Design a library management system

Prompt and scope

Members search the catalog, borrow and return copies, place holds, and pay late fees. Librarians add books and copies.

Model and invariants

Book(isbn, title, authors)            -- the work
Copy(id, isbn, status: AVAILABLE | ON_LOAN | ON_HOLD_SHELF | LOST)
Member(id, max_loans, fines_cents)
Loan(id, copy_id, member_id, due_at, returned_at)
Hold(id, isbn, member_id, created_at, status)

Separate the book from its physical copies; holds are placed on the book, loans on a copy. A copy is on at most one open loan. A member cannot exceed their loan limit or borrow with unpaid fines over a threshold.

Operations

• checkout(member, copy): validate member limits and copy status, create the loan, set the copy ON_LOAN.
• return(copy): close the loan, compute a fine from days late, then, if a hold queue exists for the book, put the copy on the hold shelf for the earliest hold and notify that member.
• place_hold(member, isbn): append to the book's FIFO hold queue.

Walkthrough and tests

Bob returns a copy of a book with Alice's hold pending. The copy goes to the hold shelf, not back to AVAILABLE, so Carol cannot check it out at the desk. Tests: limit reached, return of a lost copy, fine calculation across a month boundary, and hold expiry passing the copy to the next member.