package server

import (
	"fmt"
	"sync"

	kcp "github.com/xtaci/kcp-go/v5"

	"omnisocketgo/cmd/internal/latencylog"
	"omnisocketgo/cmd/internal/protocol"
	"omnisocketgo/cmd/internal/transport"
)

// KCPOption 用于配置 KCPHub 的可选行为。
type KCPOption func(*KCPHub)

// WithKCPLogger 为 KCP hub 注入时延日志记录器。
func WithKCPLogger(logger latencylog.Logger) KCPOption {
	return func(hub *KCPHub) {
		hub.logger = logger
	}
}

// KCPHub 管理已注册 peer 的 KCP 会话，并负责在它们之间转发消息。
type KCPHub struct {
	mu     sync.RWMutex
	peers  map[string]*transport.KCPConn
	logger latencylog.Logger
}

// NewKCPHub 创建一个空的 KCP 连接中心。
func NewKCPHub(opts ...KCPOption) *KCPHub {
	hub := &KCPHub{
		peers:  make(map[string]*transport.KCPConn),
		logger: latencylog.NoopLogger{},
	}
	for _, opt := range opts {
		opt(hub)
	}
	if hub.logger == nil {
		hub.logger = latencylog.NoopLogger{}
	}
	return hub
}

// HasPeer 返回给定 ID 是否已经注册到 hub。
func (h *KCPHub) HasPeer(peerID string) bool {
	h.mu.RLock()
	defer h.mu.RUnlock()

	_, ok := h.peers[peerID]
	return ok
}

// ServeSession 处理一条新接入的 KCP 会话。
func (h *KCPHub) ServeSession(session *kcp.UDPSession) error {
	conn, err := transport.NewKCPConn(
		session,
		transport.WithKCPLogger(h.logger, latencylog.NodeRoleServer, "hub"),
	)
	if err != nil {
		_ = session.Close()
		return fmt.Errorf("server: create kcp transport conn: %w", err)
	}

	peerID, err := h.registerConn(conn)
	if err != nil {
		_ = conn.Close()
		return err
	}
	defer h.unregister(peerID, conn)

	return h.receivePeerLoop(peerID, conn)
}

// 注册新连接时，KCPHub 期望第一条消息是一个 register 消息，包含 peer 的 ID
func (h *KCPHub) registerConn(conn *transport.KCPConn) (string, error) {
	msg, err := conn.Receive()
	if err != nil {
		return "", fmt.Errorf("server: receive kcp register: %w", err)
	}

	if msg.Type != protocol.MessageTypeRegister {
		if sendErr := sendKCPServerError(conn, msg.From, "first message must be register"); sendErr != nil {
			return "", fmt.Errorf("server: reject unregistered kcp peer: %w", sendErr)
		}
		return "", fmt.Errorf("server: first kcp message must be register, got %s", msg.Type)
	}

	h.mu.Lock()
	defer h.mu.Unlock()

	if _, exists := h.peers[msg.From]; exists {
		if sendErr := sendKCPServerError(conn, msg.From, fmt.Sprintf("duplicate peer id: %s", msg.From)); sendErr != nil {
			return "", fmt.Errorf("server: duplicate kcp peer id %s: %w", msg.From, sendErr)
		}
		return "", fmt.Errorf("server: duplicate kcp peer id: %s", msg.From)
	}

	h.peers[msg.From] = conn
	return msg.From, nil
}

// handlePeerMessage 处理已注册 peer 发来的消息，并将其转发给目标 peer。
func (h *KCPHub) handlePeerMessage(peerID string, conn *transport.KCPConn, msg protocol.Message) error {
	switch msg.Type {
	case protocol.MessageTypeText, protocol.MessageTypeFile:
		msg.From = peerID
		targetConn, ok := h.lookup(msg.To)
		if !ok {
			return sendKCPServerError(conn, peerID, fmt.Sprintf("unknown target: %s", msg.To))
		}
		if err := targetConn.Send(msg); err != nil {
			h.unregister(msg.To, targetConn)
			_ = targetConn.Close()
			return sendKCPServerError(conn, peerID, fmt.Sprintf("failed to forward to %s", msg.To))
		}
		return nil
	case protocol.MessageTypeRegister, protocol.MessageTypeError:
		if err := sendKCPServerError(conn, peerID, "registered peers can only send text or file messages"); err != nil {
			return fmt.Errorf("server: send kcp protocol error: %w", err)
		}
		return fmt.Errorf("server: unexpected kcp message type from peer %s: %s", peerID, msg.Type)
	default:
		if err := sendKCPServerError(conn, peerID, fmt.Sprintf("unsupported message type: %s", msg.Type)); err != nil {
			return fmt.Errorf("server: send unsupported kcp type error: %w", err)
		}
		return fmt.Errorf("server: unsupported kcp message type: %s", msg.Type)
	}
}

// receivePeerLoop 持续读取 peer 发来的消息，并交给 handlePeerMessage 处理，直到连接出错。
func (h *KCPHub) receivePeerLoop(peerID string, conn *transport.KCPConn) error {
	for {
		msg, err := conn.Receive()
		if err != nil {
			_ = conn.Close()
			return fmt.Errorf("transport: kcp receive loop read: %w", err)
		}

		if err := h.handlePeerMessage(peerID, conn, msg); err != nil {
			_ = conn.Close()
			return fmt.Errorf("transport: kcp receive loop handler: %w", err)
		}
	}
}

func (h *KCPHub) lookup(peerID string) (*transport.KCPConn, bool) {
	h.mu.RLock()
	defer h.mu.RUnlock()

	conn, ok := h.peers[peerID]
	return conn, ok
}

func (h *KCPHub) unregister(peerID string, conn *transport.KCPConn) {
	h.mu.Lock()
	defer h.mu.Unlock()

	current, ok := h.peers[peerID]
	if ok && current == conn {
		delete(h.peers, peerID)
	}
}

func sendKCPServerError(conn *transport.KCPConn, to, message string) error {
	return conn.Send(protocol.Message{
		Type: protocol.MessageTypeError,
		From: protocol.ServerPeerID,
		To:   to,
		Body: []byte(message),
	})
}