local scanner = peripherals.wrap("top")

local currentPlayers = {}
local playerIndexes = {}

local addPlayer = function(list, playerName)
	local player = {name = playerName, check = false}
	table.insert(list, player)
	playerIndexes[playerName] = #list
end

local removePlayer = function(list, playerName)
	local index = playerIndexes[playerName]
	table.remove(list, index)
	playerIndexes[playerName] = nil
end

local reset = function(list)
	for _,v in ipairs(list) do
		v.check = false
	end
end

local contains = function(playerName)
	return playerIndexes[playerName] ~= nil
end

while(true) do
	for _, name in ipairs(scanner.getPlayerNames()) do
		if(not contains(name)) then --new player has entered
			addPlayer(currentPlayers, name)
			local pos = sensor.getPlayerData(name).position
			print("Player: ", name, " has entered at x: ", pos.x, " y: ", pos.y, " z: ", pos.z)
		end
		currentPlayers[name].check = true
	end

	for _, player in ipairs(currentPlayers) do
		if(not player.check) then
			print("Player: ", player.name, " has left.")
			removePlayer(player.name)
		end
	end
end