Credit Card Number: Validate and Mask Credit Card Numbers Using Luhn's Algorithm

Question

## Problem

Implement two functions:
1. `is_valid(card_number: str) -> bool` — validate using the Luhn algorithm.
2. `mask(card_number: str) -> str` — replace all but the last 4 digits with `*`.

The Luhn check: from the rightmost digit, double every second digit; if doubling exceeds 9, subtract 9. Sum all digits. Valid if sum % 10 == 0.

```python
def is_valid(card_number: str) -> bool:
    pass

def mask(card_number: str) -> str:
    pass
```

**Example:**
```
card = "4532015112830366"
is_valid(card) -> True
mask(card)     -> "************0366"

card = "1234567890123456"
is_valid(card) -> False
mask(card)     -> "************3456"
```

**Luhn trace for "4532015112830366":**
```
Digits (R->L doubled): 6, 6, 3, 0, 3, 8, 2, 2, 1, 1, 5, 0, 2, 3, 5, 8
Sum = 62 -- wait, corrected sum = 60 -> 60 % 10 == 0 -> valid
```

## Follow-ups

1. How do you handle input with spaces or dashes (e.g. `"4532 0151 1283 0366"`)?
2. What are the valid length ranges for Visa, Mastercard, and Amex cards?
3. Beyond Luhn, what additional checks would you add in a production payment system?
4. How would you write a Luhn-valid card number generator for testing purposes?

Credit Card Number: Validate and Mask Credit Card Numbers Using Luhn's Algorithm

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