package oidc

import (
	"crypto"
	"crypto/rand"
	"crypto/rsa"
	"crypto/sha256"
	"crypto/subtle"
	"encoding/base64"
	"encoding/json"
	"fmt"
	"net/http"
	"net/url"
	"strings"
	"time"

	"keycape/internal/adapters/tenantengine"
	"keycape/internal/domain"
	profileerrors "keycape/internal/errors"
	"keycape/internal/server/telemetry"
)

// TokenHandler implements POST /token.
type TokenHandler struct {
	ClientConfig  map[string]*domain.Client
	Sessions      *SessionStore
	Users         domain.UserRepository
	SigningKey    *rsa.PrivateKey
	Issuer        string
	TokenLifetime time.Duration
	Emitter       telemetry.Emitter
	// TenantEngine sources the optional tenant_roles claim (KEY-WP-0005-T02).
	// Nil disables it entirely -- token issuance never depends on it.
	TenantEngine *tenantengine.Client
}

// tokenResponse is the JSON body returned on a successful token exchange.
type tokenResponse struct {
	AccessToken string `json:"access_token"`
	TokenType   string `json:"token_type"`
	ExpiresIn   int    `json:"expires_in"`
	IDToken     string `json:"id_token,omitempty"`
}

// ServeHTTP handles POST /token.
func (h *TokenHandler) ServeHTTP(w http.ResponseWriter, r *http.Request) {
	ctx := r.Context()

	if err := r.ParseForm(); err != nil {
		http.Error(w, "invalid form body", http.StatusBadRequest)
		return
	}

	grantType := r.FormValue("grant_type")
	if grantType == "client_credentials" {
		h.serveClientCredentials(w, r)
		return
	}
	clientID := r.FormValue("client_id")
	code := r.FormValue("code")
	codeVerifier := r.FormValue("code_verifier")

	// 1. Validate grant_type.
	if grantType != "authorization_code" {
		profileerrors.FeatureNotSupported(
			"only grant_type=authorization_code is supported",
			"grant_type="+grantType,
		).Write(w, http.StatusBadRequest)
		return
	}

	// 2. Validate client exists and may use this grant.
	client, ok := h.ClientConfig[clientID]
	if !ok {
		profileerrors.InvalidProfileUsage("unknown client_id", "client_id").
			Write(w, http.StatusBadRequest)
		return
	}

	// Grant-type eligibility, enforced equivalently to the service path
	// (KEY-WP-0016-T02). An empty grantTypes is an implicit authorization-code
	// client, matching config validation; a client_credentials-only client must
	// not reach the browser path.
	if len(client.GrantTypes) > 0 && !containsString(client.GrantTypes, "authorization_code") {
		profileerrors.InvalidProfileUsage(
			"client is not registered for grant_type=authorization_code",
			"grant_type",
		).Write(w, http.StatusBadRequest)
		return
	}

	// Confidential authorization-code clients authenticate with their secret,
	// using the same credential sources as the service grant. A public client
	// must not be able to present a secret and be treated as authenticated.
	if client.ClientType == "confidential" {
		presentedID, secret, ok := basicClientCredentials(r)
		if !ok || presentedID != clientID || client.ClientSecret == "" ||
			!secretsEqual(secret, client.ClientSecret) {
			profileerrors.InvalidProfileUsage(
				"client authentication failed",
				"Authorization",
			).Write(w, http.StatusUnauthorized)
			return
		}
	}

	// 3. Consume the PKCE session. Single-use and atomic: see
	// SessionStore.Consume (KEY-WP-0016-T01).
	sess, ok := h.Sessions.Consume(code)
	if !ok {
		profileerrors.InvalidProfileUsage(
			"authorization code not found or expired",
			"code",
		).Write(w, http.StatusBadRequest)
		return
	}

	// Verify client_id matches the session.
	if sess.ClientID != clientID {
		profileerrors.InvalidProfileUsage(
			"client_id does not match the authorization code",
			"client_id",
		).Write(w, http.StatusBadRequest)
		return
	}

	// Bind the exchange to the redirect URI the code was issued for
	// (RFC 6749 section 4.1.3, KEY-WP-0016-T02). /authorize always records an
	// exactly-matched registered redirect, so the parameter is always required
	// here and must be identical.
	if redirectURI := r.FormValue("redirect_uri"); redirectURI != sess.RedirectURI {
		profileerrors.InvalidProfileUsage(
			"redirect_uri does not match the authorization request",
			"redirect_uri",
		).Write(w, http.StatusBadRequest)
		return
	}

