gistfile1.rb

def compute_check_digit(str)
  str = str.strip.upcase
  values = str.chars.map do |char|
    case char
    when '<'
      0
    when 'A'..'Z'
      char.ord - 65 + 10
    when '0'..'9'
      char.ord - 48
    else
      raise "Unexpected character '#{char}'"
    end
  end

  values.zip([7,3,1].cycle).map{|(v,w)| v * w}.reduce(:+) % 10
end