# HG changeset patch
# User jbe
# Date 1407755929 -7200
# Node ID 0014a7c22013b215d8fdffa4cdf8710012eae3d3
# Parent  33c8f7029cfa22bf3c6801d719620db141ad03d0
Improved performance of JSON library by storing shadow tables directly via a lightuserdata key instead of using ephemeron tables

diff -r 33c8f7029cfa -r 0014a7c22013 libraries/json/json.c
--- a/libraries/json/json.c	Sun Aug 10 20:18:57 2014 +0200
+++ b/libraries/json/json.c	Mon Aug 11 13:18:49 2014 +0200
@@ -12,27 +12,36 @@
 //       levels for JSON documents <= 2 GiB.
 #define JSON_MAXDEPTH (1024*1024*1024)
 
-// generate dummy memory addresses that represents null values:
-char json_nullmark;
-#define json_isnullmark(L, i) (lua_touserdata((L), (i)) == &json_nullmark)
-#define json_pushnullmark(L) lua_pushlightuserdata((L), &json_nullmark)
+// define type JSON_LIGHTUSERDATA and
+// generate dummy memory addresses for lightuserdata values:
+#define JSON_LIGHTUSERDATA char
+static struct {
+  JSON_LIGHTUSERDATA nullmark;   // lightuserdata value represents a NULL value
+  JSON_LIGHTUSERDATA shadowtbl;  // lightuserdata key for shadow table
+} json_lightuserdata;
+
+// macros for special nullmark value:
+#define json_isnullmark(L, i) (lua_touserdata((L), (i)) == &json_lightuserdata.nullmark)
+#define json_pushnullmark(L) lua_pushlightuserdata((L), &json_lightuserdata.nullmark)
+
+// macros for getting and setting shadow tables
+#define json_setshadow(L, i) lua_rawsetp((L), (i), &json_lightuserdata.shadowtbl)
+#define json_getshadow(L, i) lua_rawgetp((L), (i), &json_lightuserdata.shadowtbl)
+#define json_createproxy(L) lua_createtable((L), 0, 1)
+
+// generate additional dummy memory addresses that represent Lua objects
+// via lightuserdata keys and LUA_REGISTRYINDEX:
+static struct {
+  JSON_LIGHTUSERDATA objectmt;  // metatable for JSON objects
+  JSON_LIGHTUSERDATA arraymt;   // metatable for JSON arrays
+} json_registry;
 
 // macros for usage of Lua registry:
-#define JSON_REGENT char
-#define JSON_REGPOINTER void *
 #define json_regpointer(x) (&json_registry.x)
 #define json_regfetchpointer(L, x) lua_rawgetp((L), LUA_REGISTRYINDEX, (x))
 #define json_regfetch(L, x) json_regfetchpointer(L, json_regpointer(x))
 #define json_regstore(L, x) lua_rawsetp(L, LUA_REGISTRYINDEX, json_regpointer(x))
 
