Compute the prime factors of a number

primefactors.rb

class PrimeFactors
  def self.of(n)
    factors(n).compact.reject { |x| !is_prime?(x) }
  end

  def self.factors(n)
    return [1] if n == 1
    (1..n / 2).map { |divisor| (n % divisor) == 0 ? divisor : nil }
  end

  def self.is_prime?(n)
    factors(n).compact == [1]
  end
end

number = ARGV.shift.to_i
puts "Prime factors of #{number} are #{PrimeFactors.of(number).inspect}"