// Package middleware contains the HTTP layer that runs before handlers:
// panic recovery, logging, CORS, and Basic authentication with tenant
// resolution.
package middleware

import (
	"crypto/subtle"
	"encoding/json"
	"errors"
	"log/slog"
	"net/http"
	"strings"
	"time"

	"golang.org/x/crypto/bcrypt"

	"github.com/travelswitch/travelhub/backend/internal/auth"
	"github.com/travelswitch/travelhub/backend/internal/repository"
)

// TenantHeader names the header an SPA uses to say which agency it is signing
// in to. A subdomain is used as a fallback so the same build works when
// deployed as skyline.example.com.
const TenantHeader = "X-Tenant"

// Recoverer turns a panic into a 500 instead of killing the connection.
func Recoverer(logger *slog.Logger) func(http.Handler) http.Handler {
	return func(next http.Handler) http.Handler {
		return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
			defer func() {
				if rec := recover(); rec != nil {
					logger.Error("panic serving request",
						"method", r.Method, "path", r.URL.Path, "panic", rec)
					writeError(w, http.StatusInternalServerError, "internal server error")
				}
			}()
			next.ServeHTTP(w, r)
		})
	}
}

// statusRecorder remembers the status code so the logger can report it.
type statusRecorder struct {
	http.ResponseWriter
	status int
}

func (s *statusRecorder) WriteHeader(code int) {
	s.status = code
	s.ResponseWriter.WriteHeader(code)
}

// Logger writes one line per request.
func Logger(logger *slog.Logger) func(http.Handler) http.Handler {
	return func(next http.Handler) http.Handler {
		return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
			start := time.Now()
			rec := &statusRecorder{ResponseWriter: w, status: http.StatusOK}
			next.ServeHTTP(rec, r)
			logger.Info("request",
				"method", r.Method,
				"path", r.URL.Path,
				"status", rec.status,
				"tenant", r.Header.Get(TenantHeader),
				"duration", time.Since(start).String(),
			)
		})
	}
}

// CORS answers preflights and echoes back only origins on the allow list.
func CORS(allowed []string) func(http.Handler) http.Handler {
	allowSet := make(map[string]bool, len(allowed))
	for _, o := range allowed {
		allowSet[strings.TrimSuffix(o, "/")] = true
	}

	return func(next http.Handler) http.Handler {
		return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
			origin := strings.TrimSuffix(r.Header.Get("Origin"), "/")
			if origin != "" && allowSet[origin] {
				h := w.Header()
				h.Set("Access-Control-Allow-Origin", origin)
				h.Set("Access-Control-Allow-Credentials", "true")
				h.Set("Access-Control-Allow-Headers", "Authorization, Content-Type, "+TenantHeader)
				h.Set("Access-Control-Allow-Methods", "GET, POST, PATCH, DELETE, OPTIONS")
				h.Set("Access-Control-Max-Age", "600")
				// Responses differ per origin, so caches must key on it.
				h.Add("Vary", "Origin")
			}

			if r.Method == http.MethodOptions {
				w.WriteHeader(http.StatusNoContent)
				return
			}
			next.ServeHTTP(w, r)
		})
	}
}

// BasicAuth authenticates the caller and pins them to one tenant.
//
// Both halves matter: the password is checked against the user row *inside*
// the named tenant, so identical credentials at another agency will not
// authenticate here. The resolved tenant goes into the request context and is
// the only tenant id the handlers below ever use.
func BasicAuth(repo *repository.Repository, logger *slog.Logger) func(http.Handler) http.Handler {
	return func(next http.Handler) http.Handler {
		return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
			slug := tenantSlug(r)
			if slug == "" {
				writeError(w, http.StatusBadRequest,
					"missing tenant: send the "+TenantHeader+" header")
				return
			}

			email, password, ok := r.BasicAuth()
			if !ok {
				unauthorized(w)
				return
			}

			ctx := r.Context()
			tenant, err := repo.TenantBySlug(ctx, slug)
			if err != nil {
				if errors.Is(err, repository.ErrNotFound) {
					// Same response as a bad password: do not let an
					// unauthenticated caller enumerate which tenants exist.
					unauthorized(w)
					return
				}
				logger.Error("tenant lookup failed", "error", err)
				writeError(w, http.StatusInternalServerError, "internal server error")
				return
			}

			user, err := repo.UserByEmail(ctx, tenant.ID, email)
			if err != nil {
				if errors.Is(err, repository.ErrNotFound) {
					// Hash a throwaway value so a missing user costs the same
					// time as a wrong password, which stops an attacker
					// learning valid addresses from response timing.
					bcrypt.CompareHashAndPassword(dummyHash, []byte(password))
					unauthorized(w)
					return
				}
				logger.Error("user lookup failed", "error", err)
				writeError(w, http.StatusInternalServerError, "internal server error")
				return
			}

			if bcrypt.CompareHashAndPassword([]byte(user.PasswordHash), []byte(password)) != nil {
				unauthorized(w)
				return
			}

			// Defence in depth: the user row must belong to the tenant we
			// resolved. The query already guarantees it; this catches a future
			// refactor that breaks the invariant.
			if subtle.ConstantTimeCompare(user.TenantID[:], tenant.ID[:]) != 1 {
				unauthorized(w)
				return
			}

			ctx = auth.WithIdentity(ctx, auth.Identity{User: user, Tenant: tenant})
			next.ServeHTTP(w, r.WithContext(ctx))
		})
	}
}

// RequireAdmin gates routes that only a tenant administrator may call.
func RequireAdmin(next http.Handler) http.Handler {
	return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
		id, err := auth.FromContext(r.Context())
		if err != nil || !id.IsAdmin() {
			writeError(w, http.StatusForbidden, "administrator role required")
			return
		}
		next.ServeHTTP(w, r)
	})
}

// dummyHash is a valid bcrypt hash of a value nobody uses, for timing parity.
var dummyHash = []byte("$2a$10$N9qo8uLOickgx2ZMRZoMyeIjZAgcfl7p92ldGxad68LJZdL17lhWy")

// tenantSlug reads the tenant from the header, falling back to the first label
// of the Host name for subdomain-per-tenant deployments.
func tenantSlug(r *http.Request) string {
	if v := strings.TrimSpace(r.Header.Get(TenantHeader)); v != "" {
		return strings.ToLower(v)
	}

	host := r.Host
	if h, _, found := strings.Cut(host, ":"); found {
		host = h
	}
	labels := strings.Split(host, ".")
	if len(labels) >= 3 && labels[0] != "www" {
		return strings.ToLower(labels[0])
	}
	return ""
}

// unauthorized replies 401 without a WWW-Authenticate header on purpose: that
// header makes browsers open their own credential dialog, which would fight
// with the Angular sign-in screen.
func unauthorized(w http.ResponseWriter) {
	writeError(w, http.StatusUnauthorized, "invalid credentials for this tenant")
}

func writeError(w http.ResponseWriter, status int, message string) {
	w.Header().Set("Content-Type", "application/json")
	w.WriteHeader(status)
	json.NewEncoder(w).Encode(map[string]string{"error": message})
}

// Chain applies middleware so the first argument is the outermost layer.
func Chain(h http.Handler, mw ...func(http.Handler) http.Handler) http.Handler {
	for i := len(mw) - 1; i >= 0; i-- {
		h = mw[i](h)
	}
	return h
}