--[[
-- Simplest possible tcp echo server.
-- ... Well, that's a lie. It's the simplest possible TCP server that can handle multiple clients at the same time, that I was able to write.
]]--

local socket = require("socket")
local server = socket.tcp()
-- The low timeout time is so no one client blocks any other
-- I can see that if the server needs to receive or send a lot of data, then those routines will become blocked. I _could_ use something better than coroutines to make this, but then it wouldn't have been so simple, now would it? :þ
server:settimeout(0.005, "t")

hostname = "localhost"
port = 12346

server:bind(hostname, port)
server:listen(128)

local clients = {}

co_newClient = coroutine.create(function ()
	while true do
		local nC, err = server:accept()
		if err then
			if err == "timeout" then
				coroutine.yield()
			else
				error("ERROR: "..err)
			end
		end
		if nC then
			-- Individual clients, however, need a higher timeout if we want to be able to give them time to connect to the tcp socket and actually send something.
			nC:settimeout(10)
			-- Insert the new client into the client table
			table.insert(clients, nC)
			nC = nil
			coroutine.yield()
		end
	end
end)
co_selectClient = coroutine.create(function ()
	while true do
		-- For more information, see https://lunarmodules.github.io/luasocket/socket.html#select
		r, w, e = socket.select(clients, nil, 0.005)
		if e then
			-- Timeout = No clients, keep waiting for clients
			if e == "timeout" then
				coroutine.resume(co_newClient)
			else
				error("ERROR: "..e)
			end
		end
		coroutine.yield()
	end
end)

co_receiveFromClient = coroutine.create(function ()
	while true do
		for _, client in ipairs(r) do
			local message, err, partial = client:receive("*l")
				if err then
					-- Maybe timeout should remove the client from the table too?
					if err == "timeout" then
						coroutine.yield()
					elseif err == "closed" then
						table.remove(clients, _)
						coroutine. yield()
					end
				end
				if message then
					if message == "goodbye" then
						client:send("GOODBYE\n")
						client:close()
						table.remove(clients, _)
						coroutine.yield()
					else
						client:send("Echo: "..message.."\n")
						coroutine.yield()
					end
				end
				if partial then
					coroutine.yield()
				end
		end
		coroutine.yield()
	end
end)

while true do
	coroutine.resume(co_newClient)
	coroutine.resume(co_selectClient)
	coroutine.resume(co_receiveFromClient)
end