package mcp

import (
	"crypto/rand"
	"encoding/hex"
	"sync"
	"time"
)

// Session represents an MCP client session.
type Session struct {
	ID           string
	CreatedAt    time.Time
	LastActivity time.Time
	Initialized  bool

	// notifications is a channel for server-initiated notifications.
	notifications chan *Response

	mu sync.RWMutex
}

// NewSession creates a new session with a cryptographically secure random ID.
func NewSession() (*Session, error) {
	id, err := generateSessionID()
	if err != nil {
		return nil, err
	}

	now := time.Now()
	return &Session{
		ID:            id,
		CreatedAt:     now,
		LastActivity:  now,
		notifications: make(chan *Response, 100),
	}, nil
}

// Touch updates the session's last activity time.
func (s *Session) Touch() {
	s.mu.Lock()
	defer s.mu.Unlock()
	s.LastActivity = time.Now()
}

// SetInitialized marks the session as initialized.
func (s *Session) SetInitialized() {
	s.mu.Lock()
	defer s.mu.Unlock()
	s.Initialized = true
}

// IsInitialized returns whether the session has been initialized.
func (s *Session) IsInitialized() bool {
	s.mu.RLock()
	defer s.mu.RUnlock()
	return s.Initialized
}

// Notifications returns the channel for server-initiated notifications.
func (s *Session) Notifications() <-chan *Response {
	return s.notifications
}

// SendNotification sends a notification to the session.
// Returns false if the channel is full.
func (s *Session) SendNotification(notification *Response) bool {
	select {
	case s.notifications <- notification:
		return true
	default:
		return false
	}
}

// Close closes the session's notification channel.
func (s *Session) Close() {
	close(s.notifications)
}

// SessionStore manages active sessions with TTL-based cleanup.
type SessionStore struct {
	sessions  map[string]*Session
	ttl       time.Duration
	mu        sync.RWMutex
	stopClean chan struct{}
	cleanDone chan struct{}
}

// NewSessionStore creates a new session store with the given TTL.
func NewSessionStore(ttl time.Duration) *SessionStore {
	s := &SessionStore{
		sessions:  make(map[string]*Session),
		ttl:       ttl,
		stopClean: make(chan struct{}),
		cleanDone: make(chan struct{}),
	}
	go s.cleanupLoop()
	return s
}

// Create creates a new session and adds it to the store.
func (s *SessionStore) Create() (*Session, error) {
	session, err := NewSession()
	if err != nil {
		return nil, err
	}

	s.mu.Lock()
	defer s.mu.Unlock()
	s.sessions[session.ID] = session
	return session, nil
}

// Get retrieves a session by ID. Returns nil if not found or expired.
func (s *SessionStore) Get(id string) *Session {
	s.mu.RLock()
	defer s.mu.RUnlock()

	session, ok := s.sessions[id]
	if !ok {
		return nil
	}

	// Check if expired
	session.mu.RLock()
	expired := time.Since(session.LastActivity) > s.ttl
	session.mu.RUnlock()

	if expired {
		return nil
	}

	return session
}

// Delete removes a session from the store.
func (s *SessionStore) Delete(id string) bool {
	s.mu.Lock()
	defer s.mu.Unlock()

	session, ok := s.sessions[id]
	if !ok {
		return false
	}

	session.Close()
	delete(s.sessions, id)
	return true
}

// Count returns the number of active sessions.
func (s *SessionStore) Count() int {
	s.mu.RLock()
	defer s.mu.RUnlock()
	return len(s.sessions)
}

// Stop stops the cleanup goroutine and waits for it to finish.
func (s *SessionStore) Stop() {
	close(s.stopClean)
	<-s.cleanDone
}

// cleanupLoop periodically removes expired sessions.
func (s *SessionStore) cleanupLoop() {
	defer close(s.cleanDone)

	ticker := time.NewTicker(s.ttl / 2)
	defer ticker.Stop()

	for {
		select {
		case <-s.stopClean:
			return
		case <-ticker.C:
			s.cleanup()
		}
	}
}

// cleanup removes expired sessions.
func (s *SessionStore) cleanup() {
	s.mu.Lock()
	defer s.mu.Unlock()

	now := time.Now()
	for id, session := range s.sessions {
		session.mu.RLock()
		expired := now.Sub(session.LastActivity) > s.ttl
		session.mu.RUnlock()

		if expired {
			session.Close()
			delete(s.sessions, id)
		}
	}
}

// generateSessionID generates a cryptographically secure random session ID.
func generateSessionID() (string, error) {
	bytes := make([]byte, 16)
	if _, err := rand.Read(bytes); err != nil {
		return "", err
	}
	return hex.EncodeToString(bytes), nil
}