Wallet / Payment Ledger

1import java.math.BigInteger;
2import java.util.*;
3/** One-currency ledger. Funding is supplied by constructor fixtures, not external banking rails. */
4public final class Solution {
5    private Solution() {}
6    public enum HoldState { ACTIVE, CAPTURED, RELEASED }
7    public record Posting(String id,Map<String,Long> deltas,String reverses){public Posting{deltas=Map.copyOf(deltas);}}
8    public record Hold(String id,String account,long amount,HoldState state){}
9    private record Transfer(String from,String to,long amount){}
10    private record Reserve(String account,long amount){}
11    private record Capture(String holdId,String to){}
12    private record Reverse(String originalId){}
13    private record Operation(Object fingerprint,String resultId){}
14    public static final class Ledger {
15        private static final String CLEARING="@clearing";
16        private final Map<String,Long> balances=new TreeMap<>(),reserved=new TreeMap<>();
17        private final Map<String,Posting> postings=new LinkedHashMap<>();
18        private final Map<String,Hold> holds=new HashMap<>();
19        private final Map<String,Operation> operations=new HashMap<>();
20        private final Set<String> reversed=new HashSet<>();
21        private long sequence;
22        public Ledger(Map<String,Long> funding){
23            if(funding.isEmpty()||funding.keySet().stream().anyMatch(id->id==null||id.isBlank()||id.equals(CLEARING))||funding.values().stream().anyMatch(v->v==null||v<0))throw new IllegalArgumentException("funding");
24            long total=funding.values().stream().map(BigInteger::valueOf).reduce(BigInteger.ZERO,BigInteger::add).longValueExact();balances.putAll(funding);balances.put(CLEARING,-total);funding.keySet().forEach(k->reserved.put(k,0L));var initial=new TreeMap<>(funding);initial.put(CLEARING,-total);postings.put("opening",new Posting("opening",initial,""));
25        }
26        private void account(String id){if(!reserved.containsKey(id))throw new IllegalArgumentException("wallet account");}
27        private static void key(String key){if(key==null||key.isBlank())throw new IllegalArgumentException("key");}
28        private Operation previous(String key,Object fingerprint){key(key);var old=operations.get(key);if(old!=null&&!old.fingerprint().equals(fingerprint))throw new IllegalArgumentException("key reused");return old;}
29        private Map<String,Long> nextBalances(Map<String,Long> delta,Map<String,Long> required){
30            if(delta.values().stream().map(BigInteger::valueOf).reduce(BigInteger.ZERO,BigInteger::add).signum()!=0)throw new IllegalArgumentException("unbalanced");
31            var next=new TreeMap<>(balances);delta.forEach((a,d)->{account(a);next.put(a,Math.addExact(next.get(a),d));});
32            for(String a:reserved.keySet())if(next.get(a)<required.get(a))throw new IllegalStateException("insufficient available funds");return next;
33        }
34        private Posting commit(Map<String,Long> delta,Map<String,Long> next,String reverse){var posting=new Posting("P"+(++sequence),delta,reverse);balances.clear();balances.putAll(next);postings.put(posting.id(),posting);return posting;}
35        public synchronized Posting transfer(String from,String to,long amount,String key){account(from);account(to);if(from.equals(to)||amount<1)throw new IllegalArgumentException("transfer");var fp=new Transfer(from,to,amount);var old=previous(key,fp);if(old!=null)return postings.get(old.resultId());var delta=Map.of(from,-amount,to,amount);var next=nextBalances(delta,reserved);var p=commit(delta,next,"");operations.put(key,new Operation(fp,p.id()));return p;}
36        public synchronized Hold reserve(String account,long amount,String key){account(account);if(amount<1)throw new IllegalArgumentException("amount");var fp=new Reserve(account,amount);var old=previous(key,fp);if(old!=null)return holds.get(old.resultId());long next=Math.addExact(reserved.get(account),amount);if(next>balances.get(account))throw new IllegalStateException("insufficient available funds");var h=new Hold("H"+(++sequence),account,amount,HoldState.ACTIVE);holds.put(h.id(),h);reserved.put(account,next);operations.put(key,new Operation(fp,h.id()));return h;}
37        public synchronized Posting capture(String holdId,String to,String key){account(to);var h=hold(holdId);if(h.account().equals(to))throw new IllegalArgumentException("recipient");var fp=new Capture(holdId,to);var old=previous(key,fp);if(old!=null)return postings.get(old.resultId());if(h.state()!=HoldState.ACTIVE)throw new IllegalStateException("hold not active");var remaining=new TreeMap<>(reserved);remaining.put(h.account(),reserved.get(h.account())-h.amount());var delta=Map.of(h.account(),-h.amount(),to,h.amount());var next=nextBalances(delta,remaining);var p=commit(delta,next,"");reserved.clear();reserved.putAll(remaining);holds.put(h.id(),new Hold(h.id(),h.account(),h.amount(),HoldState.CAPTURED));operations.put(key,new Operation(fp,p.id()));return p;}
38        public synchronized Hold release(String holdId){var h=hold(holdId);if(h.state()==HoldState.RELEASED)return h;if(h.state()!=HoldState.ACTIVE)throw new IllegalStateException("already captured");reserved.put(h.account(),reserved.get(h.account())-h.amount());var next=new Hold(h.id(),h.account(),h.amount(),HoldState.RELEASED);holds.put(h.id(),next);return next;}
39        public synchronized Posting reverse(String postingId,String key){var p=postings.get(postingId);if(p==null||postingId.equals("opening")||!p.reverses().isEmpty())throw new IllegalArgumentException("posting");var fp=new Reverse(postingId);var old=previous(key,fp);if(old!=null)return postings.get(old.resultId());if(reversed.contains(postingId))throw new IllegalStateException("already reversed");var delta=new TreeMap<String,Long>();p.deltas().forEach((a,v)->delta.put(a,Math.negateExact(v)));var next=nextBalances(delta,reserved);var result=commit(delta,next,postingId);reversed.add(postingId);operations.put(key,new Operation(fp,result.id()));return result;}
40        public synchronized Hold hold(String id){var h=holds.get(id);if(h==null)throw new IllegalArgumentException("hold");return h;}
41        public synchronized long available(String account){account(account);return balances.get(account)-reserved.get(account);}
42        public synchronized Map<String,Long> balances(){return Map.copyOf(balances);}
43        public synchronized List<Posting> postings(){return List.copyOf(postings.values());}
44        public synchronized Map<String,Long> rebuild(){var result=new TreeMap<String,Long>();balances.keySet().forEach(a->result.put(a,0L));for(var p:postings.values())p.deltas().forEach((a,v)->result.put(a,Math.addExact(result.get(a),v)));return Map.copyOf(result);}
45    }
46}
47