// SPDX-FileCopyrightText: 2023 The Pion community <https://pion.ly>
// SPDX-License-Identifier: MIT

// data-channels-detach is an example that shows how you can detach a data channel.
// This allows direct access the underlying [pion/datachannel](https://github.com/pion/datachannel).
// This allows you to interact with the data channel using a more idiomatic API based on
// the `io.ReadWriteCloser` interface.
package main

import (
	"bufio"
	"encoding/base64"
	"encoding/json"
	"errors"
	"fmt"
	"io"
	"os"
	"strings"
	"time"

	"github.com/pion/randutil"
	"github.com/pion/webrtc/v4"
)

const messageSize = 15

func main() {
	// Since this behavior diverges from the WebRTC API it has to be
	// enabled using a settings engine. Mixing both detached and the
	// OnMessage DataChannel API is not supported.

	// Create a SettingEngine and enable Detach
	s := webrtc.SettingEngine{}
	s.DetachDataChannels()

	// Create an API object with the engine
	api := webrtc.NewAPI(webrtc.WithSettingEngine(s))

	// Everything below is the Pion WebRTC API! Thanks for using it ❤️.

	// Prepare the configuration
	config := webrtc.Configuration{
		ICEServers: []webrtc.ICEServer{
			{
				URLs: []string{"stun:stun.l.google.com:19302"},
			},
		},
	}

	// Create a new RTCPeerConnection using the API object
	peerConnection, err := api.NewPeerConnection(config)
	if err != nil {
		panic(err)
	}
	defer func() {
		if cErr := peerConnection.Close(); cErr != nil {
			fmt.Printf("cannot close peerConnection: %v\n", cErr)
		}
	}()

	// Set the handler for Peer connection state
	// This will notify you when the peer has connected/disconnected
	peerConnection.OnConnectionStateChange(func(state webrtc.PeerConnectionState) {
		fmt.Printf("Peer Connection State has changed: %s\n", state.String())

		if state == webrtc.PeerConnectionStateFailed {
			// Wait until PeerConnection has had no network activity for 30 seconds or another failure.
			// It may be reconnected using an ICE Restart.
			// Use webrtc.PeerConnectionStateDisconnected if you are interested in detecting faster timeout.
			// Note that the PeerConnection may come back from PeerConnectionStateDisconnected.
			fmt.Println("Peer Connection has gone to failed exiting")
			os.Exit(0)
		}

		if state == webrtc.PeerConnectionStateClosed {
			// PeerConnection was explicitly closed. This usually happens from a DTLS CloseNotify
			fmt.Println("Peer Connection has gone to closed exiting")
			os.Exit(0)
		}
	})

	// Register data channel creation handling
	peerConnection.OnDataChannel(func(dataChannel *webrtc.DataChannel) {
		fmt.Printf("New DataChannel %s %d\n", dataChannel.Label(), dataChannel.ID())

		// Register channel opening handling
		dataChannel.OnOpen(func() {
			fmt.Printf("Data channel '%s'-'%d' open.\n", dataChannel.Label(), dataChannel.ID())

			// Detach the data channel
			raw, dErr := dataChannel.Detach()
			if dErr != nil {
				panic(dErr)
			}

			// Handle reading from the data channel
			go ReadLoop(raw)

			// Handle writing to the data channel
			go WriteLoop(raw)
		})
	})

	// Wait for the offer to be pasted
	offer := webrtc.SessionDescription{}
	decode(readUntilNewline(), &offer)

	// Set the remote SessionDescription
	err = peerConnection.SetRemoteDescription(offer)
	if err != nil {
		panic(err)
	}

	// Create answer
	answer, err := peerConnection.CreateAnswer(nil)
	if err != nil {
		panic(err)
	}

	// Create channel that is blocked until ICE Gathering is complete
	gatherComplete := webrtc.GatheringCompletePromise(peerConnection)

	// Sets the LocalDescription, and starts our UDP listeners
	err = peerConnection.SetLocalDescription(answer)
	if err != nil {
		panic(err)
	}

	// Block until ICE Gathering is complete, disabling trickle ICE
	// we do this because we only can exchange one signaling message
	// in a production application you should exchange ICE Candidates via OnICECandidate
	<-gatherComplete

	// Output the answer in base64 so we can paste it in browser
	fmt.Println(encode(peerConnection.LocalDescription()))

	// Block forever
	select {}
}

// ReadLoop shows how to read from the datachannel directly.
func ReadLoop(d io.Reader) {
	for {
		buffer := make([]byte, messageSize)
		n, err := d.Read(buffer)
		if err != nil {
			fmt.Println("Datachannel closed; Exit the readloop:", err)

			return
		}

		fmt.Printf("Message from DataChannel: %s\n", string(buffer[:n]))
	}
}

// WriteLoop shows how to write to the datachannel directly.
func WriteLoop(d io.Writer) {
	ticker := time.NewTicker(5 * time.Second)
	defer ticker.Stop()
	for range ticker.C {
		message, err := randutil.GenerateCryptoRandomString(
			messageSize, "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ",
		)
		if err != nil {
			panic(err)
		}

		fmt.Printf("Sending %s \n", message)
		if _, err := d.Write([]byte(message)); err != nil {
			panic(err)
		}
	}
}

// Read from stdin until we get a newline.
func readUntilNewline() (in string) {
	var err error

	r := bufio.NewReader(os.Stdin)
	for {
		in, err = r.ReadString('\n')
		if err != nil && !errors.Is(err, io.EOF) {
			panic(err)
		}

		if in = strings.TrimSpace(in); len(in) > 0 {
			break
		}
	}

	fmt.Println("")

	return
}

// JSON encode + base64 a SessionDescription.
func encode(obj *webrtc.SessionDescription) string {
	b, err := json.Marshal(obj)
	if err != nil {
		panic(err)
	}

	return base64.StdEncoding.EncodeToString(b)
}

// Decode a base64 and unmarshal JSON into a SessionDescription.
func decode(in string, obj *webrtc.SessionDescription) {
	b, err := base64.StdEncoding.DecodeString(in)
	if err != nil {
		panic(err)
	}

	if err = json.Unmarshal(b, obj); err != nil {
		panic(err)
	}
}