// Package errors implements the unsupported feature enforcement layer for KeyCape.
// Every request passes through the Registry middleware before reaching any handler.
// If a registered feature is detected the middleware writes a ProfileError JSON
// response, emits an EventUnsupportedFeature telemetry event, and short-circuits
// the handler chain. Adding a new unsupported feature requires only a call to
// Register — no handler changes are needed.
package errors

import (
	"net/http"
	"strings"
	"time"

	profileerrors "keycape/internal/errors"
	"keycape/internal/server/telemetry"
)

// UnsupportedFeature describes a profile boundary that KeyCape enforces.
type UnsupportedFeature struct {
	// Name is a stable string identifier used in telemetry and error payloads.
	Name string
	// ErrorType is the profile error category emitted when this feature is triggered.
	ErrorType profileerrors.ErrorType
	// Description is a human-readable explanation of why the feature is blocked.
	Description string
	// Detector reports whether the given request triggers this feature.
	Detector func(r *http.Request) bool
}

// Registry holds all known unsupported features and exposes middleware that
// enforces them on every incoming request.
type Registry struct {
	features []UnsupportedFeature
}

// NewRegistry returns an empty Registry. Use Register to add features and
// DefaultRegistry to obtain one pre-populated with the spec-mandated set.
func NewRegistry() *Registry {
	return &Registry{}
}

// Register appends a feature to the registry. Registered features are checked
// in insertion order; the first match wins.
func (reg *Registry) Register(f UnsupportedFeature) {
	reg.features = append(reg.features, f)
}

// Middleware returns an http.Handler that evaluates all registered features
// for every request before delegating to next.
//
// If a feature is triggered:
//   - A ProfileError JSON response is written with an appropriate HTTP status.
//   - An EventUnsupportedFeature telemetry event is emitted via the Emitter
//     stored in the request context (a NoopEmitter is used when none is set).
//   - next is NOT called.
//
// If no feature matches, next is called normally.
func (reg *Registry) Middleware(next http.Handler) http.Handler {
	return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
		for _, f := range reg.features {
			if f.Detector(r) {
				pe := &profileerrors.ProfileError{
					Error:       f.ErrorType,
					Description: f.Description,
					Feature:     f.Name,
				}
				pe.Write(w, httpStatusFor(f.ErrorType))

				em := telemetry.EmitterFromContext(r.Context())
				em.Emit(r.Context(), telemetry.Event{
					Timestamp:   time.Now().UTC(),
					EventType:   telemetry.EventUnsupportedFeature,
					Feature:     f.Name,
					ErrorType:   string(f.ErrorType),
					Endpoint:    r.URL.Path,
					Result:      "failure",
					Environment: "",
					TraceID:     "",
					ClientID:    r.URL.Query().Get("client_id"),
				})
				return
			}
		}
		next.ServeHTTP(w, r)
	})
}

// httpStatusFor maps an ErrorType to its canonical HTTP status code.
func httpStatusFor(et profileerrors.ErrorType) int {
	switch et {
	case profileerrors.ErrInvalidProfileUsage:
		return http.StatusBadRequest
	case profileerrors.ErrRejectedForSafety:
		return http.StatusForbidden
	case profileerrors.ErrKeycloakModeOnly:
		return http.StatusNotImplemented
	default: // ErrFeatureNotSupported
		return http.StatusNotImplemented
	}
}

// ---------------------------------------------------------------------------
// Default feature set (spec §4 — normative).
// ---------------------------------------------------------------------------

// DefaultRegistry returns a Registry pre-populated with all spec-mandated
// unsupported features. No handler changes are required to enforce new entries.
func DefaultRegistry() *Registry {
	reg := NewRegistry()

	// 1. Dynamic client registration (RFC 7591) — not in the profile.
	reg.Register(UnsupportedFeature{
		Name:        "dynamic_client_registration",
		ErrorType:   profileerrors.ErrFeatureNotSupported,
		Description: "Dynamic client registration is not part of the NetKingdom IAM Profile. Register clients statically in KeyCape configuration.",
		Detector: func(r *http.Request) bool {
			return (r.Method == http.MethodPost && strings.HasSuffix(r.URL.Path, "/connect/register")) ||
				strings.Contains(r.URL.Path, "registration")
		},
	})

	// 2. Implicit flow — blocked for security.
	reg.Register(UnsupportedFeature{
		Name:        "implicit_flow",
		ErrorType:   profileerrors.ErrRejectedForSafety,
		Description: "The implicit flow (response_type=token or id_token) is rejected. Use the authorization code flow with PKCE.",
		Detector: func(r *http.Request) bool {
			rt := r.URL.Query().Get("response_type")
			if rt == "" {
				return false
			}
			// Blocked when response_type contains "token" or "id_token" but NOT when it is exactly "code".
			// "code token" (hybrid) is also blocked.
			return rt == "token" || rt == "id_token" || strings.Contains(rt, "token") && rt != "code"
		},
	})

	// 3. Wildcard redirect_uri — blocked for security.
	reg.Register(UnsupportedFeature{
		Name:        "wildcard_redirect_uri",
		ErrorType:   profileerrors.ErrRejectedForSafety,
		Description: "Wildcard redirect URIs are not permitted. Register exact redirect URIs in the client configuration.",
		Detector: func(r *http.Request) bool {
			return strings.Contains(r.URL.Query().Get("redirect_uri"), "*")
		},
	})

	// 4. Identity brokering — available only in Keycloak mode.
	reg.Register(UnsupportedFeature{
		Name:        "identity_broker",
		ErrorType:   profileerrors.ErrKeycloakModeOnly,
		Description: "Identity brokering is available only in expanded (Keycloak) mode.",
		Detector: func(r *http.Request) bool {
			return strings.Contains(r.URL.Path, "/broker/")
		},
	})

	// 5. PKCE plain method — blocked for security (must use S256).
	// Registered BEFORE missing_pkce so a plain-method request is reported
	// as pkce_plain_method, not missing_pkce.
	reg.Register(UnsupportedFeature{
		Name:        "pkce_plain_method",
		ErrorType:   profileerrors.ErrRejectedForSafety,
		Description: "PKCE plain code challenge method is not allowed. Use S256.",
		Detector: func(r *http.Request) bool {
			return r.URL.Query().Get("code_challenge_method") == "plain"
		},
	})

	// 6. Missing PKCE on /authorize — invalid profile usage.
	reg.Register(UnsupportedFeature{
		Name:        "missing_pkce",
		ErrorType:   profileerrors.ErrInvalidProfileUsage,
		Description: "Requests to /authorize must include a code_challenge (PKCE S256 required).",
		Detector: func(r *http.Request) bool {
			return strings.HasSuffix(r.URL.Path, "/authorize") &&
				r.URL.Query().Get("code_challenge") == ""
		},
	})

	// 7. Unknown grant type on /token.
	reg.Register(UnsupportedFeature{
		Name:        "unknown_grant_type",
		ErrorType:   profileerrors.ErrFeatureNotSupported,
		Description: "Only authorization_code and refresh_token grant types are supported.",
		Detector: func(r *http.Request) bool {
			if r.Method != http.MethodPost || !strings.HasSuffix(r.URL.Path, "/token") {
				return false
			}
			gt := r.URL.Query().Get("grant_type")
			if gt == "" {
				// Also check form body if already parsed — callers may pre-parse.
				gt = r.FormValue("grant_type")
			}
			if gt == "" {
				return false // no grant_type present; let the handler decide
			}
			return gt != "authorization_code" && gt != "refresh_token"
		},
	})

	return reg
}