prime_factors.rb

prime_factors.rb

class PrimeFactors

  attr_accessor :n, :primes

  def initialize n
    self.n = n
    self.primes = prime_divisors
  end

  def self.for n
    return [] if n == 1
    divisors = new(n).primes
    prime_factors divisors, n
  end

  def self.prime_factors divisors, num
    return [] if num == 1
    return [num] if divisors.empty?
    if num % divisors[0] == 0
      [divisors[0]] + prime_factors(divisors, num / divisors[0])
    else
      prime_factors(divisors[1..divisors.size], num)
    end
  end

  private

  def prime_divisors
    divisors = *(2..(n ** 0.5).ceil.to_i)
    divisors.each do |x|
      divisors.delete_if { |y| x != y && y % x == 0 }
    end
    divisors
  end

end