public interface SlotMachine {

  /**
   * Insert the coin into the slot machine. If the coin is unknown,
   * return directly.
   *
   * @param coin the coin to insert
   * @return the unknown coin
   */
  Optional<Coin> insert(Coin coin);

  /**
   * Select the drink to buy. If the value of the current inserted coins
   * is larger than the slot's price and the slot still has drinks, it
   * returns the drink; otherwise {@code null}.
   *
   * @param index the slot index
   * @return the drink if available
   */
  Optional<Drink> select(int index);

  /**
   * Refund the unused fee.
   *
   * @return the unused fee in coins
   */
  Coin[] refund();

  /**
   * Get the value (not the amount) of the inserted coins. For example,
   * the value of two 10 dollar coins is 20, not 2 (amount).
   *
   * @return the inserted fee
   */
  int getInsertedFee();

  /**
   * Get the amount of the slots provided by the machine.
   *
   * @return the amounts of the slots
   */
  int getSolts();

  /**
   * Get the slot information of the specified index.
   *
   * @param index the slot index
   * @return the slot information if the index is correct
   */
  Optional<SlotInfo> getSlot(int index);
}