Library Management

1import java.time.LocalDate;
2import java.util.*;
3/** In-memory circulation aggregate. Calendar dates are supplied by a trusted library-zone adapter. */
4public final class Solution {
5    private Solution() {}
6    private static void id(String s){if(s==null||s.isBlank())throw new IllegalArgumentException("identifier");}
7    public record BookCopy(String id,String titleId){public BookCopy{Solution.id(id);Solution.id(titleId);}}
8    public record Policy(String version,int days,int maxLoans){public Policy{id(version);if(days<1||maxLoans<1)throw new IllegalArgumentException("policy");}}
9    public record Loan(String id,String copyId,String memberId,LocalDate due,Policy policy,int renewals,boolean returned){}
10    private record Request(String member,String copy,String loan){}
11    public static final class Library {
12        private final Map<String,BookCopy> copies=new HashMap<>();
13        private final Set<String> members;
14        private final Map<String,Loan> loans=new HashMap<>();
15        private final Map<String,String> activeCopies=new HashMap<>();
16        private final Map<String,Integer> counts=new HashMap<>();
17        private final Map<String,ArrayDeque<String>> holds=new HashMap<>();
18        private final Map<String,Request> requests=new HashMap<>();
19        private Policy current;
20        private long sequence;
21        public Library(List<BookCopy> copies,Set<String> members,Policy policy){
22            for(var c:copies)if(this.copies.put(c.id(),c)!=null)throw new IllegalArgumentException("duplicate copy");
23            this.members=Set.copyOf(members);members.forEach(Solution::id);current=Objects.requireNonNull(policy);
24        }
25        private void member(String member){if(!members.contains(member))throw new IllegalArgumentException("unknown member");}
26        private BookCopy copy(String copy){var c=copies.get(copy);if(c==null)throw new IllegalArgumentException("unknown copy");return c;}
27        private void title(String title){if(copies.values().stream().noneMatch(c->c.titleId().equals(title)))throw new IllegalArgumentException("unknown title");}
28        public synchronized void placeHold(String title,String member){
29            title(title);member(member);var q=holds.computeIfAbsent(title,k->new ArrayDeque<>());
30            if(!q.contains(member))q.addLast(member);
31        }
32        public synchronized void cancelHold(String title,String member){title(title);member(member);var q=holds.get(title);if(q!=null)q.remove(member);}
33        public synchronized Loan checkout(String copyId,String memberId,String requestId,LocalDate today){
34            id(requestId);Objects.requireNonNull(today);member(memberId);BookCopy c=copy(copyId);
35            var previous=requests.get(requestId);
36            if(previous!=null){if(!previous.member().equals(memberId)||!previous.copy().equals(copyId))throw new IllegalArgumentException("key reused");return loan(previous.loan());}
37            if(activeCopies.containsKey(copyId))throw new IllegalStateException("already on loan");
38            if(counts.getOrDefault(memberId,0)>=current.maxLoans())throw new IllegalStateException("member limit");
39            var q=holds.get(c.titleId());if(q!=null&&!q.isEmpty()&&!q.peekFirst().equals(memberId))throw new IllegalStateException("hold priority");
40            LocalDate due=today.plusDays(current.days()); // Validate arithmetic before mutation.
41            String loanId="L"+(++sequence);var l=new Loan(loanId,copyId,memberId,due,current,0,false);
42            loans.put(loanId,l);activeCopies.put(copyId,loanId);counts.merge(memberId,1,Integer::sum);requests.put(requestId,new Request(memberId,copyId,loanId));
43            if(q!=null&&!q.isEmpty())q.removeFirst();return l;
44        }
45        public synchronized Loan returnCopy(String loanId){
46            var l=loan(loanId);if(l.returned())return l;
47            var returned=new Loan(l.id(),l.copyId(),l.memberId(),l.due(),l.policy(),l.renewals(),true);
48            loans.put(loanId,returned);activeCopies.remove(l.copyId());counts.put(l.memberId(),counts.get(l.memberId())-1);return returned;
49        }
50        public synchronized Loan renew(String loanId,int expectedRenewals,LocalDate today){
51            Objects.requireNonNull(today);var l=loan(loanId);
52            if(l.returned()||l.renewals()!=expectedRenewals||l.renewals()>=1||today.isAfter(l.due()))throw new IllegalStateException("renewal rejected");
53            var q=holds.get(copy(l.copyId()).titleId());if(q!=null&&!q.isEmpty())throw new IllegalStateException("waiting hold");
54            // Extend original due date; old policy stays attached to this loan.
55            var updated=new Loan(l.id(),l.copyId(),l.memberId(),l.due().plusDays(l.policy().days()),l.policy(),l.renewals()+1,false);
56            loans.put(l.id(),updated);return updated;
57        }
58        public synchronized void changePolicy(Policy policy){current=Objects.requireNonNull(policy);}
59        public synchronized Loan loan(String id){var l=loans.get(id);if(l==null)throw new IllegalArgumentException("unknown loan");return l;}
60        public synchronized List<String> holdQueue(String title){title(title);var q=holds.get(title);return q==null?List.of():List.copyOf(q);}
61        public synchronized boolean invariantHolds(){
62            var actual=new HashMap<String,Integer>();int active=0;
63            for(var l:loans.values())if(!l.returned()){active++;if(!l.id().equals(activeCopies.get(l.copyId())))return false;actual.merge(l.memberId(),1,Integer::sum);}
64            return active==activeCopies.size()&&members.stream().allMatch(m->actual.getOrDefault(m,0).equals(counts.getOrDefault(m,0)));
65        }
66    }
67}
68