package main

import (
	"bytes"
	"io"
	"net"
	"net/http"
	"os"
	"strings"
	"time"
)

// OwnershipProxyMiddleware provides HTTP-layer routing to the primary owner
// of a cart before the request hits local handlers.
//
// Motivation:
//
//	In the current system SyncedPool can proxy cart mutations to remote owners
//	via remote grains (gRPC). For a simpler deployment you can instead forward
//	the incoming HTTP request directly to the owning host and let only the
//	owner execute the standard handlers (which apply mutations locally).
//
// Behavior:
//  1. Attempts to extract a cart id from (in priority order):
//     - Cookie "cartid"
//     - Path segment after "/byid/{id}" (e.g. /cart/byid/abc123/add/sku)
//  2. Resolves the primary owner host using the consistent hashing ring
//     maintained by SyncedPool.
//  3. If the owner is the local host (or no id found), the request proceeds.
//  4. If the owner is a different host, the middleware performs an in-cluster
//     HTTP proxy (single-hop) to http://<owner>:<port><original-path>?<query>
//     and streams the response back to the client.
//  5. Adds headers:
//     X-Cart-Owner: <resolved-owner>
//     X-Cart-Owner-Routed: "true" (only when proxied)
//     X-Cart-Id: <cart-id> (when available)
//     On local handling (not proxied) X-Cart-Owner-Routed is "false".
//
// Configuration:
//
//	CART_SERVICE_PORT (env) - target port for proxying (default: 8080)
//	CART_PROXY_TIMEOUT_MS (env) - timeout for outbound proxy calls (default: 800)
//
// Integration:
//
//	Wrap just the cart mux:
//
//	  cartMux := syncedServer.Serve() // existing cart handlers
//	  wrapped := OwnershipProxyMiddleware(syncedPool)(cartMux)
//	  mux.Handle("/cart/", http.StripPrefix("/cart", wrapped))
//
// Fallbacks:
//
//	If extraction or proxying fails, a 502 is returned (except missing cart id
//	which simply skips routing). Timeouts produce 504.
//
// NOTE:
//   - This does NOT (yet) support sticky upgrade / websockets.
//   - Only primary ownership is considered (replicas ignored).
//   - This keeps control plane & ring logic unmodified.
//
// You can gradually phase out remote grain logic by placing this middleware
// in front while leaving the rest of the code untouched.
func OwnershipProxyMiddleware(pool *SyncedPool) func(http.Handler) http.Handler {
	localHost := pool.Hostname()
	targetPort := envOr("CART_SERVICE_PORT", "8080")
	timeout := envDurationOr("CART_PROXY_TIMEOUT_MS", 800*time.Millisecond)

	client := &http.Client{
		Timeout: timeout,
		Transport: &http.Transport{
			MaxIdleConnsPerHost: 32,
			IdleConnTimeout:     90 * time.Second,
			// Dialer with small timeouts to fail fast inside cluster
			DialContext: (&net.Dialer{
				Timeout:   300 * time.Millisecond,
				KeepAlive: 30 * time.Second,
			}).DialContext,
		},
	}

	return func(next http.Handler) http.Handler {
		return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {

			// CORS preflight / safe methods that don't need routing without id.
			if r.Method == http.MethodOptions {
				next.ServeHTTP(w, r)
				return
			}

			cartId, ok := extractCartIdFromRequest(r)
			if !ok || cartId.String() == "" {
				// No cart id available -> cannot determine ownership; proceed locally.
				w.Header().Set("X-Cart-Owner-Routed", "false")
				next.ServeHTTP(w, r)
				return
			}

			owner := pool.OwnerHost(cartId)
			w.Header().Set("X-Cart-Id", cartId.String())
			w.Header().Set("X-Cart-Owner", owner)

			// Route locally if we're the owner or owner resolution empty.
			if owner == "" || owner == localHost {
				w.Header().Set("X-Cart-Owner-Routed", "false")
				next.ServeHTTP(w, r)
				return
			}

			// Proxy to remote owner
			proxyURL := buildProxyURL(r, owner, targetPort)
			bodyBuf, err := readBodyDuplicate(r)
			if err != nil {
				http.Error(w, "failed to read request body", http.StatusBadGateway)
				return
			}

			req, err := http.NewRequestWithContext(r.Context(), r.Method, proxyURL, bodyBuf)
			if err != nil {
				http.Error(w, "failed to create proxy request", http.StatusBadGateway)
				return
			}
			copyHeaders(req.Header, r.Header)
			// Ensure we don't forward hop-by-hop headers
			cleanHopHeaders(req.Header)
			req.Header.Set("X-Forwarded-For", appendForwardedFor(r))
			req.Header.Set("X-Forwarded-Host", r.Host)
			req.Header.Set("X-Forwarded-Proto", schemeFromRequest(r))
			req.Header.Set("X-Cart-Forwarded", "true")

			start := time.Now()
			resp, err := client.Do(req)
			if err != nil {
				if os.IsTimeout(err) || strings.Contains(err.Error(), "timeout") {
					http.Error(w, "gateway timeout contacting owner", http.StatusGatewayTimeout)
					return
				}
				http.Error(w, "upstream owner error", http.StatusBadGateway)
				return
			}
			defer resp.Body.Close()

