Validate swedish personal identity number

validate_personal_identity_number.rb

def validate_personal_identity_number(number)
  multiplier = 2
  sum = 0

  number[0..8].each_char do |digit|
    x = multiplier * digit.to_i

    if x > 9
      sum += x % 10 + 1
    else
      sum += x
    end

    multiplier = multiplier == 1 ? 2 : 1
  end

  ((sum.to_f / 10).ceil * 10 - sum) % 10 == number[9].to_i
end