Complete Luhn algorithm (#1167)

* Luhn algorithm bug fixed

* Robust tests for payment card number

* changes file

* Robust tests for Luhn algorithm

* remove bad quotes

* Styling changes done

* Some little changes

changes/1166-cuencandres.rst

@@ -0,0 +1 @@
+completing Luhn algorithm for `PaymentCardNumber`.

pydantic/types.py

@@ -651,6 +651,8 @@ def validate_luhn_check_digit(cls, card_number: str) -> str:
             digit = int(card_number[i])
             if i % 2 == parity:
                 digit *= 2
+            if digit > 9:
+                digit -= 9
             sum_ += digit
         valid = sum_ % 10 == 0
         if not valid:

tests/test_types_payment_card_number.py

@@ -9,11 +9,12 @@
 
 VALID_AMEX = '370000000000002'
 VALID_MC = '5100000000000003'
-VALID_VISA = '4000000000000002'
+VALID_VISA = '4050000000000001'
 VALID_OTHER = '2000000000000000008'
 LUHN_INVALID = '4000000000000000'
 LEN_INVALID = '40000000000000006'
 
+
 # Mock PaymentCardNumber
 PCN = namedtuple('PaymentCardNumber', ['card_number', 'brand'])
 PCN.__len__ = lambda v: len(v.card_number)
@@ -30,10 +31,43 @@ def test_validate_digits():
         PaymentCardNumber.validate_digits('hello')
 
 
-def test_validate_luhn_check_digit():
-    assert PaymentCardNumber.validate_luhn_check_digit(VALID_VISA) == VALID_VISA
-    with pytest.raises(LuhnValidationError):
-        PaymentCardNumber.validate_luhn_check_digit(LUHN_INVALID)
+@pytest.mark.parametrize(
+    'card_number, valid',
+    [
+        ('0', True),
+        ('00', True),
+        ('18', True),
+        ('0000000000000000', True),
+        ('4242424242424240', False),
+        ('4242424242424241', False),
+        ('4242424242424242', True),
+        ('4242424242424243', False),
+        ('4242424242424244', False),
+        ('4242424242424245', False),
+        ('4242424242424246', False),
+        ('4242424242424247', False),
+        ('4242424242424248', False),
+        ('4242424242424249', False),
+        ('42424242424242426', True),
+        ('424242424242424267', True),
+        ('4242424242424242675', True),
+        ('5164581347216566', True),
+        ('4345351087414150', True),
+        ('343728738009846', True),
+        ('5164581347216567', False),
+        ('4345351087414151', False),
+        ('343728738009847', False),
+        ('000000018', True),
+        ('99999999999999999999', True),
+        ('99999999999999999999999999999999999999999999999999999999999999999997', True),
+    ],
+)
+def test_validate_luhn_check_digit(card_number: str, valid: bool):
+    if valid:
+        assert PaymentCardNumber.validate_luhn_check_digit(card_number) == card_number
+    else:
+        with pytest.raises(LuhnValidationError):
+            PaymentCardNumber.validate_luhn_check_digit(card_number)
 
 
 @pytest.mark.parametrize(
@@ -71,7 +105,7 @@ def test_get_brand(card_number: str, brand: PaymentCardBrand):
 def test_valid():
     card = PaymentCard(card_number=VALID_VISA)
     assert str(card.card_number) == VALID_VISA
-    assert card.card_number.masked == '400000******0002'
+    assert card.card_number.masked == '405000******0001'
 
 
 @pytest.mark.parametrize(