package main

import (
	"crypto/sha256"
	"encoding/hex"
	"fmt"
	"log"
	"net/http"
	"strings"
	"sync"
	"time"
)

// ========== 存储与缓存结构体定义 ==========

// CacheItem 缓存条目，包含长URL和过期时间
type CacheItem struct {
	url      string
	expireAt time.Time
}

// URLStore 短链接存储，核心并发安全的键值对
type URLStore struct {
	mu      sync.RWMutex
	urls    map[string]string // 短码 -> 长URL
	counter int               // 简单的自增ID，用于生成哈希输入
}

// Cache 带过期时间的缓存
type Cache struct {
	mu      sync.RWMutex
	items   map[string]CacheItem // 短码 -> 缓存条目
	ttl     time.Duration        // 缓存存活时间
}

// ========== 核心方法实现 ==========

// NewURLStore 创建新的存储实例
func NewURLStore() *URLStore {
	return &URLStore{
		urls: make(map[string]string),
	}
}

// Set 保存长URL，返回生成的短码（并发安全）
func (s *URLStore) Set(longURL string) string {
	s.mu.Lock()
	defer s.mu.Unlock()

	// 使用SHA256生成哈希，截取前8位作为短码（极简化示例，实际需处理冲突）
	hash := sha256.Sum256([]byte(longURL + fmt.Sprintf("%d", s.counter)))
	shortCode := hex.EncodeToString(hash[:])[:8]
	s.counter++

	s.urls[shortCode] = longURL
	return shortCode
}

// Get 根据短码获取长URL（并发安全）
func (s *URLStore) Get(shortCode string) (string, bool) {
	s.mu.RLock()
	defer s.mu.RUnlock()

	url, found := s.urls[shortCode]
	return url, found
}

// NewCache 创建缓存实例，并启动后台清理协程
func NewCache(ttl time.Duration) *Cache {
	c := &Cache{
		items: make(map[string]CacheItem),
		ttl:   ttl,
	}
	// 启动一个协程定期清理过期缓存
	go c.cleanupLoop()
	return c
}

// Set 设置缓存
func (c *Cache) Set(key, url string) {
	c.mu.Lock()
	defer c.mu.Unlock()

	c.items[key] = CacheItem{
		url:      url,
		expireAt: time.Now().Add(c.ttl),
	}
}

// Get 从缓存获取，如果存在且未过期则返回
func (c *Cache) Get(key string) (string, bool) {
	c.mu.RLock()
	defer c.mu.RUnlock()

	item, found := c.items[key]
	if !found || time.Now().After(item.expireAt) {
		return "", false
	}
	return item.url, true
}

// cleanupLoop 定期清理过期缓存条目
func (c *Cache) cleanupLoop() {
	ticker := time.NewTicker(time.Minute * 1) // 每分钟清理一次
	defer ticker.Stop()

	for range ticker.C {
		c.mu.Lock()
		for key, item := range c.items {
			if time.Now().After(item.expireAt) {
				delete(c.items, key)
			}
		}
		c.mu.Unlock()
		log.Println("缓存清理完成，当前条目数:", len(c.items))
	}
}

// ========== HTTP 处理函数 ==========

func createHandler(store *URLStore, cache *Cache) http.HandlerFunc {
	return func(w http.ResponseWriter, r *http.Request) {
		// 只允许POST请求
		if r.Method != http.MethodPost {
			http.Error(w, "仅支持POST请求", http.StatusMethodNotAllowed)
			return
		}

		// 解析表单数据
		if err := r.ParseForm(); err != nil {
			http.Error(w, "参数解析失败", http.StatusBadRequest)
			return
		}
		longURL := r.FormValue("url")
		if longURL == "" {
			http.Error(w, "缺少url参数", http.StatusBadRequest)
			return
		}

		// 生成短码并存储
		shortCode := store.Set(longURL)
		// 将新生成的映射也放入缓存
		cache.Set(shortCode, longURL)

		// 返回短链接（简化处理，实际应包含域名）
		w.Header().Set("Content-Type", "application/json")
		fmt.Fprintf(w, `{"short_url": "http://localhost:8080/%s"}`, shortCode)
	}
}

func redirectHandler(store *URLStore, cache *Cache) http.HandlerFunc {
	return func(w http.ResponseWriter, r *http.Request) {
		// 提取短码（去除开头的`/`）
		shortCode := strings.TrimPrefix(r.URL.Path, "/")
		if shortCode == "" {
			http.Error(w, "缺少短码", http.StatusBadRequest)
			return
		}

		// 第一步：尝试从缓存获取
		longURL, found := cache.Get(shortCode)
		if found {
			http.Redirect(w, r, longURL, http.StatusFound) // 302临时重定向
			log.Printf("缓存命中: %s -> %s", shortCode, longURL)
			return
		}

		// 第二步：缓存未命中，从存储获取
		longURL, found = store.Get(shortCode)
		if !found {
			http.NotFound(w, r)
			return
		}

		// 将结果放入缓存，方便下次访问
		cache.Set(shortCode, longURL)
		http.Redirect(w, r, longURL, http.StatusFound)
	}
}

func main() {
	// 初始化存储和缓存
	store := NewURLStore()
	// 设置缓存存活时间为10分钟
	cache := NewCache(time.Minute * 10)

	// 注册HTTP路由
	http.HandleFunc("/create", createHandler(store, cache))
	http.HandleFunc("/", redirectHandler(store, cache))

	fmt.Println("短链接服务启动在 :8080")
	fmt.Println("创建短链接: curl -X POST -d 'url=https://www.example.com/very/long/path' http://localhost:8080/create")
	fmt.Println("访问短链接: 在浏览器打开返回的short_url")
	log.Fatal(http.ListenAndServe(":8080", nil))
}

{
  "short_code": "a1b2c3d4",
  "original_url": "https://www.example.com",
  "created_at": "2023-10-27T10:30:00Z"
}