package rules

import (
	"encoding/json"
	"os"
	"path/filepath"
	"sort"
	"sync"
	"time"

	"vpnem/internal/models"
)

const (
	sessionExpiry = 1 * time.Hour  // session considered stale after 1h
	studioExpiry  = 7 * 24 * time.Hour // studio record kept for 7 days
	defaultMaxCap = 50             // default max clients per server
)

// ConnectionStore manages active sessions and studio assignments.
type ConnectionStore struct {
	mu         sync.RWMutex
	path       string
	sessions   map[string]*models.ActiveSession // key: client_ip (one active session per studio)
	studios    map[string]*models.StudioRecord  // key: client_ip
	maxCap     int
	staleAfter time.Duration
}

// NewConnectionStore creates a store backed by a JSON file.
func NewConnectionStore(dataDir string) *ConnectionStore {
	return &ConnectionStore{
		path:       filepath.Join(dataDir, "connections.json"),
		sessions:   make(map[string]*models.ActiveSession),
		studios:    make(map[string]*models.StudioRecord),
		maxCap:     defaultMaxCap,
		staleAfter: sessionExpiry,
	}
}

// Load reads connections from disk.
func (s *ConnectionStore) Load() error {
	s.mu.Lock()
	defer s.mu.Unlock()

	data, err := os.ReadFile(s.path)
	if err != nil {
		if os.IsNotExist(err) {
			return nil
		}
		return err
	}

	var store struct {
		Sessions map[string]*models.ActiveSession `json:"sessions"`
		Studios  map[string]*models.StudioRecord  `json:"studios"`
	}
	if err := json.Unmarshal(data, &store); err != nil {
		return err
	}

	s.sessions = store.Sessions
	if s.sessions == nil {
		s.sessions = make(map[string]*models.ActiveSession)
	}
	s.studios = store.Studios
	if s.studios == nil {
		s.studios = make(map[string]*models.StudioRecord)
	}

	s.expireStaleLocked()
	return s.saveLocked()
}

// Connect records a new active session.
func (s *ConnectionStore) Connect(clientIP, serverIP, nodeID, osName, version string) {
	s.mu.Lock()
	defer s.mu.Unlock()

	now := time.Now()

	// Update or create active session
	s.sessions[clientIP] = &models.ActiveSession{
		ClientIP:      clientIP,
		ServerIP:      serverIP,
		NodeID:        nodeID,
		OS:            osName,
		Version:       version,
		ConnectedAt:   now,
		LastHeartbeat: now,
	}

	// Update or create studio record
	studio, exists := s.studios[clientIP]
	if !exists {
		studio = &models.StudioRecord{
			ClientIP:       clientIP,
			HomeServerIP:   serverIP,
			HomeNodeID:     nodeID,
			HomeAssignedAt: now,
			LastSeen:       now,
		}
		s.studios[clientIP] = studio
	}
	studio.LastSeen = now
	studio.TotalClients++

	// If studio has no home yet, assign one
	if studio.HomeServerIP == "" {
		studio.HomeServerIP = serverIP
		studio.HomeNodeID = nodeID
		studio.HomeAssignedAt = now
	}

	s.expireStaleLocked()
	_ = s.saveLocked()
}

// Disconnect marks a session as inactive.
func (s *ConnectionStore) Disconnect(clientIP string) {
	s.mu.Lock()
	defer s.mu.Unlock()

	delete(s.sessions, clientIP)
	_ = s.saveLocked()
}

// Heartbeat updates the last-seen time for a session.
func (s *ConnectionStore) Heartbeat(clientIP string) {
	s.mu.Lock()
	defer s.mu.Unlock()

	if sess, ok := s.sessions[clientIP]; ok {
		sess.LastHeartbeat = time.Now()
	}
	if studio, ok := s.studios[clientIP]; ok {
		studio.LastSeen = time.Now()
	}
}

// GetRecommendation returns the recommended server for a client IP.
// Pure load-based: always picks the least loaded available + healthy server.
// No sticky home — auto-balancing on every request.
func (s *ConnectionStore) GetRecommendation(clientIP string, availableIPs []string, healthyIPs map[string]bool) models.RecommendationResponse {
	s.mu.RLock()
	defer s.mu.RUnlock()

	resp := models.RecommendationResponse{}

	// Count active connections per server
	load := s.activeLoadLocked(availableIPs)

	// Always pick least loaded — no sticky
	bestIP := s.findLeastLoadedLocked(availableIPs, load, healthyIPs)
	if bestIP == "" {
		resp.Reason = "нет доступных серверов"
		return resp
	}

	resp.RecommendedServerIP = bestIP
	resp.LoadInfo = s.formatLoadInfo(load)

	// Count how many clients on this IP
	resp.StudioClients = load[bestIP]

