package middleware

import (
	"net/http"
	"strconv"
	"strings"
)

// CORSConfig CORS配置
type CORSConfig struct {
	// AllowedOrigins 允许的源，支持通配符 "*" 表示允许所有源
	// 例如: []string{"*"} 或 []string{"https://example.com", "https://app.example.com"}
	AllowedOrigins []string

	// AllowedMethods 允许的HTTP方法
	// 默认: []string{"GET", "POST", "PUT", "DELETE", "PATCH", "OPTIONS"}
	AllowedMethods []string

	// AllowedHeaders 允许的请求头
	// 默认: []string{"Content-Type", "Authorization", "X-Requested-With", "X-Timezone"}
	AllowedHeaders []string

	// ExposedHeaders 暴露给客户端的响应头
	ExposedHeaders []string

	// AllowCredentials 是否允许发送凭证（cookies等）
	AllowCredentials bool

	// MaxAge 预检请求的缓存时间（秒）
	// 默认: 86400 (24小时)
	MaxAge int
}

// DefaultCORSConfig 返回默认的CORS配置
func DefaultCORSConfig() *CORSConfig {
	return &CORSConfig{
		AllowedOrigins: []string{"*"},
		AllowedMethods: []string{"GET", "POST", "PUT", "DELETE", "PATCH", "OPTIONS"},
		AllowedHeaders: []string{"Content-Type", "Authorization", "X-Requested-With", "X-Timezone"},
		ExposedHeaders: []string{},
		AllowCredentials: false,
		MaxAge: 86400,
	}
}

// CORS CORS中间件
func CORS(config ...*CORSConfig) func(http.Handler) http.Handler {
	var cfg *CORSConfig
	if len(config) > 0 && config[0] != nil {
		cfg = config[0]
	} else {
		cfg = DefaultCORSConfig()
	}

	return func(next http.Handler) http.Handler {
		return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
			origin := r.Header.Get("Origin")

			// 处理预检请求
			if r.Method == http.MethodOptions {
				handlePreflight(w, r, cfg, origin)
				return
			}

			// 处理实际请求
			handleCORSHeaders(w, r, cfg, origin)
			next.ServeHTTP(w, r)
		})
	}
}

// handlePreflight 处理预检请求
func handlePreflight(w http.ResponseWriter, r *http.Request, cfg *CORSConfig, origin string) {
	// 检查源是否允许
	if !isOriginAllowed(origin, cfg.AllowedOrigins) {
		w.WriteHeader(http.StatusForbidden)
		return
	}

	// 设置CORS响应头
	setCORSHeaders(w, cfg, origin)

	// 检查请求的方法是否允许
	requestMethod := r.Header.Get("Access-Control-Request-Method")
	if requestMethod != "" && !isMethodAllowed(requestMethod, cfg.AllowedMethods) {
		w.WriteHeader(http.StatusMethodNotAllowed)
		return
	}

	// 检查请求头是否允许
	requestHeaders := r.Header.Get("Access-Control-Request-Headers")
	if requestHeaders != "" {
		headers := strings.Split(strings.ToLower(requestHeaders), ",")
		for _, header := range headers {
			header = strings.TrimSpace(header)
			if !isHeaderAllowed(header, cfg.AllowedHeaders) {
				w.WriteHeader(http.StatusForbidden)
				return
			}
		}
	}

	w.WriteHeader(http.StatusNoContent)
}

// handleCORSHeaders 处理实际请求的CORS头
func handleCORSHeaders(w http.ResponseWriter, r *http.Request, cfg *CORSConfig, origin string) {
	// 检查源是否允许
	if !isOriginAllowed(origin, cfg.AllowedOrigins) {
		return
	}

	// 设置CORS响应头
	setCORSHeaders(w, cfg, origin)
}

// setCORSHeaders 设置CORS响应头
func setCORSHeaders(w http.ResponseWriter, cfg *CORSConfig, origin string) {
	// Access-Control-Allow-Origin
	if len(cfg.AllowedOrigins) == 1 && cfg.AllowedOrigins[0] == "*" {
		if !cfg.AllowCredentials {
			w.Header().Set("Access-Control-Allow-Origin", "*")
		} else {
			// 如果允许凭证，不能使用 "*"，必须返回具体的源
			if origin != "" {
				w.Header().Set("Access-Control-Allow-Origin", origin)
			}
		}
	} else {
		if isOriginAllowed(origin, cfg.AllowedOrigins) {
			w.Header().Set("Access-Control-Allow-Origin", origin)
		}
	}

	// Access-Control-Allow-Methods
	if len(cfg.AllowedMethods) > 0 {
		w.Header().Set("Access-Control-Allow-Methods", strings.Join(cfg.AllowedMethods, ", "))
	}

	// Access-Control-Allow-Headers
	if len(cfg.AllowedHeaders) > 0 {
		w.Header().Set("Access-Control-Allow-Headers", strings.Join(cfg.AllowedHeaders, ", "))
	}

	// Access-Control-Expose-Headers
	if len(cfg.ExposedHeaders) > 0 {
		w.Header().Set("Access-Control-Expose-Headers", strings.Join(cfg.ExposedHeaders, ", "))
	}

	// Access-Control-Allow-Credentials
	if cfg.AllowCredentials {
		w.Header().Set("Access-Control-Allow-Credentials", "true")
	}

	// Access-Control-Max-Age
	if cfg.MaxAge > 0 {
		w.Header().Set("Access-Control-Max-Age", strconv.Itoa(cfg.MaxAge))
	}
}

// isOriginAllowed 检查源是否允许
func isOriginAllowed(origin string, allowedOrigins []string) bool {
	if origin == "" {
		return false
	}

	for _, allowed := range allowedOrigins {
		if allowed == "*" {
			return true
		}
		if allowed == origin {
			return true
		}
		// 支持简单的通配符匹配（如 "*.example.com"）
		if strings.HasPrefix(allowed, "*.") {
			domain := strings.TrimPrefix(allowed, "*.")
			if strings.HasSuffix(origin, domain) {
				return true
			}
		}
	}

	return false
}

// isMethodAllowed 检查方法是否允许
func isMethodAllowed(method string, allowedMethods []string) bool {
	method = strings.ToUpper(strings.TrimSpace(method))
	for _, allowed := range allowedMethods {
		if strings.ToUpper(allowed) == method {
			return true
		}
	}
	return false
}

// isHeaderAllowed 检查请求头是否允许
func isHeaderAllowed(header string, allowedHeaders []string) bool {
	header = strings.ToLower(strings.TrimSpace(header))
	for _, allowed := range allowedHeaders {
		if strings.ToLower(allowed) == header {
			return true
		}
	}
	return false
}