/*
 * ============================================================================
 * Projekt.....: rs2322tcp
 * Datei.......: control.go
 * Copyright (C) 2026 Dieter Lang
 *
 * SPDX-License-Identifier: GPL-3.0-or-later
 *
 * Beschreibung:
 * TCP-Control-Server für rs2322tcp.
 * Verwaltung der Control-Verbindungen, Client-Sessions und Bereitstellung
 * der aktuellen Geräteinformationen einschließlich dynamischer Data-Ports.
 * Verwaltung der dynamischen Datenverbindungen zwischen TCP und RS232.
 * ============================================================================
 */
package server

import (
	"bufio"
	"errors"
	"fmt"
	"io"
	"log"
	"net"
	"sync/atomic"

	"git.lang-dieter.de/rs2322tcp/internal/config"
	"git.lang-dieter.de/rs2322tcp/internal/serial"
	"git.lang-dieter.de/rs2322tcp/internal/transport"
)

///////////////////////////////////////////////////////////////////////////////
// ControlServer
///////////////////////////////////////////////////////////////////////////////

// ControlServer implements the rs2322tcp control listener.
type ControlServer struct {
	config        *config.ServerConfig
	listener      net.Listener
	nextSessionID uint64
}

///////////////////////////////////////////////////////////////////////////////
// Constructor
///////////////////////////////////////////////////////////////////////////////

// NewControlServer creates a new control server using the supplied
// server configuration.
func NewControlServer(cfg *config.ServerConfig) (*ControlServer, error) {
	if cfg == nil {
		return nil, fmt.Errorf("server configuration is nil")
	}

	if err := cfg.Validate(); err != nil {
		return nil, err
	}

	return &ControlServer{
		config: cfg,
	}, nil
}

///////////////////////////////////////////////////////////////////////////////
// Listener
///////////////////////////////////////////////////////////////////////////////

// Listen starts the TCP control listener.
func (s *ControlServer) Listen() error {
	if s == nil {
		return fmt.Errorf("control server is nil")
	}

	if s.listener != nil {
		return fmt.Errorf("control server is already listening")
	}

	address := net.JoinHostPort(
		s.config.Listen.Address,
		fmt.Sprintf("%d", s.config.Listen.Port),
	)

	listener, err := net.Listen("tcp", address)
	if err != nil {
		return fmt.Errorf("listen on %s: %w", address, err)
	}

	s.listener = listener

	return nil
}

// Addr returns the actual address of the control listener.
func (s *ControlServer) Addr() net.Addr {
	if s == nil || s.listener == nil {
		return nil
	}

	return s.listener.Addr()
}

// Close stops the control listener.
func (s *ControlServer) Close() error {
	if s == nil || s.listener == nil {
		return nil
	}

	err := s.listener.Close()
	s.listener = nil

	return err
}

///////////////////////////////////////////////////////////////////////////////
// Serve
///////////////////////////////////////////////////////////////////////////////

// Serve accepts incoming control connections.
//
// Each connection is handled independently.
func (s *ControlServer) Serve() error {
	if s == nil {
		return fmt.Errorf("control server is nil")
	}

	if s.listener == nil {
		return fmt.Errorf("control server is not listening")
	}

	for {
		conn, err := s.listener.Accept()
		if err != nil {
			if errors.Is(err, net.ErrClosed) {
				return nil
			}

			return fmt.Errorf("accept control connection: %w", err)
		}

		go s.handleConnection(conn)
	}
}

///////////////////////////////////////////////////////////////////////////////
// Connection handling
///////////////////////////////////////////////////////////////////////////////

// handleConnection creates a session for one control connection and
// handles all control messages belonging to that session.
func (s *ControlServer) handleConnection(conn net.Conn) *Session {
	session, err := s.newSession(conn)
	if err != nil {
		log.Printf("create session: %v", err)
		_ = conn.Close()
		return nil
	}

	log.Printf("session %d connected", session.ID())

	defer func() {
		if err := session.Close(); err != nil {
			log.Printf(
				"session %d close error: %v",
				session.ID(),
				err,
			)
		}

		log.Printf("session %d closed", session.ID())
	}()

	reader := bufio.NewReader(conn)

	for {
		var message transport.Message

		if err := transport.ReadMessage(reader, &message); err != nil {
			if !errors.Is(err, io.EOF) {
				log.Printf(
					"session %d control connection error: %v",
					session.ID(),
					err,
				)
			}

			return session
		}

		if err := s.handleMessage(session, message); err != nil {
			log.Printf(
				"session %d control message error: %v",
				session.ID(),
				err,
			)

			_ = transport.WriteMessage(
				conn,
				transport.NewError(
					transport.ErrorInternal,
					err.Error(),
				),
			)

			return session
		}
	}
}

// HandleConnectionForTest handles one control connection and returns
// the session after the connection has ended.
//
// This method exists only to allow package-level tests without opening
// a real TCP control listener.
func (s *ControlServer) HandleConnectionForTest(conn net.Conn) *Session {
	return s.handleConnection(conn)
}

///////////////////////////////////////////////////////////////////////////////
// Session creation
///////////////////////////////////////////////////////////////////////////////

