Decode a secondhand bookshop's pencilled cost and price codes and check each book's margin

# Decode a secondhand bookshop's pencilled cost and price codes and check each book's margin Inputs: `books.csv` (book_id, shelf_id, mark), `shelves.csv` (shelf_id, name) and `cipher.json` (code_word, repeat_letter). Each mark was pencilled inside a book's cover in the shop's private letter code and typed up by hand. The code word, books and shelves are synthetic; amounts are in dollars. These rules are the shop's; where `volunteer-notes.txt` disagrees, the rules win. Rules: - Normalising: remove every space and make letters upper case. A normalised mark is `<cost letters>/<price letters>`: exactly one `/` with at least one letter on each side. - Letter code: code_word has ten different letters. Its first letter stands for the digit 1, the second for 2, and so on up to the ninth for 9; the tenth letter stands for 0. repeat_letter stands for the same digit as the character just before it. Each side of the mark, read left to right, gives the amount in cents (so the digits 1250 mean 12.50). - Malformed: a mark that does not have the shape above, holds a character that is neither a code_word letter nor repeat_letter, or starts either side with repeat_letter is `malformed`; its cost and price are null. - Margins: a readable mark is `below_cost` when the price is less than the cost, then `thin_margin` when the price is less than 1.5 × the cost, and `ok` otherwise. Each book gets the first status that applies, in the order malformed, below_cost, thin_margin, ok. - Shelf value and margin: a shelf's value is the total price of its books whose status is `ok` or `thin_margin`. margin_total is the total of price minus cost over the `ok` books. - Write `out/decoded.json`: `{"books": {"<book_id>": {"status": "<status>", "cost": <dollars or null>, "price": <dollars or null>}, ...}, "status_counts": {"<status>": <number of books>, ...}, "shelf_value": {"<shelf_id>": <dollars>, ...}, "margin_total": <dollars>}`. `books` has one entry per row in books.csv. - Roll-ups: Statuses with no books are included at 0. Shelves in shelves.csv with no ok or thin_margin books are included at 0. ## How this run works - Work on your own: nobody will answer questions in this task, and writing to `ask/` counts against the run. - Write only under `out/`. Create `out/` if it is missing. Inputs are synthetic; don't fetch anything from the network. - Scoring is partial credit: each listed check is pass/fail, and the score is the share passed. Numbers are compared to within 0.005 unless the task says otherwise.