	// Check if this is the same as the studio's previous choice
	studio, hasStudio := s.studios[clientIP]
	if hasStudio && studio.HomeServerIP == bestIP {
		resp.Reason = "рекомендуемый сервер"
	} else {
		resp.Reason = "наименее загружен"
		resp.IsRebalance = hasStudio && studio.HomeServerIP != ""
	}

	return resp
}

// GetLoadInfo returns load information for all available servers.
func (s *ConnectionStore) GetLoadInfo(availableIPs []string) []models.ServerLoadInfo {
	s.mu.RLock()
	defer s.mu.RUnlock()

	load := s.activeLoadLocked(availableIPs)
	var infos []models.ServerLoadInfo

	for _, ip := range availableIPs {
		clients := load[ip]
		pct := 0
		if s.maxCap > 0 {
			pct = (clients * 100) / s.maxCap
		}
		infos = append(infos, models.ServerLoadInfo{
			ServerIP:      ip,
			ActiveClients: clients,
			LoadPercent:   pct,
			MaxCapacity:   s.maxCap,
		})
	}

	return infos
}

// activeLoadLocked counts active sessions per server IP. Must be called with lock held.
func (s *ConnectionStore) activeLoadLocked(availableIPs []string) map[string]int {
	load := make(map[string]int)
	for _, ip := range availableIPs {
		load[ip] = 0
	}
	now := time.Now()
	for _, sess := range s.sessions {
		if now.Sub(sess.LastHeartbeat) < s.staleAfter {
			load[sess.ServerIP]++
		}
	}
	return load
}

// findLeastLoadedLocked finds the least loaded available + healthy server.
func (s *ConnectionStore) findLeastLoadedLocked(availableIPs []string, load map[string]int, healthyIPs map[string]bool) string {
	type ipLoad struct {
		ip    string
		count int
	}
	var candidates []ipLoad

	for _, ip := range availableIPs {
		if len(healthyIPs) > 0 && !healthyIPs[ip] {
			continue
		}
		candidates = append(candidates, ipLoad{ip, load[ip]})
	}

	if len(candidates) == 0 {
		return ""
	}

	sort.Slice(candidates, func(i, j int) bool {
		return candidates[i].count < candidates[j].count
	})

	return candidates[0].ip
}

// expireStaleLocked removes stale sessions and old studio records.
func (s *ConnectionStore) expireStaleLocked() {
	now := time.Now()

	// Expire stale sessions
	for key, sess := range s.sessions {
		if now.Sub(sess.LastHeartbeat) > s.staleAfter {
			delete(s.sessions, key)
		}
	}

	// Expire old studio records (kept for reference)
	for key, studio := range s.studios {
		if now.Sub(studio.LastSeen) > studioExpiry {
			delete(s.studios, key)
		}
	}
}

// saveLocked writes state to disk.
func (s *ConnectionStore) saveLocked() error {
	if err := os.MkdirAll(filepath.Dir(s.path), 0o755); err != nil {
		return err
	}

	store := struct {
		Sessions map[string]*models.ActiveSession `json:"sessions"`
		Studios  map[string]*models.StudioRecord  `json:"studios"`
	}{
		Sessions: s.sessions,
		Studios:  s.studios,
	}

	data, err := json.MarshalIndent(store, "", "  ")
	if err != nil {
		return err
	}

	tmpPath := s.path + ".tmp"
	if err := os.WriteFile(tmpPath, data, 0o644); err != nil {
		return err
	}
	return os.Rename(tmpPath, s.path)
}

func (s *ConnectionStore) formatLoadInfo(load map[string]int) string {
	var parts []string
	// Sort for consistent output
	var ips []string
	for ip := range load {
		ips = append(ips, ip)
	}
	sort.Strings(ips)

	for _, ip := range ips {
		parts = append(parts, ip+"="+itoaStr(load[ip]))
	}
	return "нагрузка: " + joinStr(parts, ", ")
}

// SetMaxCapacity sets the max clients per server for load calculation.
func (s *ConnectionStore) SetMaxCapacity(n int) {
	s.mu.Lock()
	defer s.mu.Unlock()
	if n > 0 {
		s.maxCap = n
	}
}

// itoaStr converts int to string without fmt.
func itoaStr(n int) string {
	if n == 0 {
		return "0"
	}
	var digits []byte
	for n > 0 {
		digits = append([]byte{byte('0' + n%10)}, digits...)
		n /= 10
	}
	return string(digits)
}

// joinStr joins strings with separator without strings import.
func joinStr(parts []string, sep string) string {
	if len(parts) == 0 {
		return ""
	}
	result := parts[0]
	for _, p := range parts[1:] {
		result += sep + p
	}
	return result
}