validate USPS tracking numbers in Ruby

gistfile1.rb

def valid(tracking_number)
  chars = tracking_number.chars.to_a
  expected_check_digit = chars.pop.to_i

  total = 0
  chars.reverse.each_with_index do |digit, index|
    if index.even?
      total += digit.to_i * 3
    else
      total += digit.to_i
    end
  end

  check_digit = total % 10
  unless check_digit.zero?
    check_digit = 10 - check_digit
  end

  expected_check_digit == check_digit
end