Shopping Cart and Checkout

1import java.util.*;
2/** Local checkout state machine; external reserve/payment/refund calls are commands for adapters. */
3public final class Solution {
4    private Solution() {}
5    public enum State { RESERVING, PAYING, CONFIRMED, FAILED, RELEASE_REQUIRED, REFUND_REQUIRED, REFUND_AND_RELEASE_REQUIRED }
6    public record Item(String sku,int quantity,long unitPrice){public Item{if(sku==null||sku.isBlank()||quantity<1||unitPrice<0)throw new IllegalArgumentException("item");}}
7    public record Quote(String id,long cartVersion,List<Item> items,long amount,long expiresAt,String priceVersion){public Quote{items=List.copyOf(items);}}
8    public record Order(String id,Quote quote,State state){}
9    private record Begin(long cartVersion,String quoteId,String orderId){}
10    private record Paid(String orderId,boolean success){}
11    public static final class Checkout {
12        private final Map<String,Item> cart=new TreeMap<>();
13        private final Map<String,Quote> quotes=new HashMap<>();
14        private final Map<String,Order> orders=new HashMap<>();
15        private final Map<String,Begin> keys=new HashMap<>();
16        private final Map<String,Boolean> reservations=new HashMap<>();
17        private final Map<String,Paid> payments=new HashMap<>();
18        private long version,sequence,time;
19        private boolean locked;
20        private void now(long at){if(at<time)throw new IllegalArgumentException("time");time=at;}
21        public synchronized long set(Item item,long expectedVersion){Objects.requireNonNull(item);if(locked||expectedVersion!=version)throw new IllegalStateException("locked or stale cart");cart.put(item.sku(),item);return ++version;}
22        public synchronized long remove(String sku,long expectedVersion){if(locked||expectedVersion!=version)throw new IllegalStateException("locked or stale cart");if(cart.remove(sku)!=null)version++;return version;}
23        public synchronized Quote quote(String priceVersion,long ttl,long at){
24            if(cart.isEmpty()||priceVersion==null||priceVersion.isBlank()||ttl<1)throw new IllegalArgumentException("quote");
25            long deadline=Math.addExact(at,ttl),amount=0;for(var item:cart.values())amount=Math.addExact(amount,Math.multiplyExact(item.unitPrice(),item.quantity()));now(at);
26            var q=new Quote("Q"+(++sequence),version,new ArrayList<>(cart.values()),amount,deadline,priceVersion);quotes.put(q.id(),q);return q;
27        }
28        public synchronized Order begin(String quoteId,long expectedVersion,String key,long at){
29            if(key==null||key.isBlank())throw new IllegalArgumentException("key");var previous=keys.get(key);
30            if(previous!=null){if(previous.cartVersion()!=expectedVersion||!previous.quoteId().equals(quoteId))throw new IllegalArgumentException("key reused");return get(previous.orderId());}
31            var q=quotes.get(quoteId);if(q==null)throw new IllegalArgumentException("quote");now(at);
32            if(locked||expectedVersion!=version||q.cartVersion()!=version||at>=q.expiresAt())throw new IllegalStateException("stale, expired or locked");
33            var order=new Order("O"+(++sequence),q,State.RESERVING);orders.put(order.id(),order);keys.put(key,new Begin(expectedVersion,quoteId,order.id()));locked=true;return order;
34        }
35        public synchronized Order reserved(String orderId,boolean success){
36            var old=reservations.get(orderId);if(old!=null){if(old!=success)throw new IllegalStateException("contradictory reservation");return get(orderId);}
37            var order=get(orderId);if(order.state()!=State.RESERVING)throw new IllegalStateException("not reserving");
38            reservations.put(orderId,success);return update(order,success?State.PAYING:State.FAILED);
39        }
40        public synchronized Order paid(String orderId,String attemptId,boolean success){
41            if(attemptId==null||attemptId.isBlank())throw new IllegalArgumentException("attempt");var prior=payments.get(attemptId);
42            if(prior!=null){if(!prior.orderId().equals(orderId)||prior.success()!=success)throw new IllegalStateException("payment reused");return get(orderId);}
43            var order=get(orderId);
44            // A second, distinct attempt is not silently accepted; caller must reconcile it separately.
45            if(order.state()!=State.PAYING&&!(order.state()==State.RELEASE_REQUIRED||order.state()==State.FAILED))throw new IllegalStateException("unexpected payment");
46            payments.put(attemptId,new Paid(orderId,success));
47            if(order.state()!=State.PAYING)return success?update(order,order.state()==State.RELEASE_REQUIRED?State.REFUND_AND_RELEASE_REQUIRED:State.REFUND_REQUIRED):order;
48            return update(order,success?State.CONFIRMED:State.RELEASE_REQUIRED);
49        }
50        public synchronized Order cancel(String orderId){
51            var order=get(orderId);if(order.state()==State.PAYING)return update(order,State.RELEASE_REQUIRED);
52            if(order.state()==State.RESERVING)throw new IllegalStateException("resolve reservation first");return order;
53        }
54        public synchronized Order released(String orderId){var order=get(orderId);if(order.state()==State.RELEASE_REQUIRED)return update(order,State.FAILED);if(order.state()==State.REFUND_AND_RELEASE_REQUIRED)return update(order,State.REFUND_REQUIRED);throw new IllegalStateException("no release pending");}
55        public synchronized Order refunded(String orderId){var order=get(orderId);if(order.state()==State.REFUND_REQUIRED)return update(order,State.FAILED);if(order.state()==State.REFUND_AND_RELEASE_REQUIRED)return update(order,State.RELEASE_REQUIRED);throw new IllegalStateException("no refund pending");}
56        private Order update(Order order,State state){var next=new Order(order.id(),order.quote(),state);orders.put(next.id(),next);return next;}
57        public synchronized Order get(String id){var order=orders.get(id);if(order==null)throw new IllegalArgumentException("order");return order;}
58        public synchronized long version(){return version;}
59    }
60}
61