local net = require("internet")

local read_varint = function (conn, first_byte)
	local b
	if first_byte == nil then
		b = conn:read(1)
	else
		b = first_byte
	end

	local n, s = 0, 0
	while b ~= nil and b & 0x80 == 0x80 do
		if s > 21 then
			return 0, "number too large"
		end

		n = n + ((b & 0x7F) << s)
		s = s + 7
		b = conn:read(1)
	end

	if b == nil then
		return n, "eof"
	end

	return n + (b << s), nil
end

local MqttClient

function open (address, port)
	local conn = net.open(address, port)
	if conn == nil then
		return nil, "connection failed"
	end

	return MqttClient:new(conn), nil
end

function MqttClient:new (conn)
	local c = c or {}
	setmetatable(c, self)
	self.__index = self

	conn.read_varint = read_varint
	c.conn = conn
	c.is_connecting = false
	c.is_connected = false

	return c
end

function MqttClient:handle ()
	local data = self.conn:read(2)
	if data == nil then
		return "eof"
	end

	local ptype, length, _ = string.unpack("B B", s)

	local length, err = self.conn:read_varint(length)
	if err ~= nil then
		return err
	end

	if length > 0 then
		data = self.conn:read(length)
		if data == nil then
			return "eof"
		end
	else
		data = ""
	end

	if ptype & 0xF0 == 0x20 then -- CONNACK
		if ptype ~= 0x20 or length < 2 then
			self:disconnect(0x81)
			return "malformed packet"
		end

		local flags, reason, _ = string.unpack("B B", data)

		if flags ~= 0 then
			self:disconnect(0x81)
			return "malformed packet"
		end

		if reason > 127 then
			self.is_connecting = false
			self.is_connected = false
			self.conn:close()
			self.conn = nil
			return "connection closed by server"
		elseif reason ~= 0 then
			self:disconnect(0x81)
			return "malformed packet"
		end

		self.is_connecting = false
		self.is_connected = true
	elseif ptype == 0x40 then -- PUBACK
		-- TODO
	elseif ptype == 0xD0 then -- PINGRESP
		-- TODO
	elseif ptype == 0xE0 then -- DISCONNECT
		-- TODO
	end

	return nil
end

function MqttClient:connect (username, password)
	if self.is_connecting or self.is_connected then
		return nil
	end

	local length = 15
	local flags = 1
	if username ~= nil then
		length = length + 2 + #username
		flags = flags | 0x80
	end
	if password ~= nil then
		length = length + 2 + #password
		flags = flags | 0x40
	end

	if length > 127 then
		return "packet size exceeds current implementation capabilities"
	end

	local data = string.pack("> B B s2 B B I2 B s2", 0x10, length, "MQTT", 5, flags, 0, 0, "")
	if username ~= nil then
		data = data .. string.pack("> s2", username)
	end
	if password ~= nil then
		data = data .. string.pack("> s2", password)
	end

	local _, err = self.conn:write(data)
	if err ~= nil then
		return err
	end

	self.is_connecting = true

	return nil
end

function MqttClient:disconnect (reason)
	if not (self.is_connecting or self.is_connected) then
		return nil
	end

	if reason == nil then
		reason = 0
	end

	local data = string.pack("> B B B", 0xE0, 1, reason)

	local _, err = self.conn:write(data)
	if err ~= nil then
		return err
	end

	self.is_connecting = false
	self.is_connected = false

	self.conn:close()
	self.conn = nil

	return nil
end