luhn.format.txt

                                  ┏━━━━━━━━━━┓
                                  ┃   LUHN   ┃
                                  ┗━━━━━━━━━━┛

GOAL ==>                          #Checksum of a number (ISO/IEC 7812-1)
                                  #Used for credit card numbers and some national identifiers

CALCULATION ==>                   #Creating:
                                  #  - for every second digit, starting from right: NUM *= 2; if (NUM > 9) NUM -= 9
                                  #  - appended as last digit: 10 - (SUM(all digits) % 10)
                                  #Verifying:
                                  #  - same step 1
                                  #  - SUM(all digits) % 10 === 0
                                  #Can also use a different radix than 10 ("Luhn mod N algo")

PROPERTIES ==>                    #  - simple
                                  #     - catches one digit errors
                                  #     - but not more complex errors
                                  #  - can padd 0 to left without changing checksum