package main

// TODO learn more about chan chan's
// http://marcio.io/2015/07/handling-1-million-requests-per-minute-with-golang/

import (
	"bufio"
	"bytes"
	"crypto/rand"
	"encoding/base64"
	"flag"
	"fmt"
	"io/ioutil"
	"net"
	"net/http"
	"net/url"
	"os"
	"strconv"
	"strings"
	"sync"
	"time"

	"github.com/emicklei/go-restful"
	"gopkg.in/yaml.v2"
)

// I'm not sure how to pass nested structs, so I de-nested this.
// TODO Learn if passing nested structs is desirable?
type Conf struct {
	Addr     string `yaml:"addr,omitempty"`
	Port     uint   `yaml:"port,omitempty"`
	Mailer   ConfMailer
	RootPath string `yaml:"root_path,omitempty"`
}
type ConfMailer struct {
	Url    string `yaml:"url,omitempty"`
	ApiKey string `yaml:"api_key,omitempty"`
	From   string `yaml:"from,omitempty"`
}

// So we can peek at net.Conn, which we can't do natively
// https://stackoverflow.com/questions/51472020/how-to-get-the-size-of-available-tcp-data
type bufferedConn struct {
	r *bufio.Reader
	//rout *io.Reader // See https://github.com/polvi/sni/blob/master/sni.go#L135
	net.Conn
}

func newBufferedConn(c net.Conn) bufferedConn {
	return bufferedConn{bufio.NewReader(c), c}
}

func (b bufferedConn) Peek(n int) ([]byte, error) {
	return b.r.Peek(n)
}

func (b bufferedConn) Buffered() int {
	return b.r.Buffered()
}

func (b bufferedConn) Read(p []byte) (int, error) {
	/*
		if b.rout != nil {
			return b.rout.Read(p)
		}
	*/
	return b.r.Read(p)
}

type chatMsg struct {
	sender     net.Conn
	Message    string    `json:"message"`
	ReceivedAt time.Time `json:"received_at"`
	Channel    string    `json:"channel"`
	User       string    `json:"user"`
}

// Poor-Man's container/ring (circular buffer)
type chatHist struct {
	msgs []*chatMsg
	i    int // current index
	c    int // current count (number of elements)
}

// Multi-use
var config Conf
var virginConns chan net.Conn
var gotClientHello chan bufferedConn
var myChatHist chatHist
var broadcastMsg chan chatMsg

// Telnet
var wantsServerHello chan bufferedConn
var authTelnet chan telnetUser
var cleanTelnet chan telnetUser // intentionally blocking

// HTTP
var demuxHttpClient chan bufferedConn
var authReqs chan authReq
var valAuthReqs chan authReq
var delAuthReqs chan authReq

func usage() {
	fmt.Fprintf(os.Stderr, "\nusage: go run chatserver*.go\n")
	flag.PrintDefaults()
	fmt.Println()

	os.Exit(1)
}

// https://blog.questionable.services/article/generating-secure-random-numbers-crypto-rand/
func genAuthCode() (string, error) {
	n := 12
	b := make([]byte, n)
	_, err := rand.Read(b)
	// Note that err == nil only if we read len(b) bytes.
	if err != nil {
		return "", err
	}
	return base64.URLEncoding.EncodeToString(b), nil
}

func muxTcp(conn bufferedConn) {
	// Wish List for protocol detection
	// * PROXY protocol (and loop)
	// * HTTP CONNECT (proxy) (and loop)
	// * tls (and loop) https://github.com/polvi/sni
	// * http/ws
	// * irc
	// * fallback to telnet

	// At this piont we've already at least one byte via Peek()
	// so the first packet is available in the buffer

