import NextAuth, { type NextAuthResult } from "next-auth";
import Keycloak from "next-auth/providers/keycloak";
import type { JWT } from "next-auth/jwt";
import { redirect } from "next/navigation";

import type { CreateAuthOptions, SessionUser } from "./types";

/**
 * Refresh the Keycloak access token using the stored refresh_token. Returns
 * an updated JWT with new accessToken/expiresAt/refreshToken/idToken, or
 * the original JWT marked with `error: "RefreshAccessTokenError"` if the
 * refresh failed (caller surfaces this through the session so the client
 * can re-trigger sign-in).
 *
 * Pre-refresh skew is handled by the caller; this function unconditionally
 * exchanges the refresh_token.
 */
async function refreshAccessToken(
  token: JWT,
  keycloak: CreateAuthOptions["keycloak"],
): Promise<JWT> {
  const refreshToken = (token as { refreshToken?: string }).refreshToken;
  if (!refreshToken) {
    return { ...token, error: "RefreshTokenMissing" };
  }
  try {
    const res = await fetch(
      `${keycloak.issuer}/protocol/openid-connect/token`,
      {
        method: "POST",
        headers: { "Content-Type": "application/x-www-form-urlencoded" },
        body: new URLSearchParams({
          grant_type: "refresh_token",
          client_id: keycloak.clientId,
          client_secret: keycloak.clientSecret,
          refresh_token: refreshToken,
        }),
      },
    );
    const body = (await res.json()) as {
      access_token?: string;
      expires_in?: number;
      refresh_token?: string;
      id_token?: string;
      error?: string;
      error_description?: string;
    };
    if (!res.ok || !body.access_token) {
      console.error(
        "@gsc/web-kit: refresh_token exchange failed",
        res.status,
        body,
      );
      return { ...token, error: "RefreshAccessTokenError" };
    }
    return {
      ...token,
      accessToken: body.access_token,
      expiresAt:
        Math.floor(Date.now() / 1000) + (body.expires_in ?? 300),
      // Keycloak rotates refresh tokens by default; fall back to the
      // existing one when rotation is disabled at the realm.
      refreshToken: body.refresh_token ?? refreshToken,
      idToken: body.id_token ?? token.idToken,
      error: undefined,
    } as JWT;
  } catch (err) {
    console.error("@gsc/web-kit: refresh_token exchange threw", err);
    return { ...token, error: "RefreshAccessTokenError" };
  }
}

export type { SessionUser, CreateAuthOptions };

/**
 * Shape of the GET/POST route handlers NextAuth returns. Declared
 * structurally (Request → Response) rather than via
 * `NextAuthResult["handlers"]` — that type embeds `NextRequest` from
 * the kit's own copy of `next`, which conflicts with the consumer's
 * `next` and produces a spurious `RouteHandlerConfig` mismatch in
 * `.next/types/validator.ts` for every app's `[...nextauth]/route.ts`.
 *
 * Web-standard `Request`/`Response` are valid for Next.js route
 * handlers (NextRequest extends Request), and TS function-parameter
 * contravariance makes this assignable wherever the validator wants
 * `(request: NextRequest, ctx) => ...`.
 */
export type AuthRouteHandlers = {
  GET: (request: Request) => Promise<Response>;
  POST: (request: Request) => Promise<Response>;
};

/**
 * Result of `createAuth()`. Mirrors NextAuth's return value plus
 * `requireAuth` and the resolved `signInPath` so consumers don't have to
 * thread these around.
 */
export interface AuthBundle {
  handlers: AuthRouteHandlers;
  signIn: NextAuthResult["signIn"];
  signOut: NextAuthResult["signOut"];
  auth: NextAuthResult["auth"];

  /**
   * Server-only. Returns the current session user or redirects to the
   * sign-in path. Use as the first line of an RSC layout/page when the
   * route should be authenticated.
   */
  requireAuth: () => Promise<SessionUser>;

  /** Resolved sign-in URL; useful for client-side redirect helpers. */
  signInPath: string;
}

/**
 * One-line factory that wires NextAuth v5 to Keycloak with the canonical
 * GSC session shape.
 *
 * @example
 *   // src/auth.ts
 *   import { createAuth } from "@gsc/web-kit/auth/server";
 *   export const { handlers, signIn, signOut, auth, requireAuth } =
 *     createAuth({
 *       keycloak: {
 *         clientId: process.env.AUTH_KEYCLOAK_ID!,
 *         clientSecret: process.env.AUTH_KEYCLOAK_SECRET!,
 *         issuer: process.env.AUTH_KEYCLOAK_ISSUER!,
 *       },
 *     });
 */
/**
 * Decode the Keycloak access_token (JWS) and return its `realm_access.roles`.
 * Realm roles are only present in the access_token, never in the OIDC
 * userinfo `profile` — so callbacks that need to gate on roles must reach
 * into `account.access_token`, not `profile`. We don't verify the signature
 * here: NextAuth has already verified the ID token via the OIDC flow, the
 * access_token came back over the same TLS exchange, and the only use of
 * the decoded roles is local sign-in policy.
 */
function rolesFromAccessToken(accessToken: string | undefined): string[] {
  if (!accessToken) return [];
  const parts = accessToken.split(".");
  if (parts.length < 2) return [];
  try {
    const payload = parts[1];
    const padded = payload + "=".repeat((4 - (payload.length % 4)) % 4);
    const decoded = Buffer.from(
      padded.replace(/-/g, "+").replace(/_/g, "/"),
      "base64",
    ).toString("utf-8");
    const claims = JSON.parse(decoded) as {
      realm_access?: { roles?: string[] };
    };
    return claims.realm_access?.roles ?? [];
  } catch {
    return [];
  }
}

