shadowcopy.lua

local shadowcopy = {}

local weak = { __mode = "k" }
local NIL = {}

local queue = setmetatable({}, weak) -- object -> queue
local map = setmetatable({}, weak)	-- copy -> { object = obj, values = {} }

local direct_mt = {
	__index = function(t,k)
		return map[t].object[k]
	end,
	__newindex = function(t,k,v)
		map[t].object[k] = v
--		print("DIRECT SET", map[t].object, k,v)
	end,
}

local delay_mt = {
	__index = function(t,k)
		local v = map[t].values[k]
		if v == nil then
			return map[t].object[k]
		end
		if v == NIL then
			v = nil
		end
		return v
	end,
	__newindex = function(t,k,v)
		if v == nil then
			v = NIL
		end
		map[t].values[k] = v
--		print("DELAY SET", map[t].object, k, v)
	end,
}

function shadowcopy.grab(obj)
	local q = queue[obj]
	if q == nil then
		q = {}
		queue[obj] = q
	else
		setmetatable(q[#q], delay_mt)
	end
	local proxy = { object = obj, values = {} }
	local ret = setmetatable({}, direct_mt)
	map[ret] = proxy
	table.insert(q, ret)
	return ret
end

local function apply(proxy)
	local obj = proxy.object
	for k,v in pairs(proxy.values) do
		if v == NIL then
			v = nil
		end
		obj[k] = v
--		print("SET", obj, k, v)
	end
end

function shadowcopy.release(copy)
	local proxy = assert(map[copy])
	map[copy] = nil
	local q = queue[proxy.object]
	if q == nil then
		-- object collected
		return
	end
	local n = #q
	if q[n] == copy then
		-- release last copy
		q[n] = nil
		if n == 1 then
			queue[proxy.object] = nil
		else
			local p = q[n-1]
			setmetatable(p, direct_mt)
			apply(map[p])
		end
	else
		for i=1,n-1 do
			if q[i] == copy then
				table.remove(q, i)
				return
			end
		end
	end
end

return shadowcopy
--[[
local sc = require "shadowcopy"

local obj = { x = 1, y = 2 }


local proxy1 = sc.grab(obj)
local proxy2 = sc.grab(obj)

proxy1.x = 2
proxy2.x = 3

sc.release(proxy2)

for k,v in pairs(obj) do
	print(k,v)
end
]]