From 3b1429fabbaa6c2fc3d8c0f7c83b3810e530caf9 Mon Sep 17 00:00:00 2001
From: drdr xp <drdr.xp@gmail.com>
Date: Thu, 9 Apr 2015 17:25:47 +0800
Subject: [PATCH] option to force to encode empty table to "[]"

Since lua-cjson use table index type to decide what type a lua table is
encoded to, there is no way to encode empty lua type to list.

This patch solves this problem by assigning a special meta-table to an
empty table to force it to be encoded to "[]" instead of "{}":

    setmetatable({},cjson.as_array)
---
 lua_cjson.c    | 23 +++++++++++++++++++++--
 tests/test.lua |  7 +++++++
 2 files changed, 28 insertions(+), 2 deletions(-)

diff --git a/lua_cjson.c b/lua_cjson.c
index c14a1c5..0452bcd 100644
--- a/lua_cjson.c
+++ b/lua_cjson.c
@@ -74,6 +74,8 @@
 #define DEFAULT_DECODE_INVALID_NUMBERS 0
 #endif
 
+#define MT_AS_ARRAY "MT_AS_ARRAY"
+
 typedef enum {
     T_OBJ_BEGIN,
     T_OBJ_END,
@@ -687,8 +689,22 @@ static void json_append_data(lua_State *l, json_config_t *cfg,
         len = lua_array_length(l, cfg, json);
         if (len > 0)
             json_append_array(l, cfg, current_depth, json, len);
-        else
-            json_append_object(l, cfg, current_depth, json);
+        else {
+            int is_array = 0;
+            int got = lua_getmetatable(l, -1);
+            if (got > 0) {
+                luaL_getmetatable(l, MT_AS_ARRAY);
+                is_array = lua_rawequal(l, -1, -2);
+                lua_pop(l, 2);
+            }
+
+            if (is_array) {
+                json_append_array(l, cfg, current_depth, json, 0);
+            }
+            else {
+                json_append_object(l, cfg, current_depth, json);
+            }
+        }
         break;
     case LUA_TNIL:
         strbuf_append_mem(json, "null", 4);
@@ -1377,6 +1393,9 @@ static int lua_cjson_new(lua_State *l)
     lua_pushlightuserdata(l, NULL);
     lua_setfield(l, -2, "null");
 
+    luaL_newmetatable(l, MT_AS_ARRAY);
+    lua_setfield(l, -2, "as_array");
+
     /* Set module name / version fields */
     lua_pushliteral(l, CJSON_MODNAME);
     lua_setfield(l, -2, "_NAME");
diff --git a/tests/test.lua b/tests/test.lua
index b8fce84..19f2f26 100755
--- a/tests/test.lua
+++ b/tests/test.lua
@@ -89,6 +89,9 @@ local NaN = math.huge * 0;
 
 local testdata = load_testdata()
 
+local empty_array = {}
+setmetatable(empty_array, json.as_array)
+
 local cjson_tests = {
     -- Test API variables
     { "Check module name, version",
@@ -212,6 +215,10 @@ local cjson_tests = {
       json.encode, { 10 }, true, { '10' } },
     { "Encode string",
       json.encode, { "hello" }, true, { '"hello"' } },
+    { "Encode empty table as array",
+      json.encode, { empty_array }, true, { '[]' } },
+    { "Encode empty table as table",
+      json.encode, { {} }, true, { '{}' } },
     { "Encode Lua function [throw error]",
       json.encode, { function () end },
       false, { "Cannot serialise function: type not supported" } },