package security

import (
	"context"
	"fmt"
	"net"
	"sync"
	"time"
)

// RateLimiter provides comprehensive rate limiting and DDoS protection
type RateLimiter struct {
	// Per-IP rate limiting
	ipBuckets      map[string]*TokenBucket
	ipMutex        sync.RWMutex

	// Per-user rate limiting
	userBuckets    map[string]*TokenBucket
	userMutex      sync.RWMutex

	// Global rate limiting
	globalBucket   *TokenBucket

	// DDoS protection
	ddosDetector   *DDoSDetector

	// Configuration
	config         *RateLimiterConfig

	// Cleanup ticker
	cleanupTicker  *time.Ticker
	stopCleanup    chan struct{}
}

// TokenBucket implements the token bucket algorithm for rate limiting
type TokenBucket struct {
	Capacity       int           `json:"capacity"`
	Tokens         int           `json:"tokens"`
	RefillRate     int           `json:"refill_rate"`     // tokens per second
	LastRefill     time.Time     `json:"last_refill"`
	LastAccess     time.Time     `json:"last_access"`
	Violations     int           `json:"violations"`
	Blocked        bool          `json:"blocked"`
	BlockedUntil   time.Time     `json:"blocked_until"`
}

// DDoSDetector detects and mitigates DDoS attacks
type DDoSDetector struct {
	// Request patterns
	requestCounts    map[string]*RequestPattern
	patternMutex     sync.RWMutex

	// Anomaly detection
	baselineRPS      float64
	currentRPS       float64
	anomalyThreshold float64

	// Mitigation
	mitigationActive bool
	mitigationStart  time.Time
	blockedIPs       map[string]time.Time

	// Geolocation tracking
	geoTracker       *GeoLocationTracker
}

// RequestPattern tracks request patterns for anomaly detection
type RequestPattern struct {
	IP              string
	RequestCount    int
	LastRequest     time.Time
	RequestTimes    []time.Time
	UserAgent       string
	Endpoints       map[string]int
	Suspicious      bool
	Score          int
}

// GeoLocationTracker tracks requests by geographic location
type GeoLocationTracker struct {
	requestsByCountry map[string]int
	requestsByRegion  map[string]int
	suspiciousRegions map[string]bool
	mutex            sync.RWMutex
}

// RateLimiterConfig provides configuration for rate limiting
type RateLimiterConfig struct {
	// Per-IP limits
	IPRequestsPerSecond int           `json:"ip_requests_per_second"`
	IPBurstSize        int           `json:"ip_burst_size"`
	IPBlockDuration    time.Duration `json:"ip_block_duration"`

	// Per-user limits
	UserRequestsPerSecond int           `json:"user_requests_per_second"`
	UserBurstSize        int           `json:"user_burst_size"`
	UserBlockDuration    time.Duration `json:"user_block_duration"`

	// Global limits
	GlobalRequestsPerSecond int `json:"global_requests_per_second"`
	GlobalBurstSize        int `json:"global_burst_size"`

	// DDoS protection
	DDoSThreshold          int           `json:"ddos_threshold"`
	DDoSDetectionWindow    time.Duration `json:"ddos_detection_window"`
	DDoSMitigationDuration time.Duration `json:"ddos_mitigation_duration"`
	AnomalyThreshold       float64       `json:"anomaly_threshold"`

	// Cleanup
	CleanupInterval        time.Duration `json:"cleanup_interval"`
	BucketTTL             time.Duration `json:"bucket_ttl"`

	// Whitelisting
	WhitelistedIPs        []string `json:"whitelisted_ips"`
	WhitelistedUserAgents []string `json:"whitelisted_user_agents"`
}

// RateLimitResult represents the result of a rate limit check
type RateLimitResult struct {
	Allowed          bool          `json:"allowed"`
	RemainingTokens  int           `json:"remaining_tokens"`
	RetryAfter       time.Duration `json:"retry_after"`
	ReasonCode       string        `json:"reason_code"`
	Message          string        `json:"message"`
	Violations       int           `json:"violations"`
	DDoSDetected     bool          `json:"ddos_detected"`
	SuspiciousScore  int           `json:"suspicious_score"`
}

