Design an Extensible Cinema Ticket Pricing Engine

## Interview Prompt Design a ticket-pricing engine where a movie format supplies a base price, showtime and seat tier each supply a multiplier, and zero or more add-ons supply fixed prices. The final price is `(base price + add-on total) * showtime multiplier * seat-tier multiplier`. Explain a type-safe, testable design that can add new formats, showtimes, tiers, or services without editing a large conditional. ### Constraints & Assumptions - Represent money with integer minor units or decimal arithmetic, never binary floating point. - Multiplier and rounding policy must be explicit and consistent. - Unknown or incompatible options are rejected rather than silently defaulted. - Pricing configuration is versioned so a quote can later be audited. ### Clarifying Questions to Ask - At which step is rounding applied? - Can an add-on be restricted by format, showtime, or seat tier? - Must previously issued quotes remain reproducible after configuration changes? ### What a Strong Answer Covers - Value objects for money and multipliers with validated invariants. - Composable pricing components or policies instead of format-specific branching. - An explicit calculation order, rounding rule, and itemized quote output. - Configuration/version separation from pricing logic and audit-friendly inputs. - Unit, property, and compatibility tests for combinations and boundary values. ### Follow-up Questions - How would coupons and taxes fit without changing every price component? - How do you prevent a new add-on from being sold with an incompatible seat? - What should an idempotent quote API return when pricing configuration changes mid-request?