/**
   * Return the last valid auction id (ignoring FIRST_AUCTION_ID). Will be the current auction id if
   * due to generate an auction.
   */
  public static long lastBase0AuctionId(long eventId) {
    long epoch = eventId / GeneratorConfig.PROPORTION_DENOMINATOR;
    long offset = eventId % GeneratorConfig.PROPORTION_DENOMINATOR;
    if (offset < GeneratorConfig.PERSON_PROPORTION) {
      // About to generate a person.
      // Go back to the last auction in the last epoch.
      epoch--;
      offset = GeneratorConfig.AUCTION_PROPORTION - 1;
    } else if (offset >= GeneratorConfig.PERSON_PROPORTION + GeneratorConfig.AUCTION_PROPORTION) {
      // About to generate a bid.
      // Go back to the last auction generated in this epoch.
      offset = GeneratorConfig.AUCTION_PROPORTION - 1;
    } else {
      // About to generate an auction.
      offset -= GeneratorConfig.PERSON_PROPORTION;
    }
    return epoch * GeneratorConfig.AUCTION_PROPORTION + offset;
  }

  /**
   * Return a random auction id (base 0).
   */
  public static long nextBase0AuctionId(
      long nextEventId, Random random, GeneratorConfig config) {

    // Choose a random auction for any of those which are likely to still be in flight,
    // plus a few 'leads'.
    // Note that ideally we'd track non-expired auctions exactly, but that state
    // is difficult to split.
    long minAuction = Math.max(
        lastBase0AuctionId(nextEventId) - config.getNumInFlightAuctions(), 0);
    long maxAuction = lastBase0AuctionId(nextEventId);
    return minAuction + nextLong(random, maxAuction - minAuction + 1 + AUCTION_ID_LEAD);
  }