Convert Salesforce 15 digit id to 18

converter.rb

class InvalidIdError < RuntimeError
end

def convert_to_18(id)
  if id.nil?
    return nil
  elsif id.size != 15
    puts "INVALID ID: #{id}"
    raise InvalidIdError
  end
  suffix = ''

  3.times do |i|
    flags = 0
    # 3 chunks of 5 chars
    5.times do |j|
      c = id[(i * 5) + j]
      # add flag if c is uppercase
      if /[[:upper:]]/.match(c)
        flags += (1 << j)
      end
    end
    suffix += ['A'..'Z', '0'..'5'].map(&:to_a).flatten[flags]
  end
  id + suffix
end