Library circulation design

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A library copy can have one active borrower, while a title can have several people waiting. Mixing those identities makes a return from the wrong member appear valid or lets a repeated hold jump the queue. Start with the domain invariant.

These small low-level design tasks check loan ownership, authorized returns, fair holds and overdue policy. They are deliberately bounded components, not a complete library system. Use a class when it makes state ownership clearer, or a pure function when the supplied transition sequence is enough. The checker evaluates behavior. A return succeeds once, a repeated hold stays in its original place and the due day itself has no overdue charge. Empty inputs remain valid service calls.

import json

def process_payload(payload: str) -> str:
    data = json.loads(payload)
    available = set(data['books'])
    result = []
    for member, book in data['requests']:
        accepted = book in available
        result.append(accepted)
        if accepted:
            available.remove(book)
    return json.dumps(result,separators=(',',':'))

A physical copy has at most one active borrower. Availability belongs to a physical book copy, so a successful request must change the available set. Set membership gives average constant work per request and prevents double lending.

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