Luhn test of credit card numbers

# title: Luhn test of credit card numbers
# source: https://rosettacode.org/wiki/Luhn_test_of_credit_card_numbers
# category: Rosetta Code
# difficulty: Easy
# summary: Validate a small deterministic set of account numbers with the Luhn checksum.
# tags: popular, checksum, strings, math
# vibe: 0.2

def digit_value(char)
  "0123456789".index(char)
end

def luhn_valid?(number)
  sum = 0
  offset = 0
  index = number.length - 1

  while index >= 0
    digit = digit_value(number.slice(index))

    if offset % 2 == 1
      digit = digit * 2
      if digit > 9
        digit = digit - 9
      end
    end

    sum = sum + digit
    offset = offset + 1
    index = index - 1
  end

  sum % 10 == 0
end

def run
  {
    sample_true: luhn_valid?("49927398716"),
    sample_false: luhn_valid?("49927398717"),
    visa_true: luhn_valid?("4012888888881881"),
    visa_false: luhn_valid?("4012888888881882")
  }
end