Domain-model practice

Opening your lesson

A product team has changed the records but kept the domain policies. The new library, parking and elevator inputs let you practice recognizing the same invariant without relying on an earlier example's values. Read the units carefully before calculating anything.

The tasks remain independent components. A borrower owns a particular active loan, a parking ticket closes once and an elevator sweep uses one stated direction policy. Avoid inventing extra rules for a fictional business. The supplied contract determines fairness, rounding and tie behavior. Handle empty requests as ordinary work, preserve identifiers that look unusual and treat exact capacity as a valid boundary. The final check expects a complete result for every supplied request, including rejected transitions.

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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