	// Recheck grants in case the client registration changed after authorization.
	for _, scope := range sess.Scopes {
		if !containsString(h.ClientConfig[clientID].AllowedScopes, scope) {
			profileerrors.InvalidProfileUsage("requested scope is not allowed", "scope").Write(w, http.StatusBadRequest)
			return
		}
	}

	// 4. Verify PKCE code_verifier.
	if !verifyPKCE(codeVerifier, sess.PKCEChallenge) {
		profileerrors.InvalidProfileUsage(
			"code_verifier does not match code_challenge",
			"code_verifier",
		).Write(w, http.StatusBadRequest)
		return
	}

	// 5. Look up user.
	user, err := h.Users.LookupUser(ctx, sess.Username)
	if err != nil {
		http.Error(w, "user not found", http.StatusInternalServerError)
		return
	}
	if isSuspended(user) {
		profileerrors.RejectedForSafety(
			"account is suspended",
			"account_lifecycle",
		).Write(w, http.StatusForbidden)
		return
	}

	// 6. Build JWT claims.
	now := time.Now()
	exp := now.Add(h.TokenLifetime)

	claims := map[string]interface{}{
		"iss": h.Issuer,
		"sub": user.ID,
		"aud": clientID,
		"exp": exp.Unix(),
		"iat": now.Unix(),
	}
	if sess.Nonce != "" {
		claims["nonce"] = sess.Nonce
	}

	scopeSet := make(map[string]bool)
	for _, s := range sess.Scopes {
		scopeSet[s] = true
	}

	if scopeSet["profile"] {
		claims["preferred_username"] = user.Username
	}
	if scopeSet["email"] {
		claims["email"] = user.Email
	}

	// Core claims required by net-kingdom/canon/standards/iam-profile_v0.3.md
	// for every production token -- not scope-gated, unlike the recommended
	// human claims above (KEY-WP-0005-T01).
	tenant, tenantSource, err := humanTenant(h.ClientConfig[clientID], user)
	if err != nil {
		profileerrors.RejectedForSafety(
			"tenant binding conflict",
			"tenant_binding",
		).Write(w, http.StatusForbidden)
		return
	}
	claims["tenant"] = tenant
	claims["tenant_source"] = tenantSource
	claims["principal_type"] = "human"
	claims["groups"] = nonNilStrings(user.Groups)
	claims["roles"] = nonNilStrings(user.Roles)
	claims["assurance"] = assuranceClaim(sess.MFAVerified, sess.AuthTime, now)

	// Optional cached tenant_roles claim (KEY-WP-0005-T02). Fails open --
	// see internal/adapters/tenantengine's package doc for why this is the
	// one place in the whole tenant_roles design where that's correct.
	if roles, ok := h.TenantEngine.Roles(ctx, tenant); ok {
		claims["tenant_roles"] = roles
	}

	// 7. Sign JWT with RSA-SHA256.
	kid := "key-1" // static kid for v0.1
	jwtToken, err := buildJWT(claims, kid, h.SigningKey)
	if err != nil {
		http.Error(w, "failed to build JWT", http.StatusInternalServerError)
		return
	}

	// Access tokens target the statically registered resource server. ID tokens
	// remain bound to the OIDC relying party.
	if audience := h.ClientConfig[clientID].Audience; audience != "" {
		claims["aud"] = audience
	}
	claims["scope"] = strings.Join(sess.Scopes, " ")
	accessToken, err := buildJWT(claims, kid, h.SigningKey)
	if err != nil {
		http.Error(w, "failed to build JWT", http.StatusInternalServerError)
		return
	}

	// 8. Build response. The session was already consumed at lookup, so no
	// separate replay-prevention delete is needed here.
	resp := tokenResponse{
		AccessToken: accessToken,
		TokenType:   "Bearer",
		ExpiresIn:   int(h.TokenLifetime.Seconds()),
		IDToken:     jwtToken,
	}

	// 10. Emit token_issued telemetry.
	h.Emitter.Emit(ctx, telemetry.Event{
		Timestamp: time.Now(),
		EventType: telemetry.EventTokenIssued,
		ClientID:  clientID,
		Endpoint:  "/token",
		Result:    "success",
		Scopes:    sess.Scopes,
		GrantType: grantType,
	})

	w.Header().Set("Content-Type", "application/json")
	w.WriteHeader(http.StatusOK)
	_ = json.NewEncoder(w).Encode(resp)
}

