Credit Availability - Determine If a User Has Sufficient Credits for an Action

Question

## Round 1 - Coding

## Problem

Implement a credit availability checker for a promotional credits system. Users earn credits from various sources (referrals, promotions) and spend them on orders. Credits can have expiration dates and minimum order thresholds. Given a user's credit pool and an order total, determine how much credit can be applied.

```python
from dataclasses import dataclass
from datetime import date

@dataclass
class Credit:
    credit_id: str
    amount: float
    expires_on: date | None
    min_order_value: float

class CreditWallet:
    def __init__(self, credits: list[Credit]): ...

def available_credit(
        self,
        order_value: float,
        today: date
    ) -> float:
        # Sum credits that: are not expired AND meet min_order_value threshold
        # Cannot apply more credit than order_value (no negative orders)
        ...

def apply_credits(
        self,
        order_value: float,
        today: date
    ) -> tuple[float, list[str]]:
        # Returns (amount_applied, list of credit_ids used)
        # Use credits in expiry-first order (soonest expiring first)
        ...
```

```
Example:
credits = [
    Credit("C1", 5.00,  date(2024,12,31), min_order_value=15.0),
    Credit("C2", 10.00, None,              min_order_value=0.0),
]
wallet = CreditWallet(credits)
wallet.available_credit(order_value=20.0, today=date(2024,6,1)) -> 15.0
wallet.apply_credits(order_value=8.0, today=date(2024,6,1))    -> (8.0, ["C2"])
# C1 not eligible (order 8 < min 15); C2 eligible, but cap at order value
```

## Follow-ups

1. Why apply soonest-expiring credits first? What problem does it solve?
2. How would you handle fractional credit application (e.g. credit worth $10 on a $7 order)?
3. What happens if a user's credits total more than the order value?
4. How would you audit which credits were consumed per order for accounting purposes?

Credit Availability - Determine If a User Has Sufficient Credits for an Action

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