	// Note: Realistically no tls/http/irc client is going to send so few bytes
	//       (and no router is going to chunk so small)
	//       that it cannot reasonably detect the protocol in the first packet
	//       However, typical MTU is 1,500 and HTTP can have a 2k URL
	//       so expecting to find the "HTTP/1.1" in the Peek is not always reasonable
	n := conn.Buffered()
	firstMsg, err := conn.Peek(n)
	if nil != err {
		conn.Close()
		return
	}
	var protocol string
	// between A and z
	if firstMsg[0] >= 65 && firstMsg[0] <= 122 {
		i := bytes.Index(firstMsg, []byte(" /"))
		if -1 != i {
			protocol = "HTTP"
			// very likely HTTP
			j := bytes.IndexAny(firstMsg, "\r\n")
			if -1 != j {
				k := bytes.Index(bytes.ToLower(firstMsg[:j]), []byte("HTTP/1"))
				if -1 != k {
					// positively HTTP
				}
			}
		}
	} else if 0x16 /*22*/ == firstMsg[0] {
		// Because I don't always remember off the top of my head what the first byte is
		// http://blog.fourthbit.com/2014/12/23/traffic-analysis-of-an-ssl-slash-tls-session
		// https://tlseminar.github.io/first-few-milliseconds/
		// TODO I want to learn about ALPN
		protocol = "TLS"
	}

	if "" == protocol {
		// Throw away the first bytes
		b := make([]byte, 4096)
		conn.Read(b)
		fmt.Fprintf(conn, "\n\nWelcome to Sample Chat! You're not an HTTP client, assuming Telnet.\nYou must authenticate via email to participate\n\nEmail: ")
		wantsServerHello <- conn
		return
	} else if "HTTP" != protocol {
		defer conn.Close()
		fmt.Fprintf(conn, "\n\nNot yet supported. Try HTTP or Telnet\n\n")
		return
	}

	demuxHttpClient <- conn
}

func testForHello(netConn net.Conn) {
	ts := time.Now()
	fmt.Fprintf(os.Stdout, "[New Connection] (%s) welcome %s\n", ts, netConn.RemoteAddr().String())

	m := sync.Mutex{}
	virgin := true

	bufConn := newBufferedConn(netConn)
	go func() {
		// Cause first packet to be loaded into buffer
		_, err := bufConn.Peek(1)
		if nil != err {
			panic(err)
		}

		m.Lock()
		if virgin {
			virgin = false
			gotClientHello <- bufConn
		} else {
			wantsServerHello <- bufConn
		}
		m.Unlock()
	}()

	// Wait for a hello packet of some sort from the client
	// (obviously this wouldn't work in extremely high latency situations)
	time.Sleep(250 * 1000000)

	// If we still haven't received data from the client
	// assume that the client must be expecting a welcome from us
	m.Lock()
	if virgin {
		virgin = false
		// Defer as to not block and prolonging the mutex
		// (not that those few cycles much matter...)
		defer fmt.Fprintf(netConn,
			"\n\nWelcome to Sample Chat! You appear to be using Telnet (http is also available on this port)."+
				"\nYou must authenticate via email to participate\n\nEmail: ")
	}
	m.Unlock()
}

func sendAuthCode(cnf ConfMailer, to string) (string, error) {
	code, err := genAuthCode()
	if nil != err {
		return "", err
	}

	// TODO use go text templates with HTML escaping
	text := "Your authorization code:\n\n" + code
	html := "Your authorization code:<br><br>" + code

	// https://stackoverflow.com/questions/24493116/how-to-send-a-post-request-in-go
	// https://stackoverflow.com/questions/16673766/basic-http-auth-in-go
	client := http.Client{}

	form := url.Values{}
	form.Add("from", cnf.From)
	form.Add("to", to)
	form.Add("subject", "Sample Chat Auth Code: "+code)
	form.Add("text", text)
	form.Add("html", html)

	req, err := http.NewRequest("POST", cnf.Url, strings.NewReader(form.Encode()))
	if nil != err {
		return "", err
	}
	//req.PostForm = form ??
	req.Header.Add("User-Agent", "golang http.Client - Sample Chat App Authenticator")
	req.Header.Add("Content-Type", "application/x-www-form-urlencoded")
	req.SetBasicAuth("api", cnf.ApiKey)

	resp, err := client.Do(req)
	if nil != err {
		return "", err
	}

	defer resp.Body.Close()
	// Security XXX
	// we trust mailgun implicitly and this is just a demo
	// hence no DoS check on body size for now
	body, err := ioutil.ReadAll(resp.Body)
	if nil != err {
		return "", err
	}
	if resp.StatusCode < 200 || resp.StatusCode >= 300 || "{" != string(body[0]) {
		fmt.Fprintf(os.Stdout, "[Mailgun] Uh-oh...\n[Maigun] Baby Brent says: %s\n", body)
	}

