lua_State *L, const char *s, const char *p,
-                                                               const char *r) {
+LUALIB_API void luaL_addgsub (luaL_Buffer *b, const char *s,
+                                     const char *p, const char *r) {
   const char *wild;
   size_t l = strlen(p);
-  luaL_Buffer b;
-  luaL_buffinit(L, &b);
   while ((wild = strstr(s, p)) != NULL) {
-    luaL_addlstring(&b, s, wild - s);  /* push prefix */
-    luaL_addstring(&b, r);  /* push replacement in place of pattern */
+    luaL_addlstring(b, s, wild - s);  /* push prefix */
+    luaL_addstring(b, r);  /* push replacement in place of pattern */
     s = wild + l;  /* continue after 'p' */
   }
-  luaL_addstring(&b, s);  /* push last suffix */
+  luaL_addstring(b, s);  /* push last suffix */
+}
+
+
+LUALIB_API const char *luaL_gsub (lua_State *L, const char *s,
+                                  const char *p, const char *r) {
+  luaL_Buffer b;
+  luaL_buffinit(L, &b);
+  luaL_addgsub(&b, s, p, r);
   luaL_pushresult(&b);
   return lua_tostring(L, -1);
 }