-// generate dummy memory addresses that represent Lua objects
-// via lightuserdata keys and LUA_REGISTRYINDEX:
-static struct {
-  JSON_REGENT shadowtbl;  // ephemeron table that maps tables to their corresponding shadow table
-  JSON_REGENT objectmt;   // metatable for JSON objects
-  JSON_REGENT arraymt;    // metatable for JSON arrays
-} json_registry;
-
 // returns the string "<JSON null marker>":
 static int json_nullmark_tostring(lua_State *L) {
   lua_pushliteral(L, "<JSON null marker>");
@@ -53,35 +62,33 @@
   // determine is argument is given:
   if (lua_isnoneornil(L, json_convert_source_idx)) {
     // if no argument is given (or if argument is nil),
-    // create table with shadow table, and leave first table on top of stack:
-    json_regfetch(L, shadowtbl);
+    // create proxy table with shadow table, and leave proxy table on top of stack:
+    json_createproxy(L);
     lua_newtable(L);
-    lua_pushvalue(L, -1);
-    lua_newtable(L);
-    lua_rawset(L, -4);
+    json_setshadow(L, -2);
   } else {
     // if an argument was given,
-    // push its iterator function on stack position 2 if existent,
-    // else push null for normal tables:
+    // stack shall contain only one function argument:
     lua_settop(L, 1);
-    if (lua_getmetatable(L, json_convert_source_idx)) {
-      lua_getfield(L, -1, array ? "__ipairs" : "__pairs");
-      if (lua_isnil(L, -1)) luaL_checktype(L, 1, LUA_TTABLE);
-      else if (lua_type(L, -1) != LUA_TFUNCTION)
+    // check if there is an iterator function in its metatable:
+    if (luaL_getmetafield(L, json_convert_source_idx, array ? "__ipairs" : "__pairs")) {
+      // if there is an iterator function,
+      // leave it on stack position 2 and verify its type:
+      if (lua_type(L, json_convert_iterator_idx) != LUA_TFUNCTION)
         return luaL_error(L, "%s metamethod is not a function", array ? "__ipairs" : "__pairs");
-      lua_replace(L, -2);
     } else {
+      // if there is no iterator function,
+      // verify the type of the argument itself:
+      luaL_checktype(L, json_convert_source_idx, LUA_TTABLE);
+      // push nil onto stack position 2:
       lua_pushnil(L);
     }
     // create result table on stack position 3:
-    lua_newtable(L);
+    json_createproxy(L);
     // create shadow table on stack position 4:
-    json_regfetch(L, shadowtbl);
     lua_newtable(L);
-    lua_pushvalue(L, json_convert_output_idx);
-    lua_pushvalue(L, -2);
-    lua_rawset(L, -4);
-    lua_replace(L, -2);
+    lua_pushvalue(L, -1);
+    json_setshadow(L, -3);
     // check if iterator function exists:
     if (lua_isnil(L, json_convert_iterator_idx)) {
       // if there is no iterator function,
@@ -185,8 +192,7 @@
 // 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
-#define json_import_stackswap_idx 5
+#define json_import_stackswap_idx 4
 
 // macros for hex decoding:
 #define json_utf16_surrogate(x) ((x) >= 0xD800 && (x) <= 0xDFFF)
@@ -232,8 +238,6 @@
   json_regfetch(L, objectmt);
   // push arraymt onto stack position 3:
   json_regfetch(L, arraymt);
-  // push shadowtbl onto stack position 4:
-  json_regfetch(L, shadowtbl);
   // push table for stack swapping onto stack position 5:
   // (needed to avoid Lua stack overflows)
   lua_newtable(L);
@@ -307,9 +311,8 @@
     // create internal shadow table on stack:
     lua_newtable(L);
     // register internal shadow table:
-    lua_pushvalue(L, -2);
-    lua_pushvalue(L, -2);
-    lua_rawset(L, json_import_shadowtbl_idx);
+    lua_pushvalue(L, -1);
+    json_setshadow(L, -3);
     // distinguish between JSON objects and JSON arrays:
     if (c == '{') {
       // if JSON object,
@@ -610,38 +613,28 @@
   return 2;
 }
 