			// Copy status + headers
			copyHeaders(w.Header(), resp.Header)
			w.Header().Set("X-Cart-Owner-Routed", "true")
			w.Header().Set("X-Cart-Owner-Latency-Ms", durationMs(time.Since(start)))
			w.WriteHeader(resp.StatusCode)
			io.Copy(w, resp.Body)
		})
	}
}

// (Removed duplicate OwnerHost method; single implementation now lives in synced-pool.go)

// extractCartIdFromRequest tries cookie first, then path form /byid/{id}/...
func extractCartIdFromRequest(r *http.Request) (CartId, bool) {
	// Cookie
	if c, err := r.Cookie("cartid"); err == nil && c.Value != "" {
		if cid, _, _, err2 := CanonicalizeOrLegacy(c.Value); err2 == nil {
			return CartIDToLegacy(cid), true
		}
	}
	// Path-based: locate "byid" segment
	parts := splitPath(r.URL.Path)
	for i := 0; i < len(parts); i++ {
		if parts[i] == "byid" && i+1 < len(parts) {
			raw := parts[i+1]
			if raw != "" {
				if cid, _, _, err := CanonicalizeOrLegacy(raw); err == nil {
					return CartIDToLegacy(cid), true
				}
			}
		}
	}
	var zero CartId
	return zero, false
}

// Helpers

func envOr(key, def string) string {
	if v := os.Getenv(key); v != "" {
		return v
	}
	return def
}

func envDurationOr(key string, def time.Duration) time.Duration {
	if v := os.Getenv(key); v != "" {
		if d, err := time.ParseDuration(v); err == nil {
			return d
		}
	}
	return def
}

func buildProxyURL(r *http.Request, host, port string) string {
	sb := &strings.Builder{}
	sb.WriteString("http://")
	sb.WriteString(host)
	if port != "" {
		sb.WriteString(":")
		sb.WriteString(port)
	}
	// Preserve original path & query (already includes /cart prefix stripped? depends on where middleware placed)
	sb.WriteString(r.URL.Path)
	if rq := r.URL.RawQuery; rq != "" {
		sb.WriteString("?")
		sb.WriteString(rq)
	}
	return sb.String()
}

func readBodyDuplicate(r *http.Request) (io.ReadCloser, error) {
	if r.Body == nil {
		return http.NoBody, nil
	}
	defer r.Body.Close()
	buf, err := io.ReadAll(r.Body)
	if err != nil {
		return nil, err
	}
	// Restore original for downstream if local (we only call when proxying, but safe)
	r.Body = io.NopCloser(bytes.NewReader(buf))
	return io.NopCloser(bytes.NewReader(buf)), nil
}

func copyHeaders(dst, src http.Header) {
	for k, vv := range src {
		// Skip hop-by-hop; they'll be cleaned anyway
		for _, v := range vv {
			dst.Add(k, v)
		}
	}
}

var hopHeaders = map[string]struct{}{
	"Connection":          {},
	"Proxy-Connection":    {},
	"Keep-Alive":          {},
	"Proxy-Authenticate":  {},
	"Proxy-Authorization": {},
	"Te":                  {},
	"Trailer":             {},
	"Transfer-Encoding":   {},
	"Upgrade":             {},
}

func cleanHopHeaders(h http.Header) {
	for k := range hopHeaders {
		h.Del(k)
	}
}

func appendForwardedFor(r *http.Request) string {
	host, _, _ := net.SplitHostPort(r.RemoteAddr)
	if host == "" {
		host = r.RemoteAddr
	}
	prior := r.Header.Get("X-Forwarded-For")
	if prior == "" {
		return host
	}
	return prior + ", " + host
}

func schemeFromRequest(r *http.Request) string {
	if r.TLS != nil {
		return "https"
	}
	if proto := r.Header.Get("X-Forwarded-Proto"); proto != "" {
		return proto
	}
	return "http"
}

func splitPath(p string) []string {
	if p == "" || p == "/" {
		return nil
	}
	trimmed := strings.TrimPrefix(p, "/")
	if trimmed == "" {
		return nil
	}
	parts := strings.Split(trimmed, "/")
	return parts
}

func durationMs(d time.Duration) string {
	return strconvFormatInt(int64(d / time.Millisecond))
}

// strconvFormatInt is a tiny helper to avoid importing strconv for one use.
func strconvFormatInt(i int64) string {
	// Fast int64 -> string (base 10) without strconv for small dependency surface.
	if i == 0 {
		return "0"
	}
	neg := i < 0
	if neg {
		i = -i
	}
	var buf [20]byte
	pos := len(buf)
	for i > 0 {
		pos--
		buf[pos] = byte('0' + (i % 10))
		i /= 10
	}
	if neg {
		pos--
		buf[pos] = '-'
	}
	return string(buf[pos:])
}