// NewRateLimiter creates a new rate limiter with DDoS protection
func NewRateLimiter(config *RateLimiterConfig) *RateLimiter {
	if config == nil {
		config = &RateLimiterConfig{
			IPRequestsPerSecond:     100,
			IPBurstSize:            200,
			IPBlockDuration:        time.Hour,
			UserRequestsPerSecond:  1000,
			UserBurstSize:         2000,
			UserBlockDuration:     30 * time.Minute,
			GlobalRequestsPerSecond: 10000,
			GlobalBurstSize:       20000,
			DDoSThreshold:         1000,
			DDoSDetectionWindow:   time.Minute,
			DDoSMitigationDuration: 10 * time.Minute,
			AnomalyThreshold:      3.0,
			CleanupInterval:       5 * time.Minute,
			BucketTTL:            time.Hour,
		}
	}

	rl := &RateLimiter{
		ipBuckets:     make(map[string]*TokenBucket),
		userBuckets:   make(map[string]*TokenBucket),
		globalBucket:  newTokenBucket(config.GlobalRequestsPerSecond, config.GlobalBurstSize),
		config:        config,
		stopCleanup:   make(chan struct{}),
	}

	// Initialize DDoS detector
	rl.ddosDetector = &DDoSDetector{
		requestCounts:    make(map[string]*RequestPattern),
		anomalyThreshold: config.AnomalyThreshold,
		blockedIPs:      make(map[string]time.Time),
		geoTracker: &GeoLocationTracker{
			requestsByCountry: make(map[string]int),
			requestsByRegion:  make(map[string]int),
			suspiciousRegions: make(map[string]bool),
		},
	}

	// Start cleanup routine
	rl.cleanupTicker = time.NewTicker(config.CleanupInterval)
	go rl.cleanupRoutine()

	return rl
}

// CheckRateLimit checks if a request should be allowed
func (rl *RateLimiter) CheckRateLimit(ctx context.Context, ip, userID, userAgent, endpoint string) *RateLimitResult {
	result := &RateLimitResult{
		Allowed:     true,
		ReasonCode:  "OK",
		Message:     "Request allowed",
	}

	// Check if IP is whitelisted
	if rl.isWhitelisted(ip, userAgent) {
		return result
	}

	// Check for DDoS
	if rl.checkDDoS(ip, userAgent, endpoint, result) {
		return result
	}

	// Check global rate limit
	if !rl.checkGlobalLimit(result) {
		return result
	}

	// Check per-IP rate limit
	if !rl.checkIPLimit(ip, result) {
		return result
	}

	// Check per-user rate limit (if user is identified)
	if userID != "" && !rl.checkUserLimit(userID, result) {
		return result
	}

	// Update request pattern for anomaly detection
	rl.updateRequestPattern(ip, userAgent, endpoint)

	return result
}

// checkDDoS performs DDoS detection and mitigation
func (rl *RateLimiter) checkDDoS(ip, userAgent, endpoint string, result *RateLimitResult) bool {
	rl.ddosDetector.patternMutex.Lock()
	defer rl.ddosDetector.patternMutex.Unlock()

	now := time.Now()

	// Check if IP is currently blocked
	if blockedUntil, exists := rl.ddosDetector.blockedIPs[ip]; exists {
		if now.Before(blockedUntil) {
			result.Allowed = false
			result.ReasonCode = "DDOS_BLOCKED"
			result.Message = "IP temporarily blocked due to DDoS detection"
			result.RetryAfter = blockedUntil.Sub(now)
			result.DDoSDetected = true
			return true
		}
		// Unblock expired IPs
		delete(rl.ddosDetector.blockedIPs, ip)
	}

