package middleware

import (
	"bytes"
	"crypto/sha256"
	"encoding/hex"
	"encoding/json"
	"fmt"
	"net/http"
	"strings"
	"time"

	"github.com/gin-gonic/gin"
	"github.com/redis/go-redis/v9"
	"go.uber.org/zap"
)

// ResponseCacheConfig configures the HTTP response cache middleware
type ResponseCacheConfig struct {
	RedisClient  *redis.Client
	Logger       *zap.Logger
	DefaultTTL   time.Duration
	KeyPrefix    string
	EndpointTTLs map[string]time.Duration // path prefix → TTL override
}

// cachedResponse stores the cached HTTP response data
type cachedResponse struct {
	Status      int               `json:"status"`
	ContentType string            `json:"content_type"`
	Body        string            `json:"body"`
	Headers     map[string]string `json:"headers"`
}

// responseWriter captures the response for caching
type cacheResponseWriter struct {
	gin.ResponseWriter
	body *bytes.Buffer
}

func (w *cacheResponseWriter) Write(b []byte) (int, error) {
	w.body.Write(b)
	return w.ResponseWriter.Write(b)
}

// ResponseCache returns a middleware that caches GET responses in Redis.
// Only caches successful (2xx) GET requests for unauthenticated or public endpoints.
func ResponseCache(cfg ResponseCacheConfig) gin.HandlerFunc {
	if cfg.RedisClient == nil {
		return func(c *gin.Context) { c.Next() }
	}
	if cfg.DefaultTTL == 0 {
		cfg.DefaultTTL = 5 * time.Minute
	}
	if cfg.KeyPrefix == "" {
		cfg.KeyPrefix = "http_cache"
	}
	if cfg.Logger == nil {
		cfg.Logger = zap.NewNop()
	}

	return func(c *gin.Context) {
		// Only cache GET requests
		if c.Request.Method != http.MethodGet {
			c.Next()
			return
		}

		// Skip caching for authenticated requests (user-specific data)
		if c.GetHeader("Authorization") != "" {
			c.Next()
			return
		}

		// Skip caching for cookie-authenticated requests (httpOnly auth cookies)
		if _, err := c.Cookie("access_token"); err == nil {
			c.Next()
			return
		}

		// Skip caching for auth endpoints (must never serve cached user data)
		if strings.Contains(c.Request.URL.Path, "/auth/") {
			c.Next()
			return
		}

		// Generate cache key from URL + query params
		cacheKey := generateCacheKey(cfg.KeyPrefix, c.Request.URL.RequestURI())

		// Try to serve from cache
		ctx := c.Request.Context()
		cached, err := cfg.RedisClient.Get(ctx, cacheKey).Result()
		if err == nil {
			// Cache hit — serve from cache
			var resp cachedResponse
			if jsonErr := json.Unmarshal([]byte(cached), &resp); jsonErr == nil {
				for k, v := range resp.Headers {
					c.Header(k, v)
				}
				c.Header("X-Cache", "HIT")
				c.Data(resp.Status, resp.ContentType, []byte(resp.Body))
				c.Abort()
				return
			}
		}

		// Cache miss — capture response
		writer := &cacheResponseWriter{
			ResponseWriter: c.Writer,
			body:           &bytes.Buffer{},
		}
		c.Writer = writer

		c.Header("X-Cache", "MISS")
		c.Next()

		// Only cache successful responses
		status := writer.Status()
		if status < 200 || status >= 300 {
			return
		}

		// Determine TTL for this endpoint
		ttl := cfg.DefaultTTL
		for prefix, override := range cfg.EndpointTTLs {
			if strings.HasPrefix(c.Request.URL.Path, prefix) {
				ttl = override
				break
			}
		}

		// Store in cache
		resp := cachedResponse{
			Status:      status,
			ContentType: writer.Header().Get("Content-Type"),
			Body:        writer.body.String(),
			Headers: map[string]string{
				"Content-Type": writer.Header().Get("Content-Type"),
			},
		}

		data, err := json.Marshal(resp)
		if err != nil {
			cfg.Logger.Debug("Failed to marshal response for cache", zap.Error(err))
			return
		}

		if setErr := cfg.RedisClient.Set(ctx, cacheKey, data, ttl).Err(); setErr != nil {
			cfg.Logger.Debug("Failed to store response in cache",
				zap.String("key", cacheKey),
				zap.Error(setErr))
		}
	}
}

// generateCacheKey creates a deterministic cache key from a URI
func generateCacheKey(prefix, uri string) string {
	hash := sha256.Sum256([]byte(uri))
	return fmt.Sprintf("%s:%s", prefix, hex.EncodeToString(hash[:16]))
}