package main

import (
	"encoding/json"
	"errors"
	"fmt"
	"net"
)

const (
	HandshakePacketID = 0x00
	StatusRequestID   = 0x00
	StatusResponseID  = 0x00
)

const (
	SEGMENT_BITS = 0x7F
	CONTINUE_BIT = 0x80
)

type HandshakePacket struct {
	ProtocolVersion int
	ServerAddress   string
	ServerPort      uint16
	NextState       int
}

type StatusResponse struct {
	Version     VersionInfo `json:"version"`
	Players     PlayerInfo  `json:"players"`
	Description Chat        `json:"description"`
}

type VersionInfo struct {
	Name     string `json:"name"`
	Protocol int    `json:"protocol"`
}

type PlayerInfo struct {
	Max    int      `json:"max"`
	Online int      `json:"online"`
	Sample []Player `json:"sample"`
}

type Player struct {
	Name string `json:"name"`
	ID   string `json:"id"`
}

type Chat struct {
	Text string `json:"text"`
}

type Reader struct {
	data   []byte
	cursor int
}

func main() {
	listener, err := net.Listen("tcp", "localhost:25565")
	if err != nil {
		fmt.Println("Error starting server:", err)
		return
	}
	defer listener.Close()

	fmt.Println("Server started on port 25565")

	for {
		conn, err := listener.Accept()
		if err != nil {
			fmt.Println("Error accepting connection:", err)
			continue
		}

		go handleConnection(conn)
	}
}

func handleConnection(conn net.Conn) {
	r := &Reader{data: make([]byte, 1024)}

	// Print the whole packet
	buf := make([]byte, 1024)
	n, _ := conn.Read(buf)
	r.data = buf[:n]

	fmt.Println("Packet: ", r.data)

	_, err := r.readVarInt()
	if err != nil {
		fmt.Println("Error reading first number:", err)
		return
	}

	packetID, err := r.readVarInt()
	if err != nil {
		fmt.Println("Error reading packet ID:", err)
		return
	}

	fmt.Println("Packet ID: ", packetID)

	switch packetID {
	case HandshakePacketID:
		handleHandshake(r)
	default:
		fmt.Println("Unknown packet ID:", packetID)
	}
}

func handleHandshake(r *Reader) {
	var handshake HandshakePacket
	if err := readHandshakePacket(r, &handshake); err != nil {
		fmt.Println("Error reading handshake packet:", err)
		return
	}

	if handshake.NextState == 1 {
		handleStatusRequest(r)
	}
}

func handleStatusRequest(r *Reader) {
	packetID, err := r.readVarInt()
	if err != nil {
		fmt.Println("Error reading packet ID:", err)
		return
	}

	if packetID == StatusRequestID {
		status := StatusResponse{
			Version: VersionInfo{
				Name:     "1.17.1",
				Protocol: 756,
			},
			Players: PlayerInfo{
				Max:    100,
				Online: 5,
				Sample: []Player{},
			},
			Description: Chat{
				Text: "A Minecraft Server",
			},
		}

		response, err := json.Marshal(status)
		if err != nil {
			fmt.Println("Error marshaling status response:", err)
			return
		}

		w := &Writer{data: make([]byte, 1024)}

		if err := w.writeVarInt(StatusResponseID); err != nil {
			fmt.Println("Error writing response ID:", err)
			return
		}

		if err := w.writeVarInt(len(response)); err != nil {
			fmt.Println("Error writing response length:", err)
			return
		}

		if err := w.writeString(string(response)); err != nil {
			fmt.Println("Error writing response:", err)
			return
		}
	}
}

func (r *Reader) readByte() (byte, error) {
	if r.cursor >= len(r.data) {
		return 0, errors.New("EOF")
	}
	b := r.data[r.cursor]
	r.cursor++
	return b, nil
}

func (r *Reader) readVarInt() (int, error) {
	value := 0
	position := 0

	for {
		currentByte, err := r.readByte()
		if err != nil {
			return 0, err
		}

		value |= int(currentByte&SEGMENT_BITS) << position

		if (currentByte & CONTINUE_BIT) == 0 {
			break
		}

		position += 7
		if position >= 32 {
			return 0, errors.New("VarInt is too big")
		}
	}

	return value, nil
}

func (r *Reader) readString() (string, error) {
	length, err := r.readVarInt()
	if err != nil {
		return "", err
	}

	if r.cursor+length > len(r.data) {
		return "", errors.New("EOF: Reached String unexpectedly")
	}

	str := string(r.data[r.cursor : r.cursor+length])
	r.cursor += length
	return str, nil
}

type Writer struct {
	data   []byte
	cursor int
}

func (w *Writer) writeByte(b byte) error {
	if w.cursor >= len(w.data) {
		return errors.New("EOF")
	}
	w.data[w.cursor] = b
	w.cursor++
	return nil
}

func (w *Writer) writeVarInt(value int) error {
	for {
		temp := byte(value & SEGMENT_BITS)
		value >>= 7
		if value != 0 {
			temp |= CONTINUE_BIT
		}
		if err := w.writeByte(temp); err != nil {
			return err
		}
		if value == 0 {
			break
		}
	}
	return nil
}

func (w *Writer) writeString(s string) error {
	length := len(s)
	if err := w.writeVarInt(length); err != nil {
		return err
	}
	if w.cursor+length > len(w.data) {
		return errors.New("EOF")
	}
	copy(w.data[w.cursor:], s)
	w.cursor += length
	return nil
}

func writeByte(conn net.Conn, b byte) error {
	_, err := conn.Write([]byte{b})
	return err
}

func writeVarInt(conn net.Conn, value int) error {
	var buf [5]byte
	n := 0
	for {
		temp := byte(value & SEGMENT_BITS)
		value >>= 7
		if value != 0 {
			temp |= CONTINUE_BIT
		}
		buf[n] = temp
		n++
		if value == 0 {
			break
		}
	}
	_, err := conn.Write(buf[:n])
	return err
}

func readHandshakePacket(r *Reader, packet *HandshakePacket) error {
	// Print the whole packet
	fmt.Println("Packet: ", r.data)

	fmt.Println("Works so far")
	var err error
	packet.ProtocolVersion, err = r.readVarInt()
	fmt.Println("version: ", packet.ProtocolVersion)
	if err != nil {
		return err
	}

	packet.ServerAddress, err = r.readString()
	fmt.Println(packet.ServerAddress)
	if err != nil {
		return err
	}

	port, err := r.readByte()
	if err != nil {
		return err
	}
	packet.ServerPort = uint16(port)

	packet.NextState, err = r.readVarInt()
	fmt.Println("Next state: ", packet.NextState)
	if err != nil {
		return err
	}

	return nil
}