	// Get or create request pattern
	pattern, exists := rl.ddosDetector.requestCounts[ip]
	if !exists {
		pattern = &RequestPattern{
			IP:           ip,
			RequestCount: 0,
			RequestTimes: make([]time.Time, 0),
			Endpoints:    make(map[string]int),
			UserAgent:    userAgent,
		}
		rl.ddosDetector.requestCounts[ip] = pattern
	}

	// Update pattern
	pattern.RequestCount++
	pattern.LastRequest = now
	pattern.RequestTimes = append(pattern.RequestTimes, now)
	pattern.Endpoints[endpoint]++

	// Remove old request times (outside detection window)
	cutoff := now.Add(-rl.config.DDoSDetectionWindow)
	newTimes := make([]time.Time, 0)
	for _, t := range pattern.RequestTimes {
		if t.After(cutoff) {
			newTimes = append(newTimes, t)
		}
	}
	pattern.RequestTimes = newTimes

	// Calculate suspicious score
	pattern.Score = rl.calculateSuspiciousScore(pattern)
	result.SuspiciousScore = pattern.Score

	// Check if pattern indicates DDoS
	if len(pattern.RequestTimes) > rl.config.DDoSThreshold {
		pattern.Suspicious = true
		rl.ddosDetector.blockedIPs[ip] = now.Add(rl.config.DDoSMitigationDuration)

		result.Allowed = false
		result.ReasonCode = "DDOS_DETECTED"
		result.Message = "DDoS attack detected, IP blocked"
		result.RetryAfter = rl.config.DDoSMitigationDuration
		result.DDoSDetected = true

		return true
	}

	return false
}

// calculateSuspiciousScore calculates a suspicious score for request patterns
func (rl *RateLimiter) calculateSuspiciousScore(pattern *RequestPattern) int {
	score := 0

	// High request frequency
	if len(pattern.RequestTimes) > rl.config.DDoSThreshold/2 {
		score += 50
	}

	// Suspicious user agent patterns
	suspiciousUAs := []string{"bot", "crawler", "spider", "scraper", "automation"}
	for _, ua := range suspiciousUAs {
		if len(pattern.UserAgent) > 0 && containsIgnoreCase(pattern.UserAgent, ua) {
			score += 30
			break
		}
	}

	// Limited endpoint diversity (hitting same endpoint repeatedly)
	if len(pattern.Endpoints) == 1 && pattern.RequestCount > 100 {
		score += 40
	}

	// Very short intervals between requests
	if len(pattern.RequestTimes) >= 2 {
		intervals := make([]time.Duration, 0)
		for i := 1; i < len(pattern.RequestTimes); i++ {
			intervals = append(intervals, pattern.RequestTimes[i].Sub(pattern.RequestTimes[i-1]))
		}

		// Check for unusually consistent intervals (bot-like behavior)
		if len(intervals) > 10 {
			avgInterval := time.Duration(0)
			for _, interval := range intervals {
				avgInterval += interval
			}
			avgInterval /= time.Duration(len(intervals))

			if avgInterval < 100*time.Millisecond {
				score += 60
			}
		}
	}

	return score
}

// checkGlobalLimit checks the global rate limit
func (rl *RateLimiter) checkGlobalLimit(result *RateLimitResult) bool {
	if !rl.globalBucket.consume(1) {
		result.Allowed = false
		result.ReasonCode = "GLOBAL_LIMIT"
		result.Message = "Global rate limit exceeded"
		result.RetryAfter = time.Second
		return false
	}
	result.RemainingTokens = rl.globalBucket.Tokens
	return true
}

// checkIPLimit checks the per-IP rate limit
func (rl *RateLimiter) checkIPLimit(ip string, result *RateLimitResult) bool {
	rl.ipMutex.Lock()
	defer rl.ipMutex.Unlock()

	bucket, exists := rl.ipBuckets[ip]
	if !exists {
		bucket = newTokenBucket(rl.config.IPRequestsPerSecond, rl.config.IPBurstSize)
		rl.ipBuckets[ip] = bucket
	}

