feat: add encoding of nil values in tables and of empty objects

Author: pespinho

msgpack.lua

@@ -43,15 +43,27 @@ local type, pcall, pairs, select = type, pcall, pairs, select
 --]]----------------------------------------------------------------------------
 local encode_value -- forward declaration
 
-local function is_an_array(value)
+local NIL = {} -- unique object to represent nil
+
+local EMPTY_OBJECT = {} -- unique object to represent an empty object
+
+local function is_an_array(value, empty_as_array)
+    local empty = true
 	local expected = 1
-	for k in pairs(value) do
-		if k ~= expected then
-			return false
-		end
-		expected = expected + 1
-	end
-	return true
+
+    for k in pairs(value) do
+        empty = false
+        if k ~= expected then
+            return false
+        end
+        expected = expected + 1
+    end
+
+    if empty and not empty_as_array then
+        return false
+    end
+
+    return true
 end
 
 local encoder_functions = {
@@ -123,8 +135,9 @@ local encoder_functions = {
 			end
 		end
 	end,
-	['table'] = function(value)
-		if is_an_array(value) then -- it seems to be a proper Lua array
+	['table'] = function(value, empty_as_array)
+        if empty_as_array == nil then empty_as_array = true end
+        if is_an_array(value, empty_as_array) then -- it seems to be a proper Lua array
 			local elements = {}
 			for i, v in pairs(value) do
 				elements[i] = encode_value(v)
@@ -158,18 +171,11 @@ local encoder_functions = {
 }
 
 encode_value = function(value)
+    if value == NIL then return encoder_functions['nil']() end
+    if value == EMPTY_OBJECT then return encoder_functions['table']({}, false) end
 	return encoder_functions[type(value)](value)
 end
 
-local function encode(...)
-	local data = {}
-	for i = 1, select('#', ...) do
-		data[#data + 1] = encode_value(select(i, ...))
-	end
-	return tconcat(data)
-end
-
-
 --[[----------------------------------------------------------------------------
 
 		Decoder
@@ -368,6 +374,12 @@ return {
 			return nil, 'cannot decode MessagePack'
 		end
 	end,
+
+    --- an object that is packed as a nil value
+    NIL = NIL,
+
+    --- an object that is packed as an empty object
+    EMPTY_OBJECT = EMPTY_OBJECT
 }
 
 --[[----------------------------------------------------------------------------