package middleware

import (
	"bytes"
	"crypto/sha256"
	"encoding/hex"
	"encoding/json"
	"fmt"
	"io"
	"sync"
	"time"

	"github.com/gin-gonic/gin"

	"github.com/user-management-system/internal/config"
)

// RateLimitMiddleware provides simple in-memory sliding-window rate limiting.
type RateLimitMiddleware struct {
	cfg        config.RateLimitConfig
	limiters   map[string]*SlidingWindowLimiter
	mu         sync.RWMutex
	cleanupInt time.Duration
}

// SlidingWindowLimiter enforces a fixed-capacity sliding window.
type SlidingWindowLimiter struct {
	mu       sync.Mutex
	window   time.Duration
	capacity int64
	requests []int64
}

func NewSlidingWindowLimiter(window time.Duration, capacity int64) *SlidingWindowLimiter {
	return &SlidingWindowLimiter{
		window:   window,
		capacity: capacity,
		requests: make([]int64, 0),
	}
}

func (l *SlidingWindowLimiter) Allow() bool {
	l.mu.Lock()
	defer l.mu.Unlock()

	now := time.Now().UnixMilli()
	cutoff := now - l.window.Milliseconds()

	validRequests := make([]int64, 0, len(l.requests))
	for _, ts := range l.requests {
		if ts > cutoff {
			validRequests = append(validRequests, ts)
		}
	}
	l.requests = validRequests

	if int64(len(l.requests)) >= l.capacity {
		return false
	}

	l.requests = append(l.requests, now)
	return true
}

func NewRateLimitMiddleware(cfg config.RateLimitConfig) *RateLimitMiddleware {
	return &RateLimitMiddleware{
		cfg:        cfg,
		limiters:   make(map[string]*SlidingWindowLimiter),
		cleanupInt: 5 * time.Minute,
	}
}

func (m *RateLimitMiddleware) Register() gin.HandlerFunc {
	return m.limitForKey("register", 60, 10)
}

func (m *RateLimitMiddleware) Login() gin.HandlerFunc {
	return m.limitForKey("login", 60, 5)
}

func (m *RateLimitMiddleware) API() gin.HandlerFunc {
	return m.limitForKey("api", 60, 100)
}

func (m *RateLimitMiddleware) Refresh() gin.HandlerFunc {
	return m.limitForKey("refresh", 60, 10)
}

func (m *RateLimitMiddleware) limitForKey(bucket string, windowSeconds int, capacity int64) gin.HandlerFunc {
	window := time.Duration(windowSeconds) * time.Second

	return func(c *gin.Context) {
		limiterKey := m.resolveLimiterKey(c, bucket)
		limiter := m.getOrCreateLimiter(limiterKey, window, capacity)
		if !limiter.Allow() {
			c.JSON(429, gin.H{
				"code":    429,
				"message": "请求过于频繁，请稍后再试",
			})
			c.Abort()
			return
		}
		c.Next()
	}
}

func (m *RateLimitMiddleware) resolveLimiterKey(c *gin.Context, bucket string) string {
	if bucket == "refresh" {
		if refreshToken := extractRefreshToken(c); refreshToken != "" {
			return fmt.Sprintf("%s:token:%s", bucket, fingerprintValue(refreshToken))
		}
	}

	identity := "anonymous"
	if c != nil {
		if userID, ok := c.Get("user_id"); ok {
			identity = fmt.Sprintf("user:%v", userID)
		} else if ip := c.ClientIP(); ip != "" {
			identity = "ip:" + ip
		}
	}

	if bucket == "api" {
		method := ""
		route := ""
		if c != nil {
			if c.Request != nil {
				method = c.Request.Method
				if c.Request.URL != nil {
					route = c.Request.URL.Path
				}
			}
			if fullPath := c.FullPath(); fullPath != "" {
				route = fullPath
			}
		}
		return fmt.Sprintf("%s:%s:%s:%s", bucket, method, route, identity)
	}

	return fmt.Sprintf("%s:%s", bucket, identity)
}

func extractRefreshToken(c *gin.Context) string {
	if c == nil {
		return ""
	}

	if refreshToken, err := c.Cookie("ums_refresh_token"); err == nil && refreshToken != "" {
		return refreshToken
	}

	if c.Request == nil || c.Request.Body == nil {
		return ""
	}

	body, err := io.ReadAll(c.Request.Body)
	if err != nil {
		return ""
	}
	c.Request.Body = io.NopCloser(bytes.NewReader(body))

	if len(bytes.TrimSpace(body)) == 0 {
		return ""
	}

	var payload struct {
		RefreshToken string `json:"refresh_token"`
	}

	if err := json.Unmarshal(body, &payload); err != nil {
		return ""
	}

	return payload.RefreshToken
}

func fingerprintValue(value string) string {
	sum := sha256.Sum256([]byte(value))
	return hex.EncodeToString(sum[:12])
}

func (m *RateLimitMiddleware) getOrCreateLimiter(key string, window time.Duration, capacity int64) *SlidingWindowLimiter {
	m.mu.RLock()
	limiter, exists := m.limiters[key]
	m.mu.RUnlock()

	if exists {
		return limiter
	}

	m.mu.Lock()
	defer m.mu.Unlock()

	if limiter, exists = m.limiters[key]; exists {
		return limiter
	}

	limiter = NewSlidingWindowLimiter(window, capacity)
	m.limiters[key] = limiter
	return limiter
}

// Cleanup 清理过期的不活跃 limiter，防止 map 无界增长（P0 资源泄漏修复）
func (m *RateLimitMiddleware) Cleanup() {
	m.mu.Lock()
	defer m.mu.Unlock()

	now := time.Now().UnixMilli()
	for key, limiter := range m.limiters {
		limiter.mu.Lock()
		cutoff := now - limiter.window.Milliseconds()
		// 只保留仍在窗口内的请求时间戳
		validRequests := make([]int64, 0, len(limiter.requests))
		for _, ts := range limiter.requests {
			if ts > cutoff {
				validRequests = append(validRequests, ts)
			}
		}
		limiter.requests = validRequests
		isEmpty := len(limiter.requests) == 0
		limiter.mu.Unlock()

		if isEmpty {
			delete(m.limiters, key)
		}
	}
}

// StartCleanup 启动后台定期清理 goroutine，返回停止函数（P0 资源泄漏修复）
func (m *RateLimitMiddleware) StartCleanup() func() {
	ticker := time.NewTicker(m.cleanupInt)
	done := make(chan struct{})
	go func() {
		for {
			select {
			case <-ticker.C:
				m.Cleanup()
			case <-done:
				ticker.Stop()
				return
			}
		}
	}()
	return func() { close(done) }
}