Library Sorting and Checkout

Zendesk Guide needs a small in-memory book catalog that supports predictable browsing and checkout processing. Given a list of books, sort them by title or author, then process checkout requests in order.

Implement a function that returns the sorted catalog, the IDs successfully checked out, and the IDs that could not be checked out because the book was already unavailable.

Formal Specification

Implement process_books(books, sort_by, checkout_ids).

• books is a list of dictionaries. Each dictionary contains:
  • id: a unique string
  • title: a string
  • author: a string
  • available: a boolean
• sort_by is either "title" or "author".
• checkout_ids is a list of book IDs processed from left to right.
• Return a dictionary with:
  • books: the catalog sorted by the selected field, then by title, then by ID
  • checked_out: IDs successfully checked out
  • unavailable: IDs rejected because they were already checked out

The input list must not be modified. A repeated checkout request for the same book is rejected after its first successful checkout.