Problem 4.3 — ATM

Greedy denomination dispense + the two-invariant guard (no overdraft, no fake cash) in one method.

public enum Denomination { TWO_THOUSAND(2000), FIVE_HUNDRED(500), HUNDRED(100);
  final int value;
  Denomination(int v) { this.value = v; }
}

public class CashTray {
  private final Map<Denomination, Integer> counts = new EnumMap<>(Denomination.class);

  public CashTray(Map<Denomination, Integer> initial) { counts.putAll(initial); }

  // Greedy from largest to smallest. Returns null if the exact amount can't be made.
  public synchronized Map<Denomination, Integer> withdraw(int amount) {
    if (amount % 100 != 0) throw new IllegalArgumentException("multiples of 100 only");
    Map<Denomination, Integer> plan = new EnumMap<>(Denomination.class);
    int remaining = amount;
    for (Denomination d : Denomination.values()) {
      int available = counts.getOrDefault(d, 0);
      int use = Math.min(available, remaining / d.value);
      if (use > 0) { plan.put(d, use); remaining -= use * d.value; }
    }
    if (remaining != 0) return null;              // machine physically cannot make this amount
    plan.forEach((d, n) -> counts.merge(d, -n, Integer::sum));   // commit only if the whole plan works
    return plan;
  }
}

public class WithdrawService {
  private final AccountRepository accounts;   // @Transactional boundary lives here
  private final CashTray tray;

  public WithdrawResult withdraw(String accountId, int amount) {
    // INVARIANT 1: no overdraft — checked and debited atomically at the DB row
    boolean debited = accounts.debitIfSufficientFunds(accountId, amount);  // UPDATE ... WHERE balance >= amount
    if (!debited) return WithdrawResult.insufficientFunds();

    // INVARIANT 2: no fake cash — the tray can refuse even after the ledger says yes
    Map<Denomination, Integer> notes = tray.withdraw(amount);
    if (notes == null) {
      accounts.credit(accountId, amount);      // compensate: give the money back
      return WithdrawResult.cannotDispenseExactAmount();
    }
    return WithdrawResult.success(notes);
  }
}