Extend Rules Engine: Aggregate Trip-Level Policies

Context

You previously built an expense policy engine that evaluates per-expense rules (e.g., "a single meal cannot exceed $50"). Extend that system to support aggregate, trip-level rules that operate over groups of expenses for the same trip.

Assume each expense has at least: expenseId, tripId, employeeId, category, amount, currency, incurredAt. Currency normalization to a policy currency (e.g., USD) is available.

Requirements

Add support for aggregate rules including (but not limited to):

1. A trip’s total expenses cannot exceed $2,000.
2. Total meal expenses per trip cannot exceed $200.

In the provided sample, trip "002" violates both rules. Update the design and the evaluateRules return type so the output includes per-trip violations alongside per-expense violations.

Describe:

• Grouping/aggregation strategy (how expenses are grouped and metrics computed)
• Evaluation order (per-expense vs. per-trip and how updates flow)
• Explainability for aggregates (what evidence is returned and how it’s human-readable)
• Performance considerations (throughput, latency, incremental updates)
• How these results integrate into the expenses API used by many companies

Provide:

• Updated component design for aggregate rules
• A precise return type/schema for evaluateRules that includes per-trip violations
• A short, concrete example showing trip "002" violating both aggregates