filename=minetest.get_worldpath() .. "/teleport_tubes"

function read_file()
	local f = io.open(filename, "r")
	if f==nil then return {} end
    	local t = f:read("*all")
    	f:close()
	if t=="" or t==nil then return {} end
	return minetest.deserialize(t)
end

function write_file(tbl)
	local f = io.open(filename, "w")
    	f:write(minetest.serialize(tbl))
    	f:close()
end

function add_tube_in_file(pos,channel)
	tbl=read_file()
	for _,val in ipairs(tbl) do
		if val.x==pos.x and val.y==pos.y and val.z==pos.z then
			return
		end
	end
	table.insert(tbl,{x=pos.x,y=pos.y,z=pos.z,channel=channel})
	write_file(tbl)
end

function remove_tube_in_file(pos)
	tbl=read_file()
	newtbl={}
	for _,val in ipairs(tbl) do
		if val.x~=pos.x or val.y~=pos.y or val.z~=pos.z then
			table.insert(newtbl,val)
		end
	end
	write_file(newtbl)
end

function get_tubes_in_file(pos,channel)
	tbl=read_file()
	newtbl={}
	for _,val in ipairs(tbl) do
		if val.channel==channel and (val.x~=pos.x or val.y~=pos.y or val.z~=pos.z) then
			table.insert(newtbl,val)
		end
	end
	return newtbl
end


register_tube("pipeworks:teleport_tube","Teleporter pneumatic tube segment",plain_textures,noctr_textures,end_textures,
		short_texture,inv_texture,
		{tube={can_go=function(pos,node,velocity,stack)
			velocity.x=0
			velocity.y=0
			velocity.z=0
			local meta = minetest.env:get_meta(pos)
			channel=meta:get_string("channel")
			goto=get_tubes_in_file(pos,channel)
			if goto[1]==nil then return {} end
			pos.x=goto[1].x
			pos.y=goto[1].y
			pos.z=goto[1].z
			return meseadjlist
		end},
		on_construct = function(pos)
			local meta = minetest.env:get_meta(pos)
			meta:set_string("channel","0")
			meta:set_string("formspec","size[9,1;]field[0,0.5;9,1;channel;Channel:;${channel}]")
			add_tube_in_file(pos,"0")
		end,
		on_receive_fields = function(pos,formname,fields,sender)
			local meta = minetest.env:get_meta(pos)
			meta:set_string("channel",fields.channel)
			remove_tube_in_file(pos)
			add_tube_in_file(pos,fields.channel)
		end,
		after_dig_node = function(pos)
			remove_tube_in_file(pos)
		end})