// basicClientCredentials reads client_secret_basic credentials, applying the
// form-encoding decode RFC 6749 appendix B requires of both halves. Shared by
// the service grant and confidential authorization-code client authentication.
func basicClientCredentials(r *http.Request) (clientID, clientSecret string, ok bool) {
	clientID, clientSecret, ok = r.BasicAuth()
	if !ok {
		return "", "", false
	}
	decodedID, idErr := url.QueryUnescape(clientID)
	decodedSecret, secretErr := url.QueryUnescape(clientSecret)
	if idErr != nil || secretErr != nil {
		return "", "", false
	}
	return decodedID, decodedSecret, true
}

// secretsEqual compares two secrets in constant time. Digesting first keeps the
// comparison length-independent, so a wrong-length secret is indistinguishable
// from a wrong-value one.
func secretsEqual(presented, expected string) bool {
	presentedDigest := sha256.Sum256([]byte(presented))
	expectedDigest := sha256.Sum256([]byte(expected))
	return subtle.ConstantTimeCompare(presentedDigest[:], expectedDigest[:]) == 1
}

func (h *TokenHandler) serveClientCredentials(w http.ResponseWriter, r *http.Request) {
	ctx := r.Context()
	clientID, clientSecret, ok := r.BasicAuth()
	if !ok {
		profileerrors.InvalidProfileUsage("client_secret_basic authentication required", "Authorization").
			Write(w, http.StatusUnauthorized)
		return
	}
	clientID, idErr := url.QueryUnescape(clientID)
	clientSecret, secretErr := url.QueryUnescape(clientSecret)
	if idErr != nil || secretErr != nil {
		profileerrors.InvalidProfileUsage("invalid client authentication encoding", "Authorization").Write(w, http.StatusUnauthorized)
		return
	}
	client, ok := h.ClientConfig[clientID]
	if !ok || client.ClientType != "confidential" || !containsString(client.GrantTypes, "client_credentials") {
		profileerrors.InvalidProfileUsage("invalid confidential client", "client_id").
			Write(w, http.StatusUnauthorized)
		return
	}
	presentedDigest := sha256.Sum256([]byte(clientSecret))
	expectedDigest := sha256.Sum256([]byte(client.ClientSecret))
	if client.ClientSecret == "" ||
		subtle.ConstantTimeCompare(presentedDigest[:], expectedDigest[:]) != 1 {
		profileerrors.InvalidProfileUsage("invalid client authentication", "Authorization").
			Write(w, http.StatusUnauthorized)
		return
	}

	requestedScopes := strings.Fields(r.FormValue("scope"))
	if len(requestedScopes) == 0 {
		requestedScopes = append([]string(nil), client.AllowedScopes...)
	}
	for _, scope := range requestedScopes {
		if !containsString(client.AllowedScopes, scope) {
			profileerrors.InvalidProfileUsage("requested scope is not allowed", "scope").
				Write(w, http.StatusBadRequest)
			return
		}
	}

	now := time.Now()
	tokenLifetime := h.TokenLifetime
	if client.TokenLifetime > 0 {
		tokenLifetime = client.TokenLifetime
	}
	claims := map[string]interface{}{
		"iss":    h.Issuer,
		"sub":    client.ServiceSubject,
		"aud":    accessAudience(client),
		"exp":    now.Add(tokenLifetime).Unix(),
		"iat":    now.Unix(),
		"tenant": client.Tenant,
		// A service client's tenant is always registration-supplied: there is no
		// directory principal behind it to assert one (GH-DEC-2026-013 §5).
		"tenant_source":  TenantSourceRegistration,
		"principal_type": "service",
		"groups":         []string{},
		"roles":          nonNilStrings(client.Roles),
		"scope":          strings.Join(requestedScopes, " "),
		"assurance": map[string]interface{}{
			"level": "aal1", "methods": []string{"client_secret"},
			"mfa": false, "source": "key-cape", "at": now.Unix(),
		},
	}
	if roles, ok := h.TenantEngine.Roles(ctx, client.Tenant); ok {
		claims["tenant_roles"] = roles
	}
	jwtToken, err := buildJWT(claims, "key-1", h.SigningKey)
	if err != nil {
		http.Error(w, "failed to build JWT", http.StatusInternalServerError)
		return
	}
	h.Emitter.Emit(ctx, telemetry.Event{
		Timestamp: now, EventType: telemetry.EventTokenIssued, ClientID: clientID,
		Endpoint: "/token", Result: "success", Scopes: requestedScopes,
		GrantType: "client_credentials",
	})
	w.Header().Set("Content-Type", "application/json")
	w.WriteHeader(http.StatusOK)
	_ = json.NewEncoder(w).Encode(tokenResponse{
		AccessToken: jwtToken, TokenType: "Bearer",
		ExpiresIn: int(tokenLifetime.Seconds()),
	})
}

