Luhn (card numbers) check digit validator
From the rightmost digit, double every second digit (subtracting 9 if that exceeds 9), sum everything, and require a multiple of 10.
79927398714 fails the card number (Luhn) checksum, but swap the characters at positions 8 and 9 makes it valid — likely fix: 79927397814.
Suggested fix (transposition): swap the characters at positions 8 and 9.
79927397814
A passing checksum only proves the number is arithmetically self-consistent. It does not prove an IBAN names an open account, an ISBN is assigned to a real book, or a card is active, unstolen, or has funds. Those require asking the bank, the ISBN registry, or the card network.
Try it: a valid example · a broken example
curl 'https://check-digit.gumballtools.com/api/v1/run?input=79927398713&format=luhn'