interface Planet {
  name: string;
}

interface PlanetResult {
  count: number;
  next: string;
  results: Planet[];
}

@Injectable({
  providedIn: 'root'
})
export class PlanetService {
  constructor(private httpClient: HttpClient) {}

  getAllPlanets(): Observable<Planet[]> {
    return this.fetchPlanets().pipe(
      expand(result =>
        iif(() => !!result.next,
          this.fetchPlanets(result.next),
          EMPTY
        )
      ),
      pluck('results'),
      reduce((acc, planets) => acc.concat(planets), []),
    );
  }

  private fetchPlanets(nextUrl?: string): Observable<PlanetResult> {
    const startUrl = 'https://swapi.co/api/planets/';
    return this.httpClient.get<PlanetResult>(nextUrl || startUrl);
  }
}