# HG changeset patch
# User jbe
# Date 1406743151 -7200
# Node ID 0edc2f05c66ab52201c0f15a800e84770337a82c
# Parent  0690fe79b673129cc4b567a59c8bc0091b4df646
Use lightuserdata directly to store JSON null values in JSON library

diff -r 0690fe79b673 -r 0edc2f05c66a libraries/json/json.c
--- a/libraries/json/json.c	Wed Jul 30 19:24:06 2014 +0200
+++ b/libraries/json/json.c	Wed Jul 30 19:59:11 2014 +0200
@@ -8,46 +8,56 @@
 
 // macros for usage of Lua registry:
 #define JSON_REGENT char
-#define JSON_REGREF void *
-#define json_regref(x) (&json_registry.x)
-#define json_regfetchref(L, x) (lua_pushlightuserdata((L), (x)), lua_rawget((L), LUA_REGISTRYINDEX))
-#define json_regfetch(L, x) (json_regfetchref(L, json_regref(x)))
-#define json_regstore(L, x) (lua_pushlightuserdata(L, json_regref(x)), lua_pushvalue(L, -2), lua_rawset(L, LUA_REGISTRYINDEX));
+#define JSON_REGPOINTER void *
+#define json_pushlightref(L, x) (lua_pushlightuserdata((L), &json_reference.x))
+#define json_regpointer(x) (&json_registry.x)
+#define json_regfetchpointer(L, x) (lua_pushlightuserdata((L), (x)), lua_rawget((L), LUA_REGISTRYINDEX))
+#define json_regfetch(L, x) (json_regfetchpointer(L, json_regpointer(x)))
+#define json_regstore(L, x) (lua_pushlightuserdata(L, json_regpointer(x)), lua_pushvalue(L, -2), lua_rawset(L, LUA_REGISTRYINDEX));
+
+// generate dummy memory addresses that represent Lua objects
+// directly via lightuserdata values (not using the Lua registry):
+static struct {
+  JSON_REGENT nullmark;  // magic value to indicate JSON null value
+} json_reference;
+
 
 // generate dummy memory addresses that represent Lua objects
-// (via lightuserdata keys and LUA_REGISTRYINDEX):
+// via lightuserdata keys and LUA_REGISTRYINDEX:
 static struct {
-  JSON_REGENT nullmark;
-  JSON_REGENT shadowtbl;
-  JSON_REGENT unknownmt;
-  JSON_REGENT objectmt;
-  JSON_REGENT arraymt;
+  JSON_REGENT shadowtbl;  // ephemeron table that maps tables to their corresponding shadow table
+  JSON_REGENT unknownmt;  // metatable for tables that may be either JSON objects or JSON arrays
+  JSON_REGENT objectmt;   // metatable for JSON objects
+  JSON_REGENT arraymt;    // metatable for JSON arrays
 } json_registry;
 
-// marks a table as JSON object or JSON array:
+// marks a Lua table as JSON object or JSON array:
 // (returns its modified argument or a new table if argument is nil)
-static int json_mark(lua_State *L, JSON_REGREF mt) {
-  // if argument is nil, then create new table:
+static int json_mark(lua_State *L, JSON_REGPOINTER mt) {
+  // check if argument is nil
   if (lua_isnoneornil(L, 1)) {
+    // create new table at stack position 1:
     lua_settop(L, 0);
     lua_newtable(L);
-    // set shadow table:
+    // create shadow table (leaving previously created table on stack position 1):
     json_regfetch(L, shadowtbl);
     lua_pushvalue(L, 1);
     lua_newtable(L);
     lua_rawset(L, -3);
   } else {
-    // check if shadow table already exists:
+    // push shadow table on top of stack:
     json_regfetch(L, shadowtbl);
     lua_pushvalue(L, 1);
     lua_rawget(L, -2);
+    // if shadow table does not exist:
     if (lua_isnil(L, -1)) {
-      // set shadow table and leave it on top of stack:
+      // create shadow table and leave it on top of stack:
       lua_newtable(L);
       lua_pushvalue(L, 1);
       lua_pushvalue(L, -2);
       lua_rawset(L, -5);
     }
+    // move elements from original table to shadow table (that's expected on top of stack):
     for(lua_pushnil(L); lua_next(L, 1); lua_pop(L, 1)) {
       lua_pushvalue(L, -2);
       lua_pushnil(L);
@@ -60,7 +70,7 @@
   // discard everything but table to return:
   lua_settop(L, 1);
   // set metatable:
-  json_regfetchref(L, mt);
+  json_regfetchpointer(L, mt);
   lua_setmetatable(L, 1);
   // return table:
   return 1;
@@ -69,15 +79,16 @@
 // marks a table as JSON object:
 // (returns its modified argument or a new table if argument is nil)
 static int json_object(lua_State *L) {
-  return json_mark(L, json_regref(objectmt));
+  return json_mark(L, json_regpointer(objectmt));
 }
 