-// special Lua stack indicies for json_path function:
-#define json_path_shadowtbl_idx 1
-
-// stack offset of arguments to json_path function:
-#define json_path_idxshift 1
-
 // gets a value or its type from a JSON document (passed as first argument)
 // using a path (passed as variable number of keys after the first argument):
 static int json_path(lua_State *L, int type_mode) {
-  int stacktop;                      // stack index of top of stack (after shifting)
-  int idx = 2 + json_path_idxshift;  // stack index of current argument to process
+  int stacktop;  // number of arguments
+  int idx = 2;   // stack index of current argument to process
   // require at least one argument:
   luaL_checkany(L, 1);
-  // insert shadowtbl into stack at position 1 (shifting the arguments):
-  json_regfetch(L, shadowtbl);
-  lua_insert(L, 1);
-  // store stack index of top of stack:
+  // store stack index of top of stack (number of arguments):
   stacktop = lua_gettop(L);
   // use first argument as "current value" (stored on top of stack):
-  lua_pushvalue(L, 1 + json_path_idxshift);
+  lua_pushvalue(L, 1);
   // process each "path key" (2nd argument and following arguments):
   while (idx <= stacktop) {
     // if "current value" (on top of stack) is nil, then the path cannot be walked and nil is returned:
     if (lua_isnil(L, -1)) return 1;
     // try to get shadow table of "current value":
-    lua_pushvalue(L, -1);
-    lua_rawget(L, json_path_shadowtbl_idx);
+    json_getshadow(L, -1);
     if (lua_isnil(L, -1)) {
       // if no shadow table is found,
-      if (lua_type(L, -1) == LUA_TTABLE) {
+      if (lua_type(L, -2) == LUA_TTABLE) {
         // and if "current value" is a table,
-        // drop nil from stack:
+        // pop nil from stack:
         lua_pop(L, 1);
         // get "next value" using the "path key":
         lua_pushvalue(L, idx++);
@@ -714,30 +707,26 @@
 }
 
 // special Lua stack indicies for json_set function:
-#define json_set_shadowtbl_idx 1
-#define json_set_objectmt_idx 2
-#define json_set_arraymt_idx 3
+#define json_set_objectmt_idx 1
+#define json_set_arraymt_idx 2
 
 // stack offset of arguments to json_set function:
-#define json_set_idxshift 3
+#define json_set_idxshift 2
 
 // sets a value (passed as second argument) in a JSON document (passed as first argument)
 // using a path (passed as variable number of keys starting at third argument):
 static int json_set(lua_State *L) {
-  int stacktop;   // stack index of top of stack (after shifting)
-  int idx = 3;    // stack index of current argument to process
+  int stacktop;  // stack index of top of stack (after shifting)
+  int idx;       // stack index of current argument to process
   // require at least two arguments:
   luaL_checkany(L, 1);
   luaL_checkany(L, 2);
-  // insert shadowtbl into stack at position 1 (shifting the arguments):
-  json_regfetch(L, shadowtbl);
-  lua_insert(L, 1);
-  // insert objectmt into stack at position 2 (shifting the arguments):
+  // insert objectmt into stack at position 1 (shifting the arguments):
   json_regfetch(L, objectmt);
+  lua_insert(L, 1);
+  // insert arraymt into stack at position 2 (shifting the arguments):
+  json_regfetch(L, arraymt);
   lua_insert(L, 2);
-  // insert arraymt into stack at position 3 (shifting the arguments):
-  json_regfetch(L, arraymt);
-  lua_insert(L, 3);
   // store stack index of top of stack:
   stacktop = lua_gettop(L);
   // use nil as initial "parent value":
@@ -767,11 +756,10 @@
         // throw error if parent element does not exist:
         if (lua_isnil(L, -1)) return luaL_error(L, "Root element is not a JSON object");
         // push new JSON object as "current value" onto stack:
-        lua_newtable(L);
+        json_createproxy(L);
         // create and register shadow table:
-        lua_pushvalue(L, -1);
         lua_newtable(L);
-        lua_rawset(L, json_set_shadowtbl_idx);
+        json_setshadow(L, -2);
         // set metatable of JSON object: 
         lua_pushvalue(L, json_set_objectmt_idx);
         lua_setmetatable(L, -2);
@@ -798,11 +786,10 @@
         // throw error if parent element does not exist:
         if (lua_isnil(L, -1)) return luaL_error(L, "Root element is not a JSON array");
         // push new JSON array as "current value" onto stack:
-        lua_newtable(L);
+        json_createproxy(L);
         // create and register shadow table:
-        lua_pushvalue(L, -1);
         lua_newtable(L);
-        lua_rawset(L, json_set_shadowtbl_idx);
+        json_setshadow(L, -2);
         // set metatable of JSON array: 
         lua_pushvalue(L, json_set_arraymt_idx);
         lua_setmetatable(L, -2);
@@ -840,12 +827,12 @@
 static int json_len(lua_State *L) {
   // stack shall contain one function argument:
   lua_settop(L, 1);
-  // try to get corresponding shadow table for first argument:
-  json_regfetch(L, shadowtbl);
-  lua_pushvalue(L, 1);
-  lua_rawget(L, -2);
-  // if shadow table does not exist, return length of argument, else length of shadow table:
-  lua_pushnumber(L, lua_rawlen(L, lua_isnil(L, -1) ? 1 : -1));
+  // push shadow table or nil onto stack:
+  json_getshadow(L, 1);
+  // pop nil from stack if no shadow table has been found:
+  if (lua_isnil(L, -1)) lua_pop(L, 1);
+  // return length of argument or shadow table:
+  lua_pushnumber(L, lua_rawlen(L, -1));
   return 1;
 }
 
