Code Craft: Cart with Promotions (OOD)

OOD Code Craft round. A shopping cart contains items with prices and a shipping fee. A list of promotions applies — either a percentage discount (does not apply to shipping) or a fixed-dollar discount. Find the promotion that yields the lowest payment and return the final amount. Multiple parts, each adding edge cases. Requirements Input: items: list of (name, price). shipping: a fixed shipping fee added to the subtotal. promotions: list of promotion objects. Two types: PercentDiscount(percent) — multiplies the items subtotal by (1 - percent / 100). Does not apply to shipping. FixedDiscount(amount) — subtracts amount from the total (items + shipping). Cannot reduce the total below 0. Output: the minimum final payment achievable by applying exactly one promotion (or zero, if no promotion is better than no promotion at all — clarify). Each part adds an edge case: empty cart, negative subtotal after fixed discount, promotion that does not apply to the cart (e.g. minimum-purchase requirement), tie-breaking when two promotions yield equal totals. Notes Clean OOP structure: class Item { String name; double price; } abstract class Promotion { double apply(double subtotal, double shipping); } Concrete PercentDiscount and FixedDiscount subclasses implementing apply. class Cart holds items, shipping, and a method bestTotal(promotions) that iterates promotions and returns the minimum. The reason interviewers ask this in OOP form: they want to see that you separate the Promotion type from the cart's pricing logic. Inlining if isinstance(p, PercentDiscount) everywhere is a fail signal. Floating-point care: compute everything in integer cents internally if available, or use decimal.Decimal in Python. Naively using double can cause off-by-one-cent failures on test cases. Round to 2 decimals only at the output step. Edge cases the interviewer hits in successive parts: Empty cart → return shipping (no discount applies to shipping for percentage; fixed can still apply). Fixed discount > total → final = 0, not negative. Multiple promotions tied for minimum → return any; interviewer may probe whether you want deterministic tie-breaking. Promotion with a minimum-purchase floor → check eligibility before applying. Preparation Pre-write the Promotion class hierarchy with apply(subtotal, shipping) -> total and one concrete subclass each. Drill the integer-cents arithmetic pattern; this is the easiest way to dodge floating-point bugs in a 60-minute round. Practice growing the design incrementally: implement one promotion type, run tests, add the second, add eligibility, add tie-breaking. Each part adds 5–10 minutes.