 // marks a table as JSON array:
 // (returns its modified argument or a new table if argument is nil)
 static int json_array(lua_State *L) {
-  return json_mark(L, json_regref(arraymt));
+  return json_mark(L, json_regpointer(arraymt));
 }
 
+// internal states of JSON parser:
 #define JSON_STATE_VALUE 0
 #define JSON_STATE_OBJECT_KEY 1
 #define JSON_STATE_OBJECT_KEY_TERMINATOR 2
@@ -87,6 +98,7 @@
 #define JSON_STATE_ARRAY_SEPARATOR 6
 #define JSON_STATE_END 7
 
+// special Lua stack indicies for json_import function:
 #define json_import_objectmt_idx 2
 #define json_import_arraymt_idx 3
 #define json_import_shadowtbl_idx 4
@@ -98,21 +110,21 @@
   size_t total;      // total length of string to parse
   size_t pos = 0;    // current position in string to parse
   size_t level = 0;  // nested levels of objects/arrays currently being processed
-  int mode = JSON_STATE_VALUE;  // state of parser
+  int mode = JSON_STATE_VALUE;  // state of parser (i.e. "what's expected next?")
   char c;              // variable to store a single character to be processed
-  luaL_Buffer luabuf;  // Lua buffer to decode (possibly escaped) strings
-  char *cbuf;          // C buffer to decode (possibly escaped) strings
+  luaL_Buffer luabuf;  // Lua buffer to decode JSON string values
+  char *cbuf;          // C buffer to decode JSON string values
   size_t writepos;     // write position of decoded strings in C buffer
-  // limit stack to 1 element:
+  // stack shall only contain one function argument:
   lua_settop(L, 1);
-  // push json_objectmt on stack position 2:
+  // push objectmt on stack position 2:
   json_regfetch(L, objectmt);
-  // push json_arraymt on stack position 3:
+  // push arraymt on stack position 3:
   json_regfetch(L, arraymt);
-  // push json_shadowtbl on stack position 4:
+  // push shadowtbl on stack position 4:
   json_regfetch(L, shadowtbl);
-  // push json_nullmark on stack position 5:
-  json_regfetch(L, nullmark);
+  // push nullmark on stack position 5:
+  json_pushlightref(L, nullmark);
   // require string as first argument:
   str = luaL_checklstring(L, 1, &total);
   // if string contains a NULL byte, this is a syntax error
@@ -402,7 +414,7 @@
   json_regfetch(L, shadowtbl);
   lua_insert(L, 1);
   // insert json_nullmark on stack at position 2:
-  json_regfetch(L, nullmark);
+  json_pushlightref(L, nullmark);
   lua_insert(L, 2);
   // store number of arguments:
   stacktop = lua_gettop(L);
@@ -536,7 +548,7 @@
     lua_rawset(L, json_setnull_shadowtbl_idx);
   }
   lua_pushvalue(L, 2);
-  json_regfetch(L, nullmark);
+  json_pushlightref(L, nullmark);
   lua_rawset(L, -3);
   return 0;
 }
@@ -555,7 +567,7 @@
 
 static int json_index(lua_State *L) {
   lua_settop(L, 2);
-  json_regfetch(L, nullmark);  // on stack position 3
+  json_pushlightref(L, nullmark);  // on stack position 3
   json_regfetch(L, shadowtbl);
   lua_pushvalue(L, 1);
   lua_rawget(L, json_index_shadowtbl_idx);
@@ -583,7 +595,7 @@
 
 static int json_pairs_iterfunc(lua_State *L) {
   lua_settop(L, 2);
-  json_regfetch(L, nullmark);  // on stack position 3
+  json_pushlightref(L, nullmark);  // on stack position 3
   json_regfetch(L, shadowtbl);
   lua_pushvalue(L, 1);
   lua_rawget(L, json_pairs_iterfunc_shadowtbl_idx);
@@ -610,7 +622,7 @@
 static int json_ipairs_iterfunc(lua_State *L) {
   int idx;
   lua_settop(L, 2);
-  json_regfetch(L, nullmark);  // on stack position 3
+  json_pushlightref(L, nullmark);  // on stack position 3
   json_regfetch(L, shadowtbl);
   idx = lua_tointeger(L, 2) + 1;
   lua_pushvalue(L, 1);
@@ -673,8 +685,6 @@
   lua_rawset(L, -3);
   lua_setmetatable(L, -2);
   json_regstore(L, shadowtbl);
-  lua_newtable(L);
-  json_regstore(L, nullmark);
   lua_settop(L, 1);
   luaL_setfuncs(L, json_module_functions, 0);
   return 1;