@@ -853,17 +840,15 @@
 static int json_index(lua_State *L) {
   // stack shall contain two function arguments:
   lua_settop(L, 2);
-  // get corresponding shadow table for first argument:
-  json_regfetch(L, shadowtbl);
-  lua_pushvalue(L, 1);
-  lua_rawget(L, -2);
-  // throw error if no shadow table was found:
+  // replace first argument with its shadow table
+  // or throw error if no shadow table is found:
+  json_getshadow(L, 1);
   if (lua_isnil(L, -1)) return luaL_error(L, "Shadow table not found");
+  lua_replace(L, 1);
   // use key passed as second argument to lookup value in shadow table:
-  lua_pushvalue(L, 2);
-  lua_rawget(L, -2);
+  lua_rawget(L, 1);
   // if value is null-marker, then push nil onto stack:
-  if (json_isnullmark(L, -1)) lua_pushnil(L);
+  if (json_isnullmark(L, 2)) lua_pushnil(L);
   // return either looked up value, or nil
   return 1;
 }
@@ -872,16 +857,12 @@
 static int json_newindex(lua_State *L) {
   // stack shall contain three function arguments:
   lua_settop(L, 3);
-  // get corresponding shadow table for first argument:
-  json_regfetch(L, shadowtbl);
-  lua_pushvalue(L, 1);
-  lua_rawget(L, -2);
-  // throw error if no shadow table was found:
+  // replace first argument with its shadow table
+  // or throw error if no shadow table is found:
+  json_getshadow(L, 1);
   if (lua_isnil(L, -1)) return luaL_error(L, "Shadow table not found");
-  // replace first argument with shadow table:
   lua_replace(L, 1);
-  // reset stack and use second and third argument to write to shadow table:
-  lua_settop(L, 3);
+  // second and third argument to write to shadow table:
   lua_rawset(L, 1);
   // return nothing:
   return 0;
@@ -891,16 +872,14 @@
 static int json_pairs_iterfunc(lua_State *L) {
   // stack shall contain two function arguments:
   lua_settop(L, 2);
-  // get corresponding shadow table for first argument:
-  json_regfetch(L, shadowtbl);
-  lua_pushvalue(L, 1);
-  lua_rawget(L, -2);
-  // throw error if no shadow table was found:
+  // replace first argument with its shadow table
+  // or throw error if no shadow table is found:
+  json_getshadow(L, 1);
   if (lua_isnil(L, -1)) return luaL_error(L, "Shadow table not found");
+  lua_replace(L, 1);
   // get next key value pair from shadow table (using previous key from argument 2)
   // and return nothing if there is no next pair:
-  lua_pushvalue(L, 2);
-  if (!lua_next(L, -2)) return 0;
+  if (!lua_next(L, 1)) return 0;
   // replace null-marker with nil:
   if (json_isnullmark(L, -1)) {
     lua_pop(L, 1);
@@ -929,22 +908,20 @@
   lua_settop(L, 2);
   // calculate new index by incrementing second argument:
   idx = lua_tointeger(L, 2) + 1;
-  // get corresponding shadow table for first argument:
-  json_regfetch(L, shadowtbl);
-  lua_pushvalue(L, 1);
-  lua_rawget(L, -2);
-  // throw error if no shadow table was found:
+  // push shadow table onto stack position 3
+  // or throw error if no shadow table is found:
+  json_getshadow(L, 1);
   if (lua_isnil(L, -1)) return luaL_error(L, "Shadow table not found");
-  // do integer lookup in shadow table:
-  lua_rawgeti(L, -1, idx);
+  // do integer lookup in shadow table and store result on stack position 4:
+  lua_rawgeti(L, 3, idx);
   // return nothing if there was no value:
-  if (lua_isnil(L, -1)) return 0;
+  if (lua_isnil(L, 4)) return 0;
   // return new index and
   // either the looked up value if it is not equal to the null-marker
   // or nil instead of null-marker:
   lua_pushinteger(L, idx);
-  if (json_isnullmark(L, -2)) lua_pushnil(L);
-  else lua_pushvalue(L, -2);
+  if (json_isnullmark(L, 4)) lua_pushnil(L);
+  else lua_pushvalue(L, 4);
   return 2;
 }
 