func containsString(values []string, wanted string) bool {
	for _, value := range values {
		if value == wanted {
			return true
		}
	}
	return false
}

func isSuspended(user *domain.User) bool {
	return containsString(user.Groups, "netkingdom-suspended")
}

// ---------------------------------------------------------------------------
// IAM Profile core claims (KEY-WP-0005-T01)
// ---------------------------------------------------------------------------

// defaultTenant is the fallback tenant claim for users with no explicit
// Tenant assignment yet. This workstation currently operates a single
// tenant (tenant:coulomb, ADR-0006); later tenants (e.g. tenant:friendly:binky,
// ADR-0013) require an explicit domain.User.Tenant value -- this default
// never silently assigns a user to a tenant other than the platform's
// original one.
const defaultTenant = "tenant:coulomb"

// effectiveTenant resolves the tenant claim for a user, falling back to
// defaultTenant when the user has no explicit tenant assignment. The IAM
// Profile requires a non-empty tenant claim on every token.
func effectiveTenant(user *domain.User) string {
	if user.Tenant != "" {
		return user.Tenant
	}
	return defaultTenant
}

// humanTenant resolves the tenant claim for a human token (KEY-WP-0013-T05).
//
// A human's tenant is normally a property of the person, read from the
// directory. But the approval chain is bound to the landlord zone by decision
// 5ed3fb35-eca9-413a-82b9-95171ba85bf6, and approval-engine compares the claim
// by exact string equality, so an approver client has to be able to state the
// zone it issues into. Without this every human token fell back to
// defaultTenant and would have been refused downstream -- as a failed approval
// rather than as a registration defect.
//
// The rule is deliberately not an override:
//
//   - no client tenant declared  -> the directory answer, unchanged;
//   - declared, user unassigned  -> the declared zone;
//   - declared and equal         -> agreement, no ambiguity;
//   - declared and different     -> refuse to issue.
//
// So a registration can bind a zone for users the directory has not placed, and
// can never relabel a user the directory HAS placed into a different one. That
// last case fails closed rather than picking a winner, because either answer
// would be a silent cross-tenant assertion. It also means this stays correct if
// the directory later populates Tenant: the same code turns from supplying the
// zone into enforcing agreement with it, with no second migration.
//
// This is only safe because client registrations are static and
// deployment-owned; KeyCape excludes dynamic client registration by design. A
// self-service client that could name its users' tenant would be an escalation.
func humanTenant(client *domain.Client, user *domain.User) (string, string, error) {
	if client == nil || client.Tenant == "" {
		if user.Tenant != "" {
			return user.Tenant, TenantSourceDirectory, nil
		}
		return defaultTenant, TenantSourceDefault, nil
	}
	if user.Tenant != "" {
		if user.Tenant != client.Tenant {
			return "", "", fmt.Errorf("client %q binds tenant %q but the directory assigns this user a different tenant", client.ClientID, client.Tenant)
		}
		// Agreement: the directory did assert this about the person, so the
		// stronger provenance is the true one.
		return user.Tenant, TenantSourceDirectory, nil
	}
	return client.Tenant, TenantSourceRegistration, nil
}

// Tenant provenance values for the tenant_source claim (GH-DEC-2026-013 §5).
//
// A bare tenant string cannot tell a consumer whether the identity layer
// asserted the zone about this PERSON or a registration supplied it about the
// CLIENT they came through. approval-engine admits an approver by exact-matching
// that string while its contract reads as though it relies on the first, so the
// check is sound and the property a reader infers from it is absent. The ruling
// requires the claim to carry its provenance and forbids a consumer treating the
// two as equivalent for any decision turning on a fact about the person. It names
// the property; the field is ours.
//
// Three values, not the two the ruling names, and the third is the point. A
// tenant nobody asserted -- neither directory nor registration, just the
// profile's non-empty default -- is not directory-asserted, and labelling it so
// would reintroduce the same defect one level down: a consumer would read
// "directory" for a fact the directory never stated. That is the unknown-versus-
// absent distinction the ruling cites from GH-DEC-2026-011 §3, applied to our own
// fallback rather than only to the case we were asked about.
const (
	// TenantSourceDirectory: the identity layer asserted this zone about this
	// person. Includes the agreement case, where a registration declared the
	// same zone the directory did -- the directory still asserted it.
	TenantSourceDirectory = "directory"

