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Luhn checksum/check digit generation in Ruby.
class Luhn
def self.checksum(number)
digits = number.to_s.reverse.scan(/\d/).map { |x| x.to_i }
digits = digits.each_with_index.map { |d, i|
d *= 2 if i.even?
d > 9 ? d - 9 : d
}
sum = digits.inject(0) { |m, x| m + x }
mod = 10 - sum % 10
mod==10 ? 0 : mod
end
end
# Luhn.checksum(123) # => 0
# Luhn.checksum(456) # => 4
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