ruby 26 lines · 6 steps

Validating credit card numbers with Luhn

A Ruby module that sanitizes a card number, checks its shape, and runs the Luhn checksum.

Explained by highlit

Intermediate checksum luhn-algorithm validation module-function

1module CreditCard
module_function
3 
def valid?(number)
  digits = number.to_s.gsub(/[\s-]/, "")
  return false unless digits.match?(/\A\d{13,19}\z/)
7 
  luhn_valid?(digits)
end
10 
def luhn_valid?(digits)
  sum = digits
        .chars
        .map(&:to_i)
        .reverse
        .each_with_index
        .sum { |digit, index| index.odd? ? double(digit) : digit }
18 
  (sum % 10).zero?
end
21 
def double(digit)
  doubled = digit * 2
  doubled > 9 ? doubled - 9 : doubled
end
26end

01 / 01

STEP 01

Walkthrough

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Three takeaways

1Normalizing and shape-checking input before the real algorithm keeps the core logic clean.
2The Luhn checksum doubles every second digit from the right and folds sums over 9 back into a single digit.
3Breaking a rule into small named methods makes each numeric step self-documenting.

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