ordermap.lua

local index_cache = setmetatable({} , { __mode = "k" })

local meta = {}

function meta.__newindex(t, k, v)
	index_cache[t] = nil
	rawset(t, k, v)
end

function meta.__pairs(t)
	local index = index_cache[t]
	if not index then
		index = {}
		local i = 1
		for k in next, t do
			index[i] = k
			i=i+1
		end
		table.sort(index)
		index_cache[t] = index
	end
	local i = 0
	return function(t)
		i = i + 1
		for j = i,#index do
			local k = index[j]
			local v = t[k]
			if v then
				return k , v
			end
		end
	end, t
end

function make_order(t)
	return setmetatable(t or {}, meta)
end


function test(map)
	map.a = 1
	map.b = 2
	map.c = 3
	map.d = 4
	map.e = 5
	map.f = 6

	map.d = nil
	map.b = nil

	map.g = 7

	for k,v in pairs(map) do
		print(k,v)
	end
end

local tbl = {}

print "===table"
test(tbl)
print "===order map"
test (make_order(tbl))

--[[
===table
e       5
c       3
a       1
g       7
f       6
===order map
a       1
c       3
e       5
f       6
g       7
]]