	return code, nil
}

func main() {
	flag.Usage = usage
	port := flag.Uint("port", 0, "tcp telnet chat port")
	confname := flag.String("conf", "./config.yml", "yaml config file")
	flag.Parse()

	confstr, err := ioutil.ReadFile(*confname)
	fmt.Fprintf(os.Stdout, "-conf=%s\n", *confname)
	if nil != err {
		fmt.Fprintf(os.Stderr, "%s\nUsing defaults instead\n", err)
		confstr = []byte("{\"port\":" + strconv.Itoa(int(*port)) + "}")
	}
	err = yaml.Unmarshal(confstr, &config)
	if nil != err {
		config = Conf{}
	}
	if "" == config.RootPath {
		// TODO Maybe embed the public dir into the binary
		// (and provide a flag with path for override - like gitea)
		config.RootPath = "./public"
	}

	// The magical sorting hat
	virginConns = make(chan net.Conn, 128)

	// TCP & Authentication
	telnetConns := make(map[string]telnetUser)
	wantsServerHello = make(chan bufferedConn, 128)
	authTelnet = make(chan telnetUser, 128)

	// HTTP & Authentication
	myAuthReqs := make(map[string]authReq)
	authReqs = make(chan authReq, 128)
	valAuthReqs = make(chan authReq, 128)
	delAuthReqs = make(chan authReq, 128)
	gotClientHello = make(chan bufferedConn, 128)
	demuxHttpClient = make(chan bufferedConn, 128)

	//myRooms["general"] = make(chan chatMsg, 128)
	// Note: I had considered dynamically select on channels for rooms.
	// https://stackoverflow.com/questions/19992334/how-to-listen-to-n-channels-dynamic-select-statement
	// I don't think that's actually the best approach, but I just wanted to save the link

	broadcastMsg = make(chan chatMsg, 128)
	myChatHist.msgs = make([]*chatMsg, 128)

	var addr string
	if 0 != int(*port) {
		addr = config.Addr + ":" + strconv.Itoa(int(*port))
	} else {
		addr = config.Addr + ":" + strconv.Itoa(int(config.Port))
	}

	// https://golang.org/pkg/net/#Conn
	sock, err := net.Listen("tcp", addr)
	if nil != err {
		fmt.Fprintf(os.Stderr, "Couldn't bind to TCP socket %q: %s\n", addr, err)
		os.Exit(2)
	}
	fmt.Println("Listening on", addr)

	go func() {
		for {
			conn, err := sock.Accept()
			if err != nil {
				// Not sure what kind of error this could be or how it could happen.
				// Could a connection abort or end before it's handled?
				fmt.Fprintf(os.Stderr, "Error accepting connection:\n%s\n", err)
			}
			virginConns <- conn
		}
	}()

	// Learning by Example
	// https://github.com/emicklei/go-restful/blob/master/examples/restful-multi-containers.go
	// https://github.com/emicklei/go-restful/blob/master/examples/restful-basic-authentication.go
	// https://github.com/emicklei/go-restful/blob/master/examples/restful-serve-static.go
	// https://github.com/emicklei/go-restful/blob/master/examples/restful-pre-post-filters.go
	container := restful.NewContainer()

	wsStatic := new(restful.WebService)
	wsStatic.Path("/")
	wsStatic.Route(wsStatic.GET("/").To(serveStatic))
	wsStatic.Route(wsStatic.GET("/{subpath:*}").To(serveStatic))
	container.Add(wsStatic)

	cors := restful.CrossOriginResourceSharing{ExposeHeaders: []string{"Authorization"}, CookiesAllowed: false, Container: container}
	wsApi := new(restful.WebService)
	wsApi.Path("/api").Consumes(restful.MIME_JSON).Produces(restful.MIME_JSON).Filter(cors.Filter)
	wsApi.Route(wsApi.GET("/hello").To(serveHello))
	wsApi.Route(wsApi.POST("/sessions").To(requestAuth))
	wsApi.Route(wsApi.POST("/sessions/{cid}").To(issueToken))
	wsApi.Route(wsApi.GET("/rooms/{room}").Filter(requireToken).To(listMsgs))
	wsApi.Route(wsApi.POST("/rooms/{room}").Filter(requireToken).To(postMsg))
	container.Add(wsApi)

	server := &http.Server{
		Addr:    addr,
		Handler: container,
	}
	myHttpServer := newHttpServer(sock)
	go func() {
		server.Serve(myHttpServer)
	}()

