Add empty_table_as_array option for encode().

spec/json_encode_spec.lua

@@ -1,3 +1,6 @@
+require 'busted.runner'()
+
+
 --luacheck: ignore describe it
 describe('rapidjson.encode()', function()
   local rapidjson = require('rapidjson')
@@ -127,8 +130,11 @@ describe('rapidjson.encode()', function()
     )
   end)
 
+  it('should support empty_table_as_array options', function()
+    assert.are.equal('{"a": []}', rapidjson.encode({a={}}, {empty_table_as_array=true}))
+    assert.are.equal('{"a": {}}', rapidjson.encode({a={}}))
+  end)
   it('should support sort_keys and pretty options', function()
-
     assert.are.equal(
 [[{
     "A": true,

src/rapidjson.cpp

@@ -149,17 +149,19 @@ struct Key
 class Encoder {
 	bool pretty;
 	bool sort_keys;
+	bool empty_table_as_array;
 	int max_depth;
 	static const int MAX_DEPTH_DEFAULT = 128;
 public:
-	Encoder(lua_State*L, int opt) : pretty(false), sort_keys(false), max_depth(MAX_DEPTH_DEFAULT)
+	Encoder(lua_State*L, int opt) : pretty(false), sort_keys(false), empty_table_as_array(false), max_depth(MAX_DEPTH_DEFAULT)
 	{
 		if (lua_isnoneornil(L, opt))
 			return;
 		luaL_checktype(L, opt, LUA_TTABLE);
 
 		pretty = luax::optboolfield(L, opt, "pretty", false);
 		sort_keys = luax::optboolfield(L, opt, "sort_keys", false);
+		empty_table_as_array = luax::optboolfield(L, opt, "empty_table_as_array", false);
 		max_depth = luax::optintfield(L, opt, "max_depth", MAX_DEPTH_DEFAULT);
 	}
 
@@ -217,7 +219,7 @@ class Encoder {
 			luaL_error(L, "stack overflow");
 
 		lua_pushvalue(L, idx); // [table]
-		if (values::isarray(L, -1))
+		if (values::isarray(L, -1, empty_table_as_array))
 		{
 			encodeArray(L, writer, depth);
 			lua_pop(L, 1); // []

src/values.hpp

@@ -43,12 +43,22 @@ namespace values {
 		return has;
 	}
 
-	inline bool isarray(lua_State* L, int idx) {
+	inline bool isarray(lua_State* L, int idx, bool empty_table_as_array = false) {
 		bool arr = false;
 		if (hasJsonType(L, idx, arr)) // any table with a meta field __jsontype set to 'array' are arrays
 			return arr;
 
-		return luax::rawlen(L, idx) > 0; // any table has length > 0 are treat as array.
+		lua_pushvalue(L, idx);
+		lua_pushnil(L);
+		if (lua_next(L, -2) != 0) {
+			lua_pop(L, 3);
+
+			return luax::rawlen(L, idx) > 0; // any non empty table has length > 0 are treat as array.
+		}
+
+		lua_pop(L, 1);
+		// Now it comes empty table
+		return empty_table_as_array;
 	}