// newSession creates a new client session, assigns a unique ID and
// creates one dynamic data listener for every configured device.
//
// A data worker is started for every listener. The worker waits for an
// incoming TCP data connection and opens the corresponding serial device
// only when a client actually connects to the dynamic data port.
func (s *ControlServer) newSession(conn net.Conn) (*Session, error) {
	if conn == nil {
		return nil, fmt.Errorf("connection is nil")
	}

	id := atomic.AddUint64(&s.nextSessionID, 1)

	session, err := NewSession(id, conn)
	if err != nil {
		return nil, err
	}

	for _, device := range s.config.Devices {
		listener, err := NewDataListener()
		if err != nil {
			_ = session.Close()

			return nil, fmt.Errorf(
				"create data listener for device %q: %w",
				device.ID,
				err,
			)
		}

		if err := session.AddDataListener(
			device.ID,
			listener,
		); err != nil {
			_ = session.Close()

			return nil, fmt.Errorf(
				"add data listener for device %q: %w",
				device.ID,
				err,
			)
		}

		go s.runDataListener(session, device, listener)
	}

	return session, nil
}

///////////////////////////////////////////////////////////////////////////////
// Data connection handling
///////////////////////////////////////////////////////////////////////////////

// runDataListener waits for incoming TCP data connections for one
// configured device.
//
// The listener remains active for the lifetime of the session. A serial
// connection is opened only after a TCP client actually connects.
//
// Only one active data connection is permitted for each device.
func (s *ControlServer) runDataListener(
	session *Session,
	device config.DeviceConfig,
	listener *DataListener,
) {
	if session == nil || listener == nil {
		return
	}

	for {
		tcpConn, err := listener.Accept()
		if err != nil {
			if session.IsClosed() {
				return
			}

			log.Printf(
				"session %d device %q data accept error: %v",
				session.ID(),
				device.ID,
				err,
			)

			return
		}

		if session.IsClosed() {
			_ = tcpConn.Close()
			return
		}

		if _, exists := session.DataConnection(device.ID); exists {
			log.Printf(
				"session %d device %q already has an active data connection",
				session.ID(),
				device.ID,
			)

			_ = tcpConn.Close()
			continue
		}

		serialConn, err := serial.Open(device)
		if err != nil {
			log.Printf(
				"session %d device %q serial open error: %v",
				session.ID(),
				device.ID,
				err,
			)

			_ = tcpConn.Close()
			continue
		}

		dataConnection, err := NewDataConnection(
			tcpConn,
			serialConn,
			s.config.HardwareErrorResponse,
		)
		if err != nil {
			log.Printf(
				"session %d device %q create data connection error: %v",
				session.ID(),
				device.ID,
				err,
			)

			_ = serialConn.Close()
			_ = tcpConn.Close()

			continue
		}

		dataConnection.SetSerialMonitor(
			s.config.SerialMonitor,
			device.SerialPort,
		)

		if err := session.AddDataConnection(
			device.ID,
			dataConnection,
		); err != nil {
			log.Printf(
				"session %d device %q register data connection error: %v",
				session.ID(),
				device.ID,
				err,
			)

			_ = dataConnection.Close()
			continue
		}

		log.Printf(
			"session %d device %q data connection established",
			session.ID(),
			device.ID,
		)

		err = dataConnection.Run()

		if err != nil {
			log.Printf(
				"session %d device %q data connection error: %v",
				session.ID(),
				device.ID,
				err,
			)
		}

		if err := session.RemoveDataConnection(device.ID); err != nil {
			log.Printf(
				"session %d device %q remove data connection error: %v",
				session.ID(),
				device.ID,
				err,
			)
		}

		log.Printf(
			"session %d device %q data connection closed",
			session.ID(),
			device.ID,
		)

		if session.IsClosed() {
			return
		}
	}
}

///////////////////////////////////////////////////////////////////////////////
// Message handling
///////////////////////////////////////////////////////////////////////////////

// handleMessage processes one control message for the specified session.
func (s *ControlServer) handleMessage(
	session *Session,
	message transport.Message,
) error {
	if session == nil {
		return fmt.Errorf("session is nil")
	}

	conn := session.Conn()
	if conn == nil {
		return fmt.Errorf("session control connection is closed")
	}

	if message.Version != transport.ProtocolVersion {
		return transport.WriteMessage(
			conn,
			transport.NewError(
				transport.ErrorUnsupportedVersion,
				fmt.Sprintf(
					"unsupported protocol version: %d",
					message.Version,
				),
			),
		)
	}

	switch message.Type {
	case transport.MessageHello:
		return transport.WriteMessage(
			conn,
			transport.NewHelloResponse(),
		)

	case transport.MessageGetDevices:
		return s.writeDeviceList(session)

	default:
		return transport.WriteMessage(
			conn,
			transport.NewError(
				transport.ErrorUnknownMessage,
				fmt.Sprintf(
					"unknown message type: %q",
					message.Type,
				),
			),
		)
	}
}

///////////////////////////////////////////////////////////////////////////////
// Device list
///////////////////////////////////////////////////////////////////////////////

// writeDeviceList sends the configured remote devices together with
// their session-specific dynamic data ports.
func (s *ControlServer) writeDeviceList(session *Session) error {
	if session == nil {
		return fmt.Errorf("session is nil")
	}

	conn := session.Conn()
	if conn == nil {
		return fmt.Errorf("session control connection is closed")
	}

	remoteDevices := s.config.RemoteDevices()

	devices := make(
		[]transport.RemoteDeviceInfo,
		0,
		len(remoteDevices.Devices),
	)

	for _, device := range remoteDevices.Devices {
		dataPort := session.DataPort(device.ID)

		if dataPort == 0 {
			return fmt.Errorf(
				"no data listener for device %q",
				device.ID,
			)
		}

		devices = append(
			devices,
			transport.NewRemoteDeviceInfo(
				device,
				dataPort,
			),
		)
	}

	return transport.WriteMessage(
		conn,
		transport.NewDeviceList(devices),
	)
}