	// Main event loop handling most access to shared data
	for {
		select {
		case conn := <-virginConns:
			// This is short lived
			go testForHello(conn)
		case u := <-authTelnet:
			// allow to receive messages
			// (and be counted among the users)
			_, ok := telnetConns[u.email]
			if ok {
				// this is a blocking channel, and that's important
				cleanTelnet <- telnetConns[u.email]
			}
			telnetConns[u.email] = u
			// is chan chan the right way to handle this?
			u.userCount <- len(telnetConns)
			broadcastMsg <- chatMsg{
				sender:     nil,
				Message:    fmt.Sprintf("<%s> joined #general\r\n", u.email),
				ReceivedAt: time.Now(),
				Channel:    "general",
				User:       "system",
			}
		case ar := <-authReqs:
			myAuthReqs[ar.Cid] = ar
		case ar := <-valAuthReqs:
			// TODO In this case it's probably more conventional (and efficient) to
			// use struct with a mutex and the authReqs map than a chan chan
			av, ok := myAuthReqs[ar.Cid]
			//ar.Chan <- nil // TODO
			if ok {
				ar.Chan <- av
			} else {
				// sending empty object so that I can still send a copy
				// rather than a pointer above. Maybe not the right way
				// to do this, but it works for now.
				ar.Chan <- authReq{}
			}
		case ar := <-delAuthReqs:
			delete(myAuthReqs, ar.Cid)
		case bufConn := <-wantsServerHello:
			go handleTelnetConn(bufConn)
		case u := <-cleanTelnet:
			close(u.newMsg)
			// we can safely ignore this error, if any
			u.bufConn.Close()
			delete(telnetConns, u.email)
		case bufConn := <-gotClientHello:
			go muxTcp(bufConn)
		case bufConn := <-demuxHttpClient:
			// this will be Accept()ed immediately by the go-restful container
			// NOTE: we don't store these HTTP connections for broadcast
			// since we manage the session by HTTP Auth Bearer rather than TCP
			myHttpServer.chans <- bufConn
		case msg := <-broadcastMsg:
			// copy comes in, pointer gets saved (and not GC'd, I hope)
			myChatHist.msgs[myChatHist.i] = &msg
			myChatHist.i += 1
			if myChatHist.c < cap(myChatHist.msgs) {
				myChatHist.c += 1
			}
			myChatHist.i %= len(myChatHist.msgs)

			// print the system message (the "log")
			t := msg.ReceivedAt
			tf := "%d-%02d-%02d %02d:%02d:%02d (%s)"
			var sender string
			if nil != msg.sender {
				sender = msg.sender.RemoteAddr().String()
			} else {
				sender = "system"
			}
			// Tangential thought:
			// I wonder if we could use IP detection to get a Telnet client's tz
			// ... could probably make time for this in the authentication loop
			zone, _ := msg.ReceivedAt.Zone()
			fmt.Fprintf(os.Stdout, tf+" [%s] (%s): %s\r\n",
				t.Year(), t.Month(), t.Day(),
				t.Hour(), t.Minute(), t.Second(), zone,
				sender,
				msg.User, msg.Message)

			for _, u := range telnetConns {
				// Don't echo back to the original client
				if msg.sender == u.bufConn {
					continue
				}

				msg := fmt.Sprintf(tf+" [%s]: %s", t.Year(), t.Month(), t.Day(), t.Hour(),
					t.Minute(), t.Second(), zone, msg.User, msg.Message)
				select {
				case u.newMsg <- msg:
					// all is well, client was ready to receive
				default:
					// Rate Limit: Reasonable poor man's DoS prevention (Part 2)
					// This client's send channel buffer is full.
					// It is consuming data too slowly. It may be malicious.
					// In the case that it's experiencing network issues,
					// well, these things happen when you're having network issues.
					// It can reconnect.
					cleanTelnet <- u
				}
			}
		}
	}
}