diff --git a/tracedoc.lua b/tracedoc.lua
index 9bc254e..b45ad87 100644
--- a/tracedoc.lua
+++ b/tracedoc.lua
@@ -1,5 +1,6 @@
 local next = next
 local pairs = pairs
+local ipairs = ipairs
 local setmetatable = setmetatable
 local getmetatable = getmetatable
 local type = type
@@ -10,93 +11,17 @@ local tracedoc = {}
 local NULL = setmetatable({} , { __tostring = function() return "NULL" end })	-- nil
 tracedoc.null = NULL
 local tracedoc_type = setmetatable({}, { __tostring = function() return "TRACEDOC" end })
-local tracedoc_len = setmetatable({} , { __mode = "kv" })
 
 local function doc_next(doc, key)
-	-- at first, iterate all the keys changed
-	local change_keys = doc._keys
-	if key == nil or change_keys[key] then
-		while true do
-			key = next(change_keys, key)
-			if key == nil then
-				break
-			end
-			local v = doc[key]
-			if v ~= nil then
-				return key, v
-			end
-		end
-	end
-
-	-- and then, iterate all the keys in lastversion except keys changed
-
-	local lastversion = doc._lastversion
-
-	while true do
-		key = next(lastversion, key)
-		if key == nil then
-			return
-		end
-		if not change_keys[key] then
-			local v = doc[key]
-			if v ~= nil then
-				return key, v
-			end
-		end
-	end
+	return next(doc._stage, key)
 end
 
 local function doc_pairs(doc)
-	return doc_next, doc
-end
-
-local function find_length_after(doc, idx)
-	local v = doc[idx + 1]
-	if v == nil then
-		return idx
-	end
-	repeat
-		idx = idx + 1
-		v = doc[idx + 1]
-	until v == nil
-	tracedoc_len[doc] = idx
-	return idx
-end
-
-local function find_length_before(doc, idx)
-	if idx <= 1 then
-		tracedoc_len[doc] = nil
-		return 0
-	end
-	repeat
-		idx = idx - 1
-	until idx <=0 or doc[idx] ~= nil
-	tracedoc_len[doc] = idx
-	return idx
+	return pairs(doc._stage)
 end
 
 local function doc_len(doc)
-	local len = tracedoc_len[doc]
-	if len == nil then
-		len = #doc._lastversion
-		tracedoc_len[doc] = len
-	end
-	if len == 0 then
-		return find_length_after(doc, 0)
-	end
-	local v = doc[len]
-	if v == nil then
-		return find_length_before(doc, len)
-	end
-	return find_length_after(doc, len)
-end
-
-local function doc_read(doc, k)
-	if doc._keys[k] then
-		return doc._changes[k]
-	end
-	-- if k is not changed, return lastversion
-	return doc._lastversion[k]
+	return #doc._stage
 end
 
 local function doc_change(doc, k, v)
@@ -114,17 +39,20 @@ local function doc_change(doc, k, v)
 	if type(v) == "table" then
 		local vt = getmetatable(v)
 		if vt == nil then
-			local lv = doc._lastversion[k]
+			local lv = doc._stage[k]
 			if getmetatable(lv) ~= tracedoc_type then
-				-- last version is not a table, new a empty one
-				lv = tracedoc.new()
-				lv._parent = doc
-				doc._lastversion[k] = lv
-			elseif doc[k] == nil then
-				-- this version is clear first, deepcopy lastversion one
-				lv = tracedoc.new(lv)
-				lv._parent = doc
-				doc._lastversion[k] = lv
+				lv = doc._changed_values[k]
+				if getmetatable(lv) ~= tracedoc_type then
+					-- last version is not a table, new a empty one
+					lv = tracedoc.new()
+					lv._parent = doc
+					doc._stage[k] = lv
+				else
+					-- this version is clear first, deepcopy lastversion one
+					lv = tracedoc.new(lv)
+					lv._parent = doc
+					doc._stage[k] = lv
+				end
 			end
 			local keys = {}
 			for k in pairs(lv) do
@@ -139,33 +67,33 @@ local function doc_change(doc, k, v)
 			for k in pairs(keys) do
 				lv[k] = nil
 			end
-			-- don't cache sub table into doc._changes
-			doc._changes[k] = nil
-			doc._keys[k] = nil
+			-- don't cache sub table into doc._changed_values
+			doc._changed_values[k] = nil
+			doc._changed_keys[k] = nil
 			return
 		end
 	end
-	doc._changes[k] = v
-	doc._keys[k] = true
+	doc._changed_values[k] = doc._stage[k]
+	doc._changed_keys[k] = true
+	doc._stage[k] = v
 end
 
