Add a capped volume contract without editing the existing rules
Reseller C needs 20% off from twenty units, capped at 5,000 cents per quote. The current conditional is also used by the other resellers, whose issued quotes must remain reproducible.
- Focused work estimate
- 2h 30m + prerequisites
- Priority in the scenario
- Medium
- Engineering practice
- Strategy selection · Pure functions · Compatibility
Estimated field mix
- Backend70%
- System design30%
Field percentages are editorial estimates of the ticket's engineering focus. They total 100%; they are not measured time, proficiency scores, or ownership evidence.
Pattern topics
- StrategyApply
Isolate the capped volume calculation behind the same contract used by existing rules, without adding reseller-specific branches to the caller.
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Project context
A fictional equipment-rental service supports direct customers and two reseller contracts. Pricing now lives in a long conditional with subclasses left over from a discontinued campaign. Build a local TypeScript quoting module and synthetic fixtures; no starter repository, real charges, tax advice, or payment connection is supplied. Amounts use integer cents and the exercise supports USD only.
Setup prerequisites
- Pure functions
- Object composition
- Integer arithmetic
Preceding work
Complete these dependencies, or supply their agreed outputs before taking this ticket.
Acceptance criteria
- Select a pricing rule by explicit contract and revision, returning a typed error for an unknown combination.
- Implement the threshold and cap while preserving every existing baseline result.
- Return a stable rule identity and a breakdown showing the uncapped discount and applied cap.
Implementation constraints
- Use composition or a function-valued Strategy; adding a class hierarchy is not required. Keep rule evaluation free of I/O.
Verification to include
- Exercise quantities 19 and 20 with discounts below, at, and above 5,000 cents.
- Request a retired or unknown revision and confirm no fallback contract is used.
Deliverables
- Capped pricing rule, selector, and regression fixtures
Rollout and recovery
Enable reseller C only in the synthetic scenario set; keep the previous selector available for baseline replay.
Value of the work
For the engineer: Practice choosing, testing, and removing object-design abstractions around versioned business rules and tenant isolation.
For the team: Review changes that let a team introduce a contract without silently changing existing quotes, together with the maintenance costs of the chosen design.
Evidence boundaries
Outcome Evidence: Tests, patches, and runbooks are requested deliverables. They become Outcome Evidence only through a qualified Mission and immutable Evidence IDs.
Ownership Evidence: Independent adaptation must be observed under a declared verification policy and cite immutable Evidence IDs. Completing a planning ticket establishes no Ownership Evidence.