	// TenantSourceRegistration: supplied by the client registration for a person
	// the directory has placed nowhere. A fact about the client, not the person.
	TenantSourceRegistration = "registration"

	// TenantSourceDefault: nobody asserted a zone; this is the profile default,
	// emitted because the profile requires a non-empty tenant. Weaker than both.
	TenantSourceDefault = "default"
)

// nonNilStrings returns s, or an empty (non-nil) slice if s is nil, so the
// claim always serializes as `[]`, never `null` -- the profile requires
// groups/roles to be present, "possibly empty", not absent.
func nonNilStrings(s []string) []string {
	if s == nil {
		return []string{}
	}
	return s
}

// assuranceClaim builds the profile's `assurance` object from whether MFA
// was actually verified during this authorization (session.MFAVerified),
// not from static enrollment state -- a user who has MFA enrolled but
// wasn't challenged for it in this particular flow gets aal1, not aal2.
//
// `at` is the time the user authenticated, not the time this token was
// minted. Those differ whenever a browser session is reused, and the gap is
// the whole point: approval-engine stores this object verbatim as the only
// downstream record that MFA occurred (KEY-WP-0013-T05), so mint time would
// overstate how recently the person proved anything. issuedAt is the
// fallback for a session predating this field.
func assuranceClaim(mfaVerified bool, authTime, issuedAt time.Time) map[string]interface{} {
	at := authTime
	if at.IsZero() {
		at = issuedAt
	}
	level := "aal1"
	methods := []string{"pwd"}
	if mfaVerified {
		level = "aal2"
		methods = append(methods, "otp")
	}
	return map[string]interface{}{
		"level":   level,
		"methods": methods,
		"mfa":     mfaVerified,
		"source":  "key-cape",
		"at":      at.Unix(),
	}
}

// ---------------------------------------------------------------------------
// PKCE verification
// ---------------------------------------------------------------------------

// verifyPKCE checks BASE64URL(SHA256(verifier)) == challenge (S256 method).
func verifyPKCE(verifier, challenge string) bool {
	h := sha256.New()
	h.Write([]byte(verifier))
	computed := base64.RawURLEncoding.EncodeToString(h.Sum(nil))
	return computed == challenge
}

// ---------------------------------------------------------------------------
// JWT construction (stdlib only — no external JWT library)
// ---------------------------------------------------------------------------

type jwtHeader struct {
	Alg string `json:"alg"`
	Typ string `json:"typ"`
	Kid string `json:"kid"`
}

// buildJWT constructs and signs a JWT using RSA-SHA256 with the standard library.
// Format: base64url(header) + "." + base64url(payload) + "." + base64url(signature)
func buildJWT(claims map[string]interface{}, kid string, key *rsa.PrivateKey) (string, error) {
	// Header.
	hdr := jwtHeader{Alg: "RS256", Typ: "JWT", Kid: kid}
	hdrJSON, err := json.Marshal(hdr)
	if err != nil {
		return "", err
	}
	hdrB64 := base64.RawURLEncoding.EncodeToString(hdrJSON)

	// Payload.
	payloadJSON, err := json.Marshal(claims)
	if err != nil {
		return "", err
	}
	payloadB64 := base64.RawURLEncoding.EncodeToString(payloadJSON)

	// Signing input.
	signingInput := hdrB64 + "." + payloadB64

	// Digest.
	digest := sha256.Sum256([]byte(signingInput))

	// Sign with PKCS1v15 / SHA256.
	sig, err := rsa.SignPKCS1v15(rand.Reader, key, crypto.SHA256, digest[:])
	if err != nil {
		return "", err
	}
	sigB64 := base64.RawURLEncoding.EncodeToString(sig)

	return strings.Join([]string{hdrB64, payloadB64, sigB64}, "."), nil
}

// accessAudience is configured by the issuer, never selected by request input.
func accessAudience(client *domain.Client) string {
	if client.Audience != "" {
		return client.Audience
	}
	return client.ClientID
}