Resort booking

The obvious implementation asks whether the requested stay overlaps an existing booking, and refuses if it does. It is wrong twice. A property with four identical villas has one of them taken and the whole room type disappears from the search. And when a seven-night request fails, nobody — not the guest, not the person on the phone — can say which night was the problem.

The right question is per night: on the busiest night of the requested range, how many units are already occupied? If that count is below the inventory on every night, the stay can be sold. It costs a fold over the nights and it buys both a correct answer and an explainable one.

A stay is half-open. The guest occupies the check-in date and every night after it, but not the check-out date — they are gone by mid-morning and the room is sold again that afternoon.

Model the range as closed and the system refuses a booking the hotel can absolutely honour, on every changeover day, on every room, silently. It is not a dramatic bug: nothing errors, no log line appears, the room simply looks full and the revenue never arrives. There is a test named for exactly that case, and it is the most valuable one in the file.

Every amount is an integer number of centavos. Currency in a floating-point number is a bug waiting for a big enough invoice — seven nights, a seasonal multiplier, a percentage discount and 12% tax is more than enough arithmetic to leave a total a centavo away from the sum of its lines.

Rounding happens in exactly one place, and the tests assert the identity that matters: subtotal minus discount equals taxable, taxable plus tax equals total, across a range of stays. An invoice that does not add up is the sort of thing a guest photographs.

A refusal returns the constraining night, and the interface says it. "Not available" is a dead end; "the 14th is the only night we are full" lets a guest shift by a day and book anyway.

That is a product decision more than a technical one, but it is only available because the availability check was built to know the answer. An engine that returns a boolean cannot be asked a follow-up question later without being rewritten.