Design an ATM

1import java.util.*;2 3interface BankService {4    boolean verifyPin(String card, String pin);5    long balance(String card);6    Optional<String> hold(String card, long amount);   // returns hold id if authorized7    void commit(String holdId);8    void release(String holdId);9}10 11final class CashDispenser {12    private final TreeMap<Integer, Integer> cassettes = new TreeMap<>(Comparator.reverseOrder()); // denomination -> count13 14    void load(int denomination, int count) { cassettes.merge(denomination, count, Integer::sum); }15 16    /** Plan notes with backtracking over denominations so limited counts do not break greedy. */17    Optional<Map<Integer, Integer>> plan(int amount) {18        List<Integer> denoms = new ArrayList<>(cassettes.keySet());19        Map<Integer, Integer> chosen = new LinkedHashMap<>();20        return search(amount, 0, denoms, chosen) ? Optional.of(chosen) : Optional.empty();21    }22 23    private boolean search(int remaining, int i, List<Integer> denoms, Map<Integer, Integer> chosen) {24        if (remaining == 0) return true;25        if (i == denoms.size()) return false;26        int d = denoms.get(i);27        for (int n = Math.min(remaining / d, cassettes.get(d)); n >= 0; n--) {28            if (n > 0) chosen.put(d, n); else chosen.remove(d);29            if (search(remaining - n * d, i + 1, denoms, chosen)) return true;30        }31        chosen.remove(d);32        return false;33    }34 35    void dispense(Map<Integer, Integer> notes) {36        notes.forEach((d, n) -> cassettes.merge(d, -n, Integer::sum));37    }38}39 40final class Atm {41    enum State { IDLE, CARD_INSERTED, AUTHENTICATED }42    private static final int MAX_PIN_ATTEMPTS = 3;43    private static final long DAILY_LIMIT = 1_000;44 45    private final BankService bank;46    private final CashDispenser dispenser;47    private State state = State.IDLE;48    private String card;49    private int pinAttempts;50    private final Map<String, Long> withdrawnToday = new HashMap<>();51 52    Atm(BankService bank, CashDispenser dispenser) { this.bank = bank; this.dispenser = dispenser; }53 54    void insertCard(String card) {55        require(State.IDLE);56        this.card = card; pinAttempts = 0; state = State.CARD_INSERTED;57    }58 59    boolean enterPin(String pin) {60        require(State.CARD_INSERTED);61        if (bank.verifyPin(card, pin)) { state = State.AUTHENTICATED; return true; }62        if (++pinAttempts >= MAX_PIN_ATTEMPTS) { retainCard(); }63        return false;64    }65 66    long balance() { require(State.AUTHENTICATED); return bank.balance(card); }67 68    String withdraw(int amount) {69        require(State.AUTHENTICATED);70        if (amount <= 0) throw new IllegalArgumentException("Amount must be positive");71        if (withdrawnToday.getOrDefault(card, 0L) + amount > DAILY_LIMIT) return "Daily limit exceeded";72 73        Optional<Map<Integer, Integer>> notes = dispenser.plan(amount);74        if (notes.isEmpty()) return "Cannot dispense this amount; try another";75 76        Optional<String> hold = bank.hold(card, amount);77        if (hold.isEmpty()) return "Insufficient funds";78        try {79            dispenser.dispense(notes.get());80            bank.commit(hold.get());81            withdrawnToday.merge(card, (long) amount, Long::sum);82            return "Please take your cash: " + notes.get();83        } catch (RuntimeException jam) {84            bank.release(hold.get());                               // never charge for undispensed cash85            return "Dispense failed; you were not charged";86        }87    }88 89    void eject() { card = null; state = State.IDLE; }90    private void retainCard() { card = null; state = State.IDLE; }91    private void require(State expected) { if (state != expected) throw new IllegalStateException("Invalid in state " + state); }92}