Ruby method that implements luhn algorithm to validate a number, used by many credit card companies.

luhn_validator.rb

#!/usr/bin/env ruby
# Save as file, e.g., luhn_validator.rb, make executable, then: ./luhn_validator.rb 'number'

DIGIT_REDUX = [0, 2, 4, 6, 8, 1, 3, 5, 7, 9] # simple lookup instead of doubling and summing if > 9

def valid_luhn_number?(number)
  digits = number.to_s.split(//).map{|d| d.to_i}
  digits.reverse!.each_index{|i| digits[i] = DIGIT_REDUX[digits[i]] if i.odd?}.reduce(:+) % 10 == 0
end

number = ARGV[0]
puts "#{number} is #{'not ' unless valid_luhn_number?(number)}valid."