A predicate method that determines whether a number is prime. Useful for some of Project Euler's challenges.

prime.rb

class Integer
  def prime?
    if self.even? && self != 2 ||
      self.send(:sum_of_digits_div_by_three?) && self != 3 ||
        self.send(:last_two_digits_div_by_four?) ||
          self.send(:last_digit_div_by_five?) && self != 5
      return false
    else
      return true
    end
  end

  private

  def sum_of_digits_div_by_three?
    sum = 0
    self.to_s.split('').each do |digit|
      sum += digit.to_i
    end
    sum % 3 == 0 ? true : false
  end

  def last_two_digits_div_by_four?
    number = ''
    digits = self.to_s.split('')
    if digits.length >= 2
      number << digits[-2] << digits[-1]
      number.to_i % 4 == 0 ? true : false
    else
      false
    end
  end

  def last_digit_div_by_five?
    digits = self.to_s.split('')
    digits[-1].to_i == 0 || digits[-1].to_i == 5 ? true : false
  end
end