Parking Lot

1import java.util.*;
2
3/** One-process teaching reference. No provider I/O, persistence or gate hardware. */
4public final class Solution {
5    private Solution() {}
6    public enum Size { SMALL, LARGE }
7    public enum State { OPEN, PAID, CLOSED }
8    public record Vehicle(String id, Size size) {
9        public Vehicle { requireId(id); Objects.requireNonNull(size); }
10    }
11    public record Spot(String id, Size size) {
12        public Spot { requireId(id); Objects.requireNonNull(size); }
13    }
14    public record Rate(String version, long minorUnitsPerHour) {
15        public Rate { requireId(version); if (minorUnitsPerHour <= 0) throw new IllegalArgumentException("rate"); }
16    }
17    public record Ticket(String id, Vehicle vehicle, String spotId, long enteredAt,
18                         Rate rate, State state) {}
19    public record Quote(String id, String ticketId, long at, long amount, String rateVersion) {}
20    private record ParkResult(Vehicle vehicle, String ticketId) {}
21    private static void requireId(String value) {
22        if (value == null || value.isBlank()) throw new IllegalArgumentException("empty identifier");
23    }
24    public static final class ParkingLot {
25        private final List<Spot> spots;
26        private final Map<String, String> occupied = new HashMap<>();
27        private final Map<String, String> activeVehicles = new HashMap<>();
28        private final Map<String, Ticket> tickets = new HashMap<>();
29        private final Map<String, ParkResult> operations = new HashMap<>();
30        private final Map<String, Quote> quotes = new HashMap<>();
31        private final Map<String, String> receipts = new HashMap<>();
32        private Rate currentRate;
33        private long sequence;
34        public ParkingLot(List<Spot> spots, Rate rate) {
35            this.spots = List.copyOf(spots); this.currentRate = Objects.requireNonNull(rate);
36            if (this.spots.stream().map(Spot::id).distinct().count() != this.spots.size())
37                throw new IllegalArgumentException("duplicate spot");
38        }
39        // Linearization boundary: selection, claim and ticket creation share this monitor.
40        public synchronized Ticket park(Vehicle vehicle, String operationId, long at) {
41            Objects.requireNonNull(vehicle); requireId(operationId);
42            if (at < 0) throw new IllegalArgumentException("time");
43            ParkResult previous = operations.get(operationId);
44            if (previous != null) {
45                if (!previous.vehicle().equals(vehicle)) throw new IllegalArgumentException("key reused");
46                return tickets.get(previous.ticketId()); // Current snapshot of same ticket identity.
47            }
48            if (activeVehicles.containsKey(vehicle.id())) throw new IllegalStateException("already parked");
49            Spot selected = spots.stream().filter(s -> !occupied.containsKey(s.id()) &&
50                (s.size() == Size.LARGE || vehicle.size() == Size.SMALL)).findFirst()
51                .orElseThrow(() -> new IllegalStateException("full"));
52            String id = "T" + (++sequence);
53            Ticket ticket = new Ticket(id, vehicle, selected.id(), at, currentRate, State.OPEN);
54            occupied.put(selected.id(), id); tickets.put(id, ticket);
55            activeVehicles.put(vehicle.id(), id); operations.put(operationId, new ParkResult(vehicle, id));
56            return ticket;
57        }
58        public synchronized Quote quote(String ticketId, long at) {
59            Ticket t = ticket(ticketId);
60            if (t.state() != State.OPEN) throw new IllegalStateException("not open");
61            if (at < t.enteredAt()) throw new IllegalArgumentException("clock reversal");
62            long elapsed = Math.subtractExact(at, t.enteredAt());
63            long hours = elapsed / 3600 + (elapsed % 3600 == 0 ? 0 : 1);
64            long amount = Math.multiplyExact(hours, t.rate().minorUnitsPerHour());
65            Quote q = new Quote("Q" + (++sequence), ticketId, at, amount, t.rate().version());
66            quotes.put(q.id(), q); return q;
67        }
68        // Caller is a trusted payment adapter. A receipt string is not payment authentication.
69        public synchronized Ticket recordPayment(String quoteId, String receiptId) {
70            requireId(receiptId);
71            Quote q = quotes.get(quoteId);
72            if (q == null) throw new IllegalArgumentException("unknown quote");
73            String prior = receipts.get(receiptId);
74            if (prior != null) {
75                if (!prior.equals(quoteId)) throw new IllegalArgumentException("receipt reused");
76                return ticket(q.ticketId());
77            }
78            Ticket t = ticket(q.ticketId());
79            if (t.state() != State.OPEN) throw new IllegalStateException("not open");
80            Ticket paid = withState(t, State.PAID); tickets.put(t.id(), paid);
81            receipts.put(receiptId, quoteId); return paid;
82        }
83        // Ticket identity makes repeated exit naturally idempotent, even after spot reuse.
84        public synchronized Ticket exit(String ticketId) {
85            Ticket t = ticket(ticketId);
86            if (t.state() == State.CLOSED) return t;
87            if (t.state() != State.PAID) throw new IllegalStateException("unpaid");
88            Ticket closed = withState(t, State.CLOSED);
89            occupied.remove(t.spotId()); activeVehicles.remove(t.vehicle().id());
90            tickets.put(t.id(), closed); return closed;
91        }
92        public synchronized void changeRate(Rate rate) { currentRate = Objects.requireNonNull(rate); }
93        public synchronized Ticket ticket(String id) {
94            Ticket t = tickets.get(id); if (t == null) throw new IllegalArgumentException("unknown ticket");
95            return t;
96        }
97        public synchronized int occupiedCount() { return occupied.size(); }
98        public synchronized boolean invariantHolds() {
99            long active = tickets.values().stream().filter(t -> t.state() != State.CLOSED).count();
100            return active == occupied.size() && active == activeVehicles.size() &&
101                tickets.values().stream().filter(t -> t.state() != State.CLOSED).allMatch(t ->
102                    t.id().equals(occupied.get(t.spotId())) && t.id().equals(activeVehicles.get(t.vehicle().id())));
103        }
104        private static Ticket withState(Ticket t, State state) {
105            return new Ticket(t.id(), t.vehicle(), t.spotId(), t.enteredAt(), t.rate(), state);
106        }
107    }
108}
109