Belgium BBAN check fix (#136)

* Belgium BBAN check fix

The Belgian BBAN check has an issue when the checksum digits are exactly
"97". This is because the BBAN calculation is done using the "account
number" mod 97; normally, this would result in a checksum digit of "00",
but the standard in that case uses "97" instead.

Updated the checksum calculation to handle this case, plus updated the
related unit test.

* Docs update BE BBAN validation

Indicate in docs that Belgian BBAN is also checked.

schwifty/checksum/belgium.py

@@ -17,6 +17,8 @@ def compute(self, bban: str) -> str:
         spec = registry.get("iban")
         assert isinstance(spec, dict)
         checksum = int(bban[: spec["BE"]["bban_length"] - self.checksum_length]) % 97
+        # Specific value in case checksum is expliclity 97
+        checksum = checksum if checksum != 0 else 97
         return str(checksum).zfill(self.checksum_length)
 
     def validate(self, bban: str) -> bool:

schwifty/iban.py

@@ -179,8 +179,8 @@ def validate(self, validate_bban: bool = False) -> bool:
         """Validate the structural integrity of this IBAN.
 
         This function will verify the country specific format as well as the Luhn checksum in the
-        3rd and 4th position of the IBAN. For some countries (currently Germany and Italy) it will
-        also verify the correctness of the country specific checksum within the BBAN if the
+        3rd and 4th position of the IBAN. For some countries (currently Belgium, Germany and Italy)
+        it will also verify the correctness of the country specific checksum within the BBAN if the
         `validate_bban` parameter is set to `True`. For German banks it will pick the appropriate
         algorithm based on the bank code and verify that the account code has the correct checksum.
 

tests/test_checksum.py

@@ -97,3 +97,7 @@ def test_belgium_checksum():
 
 def test_belgium_checksum_failure():
     assert algorithms["BE:default"].validate("050000123456") is False
+
+
+def test_belgium_checksum_checksum_edge_case():
+    assert algorithms["BE:default"].validate("050000017797") is True

tests/test_iban.py

@@ -169,6 +169,7 @@ def test_iban_properties():
         (("BE", "050", "123456"), "BE45050012345689"),
         (("BE", "539", "0075470"), "BE68539007547034"),
         (("FR", "2004101005", "0500013M026"), "FR1420041010050500013M02606"),
+        (("BE", "050", "177"), "BE54050000017797"),
     ],
 )
 def test_generate_iban(components, compact):