export function createAuth(opts: CreateAuthOptions): AuthBundle {
  // NextAuth v5: provider-specific paths like /api/auth/signin/keycloak
  // are POST-only (CSRF-protected form submit). A GET redirect there
  // bounces to /api/auth/error?error=Configuration ("UnknownAction").
  // /api/auth/signin (no provider) is the GET-accessible page that
  // lists configured providers. Apps wanting one-click Keycloak can
  // override signInPath with a custom page that calls signIn('keycloak').
  const signInPath = opts.signInPath ?? "/api/auth/signin";

  const na = NextAuth({
    providers: [
      Keycloak({
        clientId: opts.keycloak.clientId,
        clientSecret: opts.keycloak.clientSecret,
        issuer: opts.keycloak.issuer,
      }),
    ],
    trustHost: true,
    pages: opts.pages,
    callbacks: {
      async signIn({ account }) {
        // Per-client access gate. The kit's consumers pass a predicate
        // (e.g. "must be in gsccrm_* group") via `requireClientRole`; we
        // run it against the access_token's `realm_access.roles`. Realm
        // roles are NOT present in the OIDC userinfo `profile`, only in
        // the access_token — so we decode it. Return false → NextAuth
        // refuses to mint a session and bounces the user to the configured
        // error page with `?error=AccessDenied`.
        if (!opts.requireClientRole) return true;
        const roles = rolesFromAccessToken(account?.access_token);
        return opts.requireClientRole(roles);
      },
      async jwt({ token, account, profile }) {
        // Initial sign-in: capture identity claims + refresh material.
        if (account && profile) {
          // Realm convention: GSC identity ships as three claims sourced
          // from FreeIPA user attributes via the gosecCloud realm's LDAP
          // federation + per-client OIDC protocol mappers.
          const p = profile as {
            gscTenantId?: string;
            gscCustomerId?: string;
            gscSID?: string;
          };
          if (!p.gscTenantId || !p.gscCustomerId || !p.gscSID) {
            throw new Error(
              `Keycloak token missing required GSC identity claims (gscTenantId=${!!p.gscTenantId}, gscCustomerId=${!!p.gscCustomerId}, gscSID=${!!p.gscSID})`,
            );
          }
          token.keycloakId = profile.sub;
          token.tenantId = p.gscTenantId;
          token.customerId = p.gscCustomerId;
          token.gscSid = p.gscSID;
          token.displayName =
            profile.name ??
            (profile as { preferred_username?: string }).preferred_username ??
            "";
          token.givenName = profile.given_name ?? "";
          token.familyName = profile.family_name ?? "";
          // Realm roles are in the access_token, not the OIDC profile.
          token.roles = rolesFromAccessToken(account.access_token);
          token.accessToken = account.access_token;
          token.idToken = account.id_token;
          token.refreshToken = account.refresh_token;
          // `expires_at` is Unix seconds (NextAuth's normalized form);
          // fall back to `expires_in` (relative) if the provider omitted it.
          token.expiresAt =
            (account.expires_at as number | undefined) ??
            Math.floor(Date.now() / 1000) +
              ((account.expires_in as number | undefined) ?? 300);
          return token;
        }

        // Subsequent calls: refresh if the access token is about to expire.
        // Skew of 30s avoids handing out a token that will time out mid-flight.
        const expiresAt = (token as { expiresAt?: number }).expiresAt ?? 0;
        if (Math.floor(Date.now() / 1000) < expiresAt - 30) {
          return token;
        }
        return refreshAccessToken(token, opts.keycloak);
      },
      async session({ session, token }) {
        if (!token) return session;
        const user: SessionUser = {
          id: token.sub as string,
          keycloakId: (token.keycloakId as string) ?? (token.sub as string),
          tenantId: token.tenantId as string,
          customerId: token.customerId as string,
          gscSid: token.gscSid as string,
          email: session.user?.email ?? "",
          displayName: (token.displayName as string) ?? "",
          givenName: (token.givenName as string) ?? "",
          familyName: (token.familyName as string) ?? "",
          roles: (token.roles as string[]) ?? [],
          accessToken: (token.accessToken as string) ?? "",
          idToken: token.idToken as string | undefined,
          error: (token as { error?: string }).error,
        };
        // NextAuth's default `session.user` type is narrow; we replace
        // it wholesale with our canonical shape. The cast keeps TS quiet
        // while preserving the runtime payload.
        (session as unknown as { user: SessionUser }).user = user;
        return session;
      },
    },
  });

  async function requireAuth(): Promise<SessionUser> {
    const session = await na.auth();
    const user = (session as unknown as { user?: SessionUser } | null)?.user;
    if (!user || !user.accessToken) {
      redirect(signInPath);
    }
    return user;
  }

  return {
    // Cast through unknown: NextAuth's handler param type references its
    // own NextRequest; the AuthRouteHandlers shape is structurally
    // compatible (NextRequest extends Request), but TS can't prove the
    // function-parameter contravariance across the two `next` copies on
    // its own.
    handlers: na.handlers as unknown as AuthRouteHandlers,
    signIn: na.signIn,
    signOut: na.signOut,
    auth: na.auth,
    requireAuth,
    signInPath,
  };
}