local setmetatable = setmetatable
 
module "safe_set"
 
local function enter(self, func)
	local count = self.count + 1
	self.count = count
	local val = {func, true}
	self[count] = val
	return function()
		val[1] = nil
		val[2] = nil
	end
end
 
local function empty(self)
	return self.count == 0
end
 
local weak_metatable = {__mode = "kv"}
 
function create()
	local que = {}
	que.enter = enter
	que.empty = empty
	que.ref = 0
	que.count = 0
	que.state = setmetatable({}, weak_metatable)
	return que
end
 
local function merge(self)
	local pos = 0
	for i = 1, self.count do
		if not self[i][2] then
			pos = pos + 1
		else
			self[i - pos] = self[i]
		end
	end
	local oldcount = self.count
	self.count = self.count - pos
	for i = oldcount, self.count + 1, -1 do
		self[i] = nil
	end
end
 
local function iter(state)
	local self = state[1]
	local current = state[2] + 1
	while current <= self.count do
		if self[current][2] then
			break
		end
		current = current + 1
	end
	if current <= self.count then
		state[2] = current
		return self[current][1]
	else
		self.ref = self.ref - 1
		if self.ref == 0 then
			merge(self)
		end
	end
end
 
function iterator(self)
	local ref = self.ref + 1
	self.ref = ref
 
	local state = self.state[ref] or {}
	self.state[ref] = state
 
	state[1] = self
	state[2] = 0
 
	return iter, state
end

require "safe_set"
 
s = safe_set.create()
del = s:enter(function() print("test") end)
s:enter(function() del() end)
 
for f in safe_set.iterator(s) do
  f()
end