Expense Policies - Validate and Categorize Expense Requests

Question

## Problem

You are building an expense reimbursement system. Given a list of expense items and a policy configuration, determine which expenses are approved, flagged for review, or rejected.

Policy rules (provided as config):
- Each `category` has a per-item limit and a monthly limit per employee.
- Receipts are required above a configurable threshold.
- Certain categories are entirely disallowed.

```python
def evaluate_expenses(
    expenses: List[dict],
    policy: dict
) -> List[dict]:
    # Each expense: {"id", "employee_id", "category", "amount", "has_receipt", "date"}
    # Returns same list with added "status": "approved"|"flagged"|"rejected" and "reason"
    ...
```

**Example:**
```
policy = {
  "per_item_limits": {"meals": 75, "travel": 500},
  "monthly_limits":  {"meals": 300},
  "receipt_threshold": 25,
  "disallowed": ["entertainment"]
}
expense = {"id": 1, "employee_id": "e1", "category": "meals", "amount": 80,
           "has_receipt": True, "date": "2025-03-15"}
-> {"id": 1, ..., "status": "rejected", "reason": "exceeds per-item limit"}
```

## Follow-ups

1. How would you handle policies that change mid-month?
2. How do you efficiently compute running monthly totals across a large batch?
3. What if multiple rules conflict - how do you define precedence?
4. How would you make the rule engine extensible for new policy types?

Expense Policies - Validate and Categorize Expense Requests

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