@@ -1009,11 +986,10 @@
 #define json_export_indentstring_idx 2
 #define json_export_objectmt_idx 3
 #define json_export_arraymt_idx 4
-#define json_export_shadowtbl_idx 5
-#define json_export_stackswap_idx 6
-#define json_export_luacontainer_idx 7
-#define json_export_ccontainer_idx 8
-#define json_export_buffer_idx 9
+#define json_export_stackswap_idx 5
+#define json_export_luacontainer_idx 6
+#define json_export_ccontainer_idx 7
+#define json_export_buffer_idx 8
 
 // encodes a JSON document (passed as first argument)
 // optionally using indentation (indentation string or true passed as second argument)
@@ -1063,11 +1039,9 @@
   json_regfetch(L, objectmt);
   // push arraymt onto stack position 4:
   json_regfetch(L, arraymt);
-  // push shadowtbl onto stack position 5:
-  json_regfetch(L, shadowtbl);
-  // push table for stack swapping onto stack position 6:
+  // push table for stack swapping onto stack position 5:
   lua_newtable(L);
-  // create placeholders on stack positions 7 through 8:
+  // create placeholders on stack positions 6 through 7:
   lua_settop(L, json_export_buffer_idx);
   // create Lua string buffer:
   luaL_buffinit(L, &buf);
@@ -1146,11 +1120,10 @@
         // reset stack (pop metatable from stack):
         lua_pop(L, 1);
       }
-      // replace table with its shadow table if existent, and reset stack:
-      lua_pushvalue(L, json_export_value_idx);
-      lua_rawget(L, json_export_shadowtbl_idx);
+      // replace table with its shadow table if existent:
+      json_getshadow(L, json_export_value_idx);
       if (lua_isnil(L, -1)) lua_pop(L, 1);
-      else lua_replace(L, json_export_value_idx);
+      else lua_replace(L, json_export_value_idx); 
       // check if type of table is still undetermined
       // and optionally calculate number of string keys (keycount)
       // or set keycount to zero:
@@ -1395,15 +1368,6 @@
   lua_newtable(L);
   luaL_setfuncs(L, json_metatable_functions, 0);
   json_regstore(L, arraymt);
-  // create and store ephemeron table to store shadow tables for each JSON object/array
-  // to allow NULL values returned as nil
-  lua_newtable(L);
-  lua_newtable(L);  // metatable for ephemeron table
-  lua_pushliteral(L, "__mode");
-  lua_pushliteral(L, "k");
-  lua_rawset(L, -3);
-  lua_setmetatable(L, -2);
-  json_regstore(L, shadowtbl);
   // set metatable of null marker and make it available through library module:
   json_pushnullmark(L);
   lua_newtable(L);