Problem 3 on Project Euler

p3.rb

class Numeric
  def divisible_by? num
    self % num == 0
  end

  def prime?
    not (2..self.sqrt.floor).any? {|i| self.divisible_by? i}
  end

  def sqrt
    Math.sqrt self
  end

  def primes
    ret = []
    self.sqrt.ceil.step(1, -2) do |test|
      if self.divisible_by? test
        multiple = self / test
        ret << multiple if multiple.prime?
        ret << test if test.prime?
      end
    end
    ret
  end
end

puts 600851475143.primes.max