## Design an Extensible Tax Policy Engine You already have a correct evaluator for one progressive-bracket policy. Design the surrounding tax engine so it can select different policies by tax year and filing status and can support approved alternatives such as a fixed-rate policy. Historical calculations must remain reproducible, and year- or status-specific conditionals must not be scattered through the evaluator. Use this service-level contract as a starting point: ```text calculate_tax(income_minor_units, tax_year, filing_status, policy_override=None) -> {tax_minor_units, policy_version, breakdown} ``` `income_minor_units` is a nonnegative integer. A progressive policy consists of ordered bracket ceilings and rates; each rate applies only to income inside that bracket. A fixed-rate override applies one stated rate to the full taxable income. Explain any rounding rule you choose. ### Part 1 — Model and Select Versioned Policies Describe the policy data, validation, and lookup for a requested year and filing status. Explain how the selected immutable policy reaches the evaluator and how the result retains an auditable per-bracket breakdown. #### What This Part Should Cover - Effective year and filing-status keys with an immutable policy version. - Ordered, nonoverlapping bracket boundaries and a final open-ended bracket. - Selection of exactly one approved version without mutable lookup during evaluation. - Integer or decimal arithmetic with an explicit rounding boundary. ```hint Separate policy data from the evaluator The calculation loop should consume a validated bracket list without knowing which year or filing status selected it. ``` ### Part 2 — Add Custom Rate Strategies Show how a caller can select a fixed-rate policy or another approved strategy without modifying the progressive evaluator. Define how an override is identified, validated, authorized, and recorded in the result. #### What This Part Should Cover - A common policy interface or tagged strategy representation. - Clear fixed-rate semantics and valid rate bounds. - No arbitrary executable caller code inside the service. - Provenance for the override and deterministic re-execution. ```hint Extend the policy, not the conditional tree Ask what all tax strategies must accept and return, then keep their internal rules separate. ``` ### Part 3 — Version, Test, and Operate the Engine Explain how policy corrections, new years, and regression tests are handled so an old calculation can be reproduced while new requests use the intended policy. #### What This Part Should Cover - Append-only or immutable policy versions and explicit selection rules. - Boundary tests at every bracket edge, status, and rounding threshold. - Properties such as nonnegative tax and monotonicity for valid progressive policies. - Logs or records containing inputs, selected policy identity, breakdown, and engine version. ```hint Test one unit below, at, and above each boundary Bracket bugs usually hide at the exact point where one marginal rate hands off to the next. ``` ### What a Strong Answer Covers - A policy-driven design that supports year and filing-status evolution. - Correct marginal-bracket semantics and explicit money precision. - Controlled extensibility for fixed or custom approved rates. - Historical reproducibility, validation, audit records, and boundary-focused tests. ### Follow-up Questions 1. How should a corrected policy affect calculations already issued for that year? 2. What should happen when a caller requests an unsupported filing status? 3. Where should rounding occur if several jurisdictions are composed? 4. How would you compare two policy versions before activating the newer one?