-local doc_mt = {
-	__newindex = doc_change,
-	__index = doc_read,
-	__pairs = doc_pairs,
-	__len = doc_len,
-	__metatable = tracedoc_type,	-- avoid copy by ref
-}
-
 function tracedoc.new(init)
+	local doc_stage = {}
 	local doc = {
 		_dirty = false,
 		_parent = false,
-		_changes = {},
-		_keys = {},
-		_lastversion = {},
+		_changed_keys = {},
+		_changed_values = {},
+		_stage = doc_stage,
 	}
-	setmetatable(doc, doc_mt)
+	setmetatable(doc, {
+		__newindex = doc_change,
+		__index = doc_stage, 
+		__pairs = doc_pairs,
+		__len = doc_len,
+		__metatable = tracedoc_type,	-- avoid copy by ref
+	})
 	if init then
 		for k,v in pairs(init) do
 			-- deepcopy v
@@ -180,19 +108,15 @@ function tracedoc.new(init)
 end
 
 function tracedoc.dump(doc)
-	local last = {}
-	for k,v in pairs(doc._lastversion) do
-		table.insert(last, string.format("%s:%s",k,v))
-	end
-	local changes = {}
-	for k,v in pairs(doc._changes) do
-		table.insert(changes, string.format("%s:%s",k,v))
+	local stage = {}
+	for k,v in pairs(doc._stage) do
+		table.insert(stage, string.format("%s:%s",k,v))
 	end
-	local keys = {}
-	for k in pairs(doc._keys) do
-		table.insert(keys, k)
+	local changed = {}
+	for k in pairs(doc._changed_keys) do
+		table.insert(changed, string.format("%s:%s",k,doc._changed_values[k]))
 	end
-	return string.format("last [%s]\nchanges [%s]\nkeys [%s]",table.concat(last, " "), table.concat(changes," "), table.concat(keys," "))
+	return string.format("content [%s]\nchanged [%s]",table.concat(stage, " "), table.concat(changed," "))
 end
 
 function tracedoc.commit(doc, result, prefix)
@@ -200,27 +124,26 @@ function tracedoc.commit(doc, result, prefix)
 		return result
 	end
 	doc._dirty = false
-	local lastversion = doc._lastversion
-	local changes = doc._changes
-	local keys = doc._keys
+	local stage = doc._stage
+	local changed_values = doc._changed_values
+	local changed_keys = doc._changed_keys
 	local dirty = false
-	if next(keys) ~= nil then
-		for k in next, keys do
-			local v = changes[k]
-			keys[k] = nil
-			changes[k] = nil
-			if lastversion[k] ~= v then
-				dirty = true
-				if result then
-					local key = prefix and prefix .. k or k
-					result[key] = v == nil and NULL or v
-					result._n = (result._n or 0) + 1
-				end
-				lastversion[k] = v
+	for k in pairs(changed_keys) do
+		local v = stage[k]
+		local lv = changed_values[k]
+		changed_keys[k] = nil
+		changed_values[k] = nil
+
+		if lv ~= v then
+			dirty = true
+			if result then
+				local key = prefix and prefix .. k or k
+				result[key] = v == nil and NULL or v
+				result._n = (result._n or 0) + 1
 			end
 		end
 	end
-	for k,v in pairs(lastversion) do
+	for k,v in pairs(stage) do
 		if getmetatable(v) == tracedoc_type and v._dirty then
 			if result then
 				local key = prefix and prefix .. k or k