/*
 * Keeps the active playlist in memory between player-state requests.
 * State, command and discovery requests share a queue so responses cannot
 * overwrite newer state or reuse tracks from a different playlist version.
 */
export class PlayerStateClient {
  #sendRequest;
  #snapshot = null;
  #pending = Promise.resolve();

  // sendRequest performs the application's JSON GET/POST transport.
  constructor(sendRequest) {
    this.#sendRequest = sendRequest;
  }

  // Queues a state-producing request and returns a complete browser snapshot.
  // A failed request does not prevent subsequent requests from being sent.
  request(path, body) {
    const result = this.#pending.then(() => this.#read(path, body));
    this.#pending = result.catch(() => {});
    return result;
  }

  // Sends the cached version and restores omitted tracks from that exact version.
  async #read(path, body) {
    if (this.#snapshot) {
      path += `?playlistVersion=${encodeURIComponent(this.#snapshot.playlistVersion)}`;
    }
    const nextState = await this.#sendRequest(path, body);
    if (nextState.tracks === null) {
      if (!this.#snapshot || nextState.playlistVersion !== this.#snapshot.playlistVersion) {
        this.#snapshot = null;
        throw new Error("Playlist cache does not match the server version.");
      }
      nextState.tracks = this.#snapshot.tracks;
    }
    this.#snapshot = nextState;
    return nextState;
  }
}