Wallet

package com.pharmeasy.demo;

import java.util.ArrayList;
import java.util.Collection;
import java.util.List;
import org.springframework.transaction.annotation.Transactional;

public class DemoApplication {

  public static void main(String[] args) {
    Wallet walletAccount = new WalletAccount();
    String input = "1,100,200,200,1000,1";
    walletAccount.load(input);
    System.out.println("Balance :" + walletAccount.getBalance());
    System.out.println("Coins :" + walletAccount.peek());

    walletAccount.spend(30L);
    System.out.println("Balance :" + walletAccount.getBalance());
    System.out.println("Coins :" + walletAccount.peek());
  }

}

interface Wallet {

  /**
   * Load money into my wallet
   */
  void load(String coins);

  /**
   * Return current balance of my wallet
   */
  Long getBalance();

  /**
   * Return the coins in the wallet. (peek does NOT need to be performant as to be used solely for
   * testing)
   */
  Collection<Long> peek();

  /**
   * Spend an [amount] of money from Wallet and ensure the wallet is updated (any change should be
   * added back to the wallet as a single amount) Return single change amount (if any change due) or
   * zero.
   */
  Long spend(Long amount);
}

class WalletAccount implements Wallet {

  Long balance = 0L;

  List<Long> coins = new ArrayList<>();

  @Transactional
  public void addCoin(String coin) {
    Long coinDenomintaion = Long.valueOf(coin);
    this.coins.add(coinDenomintaion);
    this.addBalance(coinDenomintaion);
  }

  private void addBalance(Long coinDenomintaion) {
    this.balance = (this.getBalance() + coinDenomintaion);
  }

  @Override
  public void load(String input) {
    List<String> coins = List.of(input.split(","));
    for (String coin : coins) {
      this.addCoin(coin);
    }
  }

  @Override
  public Long getBalance() {
    return this.balance;
  }

  public List<Long> peek() {

    return this.coins;
  }

  @Override
  public Long spend(Long amount) {
    Long currentAmount = 0L;
    /*
     * 30
     * [1, 100, 200, 200, 1000, 1]
     * ^
     * 101 --> return 101 --> Give us back 71
     * coins = [200, 200, 1000, 1]
     * add 71;
     * [200, 200, 1000, 1, 71]
     * */

    int i = 0;
    for (i = 0; i < this.coins.size(); i++) {
      Long currentCoin = this.coins.get(i);
      currentAmount += currentCoin;
      if (currentAmount >= amount) {
        break;
      }
    }
    this.balance -= currentAmount;
    this.coins = this.coins.subList(i + 1, this.coins.size());
    Long change = (currentAmount - amount);
    if (change > 0) {
      this.coins.add(change);
      addBalance(change);
    }

    return change;
  }
}