	// Check if IP is currently blocked
	if bucket.Blocked && time.Now().Before(bucket.BlockedUntil) {
		result.Allowed = false
		result.ReasonCode = "IP_BLOCKED"
		result.Message = "IP temporarily blocked due to rate limit violations"
		result.RetryAfter = bucket.BlockedUntil.Sub(time.Now())
		result.Violations = bucket.Violations
		return false
	}

	// Unblock if block period expired
	if bucket.Blocked && time.Now().After(bucket.BlockedUntil) {
		bucket.Blocked = false
		bucket.Violations = 0
	}

	if !bucket.consume(1) {
		bucket.Violations++
		result.Violations = bucket.Violations

		// Block IP after too many violations
		if bucket.Violations >= 5 {
			bucket.Blocked = true
			bucket.BlockedUntil = time.Now().Add(rl.config.IPBlockDuration)
			result.ReasonCode = "IP_BLOCKED"
			result.Message = "IP blocked due to repeated rate limit violations"
			result.RetryAfter = rl.config.IPBlockDuration
		} else {
			result.ReasonCode = "IP_LIMIT"
			result.Message = "IP rate limit exceeded"
			result.RetryAfter = time.Second
		}

		result.Allowed = false
		return false
	}

	result.RemainingTokens = bucket.Tokens
	return true
}

// checkUserLimit checks the per-user rate limit
func (rl *RateLimiter) checkUserLimit(userID string, result *RateLimitResult) bool {
	rl.userMutex.Lock()
	defer rl.userMutex.Unlock()

	bucket, exists := rl.userBuckets[userID]
	if !exists {
		bucket = newTokenBucket(rl.config.UserRequestsPerSecond, rl.config.UserBurstSize)
		rl.userBuckets[userID] = bucket
	}

	if !bucket.consume(1) {
		result.Allowed = false
		result.ReasonCode = "USER_LIMIT"
		result.Message = "User rate limit exceeded"
		result.RetryAfter = time.Second
		return false
	}

	return true
}

// updateRequestPattern updates request patterns for analysis
func (rl *RateLimiter) updateRequestPattern(ip, userAgent, endpoint string) {
	// Update geo-location tracking
	rl.ddosDetector.geoTracker.mutex.Lock()
	country := rl.getCountryFromIP(ip)
	rl.ddosDetector.geoTracker.requestsByCountry[country]++
	rl.ddosDetector.geoTracker.mutex.Unlock()
}

// newTokenBucket creates a new token bucket with the specified rate and capacity
func newTokenBucket(ratePerSecond, capacity int) *TokenBucket {
	return &TokenBucket{
		Capacity:   capacity,
		Tokens:     capacity,
		RefillRate: ratePerSecond,
		LastRefill: time.Now(),
		LastAccess: time.Now(),
	}
}

// consume attempts to consume tokens from the bucket
func (tb *TokenBucket) consume(tokens int) bool {
	now := time.Now()

	// Refill tokens based on elapsed time
	elapsed := now.Sub(tb.LastRefill)
	tokensToAdd := int(elapsed.Seconds()) * tb.RefillRate
	if tokensToAdd > 0 {
		tb.Tokens += tokensToAdd
		if tb.Tokens > tb.Capacity {
			tb.Tokens = tb.Capacity
		}
		tb.LastRefill = now
	}

	tb.LastAccess = now

	// Check if we have enough tokens
	if tb.Tokens >= tokens {
		tb.Tokens -= tokens
		return true
	}

	return false
}

// isWhitelisted checks if an IP or user agent is whitelisted
func (rl *RateLimiter) isWhitelisted(ip, userAgent string) bool {
	// Check IP whitelist
	for _, whitelistedIP := range rl.config.WhitelistedIPs {
		if ip == whitelistedIP {
			return true
		}
		// Check CIDR ranges
		if _, ipnet, err := net.ParseCIDR(whitelistedIP); err == nil {
			if ipnet.Contains(net.ParseIP(ip)) {
				return true
			}
		}
	}

