import {Credentials} from './credentials';
import {Headers, OAuth2Client, RequestMetadataResponse} from './oauth2client';

export interface IdTokenOptions {
  /**
   * The client to make the request to fetch an ID token.
   */
  idTokenProvider: IdTokenProvider;
  /**
   * The audience to use when requesting an ID token.
   */
  targetAudience: string;
}

export interface IdTokenProvider {
  fetchIdToken: (targetAudience: string) => Promise<string>;
}

export class IdTokenClient extends OAuth2Client {
  targetAudience: string;
  idTokenProvider: IdTokenProvider;

  /**
   * Google ID Token client
   *
   * Retrieve access token from the metadata server.
   * See: https://developers.google.com/compute/docs/authentication
   */
  constructor(options: IdTokenOptions) {
    super();
    this.targetAudience = options.targetAudience;
    this.idTokenProvider = options.idTokenProvider;
  }

  protected async getRequestMetadataAsync(
    // eslint-disable-next-line @typescript-eslint/no-unused-vars
    url?: string | null
  ): Promise<RequestMetadataResponse> {
    if (
      !this.credentials.id_token ||
      (this.credentials.expiry_date || 0) < Date.now()
    ) {
      const idToken = await this.idTokenProvider.fetchIdToken(
        this.targetAudience
      );
      this.credentials = {
        id_token: idToken,
        expiry_date: this.getIdTokenExpiryDate(idToken),
      } as Credentials;
    }

    const headers: Headers = {
      Authorization: 'Bearer ' + this.credentials.id_token,
    };
    return {headers};
  }

  private getIdTokenExpiryDate(idToken: string): number | void {
    const payloadB64 = idToken.split('.')[1];
    if (payloadB64) {
      const payload = JSON.parse(
        Buffer.from(payloadB64, 'base64').toString('ascii')
      );
      return payload.exp * 1000;
    }
  }
}