package main

import (
	_ "embed"
	"fmt"
	"html/template"
	"log"
	"net/http"
	"strconv"
	"sync/atomic"
)

//go:embed index.html
var indexTemplate string

var pageTemplate *template.Template

var open atomic.Uint64
var done atomic.Uint64

var broker *Broker

// A single Broker will be created in this program. It is responsible
// for keeping a list of which clients (browsers) are currently attached
// and broadcasting events (messages) to those clients.
type Broker struct {

	// Create a map of clients, the keys of the map are the channels
	// over which we can push messages to attached clients.  (The values
	// are just booleans and are meaningless.)
	//
	clients map[chan string]bool

	// Channel into which new clients can be pushed
	//
	newClients chan chan string

	// Channel into which disconnected clients should be pushed
	//
	defunctClients chan chan string

	// Channel into which messages are pushed to be broadcast out
	// to attahed clients.
	//
	messages chan string
}

// This Broker method starts a new goroutine.  It handles
// the addition & removal of clients, as well as the broadcasting
// of messages out to clients that are currently attached.
func (b *Broker) Start() {
	go func() {
		for {
			// Block until we receive from one of the
			// three following channels.
			select {
			case s := <-b.newClients:
				// There is a new client attached and we
				// want to start sending them messages.
				b.clients[s] = true
				log.Println("Added new client")

			case s := <-b.defunctClients:
				// A client has dettached and we want to
				// stop sending them messages.
				delete(b.clients, s)
				close(s)
				log.Println("Removed client")

			case msg := <-b.messages:
				// There is a new message to send.  For each
				// attached client, push the new message
				// into the client's message channel.
				for s := range b.clients {
					s <- msg
				}
				log.Printf("Broadcast message to %d clients", len(b.clients))
			}
		}
	}()
}

// This Broker method handles and HTTP request at the "/events" Endpoint.
func (b *Broker) ServeHTTP(w http.ResponseWriter, r *http.Request) {
	// Make sure that the writer supports flushing.
	f, ok := w.(http.Flusher)
	if !ok {
		http.Error(w, "Streaming unsupported!", http.StatusInternalServerError)
		return
	}

	// Create a new channel, over which the broker can
	// send this client messages.
	messageChan := make(chan string)

	// Add this client to the map of those that should
	// receive updates
	b.newClients <- messageChan

	// Listen to the closing of the http connection via the Request.Context
	notify := r.Context().Done()
	go func() {
		<-notify
		// Remove this client from the map of attached clients
		// when `EventHandler` exits.
		b.defunctClients <- messageChan
		log.Println("HTTP connection just closed.")
	}()

	// Set the headers related to event streaming.
	w.Header().Set("Content-Type", "text/event-stream")
	w.Header().Set("Cache-Control", "no-cache")
	w.Header().Set("Connection", "keep-alive")
	w.Header().Set("Transfer-Encoding", "chunked")

	for {
		msg, open := <-messageChan

		if !open {
			// If our messageChan was closed, this means that the client has
			// disconnected.
			break
		}

		if _, err := fmt.Fprintf(w, "data: %s\n\n", msg); err != nil {
			log.Printf("Writing SSE failed: %v\n", err)
			break
		}

		// Flush the response.  This is only possible if
		// the repsonse supports streaming.
		f.Flush()
	}

	// Done.
	log.Println("Finished HTTP request at ", r.URL.Path)
}

func pageHandler(w http.ResponseWriter, r *http.Request) {
	if r.URL.Path != "/" {
		w.WriteHeader(http.StatusNotFound)
		return
	}

	tempData := struct {
		Open    uint64
		Done    uint64
		IsAdmin bool
	}{open.Load(), done.Load() + 1, r.URL.Query().Has("admin")}

	if err := pageTemplate.Execute(w, tempData); err != nil {
		http.Error(w, err.Error(), http.StatusInternalServerError)
		return
	}
}

func newHandler(w http.ResponseWriter, r *http.Request) {
	ticket := open.Add(1)
	log.Printf("Create ticket %d\n", ticket)
	if _, err := fmt.Fprintf(w, "%d", ticket); err != nil {
		log.Printf("Writing response failed: %v", err)
	}
}

func doneHandler(w http.ResponseWriter, r *http.Request) {
	ticket := done.Add(1)
	log.Printf("Finished ticket %d\n", ticket)
	broker.messages <- strconv.FormatUint(ticket, 10)
	if _, err := fmt.Fprintf(w, "%d", ticket); err != nil {
		log.Printf("Writing response failed: %v", err)
	}
}

func main() {
	// Make a new Broker instance
	broker = &Broker{
		make(map[chan string]bool),
		make(chan (chan string)),
		make(chan (chan string)),
		make(chan string),
	}

	// Start processing events
	broker.Start()

	pageTemplate = template.Must(template.New("index").Parse(indexTemplate))

	http.HandleFunc("/", pageHandler)
	http.HandleFunc("/new", newHandler)
	http.HandleFunc("/done", doneHandler)
	http.Handle("/events", broker)

	log.Printf("Serving http on %d", 8080)
	err := http.ListenAndServe(fmt.Sprintf(":%d", 8080), nil)
	log.Panicf("Serving http failed with %s", err)
}