	// Check user agent whitelist
	for _, whitelistedUA := range rl.config.WhitelistedUserAgents {
		if containsIgnoreCase(userAgent, whitelistedUA) {
			return true
		}
	}

	return false
}

// getCountryFromIP gets country code from IP (simplified implementation)
func (rl *RateLimiter) getCountryFromIP(ip string) string {
	// In a real implementation, this would use a GeoIP database
	// For now, return a placeholder
	return "UNKNOWN"
}

// containsIgnoreCase checks if a string contains a substring (case insensitive)
func containsIgnoreCase(s, substr string) bool {
	return len(s) >= len(substr) &&
		   (s == substr ||
		    (len(s) > len(substr) &&
		     (s[:len(substr)] == substr ||
		      s[len(s)-len(substr):] == substr ||
		      findSubstring(s, substr))))
}

// findSubstring finds a substring in a string (helper function)
func findSubstring(s, substr string) bool {
	for i := 0; i <= len(s)-len(substr); i++ {
		if s[i:i+len(substr)] == substr {
			return true
		}
	}
	return false
}

// cleanupRoutine periodically cleans up old buckets and patterns
func (rl *RateLimiter) cleanupRoutine() {
	for {
		select {
		case <-rl.cleanupTicker.C:
			rl.cleanup()
		case <-rl.stopCleanup:
			return
		}
	}
}

// cleanup removes old buckets and patterns
func (rl *RateLimiter) cleanup() {
	now := time.Now()
	cutoff := now.Add(-rl.config.BucketTTL)

	// Clean up IP buckets
	rl.ipMutex.Lock()
	for ip, bucket := range rl.ipBuckets {
		if bucket.LastAccess.Before(cutoff) {
			delete(rl.ipBuckets, ip)
		}
	}
	rl.ipMutex.Unlock()

	// Clean up user buckets
	rl.userMutex.Lock()
	for user, bucket := range rl.userBuckets {
		if bucket.LastAccess.Before(cutoff) {
			delete(rl.userBuckets, user)
		}
	}
	rl.userMutex.Unlock()

	// Clean up DDoS patterns
	rl.ddosDetector.patternMutex.Lock()
	for ip, pattern := range rl.ddosDetector.requestCounts {
		if pattern.LastRequest.Before(cutoff) {
			delete(rl.ddosDetector.requestCounts, ip)
		}
	}
	rl.ddosDetector.patternMutex.Unlock()
}

// Stop stops the rate limiter and cleanup routines
func (rl *RateLimiter) Stop() {
	if rl.cleanupTicker != nil {
		rl.cleanupTicker.Stop()
	}
	close(rl.stopCleanup)
}

// GetMetrics returns current rate limiting metrics
func (rl *RateLimiter) GetMetrics() map[string]interface{} {
	rl.ipMutex.RLock()
	rl.userMutex.RLock()
	rl.ddosDetector.patternMutex.RLock()
	defer rl.ipMutex.RUnlock()
	defer rl.userMutex.RUnlock()
	defer rl.ddosDetector.patternMutex.RUnlock()

	blockedIPs := 0
	suspiciousPatterns := 0

	for _, bucket := range rl.ipBuckets {
		if bucket.Blocked {
			blockedIPs++
		}
	}

	for _, pattern := range rl.ddosDetector.requestCounts {
		if pattern.Suspicious {
			suspiciousPatterns++
		}
	}

	return map[string]interface{}{
		"active_ip_buckets":      len(rl.ipBuckets),
		"active_user_buckets":    len(rl.userBuckets),
		"blocked_ips":           blockedIPs,
		"suspicious_patterns":   suspiciousPatterns,
		"ddos_mitigation_active": rl.ddosDetector.mitigationActive,
		"global_tokens":         rl.globalBucket.Tokens,
		"global_capacity":       rl.globalBucket.Capacity,
	}
}